Audio file format
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An audio file format is a container format for storing audio data on a computer system. There are many file formats for storing audio files.
The general approach towards storing 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. 44,100 times per second for CDDA or 48,000 or 96,000 times per second for DVD video) and store the value with a certain resolution (e.g. 16 bits per sample in CDDA). Therefore sample rate, resolution and number of channels (e.g. 2 for stereo) are key parameters in audio file formats.
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Types of formats
It is important to distinguish between a file format and a codec. Even though most audio file formats support only one audio codec, a file format may support multiple codecs, as AVI does.
There are three major groups of audio file formats:
- uncompressed formats, such as WAV, AIFF and AU.
- formats with lossless compression, such as FLAC, Monkey's Audio (filename extension APE), WavPack, Shorten, TTA, Apple Lossless and lossless Windows Media Audio (WMA).
- formats with lossy compression, such as MP3, Vorbis (filename extension OGG), lossy Windows Media Audio (WMA) and AAC.
Lossy audio formats
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.
As of 2002, one of the most popular audio file formats was MP3, which uses the MPEG-1 audio layer 3 codec to provide acceptable lossy compression for music files. The compression is about 10:1 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 uncompressed PCM.
There are many newer audio formats and codecs claiming to achieve improved compression and quality over MP3. Vorbis is an unpatented, free codec. Microsoft has its Windows Media Audio format.
Lossless audio formats
Lossless audio formats (such as TTA) provide compression about 2:1, 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.
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.
Uncompressed audio formats
There are many uncompressed data formats, the 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, though less so than losslessly compressed formats, for it encodes all sounds, whether complex or absolutely silent, with the same number of bits. The WAV format is based on the RIFF file format, which is similar to the IFF format.
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. This format is the primary recording format used in many professional Audio Workstations used in the Television and Film industry. Stand-alone file based multi-track recorders from Sound Devices, Zaxcom, HHB USA, Fostex, and Aaton all use BWF as their preferred file format for recording multi-track audio files with SMPTE Time Code reference. This standardized Time Stamp in the Broadcast Wave File allows for easy synchronization with a separate picture element.
Multiple channels
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).
It is a common misconception that 5.1 Surround sound includes 2 rear speakers. In fact, a 5.1 setup includes what Dolby call Surround speakers, and are actually placed at the sides of the listener. [1] "6.1" setups do however, include a single rear speaker placed at the rear centre, behind the listener - Dolby calls this setup Dolby Digital EX .[2] A 7.1 setup has the usual front 3 (front Left, front Centre, front Right), 2 Surround sound speakers situated to the left and right of the listener, and 2 rear speakers (rear Left and rear Right); with the usual Sub-woofer for bass - Dolby call this .1 / Sub speaker by the term LFE (Low-Frequency Effects).
See also
External links
- Audio File Types Definitions of audio file extensions
- libsndfile, an LGPLd library that can read and write many audio file formats
- [3] - iTunes file format AAC
- BWF-Widget Pro Utility for working with Broadcast Wave Files. Metadata reader/editor and BWF Playback with SMPTE Time Code.de:Audiodatei