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  • Audio Video Interleave

    Audio Video Interleave (also Audio Video Interleaved), known by its initials AVI, is a multimedia container format introduced by Microsoft in November 1992 as part of its Video for Windows software. 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".

    AVI is a subformat 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.

    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 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).

    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.