Introduction
An audio file format is a file format for storing audio data on a computer system. It can be a raw bitstream, but it is usually a container format or an audio data format with defined storage layer.
The general approach towards storing digital audio is to sample the audio voltage which, on playback, would correspond to a certain level of signal in an individual channel with a certain resolution—the number of bits per sample—in regular intervals (forming the sample rate). This data can then be stored uncompressed, or compressed to reduce the file size.
Type Of Formats
It is important to distinguish between a file format and a codec. A codec performs the encoding and decoding of the raw audio data while the data itself is stored in a file with a specific audio file format. Most of the publicly documented audio file formats can be created with one of two or more encoders or codecs. Although most audio file formats support only one type of audio data (created with an audio coder), a multimedia container format (as MKV or AVI) 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 (filename extension APE), WavPack (filename extension WV), Shorten, TTA, ATRAC Advanced Lossless, Apple Lossless,MPEG-4 SLS, MPEG-4 ALS, MPEG-4 DST, Windows Media Audio Lossless (WMA Lossless).
- formats with lossy compression, such as MP3, Vorbis, Musepack, AAC, ATRAC and lossy Windows Media Audio (WMA).
Uncompressed audio formats
There is one major uncompressed audio format, PCM, which is usually stored as a .wav on Windows or as .aiff on Mac OS. WAV and AIFF are flexible file formats designed to store more or less any combination of sampling rates or bitrates. This makes them suitable file formats for storing and archiving an original recording. There is another file type .cda (Audio CD Track), which is a small file which serves as a shortcut to part of raw data on a music CD.
The AIFF format is based on the IFF format. 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 (EBU Technical document 3285, July 1997). 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 Sound Devices, Zaxcom, HHB USA, Fostex, and Aaton all use BWF as their preferred format.
Loseless compressed audio formats
A lossless compressed format requires much more processing time than an uncompressed format but is more efficient in space usage.
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 symphonic orchestra music. In a lossless compressed format, however, the music would occupy a marginally smaller file and the silence take up almost no space at all.
Lossless compression formats (such as the most widespread FLAC, WavPack, Monkey's Audio, ALAC/Apple Lossless) provide a compression ratio of about 2:1. Development in lossless compression formats aims to reduce processing time while maintaining a good compression ratio.
Free and Open Files Formats
- wav – standard audio file container format used mainly in Windows PCs. Commonly used for storing uncompressed (PCM), CD-quality sound files, which means that they can be large in size—around 10 MB per minute. Wave files can also contain data encoded with a variety of (lossy) codecs to reduce the file size (for example the GSM or mp3 codecs). Wav files use a RIFF structure.
- ogg – a free, open source container format supporting a variety of codecs, the most popular of which is the audio codec Vorbis. Vorbis offers compression similar to MP3 but is less popular.
- mpc - Musepack or MPC (formerly known as MPEGplus, MPEG+ or MP+) is an open source lossy audio codec, specifically optimized for transparent compression of stereo audio at bitrates of 160–180 kbit/s.
- flac – Free Lossless Audio Codec, a lossless compression codec.
- TTA - The True Audio, real-time lossless audio codec.
- aiff – standard audio file format used by Apple. It could be considered the Apple equivalent of wav.
- raw – a raw file can contain audio in any codec but is usually used with PCM audio data. It is rarely used except for technical tests.
- au – the standard audio file format used by Sun, Unix and Java. The audio in au files can be PCM or compressed with the μ-law, a-law or G729 codecs.
Open Files Formats
- gsm – designed for telephony use in Europe, gsm is a very practical format for telephone quality voice. It makes a good compromise between file size and quality. Note that wav files can also be encoded with the gsm codec.
- dct – A variable codec format designed for dictation. It has dictation header information and can be encrypted (often required by medical confidentiality laws).
- vox – the vox format most commonly uses the Dialogic ADPCM (Adaptive Differential Pulse Code Modulation) codec. Similar to other ADPCM formats, it compresses to 4-bits. Vox format files are similar to wave files except that the vox files contain no information about the file itself so the codec sample rate and number of channels must first be specified in order to play a vox file.
- mmf - a Samsung audio format that is used in ringtones
Proprietary Formats
- mp3 – MPEG Layer-3 format is the most popular format for downloading and storing music. By eliminating portions of the audio file that are less audible, mp3 files are compressed to roughly one-tenth the size of an equivalent PCM file sacrificing quality.
- aac – the Advanced Audio Coding format is based on the MPEG2 and MPEG4 standards. aac files are usually ADTS or ADIF containers.
- mp4 or m4a – MPEG-4 audio most often AAC but sometimes MP2/MP3, MPEG-4 SLS, CELP, HVXC and other audio object types defined in MPEG-4 Audio
- wma – the popular Windows Media Audio format owned by Microsoft. Designed with Digital Rights Management (DRM) abilities for copy protection.
- atrac (.wav) – the older style Sony ATRAC format. It always has a .wav file extension. To open these files simply install the ATRAC3 drivers.
- ra & rm – a Real Audio format designed for streaming audio over the Internet. The .ra format allows files to be stored in a self-contained fashion on a computer, with all of the audio data contained inside the file itself.
- ram – a text file that contains a link to the Internet address where the Real Audio file is stored. The .ram file contains no audio data itself.
- dss – Digital Speech Standard files are an Olympus proprietary format. It is a fairly old and poor codec. Gsm or mp3 are generally preferred where the recorder allows. It allows additional data to be held in the file header.
- msv – a Sony proprietary format for Memory Stick compressed voice files.
- dvf – a Sony proprietary format for compressed voice files; commonly used by Sony dictation recorders.
- IVS – A proprietary version with Digital Rights Management developed by 3D Solar UK Ltd for use in music downloaded from their Tronme Music Store and interactive music and video player.
- m4p – A proprietary version of AAC in MP4 with Digital Rights Management developed by Apple for use in music downloaded from their iTunes Music Store.
- iklax – An iKlax Media proprietary format, the iKlax format is a multi-track digital audio format allowing various actions on musical data, for instance on mixing and volumes arrangements.
- mxp4 – a Musinaut proprietary format allowing play of different versions (or skins) of the same song. It allows various interactivity scenarios between the artist and the end user.
- 3gp - multimedia container format can contain proprietary formats as AMR, AMR-WB or AMR-WB+, but also some open formats
- amr - AMR-NB audio, used primarily for speech
- awb - AMR-WB audio, used primarily for speech
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