Au file format

Infobox file format
name = Au
icon =
logo =


caption =
extension = .au
.snd
mime = audio/basic
type code =
uniform type =
magic = .snd
owner = Sun Microsystems
type = Audio container
container for = Audio, most often µ-law
contained by =
extended from =
extended to =
standard =
The Au file format is a simple audio file format introduced by Sun Microsystems. The format was common on NeXT systems and on early web pages. Originally it was headerless, being simply 8-bit µ-law-encoded data at an 8000 Hz sample rate. Hardware from other vendors often used sample rates as high as 8192 Hz, often integer factors of video clock signals. Newer files have a header that consists of six 32-bit words, an optional information chunk and then the data (in big endian format).

Although the format now supports many audio encoding formats, it remains associated with the µ-law logarithmic encoding. This encoding was native to the SPARCstation 1 hardware, where SunOS exposed the encoding to apps through the /dev/audio interface. This encoding and interface became a de facto standard for Unix sound.

New format

All fields are stored in big-endian format, including the sample data.

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.

Note: PCM data appears to be encoded as signed, rather than unsigned.

External links

* [http://www.opengroup.org/public/pubs/external/auformat.html Sun .au sound file format]


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