Audio file format

An audio file format is a file format for storing audio data on a computer system. There are many file formats for storing audio files.

The general approach to store digital audio formats is to sample the audio voltage (corresponding to a certain position in the membrane of a speaker) in regular intervals (e. g. 44100 times per second for CDDA or 44000 times per second for DVD video) and store the value with a certain resolution (e. g. 2 bytes (or 16 bit) per sample in CDDA). Therefore sample rate, resolution and number of channels (e. g. 2 for stereo) are key parameters in audio file formats.

It's important to distinguish file format and codec, even though most audio file formats support only one audio codec, a file format may support multiple codecs, like AVI does.

There are two major groups of audio file formats:

• lossless formats, like WAV, PCM, TTA, FLAC, AU
• formats with lossy compression, like MP3, Ogg Vorbis, Windows Media Audio, AAC

Lossy file formats are based on psychoacoustic models, that leave out sounds, that humans cannot or can hardly hear, e. g. a low volume sound after a big volume sound. MP3 is such an example.

Lossless audio formats (such as TTA) provides compression about 1:2, but no data/quality is lost in the compression - when uncompressed, the data will be identical to the original. Lossless audio codecs are a good choice to keep the music's original quality. For example, using the free TTA lossless audio codec you can store up to 20 audio CDs from your music collection on one single DVD-R for playback.

As of 2002, one of the most popular audio file formats was MP3, which uses the MPEG audio layer 3 codec to provide acceptable lossy compression for music files. The compression is about 1:10 compared with uncompressed WAV files (in a standard compression scheme), therefore a CD with MP3 files can store about 10 hours of music, compared to one hour of the standard CDDA, which uses WAV files.

There are many newer audio formats and codecs claiming to achieve improved compression and quality vs. MP3. Ogg Vorbis is an unpatented, free codec. Microsoft has its Windows Media Audio format.

Lossless compression of sound is not nearly as widely used outside of professional applications, as lossy compression can provide a much greater data compression ratio, with nearly the same apparent quality.

There are many uncompressed data formats, most popular of them being WAV, which is a flexible file format designed to store multiple types of audio data. It is a good file format for storing and archiving an original recording. The WAV format is based on the RIFF file format which is similar to the AIFF and IFF formats.

BWF (Broadcast Wave Format) is a standard audio format created by the European Broadcasting Union as a successor to WAV. BWF allows metadata to be stored in the file. See: European Broadcasting Union: Specification of the Broadcast Wave Format - A format for audio data files in broadcasting. EBU Technical document 3285, July 1997.

Since the 1990s, movie theatres have upgraded their sound systems to surround sound systems that carry more than two channels. The most popular examples are Advanced Audio Coding or AAC (used by Apple's iTunes) and Dolby Digital, also known as AC-3. Both codecs are copyrighted and encoders/decoders cannot be offered without paying a licence fee. The most popular multi-channel format is called 5.1, with 5 normal channels (front left, front middle, front right, back left, back right) and a subwoofer channel to carry low frequencies only (the human ear cannot distinguish where the low frequencies come from).

See also

• Audio compression
• QuickTime
• TTA - codec with no data/quality loss

External links

• libsndfile (http://www.mega-nerd.com/libsndfile/), an LGPLd library that can read and write many audio file formats
• [1] (http://www.apple.com/itunes/import.html) - iTunes file format AACde:Audiodatei