Why is the audio out of sync after converting?

Common causes: (1) variable framerate source rendered as constant (use -vsync vfr to preserve VFR); (2) different audio sample rates not resampled (add -ar 48000 ); (3) container limitations (MP4 with variable framerate misbehaves — prefer MKV during editing, encode to MP4 only at the end). Always run ffprobe on both source and output to compare timing.

Why is the audio out of sync after converting?

Common causes: (1) variable framerate source rendered as constant (use -vsync vfr to preserve VFR); (2) different audio sample rates not resampled (add -ar 48000 ); (3) container limitations (MP4 with variable framerate misbehaves — prefer MKV during editing, encode to MP4 only at the end). Always run ffprobe on both source and output to compare timing.

Guide

AVI Format: Audio Video Interleave — Legacy Container Explained

PC By Pablo Cirre • Updated May 7, 2026

AVI (Audio Video Interleave) — Legacy Format Complete Guide

What Is AVI?

AVI (Audio Video Interleave) is a multimedia container format introduced by Microsoft in November 1992 as part of the Video for Windows technology. AVI became the dominant Windows video format throughout the 1990s and early 2000s, defining how video was stored, edited, and shared in the pre-streaming era. Despite its age, AVI remains relevant because millions of existing video files use the format and virtually all video software can read it.

The name "Audio Video Interleave" describes the format's storage approach: audio and video data are interleaved throughout the file rather than separated into distinct sections, enabling simultaneous playback from sequential reads — critical in the CD-ROM era when seeking was expensive.

RIFF Structure

AVI is based on RIFF (Resource Interchange File Format), a chunked binary format developed by Microsoft and IBM in 1991. RIFF organises data into chunks:

RIFF chunk
├─ AVI  (identifies as AVI file)
│   ├─ LIST hdrl   — Header list
│   │   ├─ avih    — AVI main header (frame rate, frame count, dimensions)
│   │   ├─ LIST strl  — Stream list (one per track)
│   │   │   ├─ strh   — Stream header (type, codec FourCC, frame rate)
│   │   │   ├─ strf   — Stream format (BITMAPINFOHEADER for video; WAVEFORMATEX for audio)
│   │   │   └─ strd   — Stream data (codec-specific initialization data)
│   │   └─ ... (additional strl for each stream)
│   ├─ LIST movi   — Movie data (interleaved video/audio chunks)
│   │   ├─ 00dc   — Video frame (stream 0, compressed video)
│   │   ├─ 01wb   — Audio chunk (stream 1, wave audio)
│   │   ├─ 00dc   — Next video frame
│   │   └─ ... (interleaved throughout file)
│   └─ idx1        — Legacy index (byte offsets into movi list)

The idx1 chunk at the end of legacy AVI files is a critical limitation: because the index is at the file end, a damaged or incomplete file (download interrupted) cannot be seeked or repaired without it.

OpenDML / AVI 2.0 Extension

The original AVI format has a critical limitation: the RIFF header uses 32-bit size fields, capping file size at 4 GB (2³² bytes). For early 1990s content this was irrelevant; for modern HD video at 30 Mbps, 4 GB is only ~18 minutes.

OpenDML (Open Digital Media Library) extended AVI with 64-bit addressing in 1996:

Multiple RIFF chunks chained together via the AVIX extension type
indx (super index) chunks at the stream list level pointing to per-chunk indices
Practical size limit becomes the filesystem limit (FAT32 = still 4 GB per file; NTFS = 16 EB)

OpenDML AVI can hold arbitrarily large content on modern filesystems. Most modern AVI files are OpenDML-compliant.

FourCC Video Codec Identification

AVI identifies codecs using FourCC (Four Character Code) — a 4-byte identifier in the stream header:

FourCC	Codec	Notes
`XVID`	Xvid MPEG-4	Most common historical AVI codec
`DIVX`	DivX MPEG-4	Early streaming codec (DVD rips)
`H264` / `AVC1`	H.264	Less common in AVI; usually MP4/MKV
`MJPG`	Motion JPEG	Webcam and industrial camera output
`DIV3`	DivX 3	Very early DivX; highly compatible
`MP42`	Microsoft MPEG-4 v2	Legacy Windows codec
`WMV1/2/3`	Windows Media Video	Rare in AVI; usually WMV container
`HFYU`	HuffYUV	Lossless intraframe video
`ULRG`	Ut Video	Modern lossless codec for AVI
`RGBA`	Uncompressed RGBA	Maximum quality; maximum size

Audio in AVI

Audio is identified by a WAVEFORMATEX format tag (wFormatTag):

Tag	Codec	Notes
`0x0001`	PCM	Uncompressed; maximum quality
`0x0055`	MP3	Constant bitrate MP3 (CBR only in AVI)
`0x0161`	WMA v2	Windows Media Audio
`0x2000`	AC-3	Dolby Digital 5.1
`0x00FF`	AAC	Rare in AVI
`0xFFFE`	WAVE_FORMAT_EXTENSIBLE	Multi-channel PCM extension

AVI MP3 limitation: AVI's MP3 support is limited to CBR (Constant Bitrate); Variable Bitrate MP3 (VBR) causes audio desynchronisation in AVI. This is a fundamental format limitation causing many "audio out of sync" complaints with MP3-in-AVI files.

