SoX(1) Sound eXchange SoX(1)NAME
SoX - Sound eXchange, the Swiss Army knife of audio manipulation
SYNOPSISsox [global-options] [format-options] infile1
[[format-options] infile2] ... [format-options] outfile
[effect [effect-options]] ...
play [global-options] [format-options] infile1
[[format-options] infile2] ... [format-options]
[effect [effect-options]] ...
rec [global-options] [format-options] outfile
[effect [effect-options]] ...
DESCRIPTION
Introduction
SoX reads and writes audio files in most popular formats and can
optionally apply effects to them. It can combine multiple input
sources, synthesise audio, and, on many systems, act as a general pur‐
pose audio player or a multi-track audio recorder. It also has limited
ability to split the input into multiple output files.
All SoX functionality is available using just the sox command. To sim‐
plify playing and recording audio, if SoX is invoked as play, the out‐
put file is automatically set to be the default sound device, and if
invoked as rec, the default sound device is used as an input source.
Additionally, the soxi(1) command provides a convenient way to just
query audio file header information.
The heart of SoX is a library called libSoX. Those interested in
extending SoX or using it in other programs should refer to the libSoX
manual page: libsox(3).
SoX is a command-line audio processing tool, particularly suited to
making quick, simple edits and to batch processing. If you need an
interactive, graphical audio editor, use audacity(1).
* * *
The overall SoX processing chain can be summarised as follows:
Input(s) → Combiner → Effects → Output(s)
Note however, that on the SoX command line, the positions of the Out‐
put(s) and the Effects are swapped w.r.t. the logical flow just shown.
Note also that whilst options pertaining to files are placed before
their respective file name, the opposite is true for effects. To show
how this works in practice, here is a selection of examples of how SoX
might be used. The simple
sox recital.au recital.wav
translates an audio file in Sun AU format to a Microsoft WAV file,
whilst
sox recital.au -b 16 recital.wav channels 1 rate 16k fade 3 norm
performs the same format translation, but also applies four effects
(down-mix to one channel, sample rate change, fade-in, nomalize), and
stores the result at a bit-depth of 16.
sox-r 16k -e signed -b 8 -c 1 voice-memo.raw voice-memo.wav
converts `raw' (a.k.a. `headerless') audio to a self-describing file
format,
sox slow.aiff fixed.aiff speed 1.027
adjusts audio speed,
sox short.wav long.wav longer.wav
concatenates two audio files, and
sox-m music.mp3 voice.wav mixed.flac
mixes together two audio files.
play "The Moonbeams/Greatest/*.ogg" bass +3
plays a collection of audio files whilst applying a bass boosting
effect,
play -n -c1 synth sin %-12 sin %-9 sin %-5 sin %-2 fade h 0.1 1 0.1
plays a synthesised `A minor seventh' chord with a pipe-organ sound,
rec -c 2 radio.aiff trim 0 30:00
records half an hour of stereo audio, and
play -q take1.aiff & rec -M take1.aiff take1-dub.aiff
(with POSIX shell and where supported by hardware) records a new track
in a multi-track recording. Finally,
rec -r 44100 -b 16 -s -p silence 1 0.50 0.1% 1 10:00 0.1% | \
sox-p song.ogg silence 1 0.50 0.1% 1 2.0 0.1% : \
newfile : restart
records a stream of audio such as LP/cassette and splits in to multiple
audio files at points with 2 seconds of silence. Also, it does not
start recording until it detects audio is playing and stops after it
sees 10 minutes of silence.
N.B. The above is just an overview of SoX's capabilities; detailed
explanations of how to use all SoX parameters, file formats, and
effects can be found below in this manual, in soxformat(7), and in
soxi(1).
File Format Types
SoX can work with `self-describing' and `raw' audio files. `self-
describing' formats (e.g. WAV, FLAC, MP3) have a header that completely
describes the signal and encoding attributes of the audio data that
follows. `raw' or `headerless' formats do not contain this information,
so the audio characteristics of these must be described on the SoX com‐
mand line or inferred from those of the input file.
The following four characteristics are used to describe the format of
audio data such that it can be processed with SoX:
sample rate
The sample rate in samples per second (`Hertz' or `Hz'). Digi‐
tal telephony traditionally uses a sample rate of 8000 Hz
(8 kHz), though these days, 16 and even 32 kHz are becoming more
common. Audio Compact Discs use 44100 Hz (44.1 kHz). Digital
Audio Tape and many computer systems use 48 kHz. Professional
audio systems often use 96 kHz.
sample size
The number of bits used to store each sample. Today, 16-bit is
commonly used. 8-bit was popular in the early days of computer
audio. 24-bit is used in the professional audio arena. Other
sizes are also used.
data encoding
The way in which each audio sample is represented (or
`encoded'). Some encodings have variants with different byte-
orderings or bit-orderings. Some compress the audio data so
that the stored audio data takes up less space (i.e. disk space
or transmission bandwidth) than the other format parameters and
the number of samples would imply. Commonly-used encoding types
include floating-point, μ-law, ADPCM, signed-integer PCM, MP3,
and FLAC.
channels
The number of audio channels contained in the file. One
(`mono') and two (`stereo') are widely used. `Surround sound'
audio typically contains six or more channels.
The term `bit-rate' is a measure of the amount of storage occupied by
an encoded audio signal over a unit of time. It can depend on all of
the above and is typically denoted as a number of kilo-bits per second
(kbps). An A-law telephony signal has a bit-rate of 64 kbps.
MP3-encoded stereo music typically has a bit-rate of 128-196 kbps.
FLAC-encoded stereo music typically has a bit-rate of 550-760 kbps.
Most self-describing formats also allow textual `comments' to be embed‐
ded in the file that can be used to describe the audio in some way,
e.g. for music, the title, the author, etc.
One important use of audio file comments is to convey `Replay Gain'
information. SoX supports applying Replay Gain information, but not
generating it. Note that by default, SoX copies input file comments to
output files that support comments, so output files may contain Replay
Gain information if some was present in the input file. In this case,
if anything other than a simple format conversion was performed then
the output file Replay Gain information is likely to be incorrect and
so should be recalculated using a tool that supports this (not SoX).
The soxi(1) command can be used to display information from audio file
headers.
Determining & Setting The File Format
There are several mechanisms available for SoX to use to determine or
set the format characteristics of an audio file. Depending on the cir‐
cumstances, individual characteristics may be determined or set using
different mechanisms.
To determine the format of an input file, SoX will use, in order of
precedence and as given or available:
1. Command-line format options.
2. The contents of the file header.
3. The filename extension.
To set the output file format, SoX will use, in order of precedence and
as given or available:
1. Command-line format options.
2. The filename extension.
3. The input file format characteristics, or the closest that is sup‐
ported by the output file type.
For all files, SoX will exit with an error if the file type cannot be
determined. Command-line format options may need to be added or changed
to resolve the problem.
Playing & Recording Audio
The play and rec commands are provided so that basic playing and
recording is as simple as
play existing-file.wav
and
rec new-file.wav
These two commands are functionally equivalent to
sox existing-file.wav -d
and
sox-d new-file.wav
Of course, further options and effects (as described below) can be
added to the commands in either form.
* * *
Some systems provide more than one type of (SoX-compatible) audio
driver, e.g. ALSA & OSS, or SUNAU & AO. Systems can also have more
than one audio device (a.k.a. `sound card'). If more than one audio
driver has been built-in to SoX, and the default selected by SoX when
recording or playing is not the one that is wanted, then the AUDIO‐
DRIVER environment variable can be used to override the default. For
example (on many systems):
set AUDIODRIVER=oss
play ...
The AUDIODEV environment variable can be used to override the default
audio device, e.g.
set AUDIODEV=/dev/dsp2
play ...
sox ... -t oss
or
set AUDIODEV=hw:soundwave,1,2
play ...
sox ... -t alsa
Note that the way of setting environment variables varies from system
to system - for some specific examples, see `SOX_OPTS' below.
When playing a file with a sample rate that is not supported by the
audio output device, SoX will automatically invoke the rate effect to
perform the necessary sample rate conversion. For compatibility with
old hardware, the default rate quality level is set to `low'. This can
be changed by explicitly specifying the rate effect with a different
quality level, e.g.
play ... rate -m
or by using the --play-rate-arg option (see below).
* * *
On some systems, SoX allows audio playback volume to be adjusted whilst
using play. Where supported, this is achieved by tapping the `v' & `V'
keys during playback.
To help with setting a suitable recording level, SoX includes a peak-
level meter which can be invoked (before making the actual recording)
as follows:
rec -n
The recording level should be adjusted (using the system-provided mixer
program, not SoX) so that the meter is at most occasionally full scale,
and never `in the red' (an exclamation mark is shown). See also -S
below.
Accuracy
Many file formats that compress audio discard some of the audio signal
information whilst doing so. Converting to such a format and then con‐
verting back again will not produce an exact copy of the original
audio. This is the case for many formats used in telephony (e.g. A-
law, GSM) where low signal bandwidth is more important than high audio
fidelity, and for many formats used in portable music players (e.g.
MP3, Vorbis) where adequate fidelity can be retained even with the
large compression ratios that are needed to make portable players prac‐
tical.
Formats that discard audio signal information are called `lossy'. For‐
mats that do not are called `lossless'. The term `quality' is used as
a measure of how closely the original audio signal can be reproduced
when using a lossy format.
Audio file conversion with SoX is lossless when it can be, i.e. when
not using lossy compression, when not reducing the sampling rate or
number of channels, and when the number of bits used in the destination
format is not less than in the source format. E.g. converting from an
8-bit PCM format to a 16-bit PCM format is lossless but converting from
an 8-bit PCM format to (8-bit) A-law isn't.
N.B. SoX converts all audio files to an internal uncompressed format
before performing any audio processing. This means that manipulating a
file that is stored in a lossy format can cause further losses in audio
fidelity. E.g. with
sox long.mp3 short.mp3 trim 10
SoX first decompresses the input MP3 file, then applies the trim
effect, and finally creates the output MP3 file by re-compressing the
audio - with a possible reduction in fidelity above that which occurred
when the input file was created. Hence, if what is ultimately desired
is lossily compressed audio, it is highly recommended to perform all
audio processing using lossless file formats and then convert to the
lossy format only at the final stage.
N.B. Applying multiple effects with a single SoX invocation will, in
general, produce more accurate results than those produced using multi‐
ple SoX invocations.
Dithering
Dithering is a technique used to maximise the dynamic range of audio
stored at a particular bit-depth. Any distortion introduced by quanti‐
sation is decorrelated by adding a small amount of white noise to the
signal. In most cases, SoX can determine whether the selected process‐
ing requires dither and will add it during output formatting if appro‐
priate.
Specifically, by default, SoX automatically adds TPDF dither when the
output bit-depth is less than 24 and any of the following are true:
· bit-depth reduction has been specified explicitly using a command-
line option
· the output file format supports only bit-depths lower than that of
the input file format
· an effect has increased effective bit-depth within the internal
processing chain
For example, adjusting volume with vol 0.25 requires two additional
bits in which to losslessly store its results (since 0.25 decimal
equals 0.01 binary). So if the input file bit-depth is 16, then SoX's
internal representation will utilise 18 bits after processing this vol‐
ume change. In order to store the output at the same depth as the
input, dithering is used to remove the additional bits.
Use the -V option to see what processing SoX has automatically added.
The -D option may be given to override automatic dithering. To invoke
dithering manually (e.g. to select a noise-shaping curve), see the
dither effect.
Clipping
Clipping is distortion that occurs when an audio signal level (or `vol‐
ume') exceeds the range of the chosen representation. In most cases,
clipping is undesirable and so should be corrected by adjusting the
level prior to the point (in the processing chain) at which it occurs.
In SoX, clipping could occur, as you might expect, when using the vol
or gain effects to increase the audio volume. Clipping could also occur
with many other effects, when converting one format to another, and
even when simply playing the audio.
Playing an audio file often involves resampling, and processing by ana‐
logue components can introduce a small DC offset and/or amplification,
all of which can produce distortion if the audio signal level was ini‐
tially too close to the clipping point.
For these reasons, it is usual to make sure that an audio file's signal
level has some `headroom', i.e. it does not exceed a particular level
below the maximum possible level for the given representation. Some
standards bodies recommend as much as 9dB headroom, but in most cases,
3dB (≈ 70% linear) is enough. Note that this wisdom seems to have been
lost in modern music production; in fact, many CDs, MP3s, etc. are now
mastered at levels above 0dBFS i.e. the audio is clipped as delivered.
SoX's stat and stats effects can assist in determining the signal level
in an audio file. The gain or vol effect can be used to prevent clip‐
ping, e.g.
sox dull.wav bright.wav gain -6 treble +6
guarantees that the treble boost will not clip.
If clipping occurs at any point during processing, SoX will display a
warning message to that effect.
See also -G and the gain and norm effects.
Input File Combining
SoX's input combiner can be configured (see OPTIONS below) to combine
multiple files using any of the following methods: `concatenate',
`sequence', `mix', `mix-power', `merge', or `multiply'. The default
method is `sequence' for play, and `concatenate' for rec and sox.
For all methods other than `sequence', multiple input files must have
the same sampling rate. If necessary, separate SoX invocations can be
used to make sampling rate adjustments prior to combining.
If the `concatenate' combining method is selected (usually, this will
be by default) then the input files must also have the same number of
channels. The audio from each input will be concatenated in the order
given to form the output file.
The `sequence' combining method is selected automatically for play. It
is similar to `concatenate' in that the audio from each input file is
sent serially to the output file. However, here the output file may be
closed and reopened at the corresponding transition between input
files. This may be just what is needed when sending different types of
audio to an output device, but is not generally useful when the output
is a normal file.
If either the `mix' or `mix-power' combining method is selected then
two or more input files must be given and will be mixed together to
form the output file. The number of channels in each input file need
not be the same, but SoX will issue a warning if they are not and some
channels in the output file will not contain audio from every input
file. A mixed audio file cannot be un-mixed without reference to the
original input files.
If the `merge' combining method is selected then two or more input
files must be given and will be merged together to form the output
file. The number of channels in each input file need not be the same.
A merged audio file comprises all of the channels from all of the input
files. Un-merging is possible using multiple invocations of SoX with
the remix effect. For example, two mono files could be merged to form
one stereo file. The first and second mono files would become the left
and right channels of the stereo file.
The `multiply' combining method multiplies the sample values of corre‐
sponding channels (treated as numbers in the interval -1 to +1). If
the number of channels in the input files is not the same, the missing
channels are considered to contain all zero.
When combining input files, SoX applies any specified effects (includ‐
ing, for example, the vol volume adjustment effect) after the audio has
been combined. However, it is often useful to be able to set the volume
of (i.e. `balance') the inputs individually, before combining takes
place.
For all combining methods, input file volume adjustments can be made
manually using the -v option (below) which can be given for one or more
input files. If it is given for only some of the input files then the
others receive no volume adjustment. In some circumstances, automatic
volume adjustments may be applied (see below).
The -V option (below) can be used to show the input file volume adjust‐
ments that have been selected (either manually or automatically).
