AUCAT(1) OpenBSD Reference Manual AUCAT(1)NAMEaucat - audio server and stream manipulation tool
SYNOPSISaucat [-dlnu] [-a flag] [-b nframes] [-C min:max] [-c min:max] [-e enc]
[-f device] [-h fmt] [-i file] [-j flag] [-m mode] [-o file]
[-q device] [-r rate] [-s name] [-t mode] [-U unit] [-v volume]
[-x policy] [-z nframes]
DESCRIPTIONaucat is an audio utility which can simultaneously play and record any
number of audio streams on any number of audio devices, possibly
controlled through MIDI. It can also act as an audio server, in which
case streams correspond to client connections rather than plain files.
Audio devices are independent. A list of streams is attached to each
audio device, as well as an optional list of MIDI ports to control the
device. A typical invocation of aucat consists in providing streams to
play and record, and possibly the audio device to use, if the default is
This also applies to server mode, except that streams are created
dynamically when clients connect to the server. Thus, instead of actual
streams (paths to plain files), templates for client streams (sub-device
names) must be provided.
The options are as follows:
Control whether aucat opens the audio device only when needed or
keeps it open all the time. If the flag is on then the device is
kept open all the time, ensuring no other program can steal it.
If the flag is off, then it's automatically closed, allowing
other programs to have direct access to the device, or the device
to be disconnected. The default is on.
The buffer size of the audio device in frames. A frame consists
of one sample for each channel in the stream. This is the number
of frames that will be buffered before being played and thus
controls the playback latency.
-C min:max, -c min:max
The range of stream channel numbers for recording and playback
directions, respectively. The default is 0:1, i.e. stereo.
-d Increase log verbosity. aucat logs on stderr until it
-e enc Encoding of the playback or recording stream (see below). The
default is signed, 16-bit, native byte order.
Add this sndio(7) audio device to devices used for playing and/or
recording. Preceding streams (-ios), control MIDI ports (-q),
and per-device options (-abz) apply to this device. Device mode
and parameters are determined from streams attached to it.
-h fmt File format of the playback or record stream (see below). The
default is auto.
Add this file to the list of streams to play. If the option
argument is `-' then standard input will be used.
Control whether stream channels are joined or expanded if the
stream number of channels is not equal to the device number of
channels. If the flag is off then stream channels are routed to
the corresponding device channel, possibly discarding channels
not present in the device. If the flag is on, then a single
stream channel may be sent on multiple device channels, or
multiple stream channels may be sent to a single device channel.
For instance, this feature could be used to request mono streams
to be sent on multiple outputs or to record a stereo input into a
mono stream. The default is on.
-l Detach and become a daemon.
Set the stream mode. Valid modes are play, rec, and mon,
corresponding to playback, recording and monitoring. A
monitoring stream is a fake recording stream corresponding to the
mix of all playback streams. Multiple modes can be specified,
separated by commas, but the same stream cannot be used for both
recording and monitoring. The default is play,rec (i.e. full-
-n Loopback mode. Instead of using audio devices, send input
streams to the output, processing them on the fly. This mode is
useful to mix, demultiplex, resample or reencode audio files
Add this file to the list of recording streams. If the option
argument is `-' then standard output will be used.
Expose the audio device clock on this sndio(7) MIDI port and
allow audio device properties to be controlled through MIDI.
This includes per-stream volumes and the ability to synchronously
start, stop and relocate streams created in MIDI Machine Control
(MMC) slave mode (-t).
Sample rate in Hertz of the stream. The default is 44100Hz.
Add name to the list of sub-devices to expose in server mode.
This allows clients to use aucat instead of the physical audio
device for audio input and output in order to share the physical
device with other clients. Defining multiple sub-devices allows
splitting a physical audio device into logical devices having
different properties (e.g. channel ranges). The given name
corresponds to the ``option'' part of the sndio(7) device name
Select the way streams are controlled by MIDI Machine Control
(MMC) messages. If the mode is off (the default), then streams
are not affected by MMC messages. If the mode is slave, then
streams are started synchronously by MMC start messages;
additionally, the server clock is exposed as MIDI Time Code (MTC)
messages allowing MTC-capable software or hardware to be
synchronized to audio streams.
Unit number to use when running in server mode. Each aucat
server instance has an unique unit number, used in sndio(7)
device names. The default is 0.
-u Normally aucat tries to automatically determine the optimal
parameters for the audio device; if this option is specified, it
will instead use the parameters specified by the -Ccer options.
Software volume attenuation of the playback stream. The value
must be between 1 and 127, corresponding to -42dB and -0dB
attenuation. In server mode, clients inherit this parameter.
