An audio file format is a file format for storing digital audio data on a computer system.
The bit layout of the audio data is called the audio coding format and can be uncompressed,
or compressed to reduce the file size, often using lossy compression.
The data can be a raw bitstream in an audio coding format, but it is usually embedded
in a container format or an audio data format with defined storage layer.
Format types It is important to distinguish between the
audio coding format, the container containing the raw audio data, and an audio codec.
A codec performs the encoding and decoding of the raw audio data while this encoded data
is stored in a container file.
Although most audio file formats support only one type of audio coding data, a multimedia
container format may support multiple types of audio and video data.
There are three major groups of audio file formats:
Uncompressed audio formats, such as WAV, AIFF, AU or raw header-less PCM;
Formats with lossless compression, such as FLAC, Monkey's Audio, WavPack, TTA, ATRAC
Advanced Lossless, Apple Lossless, MPEG-4 SLS, MPEG-4 ALS, MPEG-4 DST, Windows Media
Audio Lossless, and Shorten.
Formats with lossy compression, such as MP3, Vorbis, Musepack, AAC, ATRAC and Windows Media
Audio Lossy.
Uncompressed audio format
There is one major uncompressed audio format, LPCM, which is the same variety of PCM as
used in Compact Disc Digital Audio.
Although LPCM can be stored on a computer as a raw audio format, it is usually stored
in a .wav file on Windows or in a .aiff file on Mac OS.
The AIFF format is based on the Interchange File Format, and the WAV format is based on
the similar Resource Interchange File Format.
WAV and AIFF are not inherently lossless; they're designed to store a wide variety of
audio formats, lossless and lossy; they just add a small, metadata-containing header before
the audio data to declare the format of the audio data, such as LPCM with a particular
sample rate, bit depth, endianness and number of channels.
Since WAV and AIFF are widely supported and can store LPCM, they are suitable file formats
for storing and archiving an original recording.
BWF is a standard audio format created by the European Broadcasting Union as a successor
to WAV.
Among other enhancements, BWF allows more robust metadata to be stored in the file.
See European Broadcasting Union: Specification of the Broadcast Wave Format.
This is the primary recording format used in many professional audio workstations in
the television and film industry.
BWF files include a standardized timestamp reference which allows for easy synchronization
with a separate picture element.
Stand-alone, file based, multi-track recorders from AETA, Sound Devices, Zaxcom, HHB Communications
Ltd, Fostex, Nagra, Aaton, and TASCAM all use BWF as their preferred format.
Lossless compressed audio format A lossless compressed format stores data in
less space without losing any information.
The original, uncompressed data can be recreated from the compressed version.
Uncompressed audio formats encode both sound and silence with the same number of bits per
unit of time.
Encoding an uncompressed minute of absolute silence produces a file of the same size as
encoding an uncompressed minute of music.
In a lossless compressed format, however, the music would occupy a smaller file than
an uncompressed format and the silence would take up almost no space at all.
Lossless compression formats include the common FLAC, WavPack, Monkey's Audio, ALAC.
They provide a compression ratio of about 2:1.
Development in lossless compression formats aims to reduce processing time while maintaining
a good compression ratio.
Lossy compressed audio format Lossy compression enables even greater reductions
in file size by removing some of the audio information and simplifying the data.
This of course results in a reduction in audio quality, but a variety of techniques are used,
mainly by exploiting psychoacoustics, to remove the parts of the sound that have the least
effect on perceived quality, and to minimize the amount of audible noise added during the
process.
The popular MP3 format is probably the best-known example, but the AAC format found on the iTunes
Music Store is also common.
Most formats offer a range of degrees of compression, generally measured in bit rate.
The lower the rate, the smaller the file and the more significant the quality loss.
List of formats
See also Audio compression
Comparison of audio coding formats Comparison of container formats
Comparison of video codecs Timeline of audio formats
References