There are some special considerations that need to made when mixing
input files:
Unlike the other methods, `mix' combining has the potential to cause
clipping in the combiner if no balancing is performed. In this case,
if manual volume adjustments are not given, SoX will try to ensure that
clipping does not occur by automatically adjusting the volume (ampli‐
tude) of each input signal by a factor of ¹/n, where n is the number of
input files. If this results in audio that is too quiet or otherwise
unbalanced then the input file volumes can be set manually as described
above. Using the norm effect on the mix is another alternative.
If mixed audio seems loud enough at some points but too quiet in others
then dynamic range compression should be applied to correct this - see
the compand effect.
With the `mix-power' combine method, the mixed volume is approximately
equal to that of one of the input signals. This is achieved by balanc‐
ing using a factor of ¹/√n instead of ¹/n. Note that this balancing
factor does not guarantee that clipping will not occur, but the number
of clips will usually be low and the resultant distortion is generally
imperceptible.
Output Files
SoX's default behaviour is to take one or more input files and write
them to a single output file.
This behaviour can be changed by specifying the pseudo-effect `newfile'
within the effects list. SoX will then enter multiple output mode.
In multiple output mode, a new file is created when the effects prior
to the `newfile' indicate they are done. The effects chain listed
after `newfile' is then started up and its output is saved to the new
file.
In multiple output mode, a unique number will automatically be appended
to the end of all filenames. If the filename has an extension then the
number is inserted before the extension. This behaviour can be custom‐
ized by placing a %n anywhere in the filename where the number should
be substituted. An optional number can be placed after the % to indi‐
cate a minimum fixed width for the number.
Multiple output mode is not very useful unless an effect that will stop
the effects chain early is specified before the `newfile'. If end of
file is reached before the effects chain stops itself then no new file
will be created as it would be empty.
The following is an example of splitting the first 60 seconds of an
input file into two 30 second files and ignoring the rest.
sox song.wav ringtone%1n.wav trim 0 30 : newfile : trim 0 30
Stopping SoX
Usually SoX will complete its processing and exit automatically once it
has read all available audio data from the input files.
If desired, it can be terminated earlier by sending an interrupt signal
to the process (usually by pressing the keyboard interrupt key which is
normally Ctrl-C). This is a natural requirement in some circumstances,
e.g. when using SoX to make a recording. Note that when using SoX to
play multiple files, Ctrl-C behaves slightly differently: pressing it
once causes SoX to skip to the next file; pressing it twice in quick
succession causes SoX to exit.
Another option to stop processing early is to use an effect that has a
time period or sample count to determine the stopping point. The trim
effect is an example of this. Once all effects chains have stopped
then SoX will also stop.
FILENAMES
Filenames can be simple file names, absolute or relative path names, or
URLs (input files only). Note that URL support requires that wget(1)
is available.
Note: Giving SoX an input or output filename that is the same as a SoX
effect-name will not work since SoX will treat it as an effect
specification. The only work-around to this is to avoid such
filenames. This is generally not difficult since most audio filenames
have a filename `extension', whilst effect-names do not.
Special Filenames
The following special filenames may be used in certain circumstances in
place of a normal filename on the command line:
- SoX can be used in simple pipeline operations by using the
special filename `-' which, if used as an input filename, will
cause SoX will read audio data from `standard input' (stdin),
and which, if used as the output filename, will cause SoX will
send audio data to `standard output' (stdout). Note that when
using this option for the output file, and sometimes when using
it for an input file, the file-type (see -t below) must also be
given.
"|program [options] ..."
This can be used in place of an input filename to specify the
the given program's standard output (stdout) be used as an input
file. Unlike - (above), this can be used for several inputs to
one SoX command. For example, if `genw' generates mono WAV
formatted signals to its standard output, then the following
command makes a stereo file from two generated signals:
sox-M "|genw --imd -" "|genw --thd -" out.wav
For headerless (raw) audio, -t (and perhaps other format
options) will need to be given, preceding the input command.
"wildcard-filename"
Specifies that filename `globbing' (wild-card matching) should
be performed by SoX instead of by the shell. This allows a sin‐
gle set of file options to be applied to a group of files. For
example, if the current directory contains three `vox' files,
file1.vox, file2.vox, and file3.vox, then
play --rate 6k *.vox
will be expanded by the `shell' (in most environments) to
play --rate 6k file1.vox file2.vox file3.vox
which will treat only the first vox file as having a sample rate
of 6k. With
play --rate 6k "*.vox"
the given sample rate option will be applied to all three vox
files.
-p, --sox-pipe
This can be used in place of an output filename to specify that
the SoX command should be used as in input pipe to another SoX
command. For example, the command:
play "|sox -n -p synth 2" "|sox -n -p synth 2 tremolo 10" stat
plays two `files' in succession, each with different effects.
-p is in fact an alias for `-t sox -'.
-d, --default-device
This can be used in place of an input or output filename to
specify that the default audio device (if one has been built
into SoX) is to be used. This is akin to invoking rec or play
(as described above).
-n, --null
This can be used in place of an input or output filename to
specify that a `null file' is to be used. Note that here, `null
file' refers to a SoX-specific mechanism and is not related to
any operating-system mechanism with a similar name.
Using a null file to input audio is equivalent to using a normal
audio file that contains an infinite amount of silence, and as
such is not generally useful unless used with an effect that
specifies a finite time length (such as trim or synth).
Using a null file to output audio amounts to discarding the
audio and is useful mainly with effects that produce information
about the audio instead of affecting it (such as noiseprof or
stat).
The sampling rate associated with a null file is by default
48 kHz, but, as with a normal file, this can be overridden if
desired using command-line format options (see below).
Supported File & Audio Device Types
See soxformat(7) for a list and description of the supported file for‐
mats and audio device drivers.
OPTIONS
Global Options
These options can be specified on the command line at any point before
the first effect name.
The SOX_OPTS environment variable can be used to provide alternative
default values for SoX's global options. For example:
SOX_OPTS="--buffer 20000 --play-rate-arg -hs --temp /mnt/temp"
Note that setting SOX_OPTS can potentially create unwanted changes in
the behaviour of scripts or other programs that invoke SoX. SOX_OPTS
might best be used for things (such as in the given example) that
reflect the environment in which SoX is being run. Enabling options
such as --no-clobber as default might be handled better using a shell
alias since a shell alias will not affect operation in scripts etc.
One way to ensure that a script cannot be affected by SOX_OPTS is to
clear SOX_OPTS at the start of the script, but this of course loses the
benefit of SOX_OPTS carrying some system-wide default options. An
alternative approach is to explicitly invoke SoX with default option
values, e.g.
SOX_OPTS="-V --no-clobber"
...
sox-V2 --clobber $input $output ...
Note that the way to set environment variables varies from system to
system. Here are some examples:
Unix bash:
export SOX_OPTS="-V --no-clobber"
Unix csh:
setenv SOX_OPTS "-V --no-clobber"
MS-DOS/MS-Windows:
set SOX_OPTS=-V --no-clobber
MS-Windows GUI: via Control Panel : System : Advanced : Environment
Variables
Mac OS X GUI: Refer to Apple's Technical Q&A QA1067 document.
--buffer BYTES, --input-buffer BYTES
Set the size in bytes of the buffers used for processing audio
(default 8192). --buffer applies to input, effects, and output
processing; --input-buffer applies only to input processing (for
which it overrides --buffer if both are given).
Be aware that large values for --buffer will cause SoX to be
become slow to respond to requests to terminate or to skip the
current input file.
--clobber
Don't prompt before overwriting an existing file with the same
name as that given for the output file. This is the default be‐
haviour.
--combine concatenate|merge|mix|mix-power|multiply|sequence
Select the input file combining method; for some of these, short
options are available: -m selects `mix', -M selects `merge', and
-T selects `multiply'.
See Input File Combining above for a description of the differ‐
ent combining methods.
-D, --no-dither
Disable automatic dither - see `Dithering' above. An example of
why this might occasionally be useful is if a file has been con‐
verted from 16 to 24 bit with the intention of doing some pro‐
cessing on it, but in fact no processing is needed after all and
the original 16 bit file has been lost, then, strictly speaking,
no dither is needed if converting the file back to 16 bit. See
also the stats effect for how to determine the actual bit depth
of the audio within a file.
--effects-file FILENAME
Use FILENAME to obtain all effects and their arguments. The
file is parsed as if the values were specified on the command
line. A new line can be used in place of the special : marker
to separate effect chains. For convenience, such markers at the
end of the file are normally ignored; if you want to specify an
empty last effects chain, use an explicit : by itself on the
last line of the file. This option causes any effects specified
on the command line to be discarded.
-G, --guard
Automatically invoke the gain effect to guard against clipping.
E.g.
sox-G infile -b 16 outfile rate 44100 dither -s
is shorthand for
sox infile -b 16 outfile gain -h rate 44100 gain -rh dither -s
See also -V, --norm, and the gain effect.
-h, --help
Show version number and usage information.
--help-effect NAME
Show usage information on the specified effect. The name all
can be used to show usage on all effects.
--help-format NAME
Show information about the specified file format. The name all
can be used to show information on all formats.
--i, --info
Only if given as the first parameter to sox, behave as soxi(1).
-m|-M Equivalent to --combine mix and --combine merge, respectively.
--magic
If SoX has been built with the optional `libmagic' library then
this option can be given to enable its use in helping to detect
audio file types.
--multi-threaded | --single-threaded
By default, SoX is `single threaded'. If the --multi-threaded
option is given however then SoX will process audio channels for
most multi-channel effects in parallel on hyper-threading/multi-
core architectures. This may reduce processing time, though
sometimes it may be necessary to use this option in conjuction
with a larger buffer size than is the default to gain any bene‐
fit from multi-threaded processing (e.g. 131072; see --buffer
above).
--no-clobber
Prompt before overwriting an existing file with the same name as
that given for the output file.
N.B. Unintentionally overwriting a file is easier than you
might think, for example, if you accidentally enter
sox file1 file2 effect1 effect2 ...
when what you really meant was
play file1 file2 effect1 effect2 ...
then, without this option, file2 will be overwritten. Hence,
using this option is recommended. SOX_OPTS (above), a `shell'
alias, script, or batch file may be an appropriate way of perma‐
nently enabling it.
--norm[=dB-level]
Automatically invoke the gain effect to guard against clipping
and to normalise the audio. E.g.
sox--norm infile -b 16 outfile rate 44100 dither -s
is shorthand for
sox infile -b 16 outfile gain -h rate 44100 gain -nh dither -s
Optionally, the audio can be normalized to a given level (usu‐
ally) below 0 dBFS:
sox --norm=-3 infile outfile
See also -V, -G, and the gain effect.
--play-rate-arg ARG
Selects a quality option to be used when the `rate' effect is
automatically invoked whilst playing audio. This option is typ‐
ically set via the SOX_OPTS environment variable (see above).
--plot gnuplot|octave|off
If not set to off (the default if --plot is not given), run in a
mode that can be used, in conjunction with the gnuplot program
or the GNU Octave program, to assist with the selection and con‐
figuration of many of the transfer-function based effects. For
the first given effect that supports the selected plotting pro‐
gram, SoX will output commands to plot the effect's transfer
function, and then exit without actually processing any audio.
E.g.
sox--plot octave input-file -n highpass 1320 > highpass.plt
octave highpass.plt
-q, --no-show-progress
Run in quiet mode when SoX wouldn't otherwise do so. This is
the opposite of the -S option.
-R Run in `repeatable' mode. When this option is given, where
applicable, SoX will embed a fixed time-stamp in the output file
(e.g. AIFF) and will `seed' pseudo random number generators
(e.g. dither) with a fixed number, thus ensuring that succes‐
sive SoX invocations with the same inputs and the same parame‐
ters yield the same output.
--replay-gain track|album|off
Select whether or not to apply replay-gain adjustment to input
files. The default is off for sox and rec, album for play where
(at least) the first two input files are tagged with the same
Artist and Album names, and track for play otherwise.
-S, --show-progress
Display input file format/header information, and processing
progress as input file(s) percentage complete, elapsed time, and
remaining time (if known; shown in brackets), and the number of
samples written to the output file. Also shown is a peak-level
meter, and an indication if clipping has occurred. The peak-
level meter shows up to two channels and is calibrated for digi‐
tal audio as follows (right channel shown):
dB FSD Display dB FSD Display
-25 - -11 ====
-23 = -9 ====-
-21 =- -7 =====
-19 == -5 =====-
-17 ==- -3 ======
-15 === -1 =====!
-13 ===-
A three-second peak-held value of headroom in dBs will be shown
to the right of the meter if this is below 6dB.
This option is enabled by default when using SoX to play or
record audio.
-T Equivalent to --combine multiply.
--temp DIRECTORY
Specify that any temporary files should be created in the given
DIRECTORY. This can be useful if there are permission or free-
space problems with the default location. In this case, using
`--temp .' (to use the current directory) is often a good solu‐
tion.
--version
Show SoX's version number and exit.
-V[level]
Set verbosity. This is particularly useful for seeing how any
automatic effects have been invoked by SoX.
SoX displays messages on the console (stderr) according to the
following verbosity levels:
0 No messages are shown at all; use the exit status to
determine if an error has occurred.
1 Only error messages are shown. These are generated if
SoX cannot complete the requested commands.
2 Warning messages are also shown. These are generated if
SoX can complete the requested commands, but not exactly
according to the requested command parameters, or if
clipping occurs.
3 Descriptions of SoX's processing phases are also shown.
Useful for seeing exactly how SoX is processing your
audio.
4 and above
Messages to help with debugging SoX are also shown.
By default, the verbosity level is set to 2 (shows errors and
warnings). Each occurrence of the -V option increases the ver‐
bosity level by 1. Alternatively, the verbosity level can be
set to an absolute number by specifying it immediately after the
-V, e.g. -V0 sets it to 0.
Input File Options
These options apply only to input files and may precede only input
filenames on the command line.
--ignore-length
Override an (incorrect) audio length given in an audio file's
header. If this option is given then SoX will keep reading audio
until it reaches the end of the input file.
-v, --volume FACTOR
Intended for use when combining multiple input files, this
option adjusts the volume of the file that follows it on the
command line by a factor of FACTOR. This allows it to be `bal‐
anced' w.r.t. the other input files. This is a linear (ampli‐
tude) adjustment, so a number less than 1 decreases the volume
and a number greater than 1 increases it. If a negative number
is given then in addition to the volume adjustment, the audio
signal will be inverted.
See also the norm, vol, and gain effects, and see Input File
Balancing above.
Input & Output File Format Options
These options apply to the input or output file whose name they immedi‐
ately precede on the command line and are used mainly when working with
headerless file formats or when specifying a format for the output file
that is different to that of the input file.
-b BITS, --bits BITS
The number of bits (a.k.a. bit-depth or sometimes word-length)
in each encoded sample. Not applicable to complex encodings
such as MP3 or GSM. Not necessary with encodings that have a
fixed number of bits, e.g. A/μ-law, ADPCM.
