CDFRAME(4)CDFRAME(4)NAMEcdframe - structure of a frame of audio data on Compact Disc (CD)
SYNOPSIS
#include <cdaudio.h>
DESCRIPTION
Audio on Compact Discs (CD) is blocked into frames. The CD has 75 frames
each second. One frame of data contains both audio and non-audio data
(known as subcodes). These frames are frequently referred to as
subcode-frames because it takes one complete frame to assemble all the
bits of a chunk of subcode. A program reading a CD in audio mode must
read a minimum of one CD frame. This is enforced by the function
CDreadda(3) in libcdaudio.
The cdframe structure, describing the content of a frame of CD data, is
defined in <cdaudio.h> as follows:
typedef struct cdframe {
char audio[CDDA_DATASIZE];
struct subcodeQ subcode;
} CDFRAME;
Audio Data
Audio samples in the audio array are linearly encoded in a 16-bit 2's
complement format. Encoding is carried out either without pre-emphasis
or with a first order 50/15 Ms pre-emphasis. The data stream contains
two interleaved channels, usually used as the left and right channels of
a stereo pair. The sample representing the left channel is first,
followed by the sample representing the right channel. The sampling
frequency is 44.1 kHz and both channels are sampled simultaneously during
recording.
The least significant byte of each 16-bit value is first. The bytes need
to be swapped in order to match the native byte ordering of the IRIS
Indigo and Personal IRIS 4D/35 and their audio hardware. So each 4-byte
chunk of the data array represents a pair of samples (1 left plus 1
right) as follows. Byte 0 is the least significant byte of the left
channel sample; byte 1 is the most significant byte of the left channel
sample. Byte 2 is the least significant byte of the simultaneous right
channel sample; byte 3 is the most significant byte of the simultaneous
right channel sample.
CDDA_DATASIZE is 2352 bytes.
Subcodes
The subcodeQ structure contains information from the Q subcode fields
recorded on the CD. CD's also have subcode fields P and R through W. P
subcodes provide a simple track separator intended for use by low cost
players. They have no useful purpose when the player is able to read and
process the Q subcodes. Subcodes R through W are not yet fully defined
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CDFRAME(4)CDFRAME(4)
and some CD-ROM drives do not read them. The subcodeQ structure is
defined in cdaudio.h as follows:
typedef struct subcodeQ {
unchar control;
unchar type;
union {
struct {
struct cdpackedbcd { unchar dhi:4, dlo:4; } track;
struct cdpackedbcd index; /* aka point during trk 0 */
struct cdtimecode {
unchar mhi:4, mlo:4;
unchar shi:4, slo:4;
unchar fhi:4, flo:4;
} ptime;
struct cdtimecode atime;
unchar fill[6];
} mode1;
struct {
unchar catalognumber[13];
struct cdpackedbcd aframe;
} mode2;
struct {
struct cdident {
unchar country[2];
unchar owner[3];
unchar year[2];
unchar serial[5];
} ident;
struct cdpackedbcd aframe;
unchar fill;
} mode3;
} data;
} CDSUBCODEQ;
The control field contains 4 flag bits in its least significant nibble to
define the kind of information in a track (program item).
0 0 X 0 - 2 audio channels without pre-emphasis
0 0 X 1 - 2 audio channels with pre-emphasis of 50/15 Ms
0 X 0 X - copy prohibited
0 X 1 X - copy permitted
The bits of the control field can only change during an actual pause
(index.dhi, index.dlo == 00) of at least 2 s or during the lead-in area.
Notes 1. -
The four bits of the control field should be copied to the control
field of the channel status of the digital audio interface for
domestic use.
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Notes 2. -
For non-audio applications of the compact disc the following control
codes have been defined:
0 1 X 0 - digital data
1 X X X - broadcasting use
All other combinations will be defined later on.
The type field contains 4 address-bits in its least significant nibble
that identify the type of the data field which is a union of three
possible types.
0001 - mode 1
0010 - mode 2
0011 - mode 3
Mode 1 Q Data
Mode 1 data contains track numbering and time code information. All
numbers are encoded as BCD digits. track is the number of the current
track. It is two BCD digits packed into a single byte. The most
significant digit, d1, is in the most significant nibble. Audio program
tracks are numbered from 01 - 99. Track number 00 is the lead-in track.
Track number AA (each BCD digit has the bit-pattern 1010) is the lead-out
track.
Tracks may be subdivided into sections; each section has a unique index
number. index is the index number of the current subdivision within the
track. Its representation is identical with track. Regular subdivisions
are numbered from 01 - 99. The first subdivision within a track must be
numbered 01. The index number increases in increments of 1. Index
number 00 indicates a pause and coincides with actual pauses (gaps
between tracks) in the audio program. During index number 00, track is
the number of the track that follows the pause. The first audio track is
preceded by a pause encoding of 2 s to 3 s.
During the lead-out track the subdivision number is 01.
During the lead-in track a table of contents is recorded in the mode 1
subcodes. During this time, the index number field, now known as Point,
records a track number. The atime field (see below) gives the starting
position of the track given by point. Several special track numbers are
also valid at this time. If point = A0 the minutes field of atime
contains the track number of the first track on the disk. The seconds
and frame fields are zero. If point = A1 the minutes field of atime
contains the track number of the last track on the disk. The seconds and
frame fields are zero. If point = A2 atime contains the starting point
of the lead-out track.
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In normal use (i.e. except during lead-in) ptime is the time into the
current program (track). atime is the time from the start of the disc.
In the cdtimecode structure, mhi and mlo are the current minute, shi and
slo are the current second and fhi and flo are the current frame. Frame
numbers run from 00 to 74.
Mode 2 Q Data
Mode 2 data contains the catalogue number of the disc. catalognumber is
expressed in 13 BCD digits according to the UPC/EAN code. The catalogue
number does not change on a disk. If no catalogue number is recorded
these BCD digits are all zero.
aframe contains the continuation of the absolute frame numbering from
mode 1. This allows an uninterrupted flow of frame numbers. It contains
two packed BCD digits.
Mode 3 Q Data
Mode 3 data is used to give a unique identification to an audio track.
This is done by means of the International Standard Recording Code (ISO
3901). ident is the ISRC and it consists of several fields. country and
owner are encoded in a six-bit format using the 6 least significant
digits of each byte according to the following table:
Octal ASCII Octal ASCII Octal ASCII
______________ _______________ ______________
000 0 023 C 037 O
001 1 024 D 040 P
002 2 025 E 041 Q
003 3 026 F 042 R
004 4 027 G 043 S
005 5 030 H 044 T
006 6 031 I 045 U
007 7 032 J 046 V
010 8 033 K 047 W
011 9 034 L 050 X
021 A 035 M 051 Y
022 B 036 N 052 Z
year and serial are BCD digits in the least significant nibbles of each
byte.
aframe contains the continuation of the absolute frame numbering from
mode 1. This allows an uninterrupted flow of frame numbers. It contains
two packed BCD digits.
SEE ALSOcdintro(3dm)
Compact Disc Digital Audio System. The International Electro-Technical
Commission (IEC). Bureau Central de la Commission Electrotechnique
Internationale, 3 rue de Varembe, Geneve, Suisse
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