File extension

`.avi`

AVI File Format Documentation

.avi is primarily used for Audio Video Interleave (AVI) multimedia container files introduced by Microsoft for Video for Windows, interleaving audio and video streams in a RIFF chunk structure.

Known meanings

Formats and reference material for `.avi`

AVI File Format Documentation

Microsoft Audio Video Interleave file format documentation describing the RIFF-based AVI container, headers, streams, and index; 1996 reference

video-animation windows
Microsoft Corporation

Download

AVI Overview FAQ

Practitioner-oriented FAQ covering AVI structure, codecs, players, and common interoperability problems

video-animation windows
John F. McGowan

Open link

OpenDML AVI File Format Extensions 1.02

OpenDML extensions to the AVI file format addressing the old AVI 1.0 size limits by chaining AVIX RIFF chunks and adding standard index chunks; version 1.02, 1996/1997

video-animation windows
OpenDML AVI M-JPEG File Format Committee (Matrox Electronic Systems Ltd.)

Open link

Details

Format notes

Audio Video Interleave (AVI)

AVI is a multimedia container format introduced by Microsoft in 1992 as part of Video for Windows. The name Audio Video Interleave, also written Audio Video Interleaved, refers to the way audio and video data are interleaved inside the file so that playback software can read both streams sequentially from disk without seeking back and forth across the whole file. The common extension is .avi.

AVI is built on RIFF, the Resource Interchange File Format, the same tagged chunk structure used by WAV audio. That makes AVI structurally simple and widely supported, but it also means AVI is only a container. The actual audio and video samples inside are encoded with separate codecs, and a file that one player opens without trouble may fail in another player that lacks the matching codec.

File Structure

An AVI file is a RIFF form. It begins with the four ASCII bytes RIFF, followed by a four-byte little-endian size, followed by the form type AVI (the letters AVI followed by a trailing space). The rest of the file is a sequence of chunks, each made of a four-character tag, a four-byte little-endian size, and a payload padded to an even byte boundary.

The main body is organized into three parts. LIST 'hdrl' is the header list and is required to appear first. It contains the avih main AVI header chunk and one LIST 'strl' sub-list per stream. Each stream list holds an strh stream header, an strf stream format chunk, and optional strd, strn, or strl-specific chunks. LIST 'movi' holds the actual interleaved media data. The optional idx1 chunk near the end is a legacy index that records the position and size of every data chunk inside movi.

The avih main header records timing and layout facts such as microseconds per frame, maximum bytes per second, total frames, number of streams, suggested buffer size, and the video width and height. Each strh stream header records a FourCC type such as vids for video, auds for audio, txts for text, or mids for MIDI, a codec handler FourCC, and a dwRate and dwScale pair whose ratio dwRate / dwScale gives the stream rate in samples per second. For video this is the frame rate; for audio it describes the timing of nBlockAlign audio units and equals the PCM sample rate only in the usual PCM case. The strf chunk follows the stream type: video streams use a BITMAPINFO / BITMAPINFOHEADER-based structure, including palette information when appropriate, while audio streams use a WAVEFORMATEX structure. Optional strd chunks carry codec-specific initialization data that some decoders require, and strn chunks carry a human-readable stream name.

Inside movi, data chunks are tagged with a two-digit stream index plus a two-character type. For example, 00dc is a compressed video frame for stream zero, 00db is an uncompressed video frame for stream zero, and 01wb is audio waveform bytes for stream one. Related chunks may be grouped inside a rec list so they can be read as a single record, which helps interleaved playback and editing.

Identification And Indexing

The most reliable identification is the RIFF signature followed by the AVI form type. AVI files using OpenDML may append additional RIFF chunks whose form type is AVIX, especially when crossing the old AVI 1.0 size limits, and may introduce standard index chunks such as indx and per-stream ix## indexes. Some tools also write JUNK chunks, which are padding used to align later structures on file offsets; these are valid and should be skipped rather than treated as corruption.

The legacy idx1 index is not always present, and when present it may be inconsistent with the actual movi layout if a file was edited or written by a non-conforming tool. Players that need robust seeking often rebuild the index from the movi chunks instead of trusting idx1.

Codecs And Compatibility

AVI does not mandate any particular codec. Common video codecs found inside AVI include DivX, Xvid, Microsoft MPEG-4, Intel Indeo, Cinepak, Motion JPEG, DV, HuffYUV, and uncompressed RGB. Common audio codecs include MP3, AC3, PCM, and Microsoft ADPCM. Because the container only describes framing and timing, two .avi files can both be valid AVI while requiring completely different decoder support.

This codec flexibility is the main source of AVI interoperability problems. A machine missing a specific codec will show video but no audio, audio but no video, or refuse the file entirely, sometimes with a misleading “corrupt file” message. For preservation or exchange, record the codec FourCCs for each stream in addition to keeping the .avi container, because decoding depends on matching external codec support that is not embedded in the file.

Limitations And AVI 2.0

Classic AVI 1.0 has practical limits. RIFF sizes are 32-bit, and many implementations treat them as signed, so individual files are effectively capped near 2 GB, with older FAT16-era tools and MCIAVI implementations hitting limits closer to 1 GB. The basic format has no standardized way to store aspect ratio, reliable variable frame rate, timecode, or modern metadata, and no broadly interoperable modern subtitle-track model despite support for txts text streams. B-frame video streams, common in later MPEG-4 codecs, are handled through non-standard workarounds that not every splitter honors.

The OpenDML AVI File Format Extensions, sometimes called AVI 2.0, address the old AVI 1.0 size limits, commonly encountered around 1–2 GB depending on implementation and filesystem, by chaining additional RIFF chunks with the AVIX form type and by adding richer index chunks. OpenDML uses a per-stream index model: an indx chunk may appear in the stream header after strf, and a super index can point to ix## standard or field index chunks interleaved inside movi. The field-level indexing matters for interlaced professional video. Most modern large AVI files employ the DML extensions, and the Library of Congress reports NTFS can carry OpenDML AVI files up to tens of terabytes. Older tools may still reject AVIX chunks or mishandle them, so the classic 2 GB boundary remains a practical compatibility threshold.

Preservation And Security Notes

AVI preservation should keep the original file when possible and validate more than the RIFF signature. Useful checks include chunk sizes and padding, the presence and ordering of the hdrl, movi, and index chunks, per-stream headers, stream counts, frame counts, and whether the idx1 index agrees with the movi data. Record the codec FourCCs for every stream, because the file is not self-describing beyond the container level.

AVI parsers need strict bounds checks. Malformed files can produce recursive or deeply nested LIST structures, chunk sizes that point outside the file, negative or zero sizes, large allocation requests from header fields, or deliberately inconsistent indexes that trick a player into seeking to attacker chosen offsets. Codec parsing bugs are a separate and historically large attack surface, so pipelines that accept arbitrary AVI files should demux in a constrained process and decode streams with sandboxed or actively maintained codecs. Modern containers such as MP4 and Matroska generally support contemporary features more cleanly, but AVI remains common in legacy archives.

Formats and reference material for .avi

AVI File Format Documentation

AVI Overview FAQ

OpenDML AVI File Format Extensions 1.02

Format notes

Audio Video Interleave (AVI)

File Structure

Identification And Indexing

Codecs And Compatibility

Limitations And AVI 2.0

Preservation And Security Notes

Further Reading

Formats and reference material for `.avi`