- Au file format
Infobox file format
name = Au
extension = .au
mime = audio/basic
type code =
uniform type =
magic = .snd
container for = Audio, most often
contained by =
extended from =
extended to =
standard = The Au file format is a simple
audio file formatintroduced by Sun Microsystems. The format was common on NeXTsystems 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-bitwords, an optional information chunk and then the data (in big endianformat).
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 1hardware, where SunOSexposed the encoding to apps through the /dev/audio interface. This encoding and interface became a de factostandard for Unixsound.
All fields are stored in
big-endianformat, 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 MusicKitsoftware.
Note: PCM data appears to be encoded as signed, rather than unsigned.
* [http://www.opengroup.org/public/pubs/external/auformat.html Sun .au sound file format]
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