Au file format

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The Au file format is a simple audio file format that consists of a header of 5 32-bit words, an optional information chunk and then the data (in big endian format). The format was introduced by Sun Microsystems and was also used on NeXT systems.

Although the format supports many audio encoding formats, it's often associated with the µ-law logarithmic encoding, which dates back at least to the Sparcstation I as the native method used by the SunOS /dev/audio interface which was widely used as a defacto "standard" for UNIX sound.

32 bit word	field	Description/Content Hexadecimal numbers in C notation
0	magic number	the value `0x2e736e64` (four ASCII characters ".snd")
1	data offset	the offset to the data in bytes. The minimum valid number is 24 (decimal), since this is the header length (5 32-bit words) plus a minimum of 4 bytes for the information chunk.
2	data size	data size in bytes. If unknown, the value `0xffffffff` should be used.
3	encoding	Data encoding format: 1 = 8-bit G.711 µ-law 2 = 8-bit linear PCM 3 = 16-bit linear PCM 4 = 24-bit linear PCM 5 = 32-bit linear PCM 6 = 32-bit IEEE floating point 7 = 64-bit IEEE floating point 8 = Fragmented sample data 9 = DSP program 10 = 8-bit fixed point 11 = 16-bit fixed point 12 = 24-bit fixed point 13 = 32-bit fixed point 18 = 16-bit linear with emphasis 19 = 16-bit linear compressed 20 = 16-bit linear with emphasis and compression 21 = Music kit DSP commands 23 = 4-bit ISDN u-law compressed using the ITU-T G.721 ADPCM voice data encoding scheme 24 = ITU-T G.722 ADPCM 25 = ITU-T G.723 3-bit ADPCM 26 = ITU-T G.723 5-bit ADPCM 27 = 8-bit G.711 A-law
4	sample rate	the number of samples/second (e.g., 8000)
5	channels	the number of interleaved channels (e.g., 1 for mono, 2 for stereo, more channels possible but may not be supported by all readers)

The type of encoding depends on the value of the 'encoding' field (word 3 of the header). Formats 2–7 are uncompressed PCM, therefore lossless. Formats 23–26 are ADPCM, which is a lossy, roughly 4:1 compression. Formats 1 and 27 are μ-law and A-law, respectively, both lossy. Several of the others are DSP commands or data, designed to be processed by the NeXT MusicKit software.