For an input file, the most common use for this option is to
inform SoX of the number of bits per sample in a `raw' (`header‐
less') audio file. For example
sox-r 16k -e signed -b 8 input.raw output.wav
converts a particular `raw' file to a self-describing `WAV'
file.
For an output file, this option can be used (perhaps along with
-e) to set the output encoding size. By default (i.e. if this
option is not given), the output encoding size will (providing
it is supported by the output file type) be set to the input
encoding size. For example
sox input.cdda -b 24 output.wav
converts raw CD digital audio (16-bit, signed-integer) to a
24-bit (signed-integer) `WAV' file.
-1/-2/-3/-4/-8
The number of bytes in each encoded sample. Deprecated aliases
for -b 8, -b 16, -b 24, -b 32, -b 64 respectively.
-c CHANNELS, --channels CHANNELS
The number of audio channels in the audio file. This can be any
number greater than zero.
For an input file, the most common use for this option is to
inform SoX of the number of channels in a `raw' (`headerless')
audio file. Occasionally, it may be useful to use this option
with a `headered' file, in order to override the (presumably
incorrect) value in the header - note that this is only sup‐
ported with certain file types. Examples:
sox-r 48k -e float -b 32 -c 2 input.raw output.wav
converts a particular `raw' file to a self-describing `WAV'
file.
play -c 1 music.wav
interprets the file data as belonging to a single channel
regardless of what is indicated in the file header. Note that
if the file does in fact have two channels, this will result in
the file playing at half speed.
For an output file, this option provides a shorthand for speci‐
fying that the channels effect should be invoked in order to
change (if necessary) the number of channels in the audio signal
to the number given. For example, the following two commands
are equivalent:
sox input.wav -c 1 output.wav bass -b 24
sox input.wav output.wav bass -b 24 channels 1
though the second form is more flexible as it allows the effects
to be ordered arbitrarily.
-e ENCODING, --encoding ENCODING
The audio encoding type. Sometimes needed with file-types that
support more than one encoding type. For example, with raw, WAV,
or AU (but not, for example, with MP3 or FLAC). The available
encoding types are as follows:
signed-integer
PCM data stored as signed (`two's complement') integers.
Commonly used with a 16 or 24 -bit encoding size. A
value of 0 represents minimum signal power.
unsigned-integer
PCM data stored as unsigned integers. Commonly used with
an 8-bit encoding size. A value of 0 represents maximum
signal power.
floating-point
PCM data stored as IEEE 753 single precision (32-bit) or
double precision (64-bit) floating-point (`real') num‐
bers. A value of 0 represents minimum signal power.
a-law International telephony standard for logarithmic encoding
to 8 bits per sample. It has a precision equivalent to
roughly 13-bit PCM and is sometimes encoded with reversed
bit-ordering (see the -X option).
u-law, mu-law
North American telephony standard for logarithmic encod‐
ing to 8 bits per sample. A.k.a. μ-law. It has a preci‐
sion equivalent to roughly 14-bit PCM and is sometimes
encoded with reversed bit-ordering (see the -X option).
oki-adpcm
OKI (a.k.a. VOX, Dialogic, or Intel) 4-bit ADPCM; it has
a precision equivalent to roughly 12-bit PCM. ADPCM is a
form of audio compression that has a good compromise
between audio quality and encoding/decoding speed.
ima-adpcm
IMA (a.k.a. DVI) 4-bit ADPCM; it has a precision equiva‐
lent to roughly 13-bit PCM.
ms-adpcm
Microsoft 4-bit ADPCM; it has a precision equivalent to
roughly 14-bit PCM.
gsm-full-rate
GSM is currently used for the vast majority of the
world's digital wireless telephone calls. It utilises
several audio formats with different bit-rates and asso‐
ciated speech quality. SoX has support for GSM's origi‐
nal 13kbps `Full Rate' audio format. It is usually CPU-
intensive to work with GSM audio.
Encoding names can be abbreviated where this would not be
ambiguous; e.g. `unsigned-integer' can be given as `un', but not
`u' (ambiguous with `u-law').
For an input file, the most common use for this option is to
inform SoX of the encoding of a `raw' (`headerless') audio file
(see the examples in -b and -c above).
For an output file, this option can be used (perhaps along with
-b) to set the output encoding type For example
sox input.cdda -e float output1.wav
sox input.cdda -b 64 -e float output2.wav
convert raw CD digital audio (16-bit, signed-integer) to float‐
ing-point `WAV' files (single & double precision respectively).
By default (i.e. if this option is not given), the output encod‐
ing type will (providing it is supported by the output file
type) be set to the input encoding type.
-s/-u/-f/-A/-U/-o/-i/-a/-g
Deprecated aliases for specifying the encoding types signed-
integer, unsigned-integer, floating-point, a-law, mu-law, oki-
adpcm, ima-adpcm, ms-adpcm, gsm-full-rate respectively (see -e
above).
--no-glob
Specifies that filename `globbing' (wild-card matching) should
not be performed by SoX on the following filename. For example,
if the current directory contains the two files `five-sec‐
onds.wav' and `five*.wav', then
play --no-glob "five*.wav"
can be used to play just the single file `five*.wav'.
-r, --rate RATE[k]
Gives the sample rate in Hz (or kHz if appended with `k') of the
file.
For an input file, the most common use for this option is to
inform SoX of the sample rate of a `raw' (`headerless') audio
file (see the examples in -b and -c above). Occasionally it may
be useful to use this option with a `headered' file, in order to
override the (presumably incorrect) value in the header - note
that this is only supported with certain file types. For exam‐
ple, if audio was recorded with a sample-rate of say 48k from a
source that played back a little, say 1.5%, too slowly, then
sox-r 48720 input.wav output.wav
effectively corrects the speed by changing only the file header
(but see also the speed effect for the more usual solution to
this problem).
For an output file, this option provides a shorthand for speci‐
fying that the rate effect should be invoked in order to change
(if necessary) the sample rate of the audio signal to the given
value. For example, the following two commands are equivalent:
sox input.wav -r 48k output.wav bass -b 24
sox input.wav output.wav bass -b 24 rate 48k
though the second form is more flexible as it allows rate
options to be given, and allows the effects to be ordered arbi‐
trarily.
-t, --type FILE-TYPE
Gives the type of the audio file. For both input and output
files, this option is commonly used to inform SoX of the type a
`headerless' audio file (e.g. raw, mp3) where the actual/desired
type cannot be determined from a given filename extension. For
example:
another-command | sox-t mp3 - output.wav
sox input.wav -t raw output.bin
It can also be used to override the type implied by an input
filename extension, but if overriding with a type that has a
header, SoX will exit with an appropriate error message if such
a header is not actually present.
See soxformat(7) for a list of supported file types.
-L, --endian little
-B, --endian big
-x, --endian swap
These options specify whether the byte-order of the audio data
is, respectively, `little endian', `big endian', or the opposite
to that of the system on which SoX is being used. Endianness
applies only to data encoded as floating-pont, or as signed or
unsigned integers of 16 or more bits. It is often necessary to
specify one of these options for headerless files, and sometimes
necessary for (otherwise) self-describing files. A given
endian-setting option may be ignored for an input file whose
header contains a specific endianness identifier, or for an out‐
put file that is actually an audio device.
N.B. Unlike other format characteristics, the endianness (byte,
nibble, & bit ordering) of the input file is not automatically
used for the output file; so, for example, when the following is
run on a little-endian system:
sox-B audio.s16 trimmed.s16 trim 2
trimmed.s16 will be created as little-endian;
sox-B audio.s16 -B trimmed.s16 trim 2
must be used to preserve big-endianness in the output file.
The -V option can be used to check the selected orderings.
-N, --reverse-nibbles
Specifies that the nibble ordering (i.e. the 2 halves of a byte)
of the samples should be reversed; sometimes useful with ADPCM-
based formats.
N.B. See also N.B. in section on -x above.
-X, --reverse-bits
Specifies that the bit ordering of the samples should be
reversed; sometimes useful with a few (mostly headerless) for‐
mats.
N.B. See also N.B. in section on -x above.
Output File Format Options
These options apply only to the output file and may precede only the
output filename on the command line.
--add-comment TEXT
Append a comment in the output file header (where applicable).
--comment TEXT
Specify the comment text to store in the output file header
(where applicable).
SoX will provide a default comment if this option (or --com‐
ment-file) is not given. To specify that no comment should be
stored in the output file, use --comment "" .
--comment-file FILENAME
Specify a file containing the comment text to store in the out‐
put file header (where applicable).
-C, --compression FACTOR
The compression factor for variably compressing output file for‐
mats. If this option is not given then a default compression
factor will apply. The compression factor is interpreted dif‐
ferently for different compressing file formats. See the
description of the file formats that use this option in soxfor‐
mat(7) for more information.
EFFECTS
In addition to converting, playing and recording audio files, SoX can
be used to invoke a number of audio `effects'. Multiple effects may be
applied by specifying them one after another at the end of the SoX com‐
mand line, forming an `effects chain'. Note that applying multiple
effects in real-time (i.e. when playing audio) is likely to require a
high performance computer. Stopping other applications may alleviate
performance issues should they occur.
Some of the SoX effects are primarily intended to be applied to a sin‐
gle instrument or `voice'. To facilitate this, the remix effect and
the global SoX option -M can be used to isolate then recombine tracks
from a multi-track recording.
Multiple Effect Chains
A single effects chain is made up of one or more effects. Audio from
the input runs through the chain until either the end of the input file
is reached or an effect in the chain requests to terminate the chain.
SoX supports running multiple effects chains over the input audio. In
this case, when one chain indicates it is done processing audio, the
audio data is then sent through the next effects chain. This continues
until either no more effects chains exist or the input has reached the
end of the file.
An effects chain is terminated by placing a : (colon) after an effect.
Any following effects are a part of a new effects chain.
It is important to place the effect that will stop the chain as the
first effect in the chain. This is because any samples that are
buffered by effects to the left of the terminating effect will be dis‐
carded. The amount of samples discarded is related to the --buffer
option and it should be kept small, relative to the sample rate, if the
terminating effect cannot be first. Further information on stopping
effects can be found in the Stopping SoX section.
There are a few pseudo-effects that aid using multiple effects chains.
These include newfile which will start writing to a new output file
before moving to the next effects chain and restart which will move
back to the first effects chain. Pseudo-effects must be specified as
the first effect in a chain and as the only effect in a chain (they
must have a : before and after they are specified).
The following is an example of multiple effects chains. It will split
the input file into multiple files of 30 seconds in length. Each out‐
put filename will have unique number in its name as documented in the
Output Files section.
sox infile.wav output.wav trim 0 30 : newfile : restart
Common Notation And Parameters
In the descriptions that follow, brackets [ ] are used to denote param‐
eters that are optional, braces { } to denote those that are both
optional and repeatable, and angle brackets < > to denote those that
are repeatable but not optional. Where applicable, default values for
optional parameters are shown in parenthesis ( ).
The following parameters are used with, and have the same meaning for,
several effects:
centre[k]
See frequency.
frequency[k]
A frequency in Hz, or, if appended with `k', kHz.
gain A power gain in dB. Zero gives no gain; less than zero gives an
attenuation.
width[h|k|o|q]
Used to specify the band-width of a filter. A number of differ‐
ent methods to specify the width are available (though not all
for every effect). One of the characters shown may be appended
to select the desired method as follows:
Method Notes
h Hz
k kHz
o Octaves
q Q-factor See [2]
For each effect that uses this parameter, the default method
(i.e. if no character is appended) is the one that it listed
first in the first line of the effect's description.
To see if SoX has support for an optional effect, enter sox-h and look
for its name under the list: `EFFECTS'.
Supported Effects
Note: a categorised list of the effects can be found in the accompany‐
ing `README' file.
allpass frequency[k] width[h|k|o|q]
Apply a two-pole all-pass filter with central frequency (in Hz)
frequency, and filter-width width. An all-pass filter changes
the audio's frequency to phase relationship without changing its
frequency to amplitude relationship. The filter is described in
detail in [1].
This effect supports the --plot global option.
band [-n] center[k] [width[h|k|o|q]]
Apply a band-pass filter. The frequency response drops loga‐
rithmically around the center frequency. The width parameter
gives the slope of the drop. The frequencies at center + width
and center - width will be half of their original amplitudes.
band defaults to a mode oriented to pitched audio, i.e. voice,
singing, or instrumental music. The -n (for noise) option uses
the alternate mode for un-pitched audio (e.g. percussion).
Warning: -n introduces a power-gain of about 11dB in the filter,
so beware of output clipping. band introduces noise in the
shape of the filter, i.e. peaking at the center frequency and
settling around it.
This effect supports the --plot global option.
See also sinc for a bandpass filter with steeper shoulders.
bandpass|bandreject [-c] frequency[k] width[h|k|o|q]
Apply a two-pole Butterworth band-pass or band-reject filter
with central frequency frequency, and (3dB-point) band-width
width. The -c option applies only to bandpass and selects a
constant skirt gain (peak gain = Q) instead of the default: con‐
stant 0dB peak gain. The filters roll off at 6dB per octave
(20dB per decade) and are described in detail in [1].
These effects support the --plot global option.
See also sinc for a bandpass filter with steeper shoulders.
bandreject frequency[k] width[h|k|o|q]
Apply a band-reject filter. See the description of the bandpass
effect for details.
bass|treble gain [frequency[k] [width[s|h|k|o|q]]]
Boost or cut the bass (lower) or treble (upper) frequencies of
the audio using a two-pole shelving filter with a response simi‐
lar to that of a standard hi-fi's tone-controls. This is also
known as shelving equalisation (EQ).
gain gives the gain at 0 Hz (for bass), or whichever is the
lower of ∼22 kHz and the Nyquist frequency (for treble). Its
useful range is about -20 (for a large cut) to +20 (for a large
boost). Beware of Clipping when using a positive gain.
If desired, the filter can be fine-tuned using the following
optional parameters:
frequency sets the filter's central frequency and so can be used
to extend or reduce the frequency range to be boosted or cut.
The default value is 100 Hz (for bass) or 3 kHz (for treble).
width determines how steep is the filter's shelf transition. In
addition to the common width specification methods described
above, `slope' (the default, or if appended with `s') may be
used. The useful range of `slope' is about 0.3, for a gentle
slope, to 1 (the maximum), for a steep slope; the default value
is 0.5.
The filters are described in detail in [1].
These effects support the --plot global option.
See also equalizer for a peaking equalisation effect.
bend [-fframe-rate(25)] [-oover-sample(16)] { delay,cents,duration }
Changes pitch by specified amounts at specified times. Each
given triple: delay,cents,duration specifies one bend. delay is
the amount of time after the start of the audio stream, or the
end of the previous bend, at which to start bending the pitch;
cents is the number of cents (100 cents = 1 semitone) by which
to bend the pitch, and duration the length of time over which
the pitch will be bent.
The pitch-bending algorithm utilises the Discrete Fourier Trans‐
form (DFT) at a particular frame rate and over-sampling rate.
The -f and -o parameters may be used to adjust these parameters
and thus control the smoothness of the changes in pitch.
