Audio Video Interleave
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| Filename extension | .avi |
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| Internet media type | video/avi |
| Type code | 'Vfw ' |
| Uniform Type Identifier | public.avi |
| Developed by | Microsoft |
| Container for | Audio, Video |
Audio Video Interleave, known by its acronym AVI, is a multimedia container format introduced by Microsoft in November 1992 as part of its Video for Windows technology. AVI files can contain both audio and video data in a file container that allows synchronous audio-with-video playback. Like the DVD video format, AVI files support multiple streaming audio and video, although these features are seldom used. Most AVI files also use the file format extensions developed by the Matrox OpenDML group in February 1996. These files are supported by Microsoft, and are unofficially called "AVI 2.0".
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[edit] Format
AVI is a derivative of the Resource Interchange File Format (RIFF), which divides a file's data into blocks, or "chunks." Each "chunk" is identified by a FourCC tag. An AVI file takes the form of a single chunk in a RIFF formatted file, which is then subdivided into two mandatory "chunks" and one optional "chunk".
The first sub-chunk is identified by the "hdrl" tag. This sub-chunk is the file header and contains metadata about the video, such as its width, height and frame rate. The second sub-chunk is identified by the "movi" tag. This chunk contains the actual audio/visual data that make up the AVI movie. The third optional sub-chunk is identified by the "idx1" tag which indexes the offsets of the data chunks within the file.
By way of the RIFF format, the audio/visual data contained in the "movi" chunk can be encoded or decoded by software called a codec, which is an abbreviation for (en)coder/decoder. Upon creation of the file, the codec translates between raw data and the (compressed) data format used inside the chunk. An AVI file may carry audio/visual data inside the chunks in virtually any compression scheme, including Full Frame (Uncompressed), Intel Real Time (Indeo), Cinepak, Motion JPEG, Editable MPEG, VDOWave, ClearVideo / RealVideo, QPEG, and MPEG-4 Video.
[edit] Continued use
There is slight overhead when used with popular MPEG-4 codecs (Xvid and DivX, for example), increasing file size more than necessary. The AVI container has no native support for modern MPEG-4 features like B-Frames. Hacks are sometimes used to enable modern MPEG-4 features and subtitles, however, this is the source of playback incompatibilities.
AVI files do not contain pixel aspect ratio information. Microsoft confirms that "many players, including Windows Media Player, render all AVI files with square pixels. Therefore, the frame appears stretched or squeezed horizontally when the file is played back."[1] There are other video container formats that allow irregular shaped pixels.
More modern container formats (such as QuickTime, Matroska, Ogg and MP4) offer more flexibility, however, the age of the AVI format, being widely supported on a vast range of operating systems and devices, and the availability of video editing and playback software like VirtualDub and Windows Media Player[citation needed] help keep the AVI file format popular amongst amateur videographers. To improve interoperability with the widest possible audience, videographers commonly install DirectShow filters like ffdshow to augment DirectShow, using AVI as a common-ground, lowest-common-denominator format.[citation needed]
Projects based on the FFmpeg project, including ffdshow, MPlayer, xine, and VLC media player, have solved most problems with viewing AVI format video files.
In June 2005, DivX, Inc. released its own container format called DivX Media Format (.divx extension) to succeed the AVI + DivX combo. However, this format is basically an enhanced AVI format (based on the same RIFF structure, for backward compatibility with existing players and devices) and so far, has gained no perceivable consumer traction[citation needed], even where the DivX codec was once popular (the Xvid codec has instead become the codec of choice among most of the file-sharing groups [2]).[dubious ]
[edit] DV AVI
DV AVI (aka DV-AVI) is a type of AVI file where the video has been compressed to conform with DV standards. Typically, a DV device such as a DV camcorder captures video in this format. Contrary to popular belief owing to the larger size of DV AVI files, the format is not uncompressed, but compresses only the individual frames (intraframe or spatial compression), and does not apply interframe compression (temporal compression) leading to relatively higher file sizes with lesser quality loss than files compressed using AVI compression codecs. There are two types of DV AVI files:
- Type 1: The multiplexed Audio-Video is kept in its original multiplexing and saved together into the Video section of the AVI file. This does not waste much space (audio is saved uncompressed, but even uncompressed audio is tiny compared to the video part of DV). Windows applications based on the Video for Windows API do not support it by default since a Type 1 DV AVI decompressor is not part of Windows.
- Type 2: Like Type 1, but audio is also saved as an additional audio stream into the file. This type supported by Video for Windows applications, at the cost of slightly increased file size.
Type 1 is the newer of the two types. Microsoft made the "type" designations, and decided to name their older Video for Windows-compatible version "Type 2"[3]. In the late 1990s through early 2000s, most professional-level DV software, including non-linear editing programs, only supported Type 1. One notable exception was Adobe Premiere, which only supported Type 2. High-end FireWire camcorders usually captured to Type 1 only[citation needed], while consumer level camcorders usually captured to Type 2 only. Software is available for converting Type 1 DV AVI files to Type 2, and vice-versa.
[edit] References
- ^ "Determining the Shape of Pixels and Frames". http://www.microsoft.com/windows/windowsmedia/howto/articles/PixelFrames.aspx.
- ^ "Scene Rules - Rules for the file-sharing scene indicating that Xvid is the preferred MPEG-4 ASP codec since 2004". http://www.aboutthescene.com/thescene/scenerules.html.
- ^ "DV Video Data and AVI Files". Microsoft. 2001-12-04. http://www.microsoft.com/whdc/archive/dvavi.mspx. Retrieved on 2008-06-30.
[edit] External links
- Myths about AVI
- John P. McGowan's AVI Overview
- Microsoft's AVI RIFF File Reference
- PVDTools' RIFF-AVI File Format Documentation
- Dave Wilson's comprehensive list of FourCC codes
- OpenDML (AVI 2.0) format specification
- AVI Implementation for .NET
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