Au (file format)

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Au
File extension : .au  .snd
MIME type : audio / basic
Magic number : 0x2e736e64 hex
.snd
Developed by: Sun Microsystems
Type: Audio container
Container for: Audio , mostly µ-law


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.

The data format consists strictly of three parts: a 24-byte header, an information block of any length (including 0 bytes), which can contain ASCII , but does not have to, and the actual data block. All numeric data is strictly stored as big-endian values.

The data block can

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

contain.

Most often the format is used for audio material compressed from 16-bit PCM to 8-bit µ-law.

construction

The Au file format is quite simple: it consists of a header of 7 32-bit words followed by the audio data.

32-bit word field Description / content ( hexadecimal numbers in C notation)
0 Magic number The value 0x2e736e64 (- the four ASCII characters ".snd")
1 Data offset The offset to the data in bytes . The smallest valid value is 28 (decimal).
2 Data size in bytes. If unknown, the value 0xffffffff should be used.
3 Coding Data format:

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 UIT-T G.721 ADPCM coding scheme for voice data.

4th Sample rate Number of samples per second (e.g. 8,000)
5 channels Number of interlaced channels (1 for mono, 2 for stereo, ...)
6th info Optional information (min. 4 bytes)

The coding is specified with the value of the "Coding" field (word 3 in the header). Formats 2 to 7 are uncompressed PCM and are therefore lossless. The formats numbered 23 to 36 are ADPCM, a lossy process that compresses in a ratio of around 4: 1. The formats with the number 1 and 27 are µ-law and A-law , both lossy formats. Among the rest are several DSP commands or data that are intended for processing by the NeXT " MusicKit " software .

Individual evidence

  1. ↑ AU Sound File Specifications. Retrieved February 27, 2017 .