For example, an initial tone is generated, then bent three
times, yielding four different notes in total:
play -n synth 2.5 sin 667 gain 1 \
bend .35,180,.25 .15,740,.53 0,-520,.3
Note that the clipping that is produced in this example is
deliberate; to remove it, use gain -5 in place of gain 1.
See also pitch.
biquad b0 b1 b2 a0 a1 a2
Apply a biquad IIR filter with the given coefficients. Where b*
and a* are the numerator and denominator coefficients respec‐
tively.
See http://en.wikipedia.org/wiki/Digital_biquad_filter (where a0
= 1).
This effect supports the --plot global option.
channels CHANNELS
Invoke a simple algorithm to change the number of channels in
the audio signal to the given number CHANNELS: mixing if
decreasing the number of channels or duplicating if increasing
the number of channels.
The channels effect is invoked automatically if SoX's -c option
specifies a number of channels that is different to that of the
input file(s). Alternatively, if this effect is given explic‐
itly, then SoX's -c option need not be given. For example, the
following two commands are equivalent:
sox input.wav -c 1 output.wav bass -b 24
sox input.wav output.wav bass -b 24 channels 1
though the second form is more flexible as it allows the effects
to be ordered arbitrarily.
See also remix for an effect that allows channels to be
mixed/selected arbitrarily.
chorus gain-in gain-out <delay decay speed depth -s|-t>
Add a chorus effect to the audio. This can make a single vocal
sound like a chorus, but can also be applied to instrumentation.
Chorus resembles an echo effect with a short delay, but whereas
with echo the delay is constant, with chorus, it is varied using
sinusoidal or triangular modulation. The modulation depth
defines the range the modulated delay is played before or after
the delay. Hence the delayed sound will sound slower or faster,
that is the delayed sound tuned around the original one, like in
a chorus where some vocals are slightly off key. See [3] for
more discussion of the chorus effect.
Each four-tuple parameter delay/decay/speed/depth gives the
delay in milliseconds and the decay (relative to gain-in) with a
modulation speed in Hz using depth in milliseconds. The modula‐
tion is either sinusoidal (-s) or triangular (-t). Gain-out is
the volume of the output.
A typical delay is around 40ms to 60ms; the modulation speed is
best near 0.25Hz and the modulation depth around 2ms. For exam‐
ple, a single delay:
play guitar1.wav chorus 0.7 0.9 55 0.4 0.25 2 -t
Two delays of the original samples:
play guitar1.wav chorus 0.6 0.9 50 0.4 0.25 2 -t \
60 0.32 0.4 1.3 -s
A fuller sounding chorus (with three additional delays):
play guitar1.wav chorus 0.5 0.9 50 0.4 0.25 2 -t \
60 0.32 0.4 2.3 -t 40 0.3 0.3 1.3 -s
compand attack1,decay1{,attack2,decay2}
[soft-knee-dB:]in-dB1[,out-dB1]{,in-dB2,out-dB2}
[gain [initial-volume-dB [delay]]]
Compand (compress or expand) the dynamic range of the audio.
The attack and decay parameters (in seconds) determine the time
over which the instantaneous level of the input signal is aver‐
aged to determine its volume; attacks refer to increases in vol‐
ume and decays refer to decreases. For most situations, the
attack time (response to the music getting louder) should be
shorter than the decay time because the human ear is more sensi‐
tive to sudden loud music than sudden soft music. Where more
than one pair of attack/decay parameters are specified, each
input channel is companded separately and the number of pairs
must agree with the number of input channels. Typical values
are 0.3,0.8 seconds.
The second parameter is a list of points on the compander's
transfer function specified in dB relative to the maximum possi‐
ble signal amplitude. The input values must be in a strictly
increasing order but the transfer function does not have to be
monotonically rising. If omitted, the value of out-dB1 defaults
to the same value as in-dB1; levels below in-dB1 are not com‐
panded (but may have gain applied to them). The point 0,0 is
assumed but may be overridden (by 0,out-dBn). If the list is
preceded by a soft-knee-dB value, then the points at where adja‐
cent line segments on the transfer function meet will be rounded
by the amount given. Typical values for the transfer function
are 6:-70,-60,-20.
The third (optional) parameter is an additional gain in dB to be
applied at all points on the transfer function and allows easy
adjustment of the overall gain.
The fourth (optional) parameter is an initial level to be
assumed for each channel when companding starts. This permits
the user to supply a nominal level initially, so that, for exam‐
ple, a very large gain is not applied to initial signal levels
before the companding action has begun to operate: it is quite
probable that in such an event, the output would be severely
clipped while the compander gain properly adjusts itself. A
typical value (for audio which is initially quiet) is -90 dB.
The fifth (optional) parameter is a delay in seconds. The input
signal is analysed immediately to control the compander, but it
is delayed before being fed to the volume adjuster. Specifying
a delay approximately equal to the attack/decay times allows the
compander to effectively operate in a `predictive' rather than a
reactive mode. A typical value is 0.2 seconds.
* * *
The following example might be used to make a piece of music
with both quiet and loud passages suitable for listening to in a
noisy environment such as a moving vehicle:
sox asz.wav asz-car.wav compand 0.3,1 6:-70,-60,-20 -5 -90 0.2
The transfer function (`6:-70,...') says that very soft sounds
(below -70dB) will remain unchanged. This will stop the compan‐
der from boosting the volume on `silent' passages such as
between movements. However, sounds in the range -60dB to 0dB
(maximum volume) will be boosted so that the 60dB dynamic range
of the original music will be compressed 3-to-1 into a 20dB
range, which is wide enough to enjoy the music but narrow enough
to get around the road noise. The `6:' selects 6dB soft-knee
companding. The -5 (dB) output gain is needed to avoid clipping
(the number is inexact, and was derived by experimentation).
The -90 (dB) for the initial volume will work fine for a clip
that starts with near silence, and the delay of 0.2 (seconds)
has the effect of causing the compander to react a bit more
quickly to sudden volume changes.
In the next example, compand is being used as a noise-gate for
when the noise is at a lower level than the signal:
play infile compand .1,.2 -inf,-50.1,-inf,-50,-50 0 -90 .1
Here is another noise-gate, this time for when the noise is at a
higher level than the signal (making it, in some ways, similar
to squelch):
play infile compand .1,.1 -45.1,-45,-inf,0,-inf 45 -90 .1
This effect supports the --plot global option (for the transfer
function).
See also mcompand for a multiple-band companding effect.
contrast [enhancement-amount(75)]
Comparable with compression, this effect modifies an audio sig‐
nal to make it sound louder. enhancement-amount controls the
amount of the enhancement and is a number in the range 0-100.
Note that enhancement-amount = 0 still gives a significant con‐
trast enhancement.
See also the compand and mcompand effects.
dcshift shift [limitergain]
Apply a DC shift to the audio. This can be useful to remove a
DC offset (caused perhaps by a hardware problem in the recording
chain) from the audio. The effect of a DC offset is reduced
headroom and hence volume. The stat or stats effect can be used
to determine if a signal has a DC offset.
The given dcshift value is a floating point number in the range
of ±2 that indicates the amount to shift the audio (which is in
the range of ±1).
An optional limitergain can be specified as well. It should
have a value much less than 1 (e.g. 0.05 or 0.02) and is used
only on peaks to prevent clipping.
* * *
An alternative approach to removing a DC offset (albeit with a
short delay) is to use the highpass filter effect at a frequency
of say 10Hz, as illustrated in the following example:
sox-n dc.wav synth 5 sin %0 50
sox dc.wav fixed.wav highpass 10
deemph Apply Compact Disc (IEC 60908) de-emphasis (a treble attenuation
shelving filter).
Pre-emphasis was applied in the mastering of some CDs issued in
the early 1980s. These included many classical music albums, as
well as now sought-after issues of albums by The Beatles, Pink
Floyd and others. Pre-emphasis should be removed at playback
time by a de-emphasis filter in the playback device. However,
not all modern CD players have this filter, and very few PC CD
drives have it; playing pre-emphasised audio without the correct
de-emphasis filter results in audio that sounds harsh and is far
from what its creators intended.
With the deemph effect, it is possible to apply the necessary
de-emphasis to audio that has been extracted from a pre-empha‐
sised CD, and then either burn the de-emphasised audio to a new
CD (which will then play correctly on any CD player), or simply
play the correctly de-emphasised audio files on the PC. For
example:
sox track1.wav track1-deemph.wav deemph
and then burn track1-deemph.wav to CD, or
play track1-deemph.wav
or simply
play track1.wav deemph
The de-emphasis filter is implemented as a biquad; its maximum
deviation from the ideal response is only 0.06dB (up to 20kHz).
This effect supports the --plot global option.
See also the bass and treble shelving equalisation effects.
delay {length}
Delay one or more audio channels. length can specify a time or,
if appended with an `s', a number of samples. Do not specify
both time and samples delays in the same command. For example,
delay 1.5 0 0.5 delays the first channel by 1.5 seconds, the
third channel by 0.5 seconds, and leaves the second channel (and
any other channels that may be present) un-delayed. The follow‐
ing (one long) command plays a chime sound:
play -n synth -j 3 sin %3 sin %-2 sin %-5 sin %-9 \
sin %-14 sin %-21 fade h .01 2 1.5 delay \
1.3 1 .76 .54 .27 remix - fade h 0 2.7 2.5 norm -1
and this plays a guitar chord:
play -n synth pl G2 pl B2 pl D3 pl G3 pl D4 pl G4 \
delay 0 .05 .1 .15 .2 .25 remix - fade 0 4 .1 norm -1
dither [-S|-s|-f filter] [-a] [-p precision]
Apply dithering to the audio. Dithering deliberately adds a
small amount of noise to the signal in order to mask audible
quantization effects that can occur if the output sample size is
less than 24 bits. With no options, this effect will add trian‐
gular (TPDF) white noise. Noise-shaping (only for certain sam‐
ple rates) can be selected with -s. With the -f option, it is
possible to select a particular noise-shaping filter from the
following list: lipshitz, f-weighted, modified-e-weighted,
improved-e-weighted, gesemann, shibata, low-shibata, high-shi‐
bata. Note that most filter types are available only with
44100Hz sample rate. The filter types are distinguished by the
following properties: audibility of noise, level of (inaudible,
but in some circumstances, otherwise problematic) shaped high
frequency noise, and processing speed.
See http://sox.sourceforge.net/SoX/NoiseShaping for graphs of
the different noise-shaping curves.
The -S option selects a slightly `sloped' TPDF, biased towards
higher frequencies. It can be used at any sampling rate but
below ≈22k, plain TPDF is probably better, and above ≈ 37k,
noise-shaped is probably better.
The -a option enables a mode where dithering (and noise-shaping
if applicable) are automatically enabled only when needed. The
most likely use for this is when applying fade in or out to an
already dithered file, so that the redithering applies only to
the faded portions. However, auto dithering is not fool-proof,
so the fades should be carefully checked for any noise modula‐
tion; if this occurs, then either re-dither the whole file, or
use trim, fade, and concatencate.
The -p option allows overriding the target precision.
If the SoX global option -R option is not given, then the
pseudo-random number generator used to generate the white noise
will be `reseeded', i.e. the generated noise will be different
between invocations.
This effect should not be followed by any other effect that
affects the audio.
See also the `Dithering' section above.
downsample [factor(2)]
Downsample the signal by an integer factor: Only the first out
of each factor samples is retained, the others are discarded.
No decimation filter is applied. If the input is not a properly
bandlimited baseband signal, aliasing will occur. This may be
desirable, e.g., for frequency translation.
For a general resampling effect with anti-aliasing, see rate.
See also upsample.
earwax Makes audio easier to listen to on headphones. Adds `cues' to
44.1kHz stereo (i.e. audio CD format) audio so that when lis‐
tened to on headphones the stereo image is moved from inside
your head (standard for headphones) to outside and in front of
the listener (standard for speakers). See http://www.geoci‐
ties.com/beinges for a full explanation.
echo gain-in gain-out <delay decay>
Add echoing to the audio. Echoes are reflected sound and can
occur naturally amongst mountains (and sometimes large build‐
ings) when talking or shouting; digital echo effects emulate
this behaviour and are often used to help fill out the sound of
a single instrument or vocal. The time difference between the
original signal and the reflection is the `delay' (time), and
the loudness of the reflected signal is the `decay'. Multiple
echoes can have different delays and decays.
Each given delay decay pair gives the delay in milliseconds and
the decay (relative to gain-in) of that echo. Gain-out is the
volume of the output. For example: This will make it sound as
if there are twice as many instruments as are actually playing:
play lead.aiff echo 0.8 0.88 60 0.4
If the delay is very short, then it sound like a (metallic) ro‐
bot playing music:
play lead.aiff echo 0.8 0.88 6 0.4
A longer delay will sound like an open air concert in the moun‐
tains:
play lead.aiff echo 0.8 0.9 1000 0.3
One mountain more, and:
play lead.aiff echo 0.8 0.9 1000 0.3 1800 0.25
echos gain-in gain-out <delay decay>
Add a sequence of echoes to the audio. Each delay decay pair
gives the delay in milliseconds and the decay (relative to gain-
in) of that echo. Gain-out is the volume of the output.
Like the echo effect, echos stand for `ECHO in Sequel', that is
the first echos takes the input, the second the input and the
first echos, the third the input and the first and the second
echos, ... and so on. Care should be taken using many echos; a
single echos has the same effect as a single echo.
The sample will be bounced twice in symmetric echos:
play lead.aiff echos 0.8 0.7 700 0.25 700 0.3
The sample will be bounced twice in asymmetric echos:
play lead.aiff echos 0.8 0.7 700 0.25 900 0.3
The sample will sound as if played in a garage:
play lead.aiff echos 0.8 0.7 40 0.25 63 0.3
equalizer frequency[k] width[q|o|h|k] gain
Apply a two-pole peaking equalisation (EQ) filter. With this
filter, the signal-level at and around a selected frequency can
be increased or decreased, whilst (unlike band-pass and band-
reject filters) that at all other frequencies is unchanged.
frequency gives the filter's central frequency in Hz, width, the
band-width, and gain the required gain or attenuation in dB.
Beware of Clipping when using a positive gain.
In order to produce complex equalisation curves, this effect can
be given several times, each with a different central frequency.
The filter is described in detail in [1].
This effect supports the --plot global option.
See also bass and treble for shelving equalisation effects.
fade [type] fade-in-length [stop-time [fade-out-length]]
Apply a fade effect to the beginning, end, or both of the audio.
An optional type can be specified to select the shape of the
fade curve: q for quarter of a sine wave, h for half a sine
wave, t for linear (`triangular') slope, l for logarithmic, and
p for inverted parabola. The default is logarithmic.
A fade-in starts from the first sample and ramps the signal
level from 0 to full volume over fade-in-length seconds. Spec‐
ify 0 seconds if no fade-in is wanted.
For fade-outs, the audio will be truncated at stop-time and the
signal level will be ramped from full volume down to 0 starting
at fade-out-length seconds before the stop-time. If fade-out-
length is not specified, it defaults to the same value as fade-
in-length. No fade-out is performed if stop-time is not speci‐
fied. If the file length can be determined from the input file
header and length-changing effects are not in effect, then 0 may
be specified for stop-time to indicate the usual case of a fade-
out that ends at the end of the input audio stream.
