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An au file is a computer file with the filename extension .au or sometimes .snd, and is an older audio file format often described as simple or basic. The au file format was commonly used by various early personal computer systems and web pages, but is not as commonly used today, so to listen to an au file on newer media players, it is often necessary to use an audio converter. This audio file format was created by the computer company Sun Microsystems, and originally used what is called the u-Law encoding method for data, referring to a logarithmic type of encoding mainly used in Japan and North America. Newer versions of the au format also support other types of audio encoding formats. Audio programs that can open an au file include QuickTime, Real Player's 32-bit version, Winamp, and Microsoft Windows Media Player.
A raw audio file created using the u-Law, sometimes called mu-Law, logarithmic encoding method is essentially identical to an au file, with the only difference being that newer au files have a header. Raw u-Law files and older au files were both headerless. This sound file format was used on computer systems developed and sold by NeXT, a computer company started by Steve Jobs in 1985. The u-Law method of logarithmic encoding of sound data was also extensively used on the Unix operating system, which was used by Sun Microsystems for some of its computer systems.
A newer au file has three basic parts: a header, an optional block of information that can vary in length, and the audio data itself. Included in the header is the sampling rate, the data length in bytes, and the number of channels. For example, if the header specifies the number of channels as one, the sound is in monaural, and if it is set to two, the sound is in stereo.
To listen to an au file using newer audio technology, conversion of the file is often necessary. Various audio converters on the market can handle the au format including Fx Audio Tools, ACDR, and Allok Audio Converter. In spite of the development of many newer computer file formats for audio, such as mp3, the au file format is still used on the Internet, and it can be used for sound by Java programs. Au files are also used by the digital audio editor Audacity, which is a freeware program.