AVI Limitations

Two-Stream Only (Legacy Limitation)

Traditional RIFF AVI supports exactly two streams: one video, one audio. Modern AVI (OpenDML) supports multiple streams, but most software and devices only recognise the first two. No subtitle tracks, no chapter support, no multiple audio languages.

4 GB Size Limit (Base Format)

Original AVI without OpenDML extension is limited to 4 GB. Transcoding long HD video to original-AVI format truncates the output.

Audio Desynchronisation

AVI stores audio in fixed-size chunks interleaved with video frames. If the audio codec uses variable frame sizes (VBR MP3, most modern codecs), the decoder must estimate timing from chunk positions rather than explicit timestamps — leading to cumulative drift in long files. Modern containers (MP4, MKV) use explicit timestamps for every audio packet, eliminating this problem.

No Streaming Optimisation

AVI lacks fast-start capability. The index is at the file end, requiring full download before playback can start. Unlike MP4's moov atom or MKV's SeekHead, there is no standardised way to move seeking metadata to the beginning of an AVI file.

No HDR or Wide Color Gamut Metadata

AVI's codec description structures have no fields for HDR10, HLG, or wide colour gamut information. Modern video workflows require containers that carry colour space metadata; AVI cannot.

Why AVI Is Still Encountered

Despite being superseded by MP4 and MKV, AVI files are still commonly found because:

Historical archival — millions of rips from DVD era (2000–2010) are in DivX/Xvid AVI
Webcam and industrial cameras — many use Motion JPEG AVI as native output
Legacy screen recording software — Camtasia and Fraps historically used AVI
Windows legacy compatibility — AVI plays in Windows Media Player without codecs for specific formats
Scientific instrumentation — microscopes, spectrometers, and other devices output Motion JPEG AVI as a simple, universally readable format

Converting AVI Files

AVI → MP4 (Most Common)

The standard modernisation path. H.264 or H.265 MP4 offers:

Equivalent or better quality at 50–70 % smaller file size
Universal device compatibility
Streaming and web playback
Proper timestamp-based audio sync

When the AVI source uses Xvid/DivX (MPEG-4 SP), re-encoding to H.264 provides significant quality improvement at smaller sizes.

AVI → MKV

For archival with additional tracks (subtitles, multiple audio), MKV is preferred over MP4. AVI video can often be remuxed into MKV without re-encoding if the codec is supported.

Preserving AVI Archives

For archives where the original encoding is valuable (e.g., historical preservation), consider remuxing to MKV with the original video codec rather than re-encoding, preserving bit-for-bit identical video data in a more capable container.

Related conversions

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Frequently Asked Questions

No — AVI is an older format with significant limitations: 4 GB size cap (base format), audio sync problems with VBR audio, no subtitle support, no streaming optimisation, and no HDR metadata. MP4 handles all these correctly. The only AVI advantage is legacy compatibility with very old software that predates MP4 support.

No — AVI is an older formato com significant limitations: 4 GB size cap (base formato), audio sync problems com VBR audio, no subtitle support, no streaming optimisation, e no HDR metadata. MP4 handles all these correctly. The only AVI advantage is legacy compatibilidade com very old software that predates MP4 support.

No — AVI is an older Format mit significant limitations: 4 GB size cap (base Format), audio sync problems mit VBR audio, no subtitle support, no streaming optimisation, und no HDR metadata. MP4 handles all these correctly. The only AVI advantage is legacy Kompatibilität mit very old Software that predates MP4 support.

No — AVI is an older formato con significant limitations: 4 GB size cap (base formato), audio sync problems con VBR audio, no subtitle support, no streaming optimisation, y no HDR metadata. MP4 handles all these correctly. The only AVI advantage is legacy compatibilidad con very old software that predates MP4 support.

AV1 is the most efficient (royalty-free, ~30% smaller than H.265) but encoding is slow. H.265 (HEVC) saves ~30–50% over H.264 and is supported by every modern phone and desktop. H.264 remains the safest baseline for legacy compatibility. Rule of thumb: archives → AV1, daily use → H.265, broadest reach → H.264.

The most common cause is VBR MP3 audio in an AVI container. AVI's MP3 support is designed for CBR (constant bitrate) only. VBR MP3 packets have variable sizes that confuse AVI's chunk-based timing, causing cumulative drift. Fix by re-encoding to MP4 (which uses explicit timestamps), or re-encoding the audio to PCM or CBR MP3 before muxing into AVI.