All times can be specified in either periods of time or sample
counts. To specify time periods use the format hh:mm:ss.frac
format. To specify using sample counts, specify the number of
samples and append the letter `s' to the sample count (for exam‐
ple `8000s').
See also the splice effect.
fir [coefs-file|coefs]
Use SoX's FFT convolution engine with given FIR filter coeffi‐
cients. If a single argument is given then this is treated as
the name of a file containing the filter coefficients (white-
space separated; may contain `#' comments). If the given file‐
name is `-', or if no argument is given, then the coefficients
are read from the `standard input' (stdin); otherwise, coeffi‐
cients may be given on the command line. Examples:
sox infile outfile fir 0.0195 -0.082 0.234 0.891 -0.145 0.043
sox infile outfile fir coefs.txt
with coefs.txt containing
# HP filter
# freq=10000
1.2311233052619888e-01
-4.4777096106211783e-01
5.1031563346705155e-01
-6.6502926320995331e-02
...
This effect supports the --plot global option.
flanger [delay depth regen width speed shape phase interp]
Apply a flanging effect to the audio. See [3] for a detailed
description of flanging.
All parameters are optional (right to left).
Range Default Description
delay 0 - 30 0 Base delay in milliseconds.
depth 0 - 10 2 Added swept delay in milliseconds.
regen -95 - 95 0 Percentage regeneration (delayed
signal feedback).
width 0 - 100 71 Percentage of delayed signal mixed
with original.
speed 0.1 - 10 0.5 Sweeps per second (Hz).
shape sin Swept wave shape: sine|triangle.
phase 0 - 100 25 Swept wave percentage phase-shift
for multi-channel (e.g. stereo)
flange; 0 = 100 = same phase on
each channel.
interp lin Digital delay-line interpolation:
linear|quadratic.
gain [-e|-B|-b|-r] [-n] [-l|-h] [gain-dB]
Apply amplification or attenuation to the audio signal, or, in
some cases, to some of its channels. Note that use of any of
-e, -B, -b, -r, or -n requires temporary file space to store the
audio to be processed, so may be unsuitable for use with
`streamed' audio.
Without other options, gain-dB is used to adjust the signal
power level by the given number of dB: positive amplifies
(beware of Clipping), negative attenuates. With other options,
the gain-dB amplification or attenuation is (logically) applied
after the processing due to those options.
Given the -e option, the levels of the audio channels of a
multi-channel file are `equalised', i.e. gain is applied to all
channels other than that with the highest peak level, such that
all channels attain the same peak level (but, without also giv‐
ing -n, the audio is not `normalised').
The -B (balance) option is similar to -e, but with -B, the RMS
level is used instead of the peak level. -B might be used to
correct stereo imbalance caused by an imperfect record turntable
cartridge. Note that unlike -e, -B might cause some clipping.
-b is similar to -B but has clipping protection, i.e. if neces‐
sary to prevent clipping whilst balancing, attenuation is
applied to all channels. Note, however, that in conjunction
with -n, -B and -b are synonymous.
The -r option is used in conjunction with a prior invocation of
gain with the -h option - see below for details.
The -n option normalises the audio to 0dB FSD; it is often used
in conjunction with a negative gain-dB to the effect that the
audio is normalised to a given level below 0dB. For example,
sox infile outfile gain -n
normalises to 0dB, and
sox infile outfile gain -n -3
normalises to -3dB.
The -l option invokes a simple limiter, e.g.
sox infile outfile gain -l 6
will apply 6dB of gain but never clip. Note that limiting more
than a few dBs more than occasionally (in a piece of audio) is
not recommended as it can cause audible distortion. See the
compand effect for a more capable limiter.
The -h option is used to apply gain to provide head-room for
subsequent processing. For example, with
sox infile outfile gain -h bass +6
6dB of attenuation will be applied prior to the bass boosting
effect thus ensuring that it will not clip. Of course, with
bass, it is obvious how much headroom will be needed, but with
other effects (e.g. rate, dither) it is not always as clear.
Another advantage of using gain -h rather than an explicit
attenuation, is that if the headroom is not used by subsequent
effects, it can be reclaimed with gain -r, for example:
sox infile outfile gain -h bass +6 rate 44100 gain -r
The above effects chain guarantees never to clip nor amplify; it
attenuates if necessary to prevent clipping, but by only as much
as is needed to do so.
Output formatting (dithering and bit-depth reduction) also
requires headroom (which cannot be `reclaimed'), e.g.
sox infile outfile gain -h bass +6 rate 44100 gain -rh dither
Here, the second gain invocation, reclaims as much of the head‐
room as it can from the preceding effects, but retains as much
headroom as is needed for subsequent processing. The SoX global
option -G can be given to automatically invoke gain -h and gain
-r.
See also the norm and vol effects.
highpass|lowpass [-1|-2] frequency[k] [width[q|o|h|k]]
Apply a high-pass or low-pass filter with 3dB point frequency.
The filter can be either single-pole (with -1), or double-pole
(the default, or with -2). width applies only to double-pole
filters; the default is Q = 0.707 and gives a Butterworth
response. The filters roll off at 6dB per pole per octave (20dB
per pole per decade). The double-pole filters are described in
detail in [1].
These effects support the --plot global option.
See also sinc for filters with a steeper roll-off.
hilbert [-n taps]
Apply an odd-tap Hilbert transform filter, phase-shifting the
signal by 90 degrees.
This is used in many matrix coding schemes and for analytic sig‐
nal generation. The process is often written as a multiplica‐
tion by i (or j), the imaginary unit.
An odd-tap Hilbert transform filter has a bandpass characteris‐
tic, attenuating the lowest and highest frequencies. Its band‐
width can be controlled by the number of filter taps, which can
be specified with -n. By default, the number of taps is chosen
for a cutoff frequency of about 75 Hz.
This effect supports the --plot global option.
ladspa module [plugin] [argument...]
Apply a LADSPA [5] (Linux Audio Developer's Simple Plugin API)
plugin. Despite the name, LADSPA is not Linux-specific, and a
wide range of effects is available as LADSPA plugins, such as
cmt [6] (the Computer Music Toolkit) and Steve Harris's plugin
collection [7]. The first argument is the plugin module, the
second the name of the plugin (a module can contain more than
one plugin) and any other arguments are for the control ports of
the plugin. Missing arguments are supplied by default values if
possible. Only plugins with at most one audio input and one
audio output port can be used. If found, the environment vari‐
able LADSPA_PATH will be used as search path for plugins.
loudness [gain [reference]]
Loudness control - similar to the gain effect, but provides
equalisation for the human auditory system. See
http://en.wikipedia.org/wiki/Loudness for a detailed description
of loudness. The gain is adjusted by the given gain parameter
(usually negative) and the signal equalised according to ISO 226
w.r.t. a reference level of 65dB, though an alternative refer‐
ence level may be given if the original audio has been equalised
for some other optimal level. A default gain of -10dB is used
if a gain value is not given.
See also the gain effect.
lowpass [-1|-2] frequency[k] [width[q|o|h|k]]
Apply a low-pass filter. See the description of the highpass
effect for details.
mcompand "attack1,decay1{,attack2,decay2}
[soft-knee-dB:]in-dB1[,out-dB1]{,in-dB2,out-dB2}
[gain [initial-volume-dB [delay]]]" {crossover-freq[k]
"attack1,..."}
The multi-band compander is similar to the single-band compander
but the audio is first divided into bands using Linkwitz-Riley
cross-over filters and a separately specifiable compander run on
each band. See the compand effect for the definition of its
parameters. Compand parameters are specified between double
quotes and the crossover frequency for that band is given by
crossover-freq; these can be repeated to create multiple bands.
For example, the following (one long) command shows how multi-
band companding is typically used in FM radio:
play track1.wav gain -3 sinc 8000- 29 100 mcompand \
"0.005,0.1 -47,-40,-34,-34,-17,-33" 100 \
"0.003,0.05 -47,-40,-34,-34,-17,-33" 400 \
"0.000625,0.0125 -47,-40,-34,-34,-15,-33" 1600 \
"0.0001,0.025 -47,-40,-34,-34,-31,-31,-0,-30" 6400 \
"0,0.025 -38,-31,-28,-28,-0,-25" \
gain 15 highpass 22 highpass 22 sinc -n 255 -b 16 -17500 \
gain 9 lowpass -1 17801
The audio file is played with a simulated FM radio sound (or
broadcast signal condition if the lowpass filter at the end is
skipped). Note that the pipeline is set up with US-style 75us
pre-emphasis.
See also compand for a single-band companding effect.
noiseprof [profile-file]
Calculate a profile of the audio for use in noise reduction.
See the description of the noisered effect for details.
noisered [profile-file [amount]]
Reduce noise in the audio signal by profiling and filtering.
This effect is moderately effective at removing consistent back‐
ground noise such as hiss or hum. To use it, first run SoX with
the noiseprof effect on a section of audio that ideally would
contain silence but in fact contains noise - such sections are
typically found at the beginning or the end of a recording.
noiseprof will write out a noise profile to profile-file, or to
stdout if no profile-file or if `-' is given. E.g.
sox speech.wav -n trim 0 1.5 noiseprof speech.noise-profile
To actually remove the noise, run SoX again, this time with the
noisered effect; noisered will reduce noise according to a noise
profile (which was generated by noiseprof), from profile-file,
or from stdin if no profile-file or if `-' is given. E.g.
sox speech.wav cleaned.wav noisered speech.noise-profile 0.3
How much noise should be removed is specified by amount-a number
between 0 and 1 with a default of 0.5. Higher numbers will
remove more noise but present a greater likelihood of removing
wanted components of the audio signal. Before replacing an
original recording with a noise-reduced version, experiment with
different amount values to find the optimal one for your audio;
use headphones to check that you are happy with the results,
paying particular attention to quieter sections of the audio.
On most systems, the two stages - profiling and reduction - can
be combined using a pipe, e.g.
sox noisy.wav -n trim 0 1 noiseprof | play noisy.wav noisered
norm [dB-level]
Normalise the audio. norm is just an alias for gain -n; see the
gain effect for details.
oops Out Of Phase Stereo effect. Mixes stereo to twin-mono where
each mono channel contains the difference between the left and
right stereo channels. This is sometimes known as the `karaoke'
effect as it often has the effect of removing most or all of the
vocals from a recording. It is equivalent to remix 1,2i 1,2i.
overdrive [gain(20) [colour(20)]]
Non linear distortion. The colour parameter controls the amount
of even harmonic content in the over-driven output.
pad { length[@position] }
Pad the audio with silence, at the beginning, the end, or any
specified points through the audio. Both length and position
can specify a time or, if appended with an `s', a number of sam‐
ples. length is the amount of silence to insert and position
the position in the input audio stream at which to insert it.
Any number of lengths and positions may be specified, provided
that a specified position is not less that the previous one.
position is optional for the first and last lengths specified
and if omitted correspond to the beginning and the end of the
audio respectively. For example, pad 1.5 1.5 adds 1.5 seconds
of silence padding at each end of the audio, whilst pad
4000s@3:00 inserts 4000 samples of silence 3 minutes into the
audio. If silence is wanted only at the end of the audio, spec‐
ify either the end position or specify a zero-length pad at the
start.
See also delay for an effect that can add silence at the begin‐
ning of the audio on a channel-by-channel basis.
phaser gain-in gain-out delay decay speed [-s|-t]
Add a phasing effect to the audio. See [3] for a detailed
description of phasing.
delay/decay/speed gives the delay in milliseconds and the decay
(relative to gain-in) with a modulation speed in Hz. The modu‐
lation is either sinusoidal (-s) - preferable for multiple
instruments, or triangular (-t) - gives single instruments a
sharper phasing effect. The decay should be less than 0.5 to
avoid feedback, and usually no less than 0.1. Gain-out is the
volume of the output.
For example:
play snare.flac phaser 0.8 0.74 3 0.4 0.5 -t
Gentler:
play snare.flac phaser 0.9 0.85 4 0.23 1.3 -s
A popular sound:
play snare.flac phaser 0.89 0.85 1 0.24 2 -t
More severe:
play snare.flac phaser 0.6 0.66 3 0.6 2 -t
pitch [-q] shift [segment [search [overlap]]]
Change the audio pitch (but not tempo).
shift gives the pitch shift as positive or negative `cents'
(i.e. 100ths of a semitone). See the tempo effect for a
description of the other parameters.
See also the bend, speed, and tempo effects.
rate [-q|-l|-m|-h|-v] [override-options] RATE[k]
Change the audio sampling rate (i.e. resample the audio) to any
given RATE (even non-integer if this is supported by the output
file format) using a quality level defined as follows:
Quality Band- Rej dB Typical Use
width
-q quick n/a ≈30 @ playback on
Fs/4 ancient hardware
-l low 80% 100 playback on old
hardware
-m medium 95% 100 audio playback
-h high 95% 125 16-bit mastering
(use with dither)
-v very high 95% 175 24-bit mastering
where Band-width is the percentage of the audio frequency band
that is preserved and Rej dB is the level of noise rejection.
Increasing levels of resampling quality come at the expense of
increasing amounts of time to process the audio. If no quality
option is given, the quality level used is `high' (but see
`Playing & Recording Audio' above regarding playback).
The `quick' algorithm uses cubic interpolation; all others use
band-limited interpolation. By default, all algorithms have a
`linear' phase response; for `medium', `high' and `very high',
the phase response is configurable (see below).
The rate effect is invoked automatically if SoX's -r option
specifies a rate that is different to that of the input file(s).
Alternatively, if this effect is given explicitly, then SoX's -r
option need not be given. For example, the following two com‐
mands are equivalent:
sox input.wav -r 48k output.wav bass -b 24
sox input.wav output.wav bass -b 24 rate 48k
though the second command is more flexible as it allows rate
options to be given, and allows the effects to be ordered arbi‐
trarily.
* * *
Warning: technically detailed discussion follows.
The simple quality selection described above provides settings
that satisfy the needs of the vast majority of resampling tasks.
Occasionally, however, it may be desirable to fine-tune the
resampler's filter response; this can be achieved using over‐
ride options, as detailed in the following table:
-M/-I/-L Phase response = minimum/intermediate/linear
-s Steep filter (band-width = 99%)
-a Allow aliasing/imaging above the pass-band
-b 74-99.7 Any band-width %
-p 0-100 Any phase response (0 = minimum, 25 = intermediate,
50 = linear, 100 = maximum)
N.B. Override options cannot be used with the `quick' or `low'
quality algorithms.
All resamplers use filters that can sometimes create `echo'
(a.k.a. `ringing') artefacts with transient signals such as
those that occur with `finger snaps' or other highly percussive
sounds. Such artefacts are much more noticeable to the human
ear if they occur before the transient (`pre-echo') than if they
occur after it (`post-echo'). Note that frequency of any such
artefacts is related to the smaller of the original and new sam‐
pling rates but that if this is at least 44.1kHz, then the arte‐
facts will lie outside the range of human hearing.
