Tribblix: manual page: au.5

AU(5) File Formats and Configurations AU(5)

NAME

au - AU audio file format

SYNOPSIS

#include <audio/au.h>

DESCRIPTION

An AU audio file is composed of three parts: a header, an optional
description field, and a contiguous segment of audio data. The header
is 24 bytes, and the description field is at least 4 bytes.
Therefore, the offset for most AU files is 28 bytes. However, some
people store additional data in the AU header.

The AU audio structure members and audio data are stored big endian.
That is, it starts with the most significant byte, regardless of the
native byte order of the machine architecture on which an application
may be running. Therefore, multi-byte audio data may require byte
reversal for proper playback on different processor architectures.
See the macro section for properly reading and writing the AU audio
structure members.

The AU header is defined by the following structure:

struct au_filehdr {
uint32_t au_magic; /* magic number (.snd) */
uint32_t au_offset; /* byte offset to start of audio data */
uint32_t au_data_size; /* data length in bytes */
uint32_t au_encoding; /* data encoding */
uint32_t au_sample_rate; /* samples per second */
uint32_t au_channels; /* number of interleaved channels */
};
typedef struct au_filehdr au_filehdr_t;

The au_magic field always contains the following constant for an AU
audio file:

AUDIO_AU_FILE_MAGIC ( 0x2e736e64 ) /* ".snd" */

The au_offset field contains the length of the audio file header plus
the variable length info field. Consequently, it can be interpreted
as the offset from the start of the file to the start of the audio
data.

The au_data_size field contains the length, in bytes, of the audio
data segment. If this length is not known when the header is written,
it should be set to AUDIO_AU_UNKNOWN_SIZE, defined as follows:

AUDIO_AU_UNKNOWN_SIZE ( ~0 ) /* (unsigned) -1 */

When the au_data_size field contains AUDIO_AU_UNKNOWN_SIZE, the
length of the audio data can be determined by subtracting au_offset
from the total length of the file.

The encoding field contains one of the following enumerated keys:

AUDIO_AU_ENCODING_ULAW /* 8-bit u-law */
AUDIO_AU_ENCODING_LINEAR_8 /* 8-bit linear PCM */
AUDIO_AU_ENCODING_LINEAR_16 /* 16-bit linear PCM */
AUDIO_AU_ENCODING_LINEAR_24 /* 24-bit linear PCM */
AUDIO_AU_ENCODING_LINEAR_32 /* 32-bit linear PCM */
AUDIO_AU_ENCODING_FLOAT /* Floating point */
AUDIO_AU_ENCODING_DOUBLE /* Double precision float */
AUDIO_AU_ENCODING_FRAGMENTED /* Fragmented sample data */
AUDIO_AU_ENCODING_DSP /* DSP program */
AUDIO_AU_ENCODING_FIXED_8 /* 8-bit fixed point */
AUDIO_AU_ENCODING_FIXED_16 /* 16-bit fixed point */
AUDIO_AU_ENCODING_FIXED_24 /* 24-bit fixed point */
AUDIO_AU_ENCODING_FIXED_32 /* 32-bit fixed point */
AUDIO_AU_ENCODING_EMPHASIS /* 16-bit linear with emphasis */
AUDIO_AU_ENCODING_COMPRESSED /* 16-bit linear compressed */
AUDIO_AU_ENCODING_EMP_COMP /* 16-bit linear with emphasis
and compression */
AUDIO_AU_ENCODING_MUSIC_KIT /* Music kit DSP commands */
AUDIO_AU_ENCODING_ADPCM_G721 /* CCITT G.721 ADPCM */
AUDIO_AU_ENCODING_ADPCM_G722 /* CCITT G.722 ADPCM */
AUDIO_AU_ENCODING_ADPCM_G723_3 /* CCITT G.723.3 ADPCM */
AUDIO_AU_ENCODING_ADPCM_G723_5 /* CCITT G.723.5 ADPCM */
AUDIO_AU_ENCODING_ALAW /* 8-bit A-law G.711 */

All of the linear encoding formats are signed integers centered at
zero.

The au_sample_rate field contains the audio file's sampling rate in
samples per second. Some common sample rates include 8000, 11025,
22050, 44100, and 48000 samples per second.

The au_channels field contains the number of interleaved data
channels. For monaural data, this value is set to one. For stereo
data, this value is set to two. More than two data channels can be
interleaved, but such formats are currently unsupported by the
Solaris audio driver architecture. For a stereo sound file, the first
sample is the left track and the second sample is the right track.

The optional info field is a variable length annotation field that
can be either text or data. If it is a text description of the sound,
then it should be NULL terminated. However, some older files might
not be terminated properly. The size of the info field is set when
the structure is created and cannot be enlarged later.

Macros

Accessing all of the AU audio structure members should be done
through the supplied AUDIO_AU_FILE2HOST and AUDIO_AU_HOST2FILE
macros. By always using these macros, code will be byte-order
independent. See the example below.

EXAMPLES

Example 1: Displaying Header Information for a Sound File

The following program reads and displays the header information for
an AU sound file. The AUDIO_AU_FILE2HOST macro ensures that this
information will always be in the proper byte order.

void main(void)
{
au_filehdr_t hdr;
au_filehdr_t local;
int fd;
char *name = "bark.au";

if ((fd = open(name, O_RDONLY)) < 0) {
printf("can't open file %s\n", name);
exit(1);
}

(void) read(fd, &hdr, sizeof (hdr));

AUDIO_AU_FILE2HOST(&hdr.au_magic, &local.au_magic);
AUDIO_AU_FILE2HOST(&hdr.au_offset, &local.au_offset);
AUDIO_AU_FILE2HOST(&hdr.au_data_size, &local.au_data_size);
AUDIO_AU_FILE2HOST(&hdr.au_encoding, &local.au_encoding);
AUDIO_AU_FILE2HOST(&hdr.au_sample_rate, &local.au_sample_rate);
AUDIO_AU_FILE2HOST(&hdr.au_channels, &local.au_channels);

printf("Magic = %x\n", local.au_magic);
printf("Offset = %d\n", local.au_offset);
printf("Number of data bytes = %d\n", local.au_data_size);
printf("Sound format = %d\n", local.au_encoding);
printf("Sample rate = %d\n", local.au_sample_rate);
printf("Number of channels = %d\n", local.au_channels);

(void) close(fd);
}

ATTRIBUTES

NOTES

Some older AU audio files are incorrectly coded with info strings
that are not properly NULL-terminated. Thus, applications should
always use the au_offset value to find the end of the info data and
the beginning of the audio data.

January 15, 2001 AU(5)