The Au file format is a simple

audio file format An audio file format is a file format for storing digital audio data on a computer system. The bit layout of the audio data (excluding metadata) is called the audio coding format and can be uncompressed, or audio compression (data), compressed t ...

introduced by

Sun Microsystems Sun Microsystems, Inc., often known as Sun for short, was an American technology company that existed from 1982 to 2010 which developed and sold computers, computer components, software, and information technology services. Sun contributed sig ...

. The format was common on

NeXT NeXT, Inc. (later NeXT Computer, Inc. and NeXT Software, Inc.) was an American technology company headquartered in Redwood City, California that specialized in computer workstations for higher education and business markets, and later develope ...

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 In computer architecture, 32-bit computing refers to computer systems with a processor, memory, and other major system components that operate on data in a maximum of 32- bit units. Compared to smaller bit widths, 32-bit computers can perform la ...

words, an optional information chunk which is always of non-zero size, and then the data (in

big-endian '' Jonathan_Swift.html" ;"title="Gulliver's Travels'' by Jonathan Swift">Gulliver's Travels'' by Jonathan Swift, the novel from which the term was coined In computing, endianness is the order in which bytes within a word (data type), word of d ...

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 The SPARCstation 1 (Sun 4/60, code-named ''Campus'') is the first of the SPARCstation series of SPARC-based workstations sold by Sun Microsystems. The design originated in 1987 by a Sun spin-off company, UniSun, which was soon re-acquired. The ...

hardware, where

SunOS SunOS is a Unix-branded operating system developed by Sun Microsystems for their workstation and server computer systems from 1982 until the mid-1990s. The ''SunOS'' name is usually only used to refer to versions 1.0 to 4.1.4, which were based ...

exposed the encoding to application programs through the /dev/audio

device file In Unix-like operating systems, a device file, device node, or special file is an interface to a device driver that appears in a file system as if it were an ordinary file. There are also special files in DOS, OS/2, and Windows. These s ...

interface. This encoding and interface became a de facto standard for

Unix Unix (, ; trademarked as UNIX) is a family of multitasking, multi-user computer operating systems that derive from the original AT&T Unix, whose development started in 1969 at the Bell Labs research center by Ken Thompson, Dennis Ritchie, a ...

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 through 7 are uncompressed linear

PCM Pulse-code modulation (PCM) is a method used to Digital signal (signal processing), digitally represent analog signals. It is the standard form of digital audio in computers, compact discs, digital telephony and other digital audio application ...

, therefore technically

lossless Lossless compression is a class of data compression that allows the original data to be perfectly reconstructed from the compressed data with no loss of information. Lossless compression is possible because most real-world data exhibits statisti ...

(although not necessarily free of

quantization error Quantization, in mathematics and digital signal processing, is the process of mapping input values from a large set (often a continuous set) to output values in a (countable) smaller set, often with a finite number of elements. Rounding and ...

, especially in 8-bit form). Formats 1 and 27 are μ-law and

A-law An A-law algorithm is a standard companding algorithm, used in European 8-bit PCM digital communications systems to optimize, i.e. modify, the dynamic range of an analog signal for digitizing. It is one of the two companding algorithms in the ...

, respectively, both

companding In telecommunications and signal processing, companding (occasionally called compansion) is a method of mitigating the detrimental effects of a channel with limited dynamic range. The name is a portmanteau of the words compressing and expandi ...

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 In information technology, lossy compression or irreversible compression is the class of data compression methods that uses inexact approximations and partial data discarding to represent the content. These techniques are used to reduce data size ...

, 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

Music Kit The Music Kit was a software package for the NeXT Computer system. First developed by David A. Jaffe and Julius O. Smith, it supported the Motorola 56001 DSP that was included on the NeXT Computer's motherboard. It was also the first architectur ...

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

External links

Oracle man pages: audio(7i) — generic audio device interface
(for information on the /dev/audio interface) {{DEFAULTSORT:Au File Format Computer file formats Digital container formats Audio codecs