A phase response setting may be used to control the distribution
of any transient echo between `pre' and `post': with minimum
phase, there is no pre-echo but the longest post-echo; with lin‐
ear phase, pre and post echo are in equal amounts (in signal
terms, but not audibility terms); the intermediate phase setting
attempts to find the best compromise by selecting a small length
(and level) of pre-echo and a medium lengthed post-echo.
Minimum, intermediate, or linear phase response is selected
using the -M, -I, or -L option; a custom phase response can be
created with the -p option. Note that phase responses between
`linear' and `maximum' (greater than 50) are rarely useful.
A resampler's band-width setting determines how much of the fre‐
quency content of the original signal (w.r.t. the original sam‐
ple rate when up-sampling, or the new sample rate when down-sam‐
pling) is preserved during conversion. The term `pass-band' is
used to refer to all frequencies up to the band-width point
(e.g. for 44.1kHz sampling rate, and a resampling band-width of
95%, the pass-band represents frequencies from 0Hz (D.C.) to
circa 21kHz). Increasing the resampler's band-width results in
a slower conversion and can increase transient echo artefacts
(and vice versa).
The -s `steep filter' option changes resampling band-width from
the default 95% (based on the 3dB point), to 99%. The -b option
allows the band-width to be set to any value in the range
74-99.7 %, but note that band-width values greater than 99% are
not recommended for normal use as they can cause excessive tran‐
sient echo.
If the -a option is given, then aliasing/imaging above the pass-
band is allowed. For example, with 44.1kHz sampling rate, and a
resampling band-width of 95%, this means that frequency content
above 21kHz can be distorted; however, since this is above the
pass-band (i.e. above the highest frequency of interest/audi‐
bility), this may not be a problem. The benefits of allowing
aliasing/imaging are reduced processing time, and reduced (by
almost half) transient echo artefacts. Note that if this option
is given, then the minimum band-width allowable with -b
increases to 85%.
Examples:
sox input.wav -b 16 output.wav rate -s -a 44100 dither -s
default (high) quality resampling; overrides: steep filter,
allow aliasing; to 44.1kHz sample rate; noise-shaped dither to
16-bit WAV file.
sox input.wav -b 24 output.aiff rate -v -I -b 90 48k
very high quality resampling; overrides: intermediate phase,
band-width 90%; to 48k sample rate; store output to 24-bit AIFF
file.
* * *
The pitch and speed effects use the rate effect at their core.
remix [-a|-m|-p] <out-spec>
out-spec = in-spec{,in-spec} | 0
in-spec = [in-chan][-[in-chan2]][vol-spec]
vol-spec = p|i|v[volume]
Select and mix input audio channels into output audio channels.
Each output channel is specified, in turn, by a given out-spec:
a list of contributing input channels and volume specifications.
Note that this effect operates on the audio channels within the
SoX effects processing chain; it should not be confused with the
-m global option (where multiple files are mix-combined before
entering the effects chain).
An out-spec contains comma-separated input channel-numbers and
hyphen-delimited channel-number ranges; alternatively, 0 may be
given to create a silent output channel. For example,
sox input.wav output.wav remix 6 7 8 0
creates an output file with four channels, where channels 1, 2,
and 3 are copies of channels 6, 7, and 8 in the input file, and
channel 4 is silent. Whereas
sox input.wav output.wav remix 1-3,7 3
creates a (somewhat bizarre) stereo output file where the left
channel is a mix-down of input channels 1, 2, 3, and 7, and the
right channel is a copy of input channel 3.
Where a range of channels is specified, the channel numbers to
the left and right of the hyphen are optional and default to 1
and to the number of input channels respectively. Thus
sox input.wav output.wav remix -
performs a mix-down of all input channels to mono.
By default, where an output channel is mixed from multiple (n)
input channels, each input channel will be scaled by a factor of
¹/n. Custom mixing volumes can be set by following a given
input channel or range of input channels with a vol-spec (volume
specification). This is one of the letters p, i, or v, followed
by a volume number, the meaning of which depends on the given
letter and is defined as follows:
Letter Volume number Notes
p power adjust in dB 0 = no change
i power adjust in dB As `p', but invert
the audio
v voltage multiplier 1 = no change, 0.5
≈ 6dB attenuation,
2 ≈ 6dB gain, -1 =
invert
If an out-spec includes at least one vol-spec then, by default,
¹/n scaling is not applied to any other channels in the same
out-spec (though may be in other out-specs). The -a (automatic)
option however, can be given to retain the automatic scaling in
this case. For example,
sox input.wav output.wav remix 1,2 3,4v0.8
results in channel level multipliers of 0.5,0.5 1,0.8, whereas
sox input.wav output.wav remix -a 1,2 3,4v0.8
results in channel level multipliers of 0.5,0.5 0.5,0.8.
The -m (manual) option disables all automatic volume adjust‐
ments, so
sox input.wav output.wav remix -m 1,2 3,4v0.8
results in channel level multipliers of 1,1 1,0.8.
The volume number is optional and omitting it corresponds to no
volume change; however, the only case in which this is useful is
in conjunction with i. For example, if input.wav is stereo,
then
sox input.wav output.wav remix 1,2i
is a mono equivalent of the oops effect.
If the -p option is given, then any automatic ¹/n scaling is
replaced by ¹/√n (`power') scaling; this gives a louder mix but
one that might occasionally clip.
* * *
One use of the remix effect is to split an audio file into a set
of files, each containing one of the constituent channels (in
order to perform subsequent processing on individual audio chan‐
nels). Where more than a few channels are involved, a script
such as the following (Bourne shell script) is useful:
#!/bin/sh
chans=`soxi -c "$1"`
while [ $chans -ge 1 ]; do
chans0=`printf %02i $chans` # 2 digits hence up to 99 chans
out=`echo "$1"|sed "s/\(.*\)\.\(.*\)/\1-$chans0.\2/"`
sox "$1" "$out" remix $chans
chans=`expr $chans - 1`
done
If a file input.wav containing six audio channels were given,
the script would produce six output files: input-01.wav,
input-02.wav, ..., input-06.wav.
See also the swap effect.
repeat [count (1)]
Repeat the entire audio count times, or once if count is not
given. Requires temporary file space to store the audio to be
repeated. Note that repeating once yields two copies: the orig‐
inal audio and the repeated audio.
reverb [-w|--wet-only] [reverberance (50%) [HF-damping (50%)
[room-scale (100%) [stereo-depth (100%)
[pre-delay (0ms) [wet-gain (0dB)]]]]]]
Add reverberation to the audio using the `freeverb' algorithm.
A reverberation effect is sometimes desirable for concert halls
that are too small or contain so many people that the hall's
natural reverberance is diminished. Applying a small amount of
stereo reverb to a (dry) mono signal will usually make it sound
more natural. See [3] for a detailed description of reverbera‐
tion.
Note that this effect increases both the volume and the length
of the audio, so to prevent clipping in these domains, a typical
invocation might be:
play dry.wav gain -3 pad 0 3 reverb
The -w option can be given to select only the `wet' signal, thus
allowing it to be processed further, independently of the `dry'
signal. E.g.
play -m voice.wav "|sox voice.wav -p reverse reverb -w reverse"
for a reverse reverb effect.
reverse
Reverse the audio completely. Requires temporary file space to
store the audio to be reversed.
riaa Apply RIAA vinyl playback equalisation. The sampling rate must
be one of: 44.1, 48, 88.2, 96 kHz.
This effect supports the --plot global option.
silence [-l] above-periods [duration threshold[d|%]
[below-periods duration threshold[d|%]]
Removes silence from the beginning, middle, or end of the audio.
`Silence' is determined by a specified threshold.
The above-periods value is used to indicate if audio should be
trimmed at the beginning of the audio. A value of zero indicates
no silence should be trimmed from the beginning. When specifying
an non-zero above-periods, it trims audio up until it finds non-
silence. Normally, when trimming silence from beginning of audio
the above-periods will be 1 but it can be increased to higher
values to trim all audio up to a specific count of non-silence
periods. For example, if you had an audio file with two songs
that each contained 2 seconds of silence before the song, you
could specify an above-period of 2 to strip out both silence
periods and the first song.
When above-periods is non-zero, you must also specify a duration
and threshold. Duration indications the amount of time that non-
silence must be detected before it stops trimming audio. By
increasing the duration, burst of noise can be treated as
silence and trimmed off.
Threshold is used to indicate what sample value you should treat
as silence. For digital audio, a value of 0 may be fine but for
audio recorded from analog, you may wish to increase the value
to account for background noise.
When optionally trimming silence from the end of the audio, you
specify a below-periods count. In this case, below-period means
to remove all audio after silence is detected. Normally, this
will be a value 1 of but it can be increased to skip over peri‐
ods of silence that are wanted. For example, if you have a song
with 2 seconds of silence in the middle and 2 second at the end,
you could set below-period to a value of 2 to skip over the
silence in the middle of the audio.
For below-periods, duration specifies a period of silence that
must exist before audio is not copied any more. By specifying a
higher duration, silence that is wanted can be left in the
audio. For example, if you have a song with an expected 1 sec‐
ond of silence in the middle and 2 seconds of silence at the
end, a duration of 2 seconds could be used to skip over the mid‐
dle silence.
Unfortunately, you must know the length of the silence at the
end of your audio file to trim off silence reliably. A work
around is to use the silence effect in combination with the
reverse effect. By first reversing the audio, you can use the
above-periods to reliably trim all audio from what looks like
the front of the file. Then reverse the file again to get back
to normal.
To remove silence from the middle of a file, specify a below-
periods that is negative. This value is then treated as a posi‐
tive value and is also used to indicate the effect should
restart processing as specified by the above-periods, making it
suitable for removing periods of silence in the middle of the
audio.
The option -l indicates that below-periods duration length of
audio should be left intact at the beginning of each period of
silence. For example, if you want to remove long pauses between
words but do not want to remove the pauses completely.
The period counts are in units of samples. Duration counts may
be in the format of hh:mm:ss.frac, or the exact count of sam‐
ples. Threshold numbers may be suffixed with d to indicate the
value is in decibels, or % to indicate a percentage of maximum
value of the sample value (0% specifies pure digital silence).
The following example shows how this effect can be used to start
a recording that does not contain the delay at the start which
usually occurs between `pressing the record button' and the
start of the performance:
rec parameters filename other-effects silence 1 5 2%
sinc [-a att|-b beta] [-p phase|-M|-I|-L] [-t tbw|-n taps] [freqHP]
[-freqLP [-t tbw|-n taps]]
Apply a sinc kaiser-windowed low-pass, high-pass, band-pass, or
band-reject filter to the signal. The freqHP and freqLP parame‐
ters give the frequencies of the 6dB points of a high-pass and
low-pass filter that may be invoked individually, or together.
If both are given, then freqHP less than freqLP creates a band-
pass filter, freqHP greater than freqLP creates a band-reject
filter. For example, the invocations
sinc 3k
sinc -4k
sinc 3k-4k
sinc 4k-3k
create a high-pass, low-pass, band-pass, and band-reject filter
respectively.
The default stop-band attenuation of 120dB can be overridden
with -a; alternatively, the kaiser-window `beta' parameter can
be given directly with -b.
The default transition band-width of 5% of the total band can be
overridden with -t (and tbw in Hertz); alternatively, the number
of filter taps can be given directly with -n.
If both freqHP and freqLP are given, then a -t or -n option
given to the left of the frequencies applies to both frequen‐
cies; one of these options given to the right of the frequencies
applies only to freqLP.
The -p, -M, -I, and -L options control the filter's phase
response; see the rate effect for details.
This effect supports the --plot global option.
spectrogram [options]
Create a spectrogram of the audio; the audio is passed unmodi‐
fied through the SoX processing chain. This effect is optional
- type sox--help and check the list of supported effects to see
if it has been included.
The spectrogram is rendered in a Portable Network Graphic (PNG)
file, and shows time in the X-axis, frequency in the Y-axis, and
audio signal magnitude in the Z-axis. Z-axis values are repre‐
sented by the colour (or optionally the intensity) of the pixels
in the X-Y plane. If the audio signal contains multiple chan‐
nels then these are shown from top to bottom starting from chan‐
nel 1 (which is the left channel for stereo audio).
For example, if `my.wav' is a stereo file, then with
sox my.wav -n spectrogram
a spectrogram of the entire file will be created in the file
`spectrogram.png'. More often though, analysis of a smaller
portion of the audio is required; e.g. with
sox my.wav -n remix 2 trim 20 30 spectrogram
the spectrogram shows information only from the second (right)
channel, and of thirty seconds of audio starting from twenty
seconds in. To analyse a small portion of the frequency domain,
the rate effect may be used, e.g.
sox my.wav -n rate 6k spectrogram
allows detailed analysis of frequencies up to 3kHz (half the
sampling rate) i.e. where the human auditory system is most sen‐
sitive. With
sox my.wav -n trim 0 10 spectrogram -x 600 -y 200 -z 100
the given options control the size of the spectrogram's X, Y & Z
axes (in this case, the spectrogram area of the produced image
will be 600 by 200 pixels in size and the Z-axis range will be
100 dB). Note that the produced image includes axes legends
etc. and so will be a little larger than the specified spectro‐
gram size. In this example:
sox-n -n synth 6 tri 10k:14k spectrogram -z 100 -w kaiser
an analysis `window' with high dynamic range is selected to best
display the spectrogram of a swept triangular wave. For a smi‐
lar example, append the following to the `chime' command in the
description of the delay effect (above):
rate 2k spectrogram -X 200 -Z -10 -w kaiser
Options are also avaliable to control the appearance (colour-
set, brightness, contrast, etc.) and filename of the spectro‐
gram; e.g. with
sox my.wav -n spectrogram -m -l -o print.png
a spectrogram is created suitable for printing on a `black and
white' printer.
Options:
-x num Change the (maximum) width (X-axis) of the spectrogram
from its default value of 800 pixels to a given number
between 100 and 5000. See also -X and -d.
-X num X-axis pixels/second; the default is auto-calculated to
fit the given or known audio duration to the X-axis size,
or 100 otherwise. If given in conjunction with -d, this
option affects the width of the spectrogram; otherwise,
it affects the duration of the spectrogram. num can be
from 1 (low time resolution) to 5000 (high time resolu‐
tion) and need not be an integer. SoX may make a slight
adjustment to the given number for processing quantisa‐
tion reasons; if so, SoX will report the actual number
used (viewable when the SoX global option -V is in
effect). See also -x and -d.
-y num Sets the Y-axis size in pixels (per channel); this is the
number of frequency `bins' used in the Fourier analysis
that produces the spectrogram. N.B. it can be slow to
produce the spectrogram if this number is not one more
than a power of two (e.g. 129). By default the Y-axis
size is chosen automatically (depending on the number of
channels). See -Y for alternative way of setting spec‐
trogram height.
-Y num Sets the target total height of the spectrogram(s). The
default value is 550 pixels. Using this option (and by
default), SoX will choose a height for individual spec‐
trogram channels that is one more than a power of two, so
the actual total height may fall short of the given num‐
ber. However, there is also a minimum height per channel
so if there are many channels, the number may be
exceeded. See -y for alternative way of setting spectro‐
gram height.
