Au file format
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The Au file format is a simple audio file format that consists of a header of 6 32-bit words and then the data. The format was introduced by Sun Microsystems.
| 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). |
| 2 | data size | data size in bytes. If unknown, the value 0xffffffff should be used. |
| 3 | encoding | Data encoding format:
1=8-bit ISDN u-law, 2=8-bit linear PCM [REF-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, 23=8-bit ISDN u-law compressed using the UIT-T G.721 ADPCM voice data encoding scheme. |
| 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) |
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 23-36 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.