Au file format

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The Au file format is a container format for audio data. It was introduced by Sun Microsystems and is the standard audio format on NeXT and Sun computers dar.

The data format consists strictly of three parts: a 24 -byte header, a block of information of any length (even 0 bytes ), which may contain ASCII, but need not be, and the actual data block. All numeric data are stored strictly as big-endian values.

The data block may

  • Uncompressed linear PCM ( Pulse Code Modulation) in mono or stereo, 16-bit or 8 bits per sample and at different sampling rates,
  • 4 bits / sample compressed (32 kbits / sec ) to CCITT G.721 ADPCM ( Adaptive Pulse Code Modulation),
  • Compressed 8 -bit A-law,
  • Compressed 8-bit μ -law
  • Or other types of data


Mostly, the format for 16- bit PCM is used on 8 -bit μ -law compressed audio.


The Au file format is quite simple: It consists of a header from six 32- bit words followed by the audio data.

1: 8-bit ISDN μ -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 μ -law compressed according to the encoding scheme UIT - T G.721 ADPCM for speech data.

The encoding is the value of the " encoding " specified ( word 3 in the header). The formats 2-7 are uncompressed PCM, therefore lossless. The formats of the numbers 23 to 36 are ADPCM, a lossy, compressed is about a 4:1 ratio method. For the formats with the number 1 and 27 are μ -law and A-law, both lossy formats. Among the rest are multiple DSP commands or data are provided for processing by the NeXT software " MusicKit ".

  • Container format