-z num Z-axis (colour) range in dB, default 120. This sets the
dynamic-range of the spectrogram to be -num dBFS to
0 dBFS. Num may range from 20 to 180. Decreasing
dynamic-range effectively increases the `contrast' of the
spectrogram display, and vice versa.
-Z num Sets the upper limit of the Z-axis in dBFS. A negative
num effectively increases the `brightness' of the spec‐
trogram display, and vice versa.
-q num Sets the Z-axis quantisation, i.e. the number of differ‐
ent colours (or intensities) in which to render Z-axis
values. A small number (e.g. 4) will give a
`poster'-like effect making it easier to discern magni‐
tude bands of similar level. Small numbers also usually
result in small PNG files. The number given specifies
the number of colours to use inside the Z-axis range; two
colours are reserved to represent out-of-range values.
-w name
Window: Hann (default), Hamming, Bartlett, Rectangular or
Kaiser. The spectrogram is produced using the Discrete
Fourier Transform (DFT) algorithm. A significant parame‐
ter to this algorithm is the choice of `window function'.
By default, SoX uses the Hann window which has good all-
round frequency-resolution and dynamic-range properties.
For better frequency resolution (but lower dynamic-
range), select a Hamming window; for higher dynamic-range
(but poorer frequency-resolution), select a Kaiser win‐
dow. Bartlett and Rectangular windows are also avail‐
able.
-W num Window adjustment parameter. This can be used to make
small adjustments to the Kaiser window shape. A positive
number (up to ten) increases its dynamic range, a nega‐
tive number decreases it.
-s Allow slack overlapping of DFT windows. This can, in
some cases, increase image sharpness and give greater
adherence to the -x value, but at the expense of a little
spectral loss.
-m Creates a monochrome spectrogram (the default is colour).
-h Selects a high-colour palette - less visually pleasing
than the default colour palette, but it may make it eas‐
ier to differentiate different levels. If this option is
used in conjunction with -m, the result will be a hybrid
monochrome/colour palette.
-p num Permute the colours in a colour or hybrid palette. The
num parameter, from 1 (the default) to 6, selects the
permutation.
-l Creates a `printer friendly' spectrogram with a light
background (the default has a dark background).
-a Suppress the display of the axis lines. This is some‐
times useful in helping to discern artefacts at the spec‐
trogram edges.
-r Raw spectrogram: suppress the display of axes and leg‐
ends.
-A Selects an alternative, fixed colour-set. This is pro‐
vided only for compatibility with spectrograms produced
by another package. It should not normally be used as it
has some problems, not least, a lack of differentiation
at the bottom end which results in masking of low-level
artefacts.
-t text
Set the image title - text to display above the spectro‐
gram.
-c text
Set (or clear) the image comment - text to display below
and to the left of the spectrogram.
-o text
Name of the spectrogram output PNG file, default `spec‐
trogram.png'.
Advanced Options:
In order to process a smaller section of audio without affecting
other effects or the output signal (unlike when the trim effect
is used), the following options may be used.
-d duration
This option sets the X-axis resolution such that audio
with the given duration ([[HH:]MM:]SS) fits the selected
(or default) X-axis width. For example,
sox input.mp3 output.wav -n spectrogram -d 1:00 stats
creates a spectrogram showing the first minute of the
audio, whilst
the stats effect is applied to the entire audio signal.
See also -X for an alternative way of setting the X-axis
resolution.
-S time
Start the spectrogram at the given point in the audio
stream. For example
sox input.aiff output.wav spectrogram -S 1:00
creates a spectrogram showing all but the first minute of
the audio (the output file however, receives the entire
audio stream).
For the ability to perform off-line processing of spectral data,
see the stat effect.
speed factor[c]
Adjust the audio speed (pitch and tempo together). factor is
either the ratio of the new speed to the old speed: greater than
1 speeds up, less than 1 slows down, or, if appended with the
letter `c', the number of cents (i.e. 100ths of a semitone) by
which the pitch (and tempo) should be adjusted: greater than 0
increases, less than 0 decreases.
By default, the speed change is performed by resampling with the
rate effect using its default quality/speed. For higher quality
or higher speed resampling, in addition to the speed effect,
specify the rate effect with the desired quality option.
See also the bend, pitch, and tempo effects.
splice [-h|-t|-q] { position[,excess[,leeway]] }
Splice together audio sections. This effect provides two things
over simple audio concatenation: a (usually short) cross-fade is
applied at the join, and a wave similarity comparison is made to
help determine the best place at which to make the join.
One of the options -h, -t, or -q may be given to select the fade
envelope as half-cosine wave (the default), triangular (a.k.a.
linear), or quarter-cosine wave respectively.
Type Audio Fade level Transitions
t correlated constant gain abrupt
h correlated constant gain smooth
q uncorrelated constant power smooth
To perform a splice, first use the trim effect to select the
audio sections to be joined together. As when performing a tape
splice, the end of the section to be spliced onto should be
trimmed with a small excess (default 0.005 seconds) of audio
after the ideal joining point. The beginning of the audio sec‐
tion to splice on should be trimmed with the same excess (before
the ideal joining point), plus an additional leeway (default
0.005 seconds). SoX should then be invoked with the two audio
sections as input files and the splice effect given with the
position at which to perform the splice - this is length of the
first audio section (including the excess).
The following diagram uses the tape analogy to illustrate the
splice operation. The effect simulates the diagonal cuts and
joins the two pieces:
length1 excess
-----------><--->
_________ : : _________________
\ : : :\ `
\ : : : \ `
\: : : \ `
* : : * - - *
\ : : :\ `
\ : : : \ `
_______________\: : : \_____`____
: : : :
<---> <----->
excess leeway
where * indicates the joining points.
For example, a long song begins with two verses which start (as
determined e.g. by using the play command with the trim (start)
effect) at times 0:30.125 and 1:03.432. The following commands
cut out the first verse:
sox too-long.wav part1.wav trim 0 30.130
(5 ms excess, after the first verse starts)
sox too-long.wav part2.wav trim 1:03.422
(5 ms excess plus 5 ms leeway, before the second verse starts)
sox part1.wav part2.wav just-right.wav splice 30.130
For another example, the SoX command
play "|sox -n -p synth 1 sin %1" "|sox -n -p synth 1 sin %3"
generates and plays two notes, but there is a nasty click at the
transition; the click can be removed by splicing instead of con‐
catenating the audio, i.e. by appending splice 1 to the command.
(Clicks at the beginning and end of the audio can be removed by
preceding the splice effect with fade q .01 2 .01).
Provided your arithmetic is good enough, multiple splices can be
performed with a single splice invocation. For example:
#!/bin/sh
# Audio Copy and Paste Over
# acpo infile copy-start copy-stop paste-over-start outfile
# All times measured in samples.
rate=`soxi -r "$1"`
e=`expr $rate '*' 5 / 1000` # Using default excess
l=$e # and leeway.
sox "$1" piece.wav trim `expr $2 - $e - $l`s \
`expr $3 - $2 + $e + $l + $e`s
sox "$1" part1.wav trim 0 `expr $4 + $e`s
sox "$1" part2.wav trim `expr $4 + $3 - $2 - $e - $l`s
sox part1.wav piece.wav part2.wav "$5" splice \
`expr $4 + $e`s \
`expr $4 + $e + $3 - $2 + $e + $l + $e`s
In the above Bourne shell script, two splices are used to `copy
and paste' audio.
* * *
It is also possible to use this effect to perform general cross-
fades, e.g. to join two songs. In this case, excess would typi‐
cally be an number of seconds, the -q option would typically be
given (to select an `equal power' cross-fade), and leeway should
be zero (which is the default if -q is given). For example, if
f1.wav and f2.wav are audio files to be cross-faded, then
sox f1.wav f2.wav out.wav splice -q $(soxi -D f1.wav),3
cross-fades the files where the point of equal loudness is 3
seconds before the end of f1.wav, i.e. the total length of the
cross-fade is 2 × 3 = 6 seconds (Note: the $(...) notation is
POSIX shell).
stat [-s scale] [-rms] [-freq] [-v] [-d]
Display time and frequency domain statistical information about
the audio. Audio is passed unmodified through the SoX process‐
ing chain.
The information is output to the `standard error' (stderr)
stream and is calculated, where n is the duration of the audio
in samples, c is the number of audio channels, r is the audio
sample rate, and xk represents the PCM value (in the range -1 to
+1 by default) of each successive sample in the audio, as fol‐
lows:
Samples read n×c
Length (seconds) n÷r
Scaled by See -s below.
Maximum amplitude max(xk) The maximum sample
value in the audio;
usually this will
be a positive num‐
ber.
Minimum amplitude min(xk) The minimum sample
value in the audio;
usually this will
be a negative num‐
ber.
Midline amplitude ½min(xk)+½max(xk)
Mean norm ¹/nΣ│xk│ The average of the
absolute value of
each sample in the
audio.
Mean amplitude ¹/nΣxk The average of each
sample in the
audio. If this
figure is non-zero,
then it indicates
the presence of a
D.C. offset (which
could be removed
using the dcshift
effect).
RMS amplitude √(¹/nΣxk²) The level of a D.C.
signal that would
have the same power
as the audio's
average power.
Maximum delta max(│xk-xk-1│)
Minimum delta min(│xk-xk-1│)
Mean delta ¹/n-1Σ│xk-xk-1│
RMS delta √(¹/n-1Σ(xk-xk-1)²)
Rough frequency In Hz.
Volume Adjustment The parameter to
the vol effect
which would make
the audio as loud
as possible without
clipping. Note:
See the discussion
on Clipping above
for reasons why it
is rarely a good
idea actually to do
this.
Note that the delta measurements are not applicable for multi-
channel audio.
The -s option can be used to scale the input data by a given
factor. The default value of scale is 2147483647 (i.e. the max‐
imum value of a 32-bit signed integer). Internal effects always
work with signed long PCM data and so the value should relate to
this fact.
The -rms option will convert all output average values to `root
mean square' format.
The -v option displays only the `Volume Adjustment' value.
The -freq option calculates the input's power spectrum (4096
point DFT) instead of the statistics listed above. This should
only be used with a single channel audio file.
The -d option displays a hex dump of the 32-bit signed PCM data
audio in SoX's internal buffer. This is mainly used to help
track down endian problems that sometimes occur in cross-plat‐
form versions of SoX.
See also the stats effect.
stats [-b bits|-x bits|-s scale] [-w window-time]
Display time domain statistical information about the audio
channels; audio is passed unmodified through the SoX processing
chain. Statistics are calculated and displayed for each audio
channel and, where applicable, an overall figure is also given.
For example, for a typical well-mastered stereo music file:
Overall Left Right
DC offset 0.000803 -0.000391 0.000803
Min level -0.750977 -0.750977 -0.653412
Max level 0.708801 0.708801 0.653534
Pk lev dB -2.49-2.49-3.69
RMS lev dB -19.41-19.13-19.71
RMS Pk dB -13.82-13.82-14.38
RMS Tr dB -85.25-85.25-82.66
Crest factor - 6.79 6.32
Flat factor 0.00 0.00 0.00
Pk count 2 2 2
Bit-depth 16/16 16/16 16/16
Num samples 7.72M
Length s 174.973
Scale max 1.000000
Window s 0.050
DC offset, Min level, and Max level are shown, by default, in
the range ±1. If the -b (bits) options is given, then these
three measurements will be scaled to a signed integer with the
given number of bits; for example, for 16 bits, the scale would
be -32768 to +32767. The -x option behaves the same way as -b
except that the signed integer values are displayed in hexadeci‐
mal. The -s option scales the three measurements by a given
floating-point number.
Pk lev dB and RMS lev dB are standard peak and RMS level mea‐
sured in dBFS. RMS Pk dB and RMS Tr dB are peak and trough val‐
ues for RMS level measured over a short window (default 50ms).
Crest factor is the standard ratio of peak to RMS level (note:
not in dB).
Flat factor is a measure of the flatness (i.e. consecutive sam‐
ples with the same value) of the signal at its peak levels (i.e.
either Min level, or Max level). Pk count is the number of
occasions (not the number of samples) that the signal attained
either Min level, or Max level.
The right-hand Bit-depth figure is the standard definition of
bit-depth i.e. bits less significant than the given number are
fixed at zero. The left-hand figure is the number of most sig‐
nificant bits that are fixed at zero (or one for negative num‐
bers) subtracted from the right-hand figure (the number sub‐
tracted is directly related to Pk lev dB).
For multi-channel audio, an overall figure for each of the above
measurements is given and derived from the channel figures as
follows: DC offset: maximum magnitude; Max level, Pk lev dB,
RMS Pk dB, Bit-depth: maximum; Min level, RMS Tr dB: minimum;
RMS lev dB, Flat factor, Pk count: average; Crest factor: not
applicable.
Length s is the duration in seconds of the audio, and Num sam‐
ples is equal to the sample-rate multiplied by Length.
Scale Max is the scaling applied to the first three measure‐
ments; specifically, it is the maximum value that could apply to
Max level. Window s is the length of the window used for the
peak and trough RMS measurements.
See also the stat effect.
swap Swap stereo channels. See also remix for an effect that allows
arbitrary channel selection and ordering (and mixing).
stretch factor [window fade shift fading]
Change the audio duration (but not its pitch). This effect is
broadly equivalent to the tempo effect with (factor inverted
and) search set to zero, so in general, its results are compara‐
tively poor; it is retained as it can sometimes out-perform
tempo for small factors.
factor of stretching: >1 lengthen, <1 shorten duration. window
size is in ms. Default is 20ms. The fade option, can be `lin'.
shift ratio, in [0 1]. Default depends on stretch factor. 1 to
shorten, 0.8 to lengthen. The fading ratio, in [0 0.5]. The
amount of a fade's default depends on factor and shift.
See also the tempo effect.
synth [-j KEY] [-n] [len [off [ph [p1 [p2 [p3]]]]]] {[type] [combine]
[[%]freq[k][:|+|/|-[%]freq2[k]]] [off [ph [p1 [p2 [p3]]]]]}
This effect can be used to generate fixed or swept frequency
audio tones with various wave shapes, or to generate wide-band
noise of various `colours'. Multiple synth effects can be cas‐
caded to produce more complex waveforms; at each stage it is
possible to choose whether the generated waveform will be mixed
with, or modulated onto the output from the previous stage.
Audio for each channel in a multi-channel audio file can be syn‐
thesised independently.
Though this effect is used to generate audio, an input file must
still be given, the characteristics of which will be used to set
the synthesised audio length, the number of channels, and the
sampling rate; however, since the input file's audio is not nor‐
mally needed, a `null file' (with the special name -n) is often
given instead (and the length specified as a parameter to synth
or by another given effect that can has an associated length).
