Audio file format

An audio file format is a file format for storing digital audio data on a computer system. This data can be stored uncompressed, or compressed to reduce the file size. It can be a raw bitstream, but it is usually a container format or an audio data format with defined storage layer.

Types of formats

It is important to distinguish between a file format and an audio 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. Although most audio file formats support only one type of audio data (created with an audio coder), a multimedia container format (as Matroska 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), TTA, ATRAC Advanced Lossless, Apple Lossless (filename extension m4a), MPEG-4 SLS, MPEG-4 ALS, MPEG-4 DST, Windows Media Audio Lossless (WMA Lossless), and Shorten (SHN).
• Formats with lossy compression, such as MP3, Vorbis, Musepack, AAC, ATRAC and Windows Media Audio Lossy (WMA lossy)).

Uncompressed audio formats

There is one major uncompressed audio format, PCM, which 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 (IFF). The WAV format is based on the Resource Interchange File Format (RIFF), which is similar to IFF. 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.

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,^[1] Zaxcom,^[2] HHB USA,^[3] Fostex, and Aaton^[4] all use BWF as their preferred format.

The .cda (Compact Disk Audio Track) is a small file that serves as a shortcut to the audio data for a track on a music CD. It does not contain audio data and is therefore not considered to be a proper audio file format.

Lossless compressed audio formats

A lossless compressed format stores data in less space by eliminating unnecessary data.

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 portion of the file and the silence would take up almost no space at all.

Lossless compression formats enable the original uncompressed data to be recreated exactly. They include the common^[5] FLAC, WavPack, Monkey's Audio, ALAC (Apple Lossless). They provide a compression ratio of about 2:1 (i.e. their files take up half the space of the originals). Development in lossless compression formats aims to reduce processing time while maintaining a good compression ratio.

Lossy compressed audio formats

Lossy compression enables even greater reductions in file size by removing some of the data. Lossy compression typically achieves far greater compression but somewhat reduced quality than lossless compression by simplifying the complexities of the data.^[6] A variety of techniques are used, mainly by exploiting psychoacoustics, to remove data with minimal reduction in the quality of reproduction. For many everyday listening situations, the loss in data (and thus quality) is imperceptible. 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: Comparison of audio formats and List of codecs

• 3gp - multimedia container format can contain proprietary formats as AMR, AMR-WB or AMR-WB+, but also some open formats
• act - ACT is a lossy ADPCM 8 kbit/s compressed audio format recorded by most Chinese MP3 and MP4 players with a recording function, and voice recorders
• AIFF – standard audio file format used by Apple. It could be considered the Apple equivalent of wav.
• aac – the Advanced Audio Coding format is based on the MPEG2 and MPEG4 standards. aac files are usually ADTS or ADIF containers.
• ALAC - Apple Lossless compression, a lossless compression format from Apple.
• amr - AMR-NB audio, used primarily for speech.
• atrac (.wav) – the older style Sony ATRAC format. It always has a .wav file extension. To open these files, install the ATRAC3 drivers.
• 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.
• awb - AMR-WB audio, used primarily for speech, same as the ITU-T's G.722.2 specification.
• dct – A variable codec format designed for dictation. It has dictation header information and can be encrypted (as may be required by medical confidentiality laws). A proprietary format of NCH Software.
• 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.
• dvf – a Sony proprietary format for compressed voice files; commonly used by Sony dictation recorders.
• flac – File format for the Free Lossless Audio Codec, a lossless compression codec.
• 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.
• 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.
• 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.
• mmf - a Samsung audio format that is used in ringtones.
• 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.
• msv – a Sony proprietary format for Memory Stick compressed voice files.
• 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.
• ogg – a free, open source container format supporting a variety of formats, the most popular of which is the audio format Vorbis. Vorbis offers compression similar to MP3 but is less popular.
• 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.
• raw – a raw file can contain audio in any format but is usually used with PCM audio data. It is rarely used except for technical tests.
• TTA - The True Audio, real-time lossless audio codec.
• 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.
• 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 formats). Wav files use a RIFF structure.
• wma – the popular Windows Media Audio format owned by Microsoft. Designed with Digital Rights Management (DRM) abilities for copy protection.

See also

• Red Book (audio CD standard)
• Container format (digital) definition and article
• Comparison of container formats
• Audio compression (data)
• Comparison of video codecs

References

1. ^ Sound Devices, LLC | Professional, Portable Audio Products
2. ^ Zaxcom
3. ^ HHB USA | Essential Tools for Audio Professionals
4. ^ http://aaton.com
5. ^ 2008 ripping/encoding general poll - Hydrogenaudio Forums^{[unreliable source?]}
6. ^ Lossy Data^{[dead link]}

External links

• WikiRecording's Guide to Audio File Formats for Recording
• WikiRecording's Sound Designer II (SDII) File Format Article