Reducing the volume in advance reduces a client's dynamic range,
but allows client volume to stay independent from the number of
clients as long as their number is small enough. A good
compromise is to use -4dB attenuation (12 volume units) for each
additional client expected (115 if 2 clients are expected, 103
for 3 clients, and so on).
Action when the output stream cannot accept recorded data fast
enough or the input stream cannot provide data to play fast
enough. If the policy is ``ignore'' (the default) then samples
that cannot be written are discarded and samples that cannot be
read are replaced by silence. If the policy is ``sync'' then
recorded samples are discarded, but the same amount of silence
will be written once the stream is unblocked, in order to reach
the right position in time. Similarly silence is played, but the
same amount of samples will be discarded once the stream is
unblocked. If the policy is ``error'' then the stream is closed
If a stream is created with the -t option, the ``ignore'' action
is disabled for any stream connected to it to ensure proper
The audio device block size in frames. This is the number of
frames between audio clock ticks, i.e. the clock resolution. If
a stream is created with the -t option, and MTC is used for
synchronization, the clock resolution must be 96, 100 or 120
ticks per second for maximum accuracy. For instance, 120 ticks
per second at 48000Hz corresponds to a 400 frame block size.
On the command line, per-device parameters (-abz) must precede the device
definition (-f), and per-stream parameters (-Ccehjmrtvx) must precede the
stream definition (-ios). MIDI ports (-q) and streams definitions (-ios)
must precede the definition of the device (-f) to which they are
attached. Global parameters (-dlnu) are position-independent.
If no audio devices (-f) are specified, settings are applied as if the
default device is specified as the last argument. If no streams (-ios)
are specified for a device, a default server sub-device is created
attached to it, meaning that aucat behaves as an audio server. The
default sndio(7) device is aucat:0 (also known as aucat:0.default)
If aucat is sent SIGHUP, SIGINT or SIGTERM, it terminates recording to
File formats are specified using the -h option. The following file
formats are supported:
raw Headerless file. This format is recommended since it has
wav Microsoft WAVE file format. There are limitations
inherent to the file format itself: not all encodings are
supported, file sizes are limited to 2GB, and the file
must support the lseek(2) operation (e.g. pipes do not
auto Try to guess, depending on the file name.
Encodings are specified using the -e option. The following encodings are
s8 signed 8-bit
u8 unsigned 8-bit
s16le signed 16-bit, little endian
u16le unsigned 16-bit, little endian
s16be signed 16-bit, big endian
u16be unsigned 16-bit, big endian
s24le signed 24-bit, stored in 4 bytes, little endian
u24le unsigned 24-bit, stored in 4 bytes, little endian
s24be signed 24-bit, stored in 4 bytes, big endian
u24be unsigned 24-bit, stored in 4 bytes, big endian
s32le signed 32-bit, little endian
u32le unsigned 32-bit, little endian
s32be signed 32-bit, big endian
u32be unsigned 32-bit, big endian
s24le3 signed 24-bit, packed in 3 bytes, little endian
u24le3 unsigned 24-bit, packed in 3 bytes, big endian
s24be3 signed 24-bit, packed in 3 bytes, little endian
u24be3 unsigned 24-bit, packed in 3 bytes, big endian
s20le3 signed 20-bit, packed in 3 bytes, little endian
u20le3 unsigned 20-bit, packed in 3 bytes, big endian
s20be3 signed 20-bit, packed in 3 bytes, little endian
u20be3 unsigned 20-bit, packed in 3 bytes, big endian
s18le3 signed 18-bit, packed in 3 bytes, little endian
u18le3 unsigned 18-bit, packed in 3 bytes, big endian
s18be3 signed 18-bit, packed in 3 bytes, little endian
u18be3 unsigned 18-bit, packed in 3 bytes, big endian
If at least one sub-device (-s) is exposed by aucat, including the case
when no stream options are given, then aucat can be used as a server to
overcome hardware limitations and allow applications to run on fixed
sample rate devices or on devices supporting only unusual encodings.
Certain applications, such as synthesis software, require a low latency
audio setup. To reduce the probability of buffer underruns or overruns,
especially on busy machines, the server can be started by the super-user,
in which case it will run with higher priority. Any user will still be
able to connect to it, but for privacy reasons only one user may have
connections to it at a given time.
MIDI CONTROLaucat can expose the audio device clock on registered MIDI ports (-q) and
allows audio device properties to be controlled through MIDI. If running
in server mode aucat creates a MIDI port with the same name as the
default audio device to which MIDI programs can connect.