For example, the following produces a 3 second, 48kHz, audio
file containing a sine-wave swept from 300 to 3300 Hz:
sox-n output.wav synth 3 sine 300-3300
and this produces an 8 kHz version:
sox-r 8000 -n output.wav synth 3 sine 300-3300
Multiple channels can be synthesised by specifying the set of
parameters shown between braces multiple times; the following
puts the swept tone in the left channel and adds `brown' noise
in the right:
sox-n output.wav synth 3 sine 300-3300 brownnoise
The following example shows how two synth effects can be cas‐
caded to create a more complex waveform:
play -n synth 0.5 sine 200-500 synth 0.5 sine fmod 700-100
Frequencies can also be given in `scientific' note notation, or,
by prefixing a `%' character, as a number of semitones relative
to `middle A' (440 Hz). For example, the following could be
used to help tune a guitar's low `E' string:
play -n synth 4 pluck %-29
or with a (Bourne shell) loop, the whole guitar:
for n in E2 A2 D3 G3 B3 E4; do
play -n synth 4 pluck $n repeat 2; done
See the delay effect (above) and the reference to `SoX scripting
examples' (below) for more synth examples.
N.B. This effect generates audio at maximum volume (0dBFS),
which means that there is a high chance of clipping when using
the audio subsequently, so in many cases, you will want to fol‐
low this effect with the gain effect to prevent this from hap‐
pening. (See also Clipping above.) Note that, by default, the
synth effect incorporates the functionality of gain -h (see the
gain effect for details); synth's -n option may be given to dis‐
able this behaviour.
A detailed description of each synth parameter follows:
len is the length of audio to synthesise expressed as a time or
as a number of samples; 0=inputlength, default=0.
The format for specifying lengths in time is hh:mm:ss.frac. The
format for specifying sample counts is the number of samples
with the letter `s' appended to it.
type is one of sine, square, triangle, sawtooth, trapezium, exp,
[white]noise, tpdfnoise pinknoise, brownnoise, pluck;
default=sine.
combine is one of create, mix, amod (amplitude modulation), fmod
(frequency modulation); default=create.
freq/freq2 are the frequencies at the beginning/end of synthesis
in Hz or, if preceded with `%', semitones relative to A
(440 Hz); alternatively, `scientific' note notation (e.g. E2)
may be used. The default frequency is 440Hz. By default, the
tuning used with the note notations is `equal temperament'; the
-j KEY option selects `just intonation', where KEY is an integer
number of semitones relative to A (so for example, -9 or 3
selects the key of C), or a note in scientific notation.
If freq2 is given, then len must also have been given and the
generated tone will be swept between the given frequencies. The
two given frequencies must be separated by one of the characters
`:', `+', `/', or `-'. This character is used to specify the
sweep function as follows:
: Linear: the tone will change by a fixed number of hertz
per second.
+ Square: a second-order function is used to change the
tone.
/ Exponential: the tone will change by a fixed number of
semitones per second.
- Exponential: as `/', but initial phase always zero, and
stepped (less smooth) frequency changes.
Not used for noise.
off is the bias (DC-offset) of the signal in percent; default=0.
ph is the phase shift in percentage of 1 cycle; default=0. Not
used for noise.
p1 is the percentage of each cycle that is `on' (square), or
`rising' (triangle, exp, trapezium); default=50 (square, trian‐
gle, exp), default=10 (trapezium), or sustain (pluck);
default=40.
p2 (trapezium): the percentage through each cycle at which
`falling' begins; default=50. exp: the amplitude in multiples of
2dB; default=50, or tone-1 (pluck); default=20.
p3 (trapezium): the percentage through each cycle at which
`falling' ends; default=60, or tone-2 (pluck); default=90.
tempo [-q] [-m|-s|-l] factor [segment [search [overlap]]]
Change the audio playback speed but not its pitch. This effect
uses the WSOLA algorithm. The audio is chopped up into segments
which are then shifted in the time domain and overlapped (cross-
faded) at points where their waveforms are most similar as
determined by measurement of `least squares'.
By default, linear searches are used to find the best overlap‐
ping points. If the optional -q parameter is given, tree
searches are used instead. This makes the effect work more
quickly, but the result may not sound as good. However, if you
must improve the processing speed, this generally reduces the
sound quality less than reducing the search or overlap values.
The -m option is used to optimize default values of segment,
search and overlap for music processing.
The -s option is used to optimize default values of segment,
search and overlap for speech processing.
The -l option is used to optimize default values of segment,
search and overlap for `linear' processing that tends to cause
more noticeable distortion but may be useful when factor is
close to 1.
If -m, -s, or -l is specified, the default value of segment will
be calculated based on factor, while default search and overlap
values are based on segment. Any values you provide still over‐
ride these default values.
factor gives the ratio of new tempo to the old tempo, so e.g.
1.1 speeds up the tempo by 10%, and 0.9 slows it down by 10%.
The optional segment parameter selects the algorithm's segment
size in milliseconds. If no other flags are specified, the
default value is 82 and is typically suited to making small
changes to the tempo of music. For larger changes (e.g. a factor
of 2), 41 ms may give a better result. The -m, -s, and -l flags
will cause the segment default to be automatically adjusted
based on factor. For example using -s (for speech) with a tempo
of 1.25 will calculate a default segment value of 32.
The optional search parameter gives the audio length in mil‐
liseconds over which the algorithm will search for overlapping
points. If no other flags are specified, the default value is
14.68. Larger values use more processing time and may or may
not produce better results. A practical maximum is half the
value of segment. Search can be reduced to cut processing time
at the risk of degrading output quality. The -m, -s, and -l
flags will cause the search default to be automatically adjusted
based on segment.
The optional overlap parameter gives the segment overlap length
in milliseconds. Default value is 12, but -m, -s, or -l flags
automatically adjust overlap based on segment size. Increasing
overlap increases processing time and may increase quality. A
practical maximum for overlap is the value of search, with over‐
lap typically being (at least) a little smaller then search.
See also speed for an effect that changes tempo and pitch
together, pitch and bend for effects that change pitch only, and
stretch for an effect that changes tempo using a different algo‐
rithm.
treble gain [frequency[k] [width[s|h|k|o|q]]]
Apply a treble tone-control effect. See the description of the
bass effect for details.
tremolo speed [depth]
Apply a tremolo (low frequency amplitude modulation) effect to
the audio. The tremolo frequency in Hz is given by speed, and
the depth as a percentage by depth (default 40).
trim {[=|-]position}
Cuts portions out of the audio. Any number of positions may be
given; audio is not sent to the output until the first position
is reached. The effect then alternates between copying and dis‐
carding audio at each position.
If a position is preceded by an equals or minus sign, it is
interpreted relative to the beginning or the end of the audio,
respectively. (The audio length must be known for end-relative
locations to work.) Otherwise, it is considered an offset from
the last position, or from the start of audio for the first
parameter. Using a value of 0 for the first position parameter
allows copying from the beginning of the audio.
All parameters can be specified using either an amount of time
or an exact count of samples. The format for specifying lengths
in time is hh:mm:ss.frac. A value of 1:30.5 for the first
parameter will not start until 1 minute, thirty and ½ seconds
into the audio. The format for specifying sample counts is the
number of samples with the letter `s' appended to it. A value
of 8000s for the first parameter will wait until 8000 samples
are read before starting to process audio.
For example,
sox infile outfile trim 0 10
will copy the first ten seconds, while
play infile trim 12:34 =15:00 -2:00
will play from 12 minutes 34 seconds into the audio up to 15
minutes into the audio (i.e. 2 minutes and 26 seconds long),
then resume playing two minutes before the end of audio.
upsample [factor]
Upsample the signal by an integer factor: factor-1 zero-value
samples are inserted between each pair of input samples. As a
result, the original spectrum is replicated into the new fre‐
quency space (aliasing) and attenuated. This attenuation can be
compensated for by adding vol factor after any further process‐
ing. The upsample effect is typically used in combination with
filtering effects.
For a general resampling effect with anti-aliasing, see rate.
See also downsample.
vad [options]
Voice Activity Detector. Attempts to trim silence and quiet
background sounds from the ends of (fairly high resolution i.e.
16-bit, 44-48kHz) recordings of speech. The algorithm currently
uses a simple cepstral power measurement to detect voice, so may
be fooled by other things, especially music. The effect can
trim only from the front of the audio, so in order to trim from
the back, the reverse effect must also be used. E.g.
play speech.wav norm vad
to trim from the front,
play speech.wav norm reverse vad reverse
to trim from the back, and
play speech.wav norm vad reverse vad reverse
to trim from both ends. The use of the norm effect is recom‐
mended, but remember that neither reverse nor norm is suitable
for use with streamed audio.
Options:
Default values are shown in parenthesis.
-t num (7)
The measurement level used to trigger activity detection.
This might need to be changed depending on the noise
level, signal level and other charactistics of the input
audio.
-T num (0.25)
The time constant (in seconds) used to help ignore short
bursts of sound.
-s num (1)
The amount of audio (in seconds) to search for qui‐
eter/shorter bursts of audio to include prior to the
detected trigger point.
-g num (0.25)
Allowed gap (in seconds) between quieter/shorter bursts
of audio to include prior to the detected trigger point.
-p num (0)
The amount of audio (in seconds) to preserve before the
trigger point and any found quieter/shorter bursts.
Advanced Options:
These allow fine tuning of the algorithm's internal parameters.
-b num The algorithm (internally) uses adaptive noise estima‐
tion/reduction in order to detect the start of the wanted
audio. This option sets the time for the initial noise
estimate.
-N num Time constant used by the adaptive noise estimator for
when the noise level is increasing.
-n num Time constant used by the adaptive noise estimator for
when the noise level is decreasing.
-r num Amount of noise reduction to use in the detection algo‐
rithm (e.g. 0, 0.5, ...).
-f num Frequency of the algorithm's processing/measurements.
-m num Measurement duration; by default, twice the measurement
period; i.e. with overlap.
-M num Time constant used to smooth spectral measurements.
-h num `Brick-wall' frequency of high-pass filter applied at the
input to the detector algorithm.
-l num `Brick-wall' frequency of low-pass filter applied at the
input to the detector algorithm.
-H num `Brick-wall' frequency of high-pass lifter used in the
detector algorithm.
-L num `Brick-wall' frequency of low-pass lifter used in the
detector algorithm.
See also the silence effect.
vol gain [type [limitergain]]
Apply an amplification or an attenuation to the audio signal.
Unlike the -v option (which is used for balancing multiple input
files as they enter the SoX effects processing chain), vol is an
effect like any other so can be applied anywhere, and several
times if necessary, during the processing chain.
The amount to change the volume is given by gain which is inter‐
preted, according to the given type, as follows: if type is
amplitude (or is omitted), then gain is an amplitude (i.e. volt‐
age or linear) ratio, if power, then a power (i.e. wattage or
voltage-squared) ratio, and if dB, then a power change in dB.
When type is amplitude or power, a gain of 1 leaves the volume
unchanged, less than 1 decreases it, and greater than 1
increases it; a negative gain inverts the audio signal in addi‐
tion to adjusting its volume.
When type is dB, a gain of 0 leaves the volume unchanged, less
than 0 decreases it, and greater than 0 increases it.
See [4] for a detailed discussion on electrical (and hence audio
signal) voltage and power ratios.
Beware of Clipping when the increasing the volume.
The gain and the type parameters can be concatenated if desired,
e.g. vol 10dB.
An optional limitergain value can be specified and should be a
value much less than 1 (e.g. 0.05 or 0.02) and is used only on
peaks to prevent clipping. Not specifying this parameter will
cause no limiter to be used. In verbose mode, this effect will
display the percentage of the audio that needed to be limited.
See also gain for a volume-changing effect with different capa‐
bilities, and compand for a dynamic-range compression/expan‐
sion/limiting effect.
Deprecated Effects
The following effects have been renamed or have their functionality
included in another effect; they continue to work in this version of
SoX but may be removed in future.
mixer [ -l|-r|-f|-b|-1|-2|-3|-4|n{,n} ]
Reduce the number of audio channels by mixing or selecting chan‐
nels, or increase the number of channels by duplicating chan‐
nels. Note: this effect operates on the audio channels within
the SoX effects processing chain; it should not be confused with
the -m global option (where multiple files are mix-combined
before entering the effects chain).
When reducing the number of channels it is possible to use the
-l, -r, -f, -b, -1, -2, -3, -4, options to select only the left,
right, front, back channel(s) or specific channel for the output
instead of averaging the channels. The -l, and -r options will
do averaging in quad-channel files so select the exact channel
to prevent this.
The mixer effect can also be invoked with up to 16 numbers, sep‐
arated by commas, which specify the proportion (0 = 0% and 1 =
100%) of each input channel that is to be mixed into each output
channel. In two-channel mode, 4 numbers are given: l → l, l →
r, r → l, and r → r, respectively. In four-channel mode, the
first 4 numbers give the proportions for the left-front output
channel, as follows: lf → lf, rf → lf, lb → lf, and rb → rf.
The next 4 give the right-front output in the same order, then
left-back and right-back.
It is also possible to use the 16 numbers to expand or reduce
the channel count; just specify 0 for unused channels.
Finally, certain reduced combination of numbers can be specified
for certain input/output channel combinations.
In Ch Out Ch Num Mappings
2 1 2 l → l, r → l
2 2 1 adjust balance
4 1 4 lf → l, rf → l, lb → l, rb → l
4 2 2 lf → l&rf → r, lb → l&rb → r
4 4 1 adjust balance
4 4 2 front balance, back balance
This effect has been superseded by the remix effect that handles
any number of channels.
DIAGNOSTICS
Exit status is 0 for no error, 1 if there is a problem with the com‐
mand-line parameters, or 2 if an error occurs during file processing.
BUGS
Please report any bugs found in this version of SoX to the mailing list
(sox-users@lists.sourceforge.net).
SEE ALSOsoxi(1), soxformat(7), libsox(3)audacity(1), gnuplot(1), octave(1), wget(1)
The SoX web site at http://sox.sourceforge.net
SoX scripting examples at http://sox.sourceforge.net/Docs/Scripts
References
[1] R. Bristow-Johnson, Cookbook formulae for audio EQ biquad filter
coefficients, http://musicdsp.org/files/Audio-EQ-Cookbook.txt
[2] Wikipedia, Q-factor, http://en.wikipedia.org/wiki/Q_factor
[3] Scott Lehman, Effects Explained, http://harmony-cen‐
tral.com/Effects/effects-explained.html
[4] Wikipedia, Decibel, http://en.wikipedia.org/wiki/Decibel
[5] Richard Furse, Linux Audio Developer's Simple Plugin API,
http://www.ladspa.org
[6] Richard Furse, Computer Music Toolkit, http://www.ladspa.org/cmt
[7] Steve Harris, LADSPA plugins, http://plugin.org.uk
LICENSE
Copyright 1998-2011 Chris Bagwell and SoX Contributors.
Copyright 1991 Lance Norskog and Sundry Contributors.
This program is free software; you can redistribute it and/or modify it
under the terms of the GNU General Public License as published by the
Free Software Foundation; either version 2, or (at your option) any
later version.
This program is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of MER‐
CHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General
Public License for more details.
AUTHORS
Chris Bagwell (cbagwell@users.sourceforge.net). Other authors and con‐
tributors are listed in the ChangeLog file that is distributed with the
source code.
sox February 19, 2011 SoX(1)