The most common cause is VBR MP3 audio in an AVI container. AVI's MP3 support is designed para CBR (constant bitrate) only. VBR MP3 packets have variable sizes that confuse AVI's chunk-based timing, causing cumulative drift. Fix by re-encoding to MP4 (which uses explicit timestamps), ou re-encoding the audio to PCM ou CBR MP3 antes muxing em AVI.

The most common cause is VBR MP3 audio in an AVI Container. AVI's MP3 support is designed für CBR (constant Bitrate) only. VBR MP3 packets have variable sizes that confuse AVI's chunk-based timing, causing cumulative drift. Fix by re-encoding to MP4 (which uses explicit timestamps), oder re-encoding the audio to PCM oder CBR MP3 vor muxing in AVI.

The most common cause is VBR MP3 audio in an AVI contenedor. AVI's MP3 support is designed para CBR (constant bitrate) only. VBR MP3 packets have variable sizes that confuse AVI's chunk-based timing, causing cumulative drift. Fix by re-encoding to MP4 (which uses explicit timestamps), o re-encoding the audio to PCM o CBR MP3 antes muxing en AVI.

CRF (Constant Rate Factor) is the best default for offline files: ffmpeg picks the bitrate frame-by-frame to maintain perceived quality. Two-pass is only better when you must hit an exact final size (DVD targets). Constant bitrate is for streaming with a fixed channel. For "smallest at quality X" always use CRF.

Yes, with the OpenDML extension (AVI 2.0). OpenDML uses 64-bit addressing and can create AVI files of any size on NTFS or ext4 filesystems. The original RIFF-based AVI is limited to 4 GB. Most modern encoding software creates OpenDML-compliant AVI automatically for large files.

Sim, com the OpenDML extension (AVI 2.0). OpenDML uses 64-bit addressing e can create AVI arquivos of any size on NTFS ou ext4 filesystems. The original RIFF-based AVI is limited to 4 GB. Most moderno codificação software creates OpenDML-compliant AVI automatically para large files.

Ja, mit the OpenDML extension (AVI 2.0). OpenDML uses 64-bit addressing und can create AVI Dateien von any size on NTFS oder ext4 filesystems. The original RIFF-based AVI is limited to 4 GB. Most modern Codierung Software creates OpenDML-compliant AVI automatically für large files.

Sí, con the OpenDML extension (AVI 2.0). OpenDML uses 64-bit addressing y can create AVI archivos de any size on NTFS o ext4 filesystems. The original RIFF-based AVI is limited to 4 GB. Most moderno codificación software creates OpenDML-compliant AVI automatically para large files.

Common causes: (1) variable framerate source rendered as constant (use <code>-vsync vfr</code> to preserve VFR); (2) different audio sample rates not resampled (add <code>-ar 48000</code>); (3) container limitations (MP4 with variable framerate misbehaves — prefer MKV during editing, encode to MP4 only at the end). Always run <code>ffprobe</code> on both source and output to compare timing.

The dominant codecs for DVD rips in AVI format (2001–2010) were DivX and Xvid — both implementations of MPEG-4 Simple Profile (MPEG-4 SP, also called MPEG-4 Part 2). These produced acceptable quality at 700 MB per movie (to fit on a CD-R). They are now superseded by H.264 and H.265 which offer significantly better quality at similar or smaller file sizes.

The dominant codecs para DVD rips in AVI formato (2001–2010) were DivX e Xvid — both implementations of MPEG-4 simples Profile (MPEG-4 SP, also called MPEG-4 Part 2). These produced acceptable quality at 700 MB per movie (to fit em um CD-R). They are now superseded by H.264 e H.265 which offer significantly better quality at similar ou smaller tamanho do arquivos.

The dominant Codecs für DVD rips in AVI Format (2001–2010) were DivX und Xvid — both implementations von MPEG-4 einfach Profile (MPEG-4 SP, also called MPEG-4 Part 2). These produced acceptable quality at 700 MB per movie (to fit auf einem CD-R). They are now superseded by H.264 und H.265 which offer significantly better quality at similar oder smaller Dateigrößes.

The dominant codecs para DVD rips in AVI formato (2001–2010) were DivX y Xvid — both implementations de MPEG-4 simple Profile (MPEG-4 SP, also called MPEG-4 Part 2). These produced acceptable quality at 700 MB per movie (to fit en un CD-R). They are now superseded by H.264 y H.265 which offer significantly better quality at similar o smaller tamaño de archivos.

Yes if you only change the container: <code>ffmpeg -i in.mkv -c copy out.mp4</code>. This remuxes the stream without re-encoding, takes seconds even for hours of footage. Limitations: codec must be supported by the target container (e.g. you cannot put H.264 in WebM, only VP8/VP9/AV1). To shrink size you must re-encode.