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 specifying pixel aspect ratio.
More modern container formats (such as QuickTime, Matroska, Ogg and MP4) offer more flexibility, however, 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. 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
- Type 2: Like type 1, but audio is also saved as an additional audio stream into the file.
- Supported by VfW applications, at the price of little increased file size.
Type 1 is actually the newer of the two types. Microsoft made the "type" designations, and decided to name their older VfW-compatible version "Type 2", which only furthered confusion about the two types. 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 controllers usually captured to Type 1 only, while "consumer" level controllers usually captured to Type 2 only. Software is and was available for converting Type 1 AVIs to Type 2, and vice-versa, but this is a time-consuming process.
Many current FireWire controllers still only capture to one or the other type. However, almost all current DV software supports both Type 1 and Type 2 editing and rendering, including Adobe Premiere. Thus, many of today's users are unaware of the fact that there are two types of DV AVI files. In any event, the debate continues as to which – Type 1 or Type 2 – if either, is better.
[edit] References
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This section needs additional citations for verification. Please help improve this article by adding reliable references. Unsourced material may be challenged and removed. (December 2007) |
[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
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