A MIDI channel is assigned to each stream, and the volume is changed
using the standard volume controller (number 7). Similarly, when the
audio client changes its volume, the same MIDI controller message is sent
out; it can be used for instance for monitoring or as feedback for
Streams created with the -t option are controlled by the following MMC
relocate Streams are relocated to the requested time postion
relative to the beginning of the stream, at which
playback and recording must start. If the requested
position is beyond the end of file, the stream is
temporarly disabled until a valid postion is
requested. This message is ignored by client streams
(server mode). The given time position is sent to
MIDI ports as an MTC ``full frame'' message forcing
all MTC-slaves to relocate to the given position (see
start Put all streams in starting mode. In this mode, aucat
waits for all streams to become ready to start, and
then starts them synchronously. Once started, new
streams can be created (server mode) but they will be
blocked until the next stop-to-start transition.
stop Put all streams in stopped mode (the default). In
this mode, any stream attempting to start playback or
recording is paused. Files are stopped and rewound
back to the starting position, while client streams
(server mode) that are already started are not
affected until they stop and try to start again.
Streams created with the -t option export the server clock using MTC,
allowing non-audio software or hardware to be synchronized to the audio
stream. The following sample rates (-r) and block sizes (-z) are
recommended for maximum accuracy:
o 44100Hz, 441 frames
o 48000Hz, 400 frames
o 48000Hz, 480 frames
o 48000Hz, 500 frames
For instance, the following command will create two devices: the default
aucat:0 and a MIDI-controlled aucat:0.mmc:
$ aucat-l -r 48000 -z 400 -s default -t slave -s mmc
Streams connected to aucat:0 behave normally, while streams connected to
aucat:0.mmc wait for the MMC start signal and start synchronously.
Regardless of which device a stream is connected to, its playback volume
knob is exposed.
For instance, the following command will play a file on the aucat:0.mmc
audio device, and give full control to MIDI software or hardware
connected to the midithru:0 MIDI device:
$ aucat-t slave -q midithru:0 -i file.wav -f aucat:0.mmc
At this stage, aucat will start, stop and relocate automatically
following all user actions in the MIDI sequencer. Note that the
sequencer must use aucat:0 as the MTC source, i.e. the audio server, not
the audio player.
AUDIODEVICE sndio(7) audio device to use if the -f option is not
The following will mix and play two stereo streams, the first at 48kHz
and the second at 44.1kHz:
$ aucat-r 48000 -i file1.raw -r 44100 -i file2.raw
The following will record channels 2 and 3 into one stereo file and
channels 6 and 7 into another stereo file using a 96kHz sampling rate for
$ aucat-r 96000 -C 2:3 -o file1.raw -C 6:7 -o file2.raw
The following will split a stereo file into two mono files:
$ aucat-n -i stereo.wav -C 0:0 -o left.wav -C 1:1 -o right.wav
The following will start aucat in server mode using default parameters,
but will create an additional sub-device for output to channels 2:3 only
(rear speakers on most cards), exposing the aucat:0 and aucat:0.rear
$ aucat-l -s default -c 2:3 -s rear
The following will start aucat in server mode creating the default sub-
device with low volume and an additional sub-device for high volume
output, exposing the aucat:0 and aucat:0.max devices:
$ aucat-l -v 65 -s default -v 127 -s max
The following will start aucat in server mode configuring the audio
device to use a 48kHz sample frequency, 240-frame block size, and 2-block
buffers. The corresponding latency is 10ms, which is the time it takes
the sound to propagate 3.5 meters.
$ aucat-l -r 48000 -b 480 -z 240
SEE ALSOaudioctl(1), cdio(1), mixerctl(1), audio(4), sndio(7)BUGS
The aucat utility assumes non-blocking I/O for input and output streams.
It will not work reliably on files that may block (ordinary files block,
pipes don't). To avoid audio underruns/overruns or MIDI jitter caused by
file I/O, it's recommended to use two aucat processes: a server handling
audio and MIDI I/O and a client handling disk I/O.
Resampling is low quality; down-sampling especially should be avoided
Processing is done using 16-bit arithmetic, thus samples with more than
16 bits are rounded. 16 bits (i.e. 97dB dynamic) are largely enough for
most applications though.
If -a off is used in server mode, aucat creates sub-devices to expose
first and then opens the audio hardware on demand. Technically, this
allows aucat to attempt to use one of the sub-devices it exposes as audio
device, creating a deadlock. To avoid this, -a off is disabled for the
default audio device, but nothing prevents the user from shooting himself
in the foot by creating a similar deadlock.
OpenBSD 4.9 July 31, 2010 OpenBSD 4.9