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Au file format

Simple audio file format

Field	Value
name	Au
extension	.au
.snd
_nomimecode	yes
mime	(headerless format)
type code	{{plainlist
* public.au-audio<ref>{{cite web	url	https://developer.apple.com/documentation/avfoundation/avfiletypesunau	title=AVFileTypeSunAU	work=Apple Developer Documentation	publisher=Apple Inc.}}
* public.ulaw-audio<ref>{{cite web	url	https://developer.apple.com/library/archive/documentation/Miscellaneous/Reference/UTIRef/Articles/System-DeclaredUniformTypeIdentifiers.html	title=System-Declared Uniform Type Identifiers	work=Uniform Type Identifiers Reference	publisher=Apple Inc.}} (headerless variant)
magic	(newer versions)
owner	Sun Microsystems
genre	audio file format, container format
container for	Audio, most often μ-law

.snd

public.au-audio
public.ulaw-audio (headerless variant) 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 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 multiples of video clock signal frequencies. Newer files have a header that consists of six unsigned 32-bit words, an optional information chunk which is always of non-zero size, 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 application programs through the /dev/audio device file 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.

uint32 word	field	Description
0	Magic number	The value (four ASCII characters ".snd")
1	Data offset	The offset to the data in bytes. (In the older Sun version, this had to be a multiple of 8.) The minimum valid number is 28 (decimal), since this is the header length (six 32-bit words) plus a minimal annotation size (4 bytes, another 32-bit word).
2	data size	Data size in bytes, not including the header. If unknown, the value should be used.
3	Encoding	Data encoding format:
4	Sample rate	The number of samples/second, e.g., 8000, 11025, 22050, 44100, and 48000. NeXT may use 8013.
5	Channels	The number of interleaved channels, e.g., 1 for mono, 2 for stereo; more channels possible, but may not be supported by all readers.
6	–	Optional annotation or description string, NULL-terminated. A minimum of 4 bytes must be stored even if unused.

The type of encoding depends on the value of the "encoding" field (word 3 of the header). Formats 2 through 7 are uncompressed linear PCM, therefore technically lossless (although not necessarily free of quantization error, especially in 8-bit form). Formats 1 and 27 are μ-law and A-law, respectively, both companding logarithmic representations of PCM, and arguably lossy, as they pack what would otherwise be almost 16 bits of dynamic range into 8 bits of encoded data, even though this is achieved by an altered dynamic response and no data are discarded. Formats 23 through 26 are ADPCM, which is an early form of lossy compression, usually with four bits of encoded data per audio sample (for 4:1 efficiency with 16-bit input, or 2:1 with 8-bit input). Several of the others (number 8 through 22) are DSP commands or data, designed to be processed by the NeXT Music Kit software.

Note: PCM formats are encoded as signed data, as opposed to unsigned.

The current format supports only a single audio data segment per file. The variable-length annotation field is currently ignored by most audio applications.

References

"audio/basic".
"AVFileTypeSunAU". [[Apple Inc.]].
"System-Declared Uniform Type Identifiers". [[Apple Inc.]].
"Audio File Formats FAQ: File Formats.".
"Audio File and Compression Formats".
[https://docs.oracle.com/cd/E36784_01/html/E36882/au-4.html Oracle man pages: au(4) - AU audio file format] (current specification)

Info: Wikipedia Source

This article was imported from Wikipedia and is available under the Creative Commons Attribution-ShareAlike 4.0 License. Content has been adapted to SurfDoc format. Original contributors can be found on the article history page.

computer-file-formats digital-container-formats audio-codecs

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