View source for Au file format

{{Short description|Simple audio file format}}
{{Infobox file format
| name = Au
| icon =
| logo =
| screenshot =
| caption =
| extension = .au<br />.snd
| _nomimecode = yes
| mime = {{code|audio/basic}} (headerless format)<ref name=IANA>{{cite web |title=audio/basic |url=https://www.iana.org/assignments/media-types/audio/basic |website=IANA.org |access-date=23 February 2023}}</ref>
| 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.]]}}</ref>
* 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.]]}}</ref> (headerless variant)
}}
| uniform type =
| magic = {{code|.snd}} (newer versions)
| owner = [[Sun Microsystems]]
| genre = [[audio file format]], [[container format]]
| container for = [[Audio frequency|Audio]], most often [[μ-law algorithm|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 8-bit [[μ-law algorithm|μ-law]]-encoded data at an 8000 Hz sample rate.<ref name=IANA/> Hardware from other vendors often used sample rates as high as 8192&nbsp;Hz, often integer multiples of video clock signal frequencies. Newer files have a header that consists of six [[Signedness|unsigned]] [[32-bit computing|32-bit]] words, an optional information chunk which is always of non-zero size, and then the data (in [[Endianness|big-endian]] format).

Although the format now supports many [[digital audio|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.<ref name=spec/><ref name=sox>{{cite web |title=Audio File Formats FAQ: File Formats. |url=https://sox.sourceforge.net/AudioFormats-11.html#ss11.2 |website=sox.sourceforge.net |archive-url=https://web.archive.org/web/20230223152815/https://sox.sourceforge.net/AudioFormats-11.html#ss11.2 |archive-date=23 February 2023}}</ref>
{{Clear}}
{| class="wikitable" width="100%"
! uint32 word
! field
! Description
|- valign = top
| 0
| [[Magic number (programming)|Magic number]]
| The value {{mono|0x2e736e64}} (four ASCII characters ".snd")
|- valign = top
| 1 || Data offset
| The offset to the data in [[byte]]s. (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).
|- valign = top
| 2 || data size
| Data size in bytes, not including the header. If unknown, the value {{mono|0xffffffff}} should be used.
|- valign = top
| 3 || Encoding || Data encoding format:
{{div col|colwidth=15em}}
{{Ordered list|start=0
| Unspecified
| 8-bit [[G.711]] [[μ-law algorithm|μ-law]]
| 8-bit linear [[Pulse-code modulation|PCM]]
| 16-bit linear PCM
| 24-bit linear PCM
| 32-bit linear PCM
| 32-bit [[IEEE 754|IEEE floating point]]
| 64-bit IEEE floating point
| Fragmented sample data
| Nested (unclear format)
| DSP program
| 8-bit [[Fixed-point arithmetic|fixed point]]
| 16-bit fixed point
| 24-bit fixed point
| 32-bit fixed point
| (Unassigned)
| non-audio display data
| μ-law [[Squelch]] format<ref>{{cite web |title=Audio File and Compression Formats |url=https://docs.oracle.com/cd/A87860_01/doc/inter.817/a85336/mm_audfm.htm |website=docs.oracle.com}}</ref>
| 16-bit linear with emphasis
| 16-bit linear compressed
| 16-bit linear with emphasis and compression
| Music kit DSP commands 
| Music kit DSP commands samples
| [[G.726|ITU-T G.721]] 4-bit [[Adaptive differential pulse-code modulation|ADPCM]]
| [[G.722|ITU-T G.722]] [[Adaptive differential pulse-code modulation#SB-ADPCM|SB-ADPCM]]
| [[G.723|ITU-T G.723]] 3-bit ADPCM
| [[G.723|ITU-T G.723]] 5-bit ADPCM
| 8-bit [[G.711]] [[A-law algorithm|A-law]]
}}{{div col end}}

Values 0 through 255 are supposed to be assigned by a file format authority (was NeXT, now Oracle). Other values can be used for custom formats.<ref name=sox/>
|- valign = top
| 4 || Sample rate
| The number of samples/second, e.g., 8000, 11025, 22050, 44100, and 48000.<ref name=spec/> NeXT may use 8013.<ref name=sox/>
|- valign = top
| 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.
|- valign = top
| 6 || &ndash; 
| Optional annotation or description string, NULL-terminated. A minimum of 4 bytes must be stored even if unused.

In the older Sun version, its length had to be a non-zero multiple of 8 bytes. In some older implementations, the string is not properly NULL-terminated, but the offset remains reliable.<ref name=spec>[https://docs.oracle.com/cd/E36784_01/html/E36882/au-4.html Oracle man pages: au(4) - AU audio file format] (current specification)</ref>
|}

The type of encoding depends on the value of the "encoding" field (word 3 of the header). Formats 2 through 7 are uncompressed linear [[Pulse-code modulation|PCM]], therefore technically [[Lossless compression|lossless]] (although not necessarily free of [[Quantization (signal_processing)|quantization error]], especially in 8-bit form). Formats 1 and 27 are [[μ-law algorithm|μ-law]] and [[A-law algorithm|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; equivalent to, ''e.g.,'' encoding CD quality MP3 at a 352&nbsp;kbit rate using a low quality encoder). Several of the others (number 8 through 22) are [[digital signal processing|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 ==
{{Reflist}}

== External links ==
*[https://docs.oracle.com/cd/E26505_01/html/816-5177/audio-7i.html Oracle man pages: audio(7i) — generic audio device interface] (for information on the /dev/audio interface)

{{Compression formats|state=collapsed}}

{{DEFAULTSORT:Au File Format}}
[[Category:Computer file formats]]
[[Category:Digital container formats]]
[[Category:Audio codecs]]