Audio Video Interleave

Audio Video Interleave

Filename extension	.avi
Internet media type	video/vnd.avi[1] video/avi video/msvideo video/x-msvideo
Type code	'Vfw '
Uniform Type Identifier	public.avi
Developed by	Microsoft
Container for	Audio, Video

Audio Video Interleave (also Audio Video Interleaved), 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".

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.

Metadata

As a derivative of the Resource Interchange File Format (RIFF), AVI files are commonly tagged with metadata in the INFO chunk. In addition, AVI files can embed Extensible Metadata Platform (XMP). By design, any RIFF file can legally include additional chunks of data, each identified by a four-character code; software which does not understand that particular code should simply skip the chunk. As such, it is theoretically possible to expand any RIFF file format, including AVI, to support almost any conceivable metadata. Some of the limitations of AVI in modern use relate to a lack of standardization in this metadata (see Limitations below).

Limitations

Since its introduction in the early 90s, new computer video techniques have been introduced which the original AVI specification did not anticipate:

• AVI does not provide a standardized way to encode aspect ratio information, with the result that players cannot select the right one automatically (though it may be possible to do so manually).^[2]
• There are several competing approaches to including a time code in AVI files, which affects usability of the format in film and television post-production, although it is widely used. For WAV audio files, Broadcast Wave extensions were designed to standardize post-production metadata, but an equivalent for AVI files has not emerged.
• AVI is not intended to contain variable frame rate material. Workarounds for this limitation increase overhead dramatically.
• AVI was not intended to contain video using any compression technique which requires access to future video frame data beyond the current frame. Approaches exist to support modern video compression techniques (such as MPEG-4) which rely on this function, although this is beyond the intent of the original specification and may cause problems with playback software which does not anticipate this use.
• AVI cannot contain some specific types of variable bitrate (VBR) data reliably (such as MP3 audio at sample rates below 32 kHz).
• Overhead for AVI files at the resolutions and frame rates normally used to encode standard definition feature films is about 5 MB per hour of video, the significance of which varies with the application.

More recent container formats (such as Matroska, Ogg and MP4) solve all these problems, although software is freely available to both create and correctly replay AVI files which use these more recent techniques.

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
  • Does not waste much space (audio is saved uncompressed, but even uncompressed audio is tiny compared to the video part of DV), but Windows applications based on the VfW API do not support it.
• 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 a small increase in 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.

See also

• Comparison of container formats
• List of video editing software

References

1. ^ Microsoft Corporation (June 1998). "WAVE and AVI Codec Registries - RFC 2361". IETF. http://tools.ietf.org/html/rfc2361. Retrieved 2009-12-06. 
2. ^ "Determining the Shape of Pixels and Frames". http://www.microsoft.com/windows/windowsmedia/howto/articles/PixelFrames.aspx. 

External links

• 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 file format documentation