Sidst opdateret: 30 Jun, 2026
TL;DR – AVI (Audio Video Interleave) er Microsofts første “digital video” container, født med Windows 95. Det er en simpel RIFF‑baseret fil, der interleaverer komprimerede video‑ og lyd‑chunks, så en afspiller kan læse dem i takt. Formatet forstås stadig af Windows Media Player og en række open‑source‑værktøjer, men det mangler moderne funktioner som HDR, 10‑bit farve og robust streaming‑metadata. Hvis du nogensinde har brug for at grave i ældre optagelser, forstå chunk‑layoutet, FourCC‑koderne og OpenDML‑udvidelserne, der løfter 2 GB‑grænsen – det er kernen i AVI.
1. Hvad er en AVI-fil egentlig?
- Fuldt navn: Audio Video Interleave
- Første optræden: Windows 95 (1995) – Microsofts svar på “digital video” på forbruger‑PC’en.
- Specifikation: Bygget på RIFF (Resource Interchange File Format) specifikationen fra 1991. RIFF er en generisk “chunk‑baseret” container; AVI definerer blot et sæt af chunks, der indeholder lyd, video og indeksdata.
- Filudvidelse / MIME:
.avi–video/x-msvideo. - Primært mål: Hold lyd- og videostrømme tæt sammenflettet på disken, så en simpel afspilningsløkke kan læse en videoramme, derefter den tilsvarende lydprøve, uden dyre søgninger.
- Legacy-status: Stadig afspilningsbar i Windows Media Player, DirectShow, VLC og utallige andre afspillere, men det er ikke en “moderne” container. Ingen indbygget HDR, 10‑bit, variabel bitrate eller rig metadataunderstøttelse.
2. Inde i boksen – Sådan fungerer AVI
RIFF-chunk-skelettet
En AVI-fil er blot en række af chunks:
RIFF <size> "AVI " ; file header
LIST "hdrl" ; header list
avih ... ; main AVI header (global info)
LIST "strl" ; stream list (one per stream)
strh ... ; stream header (type, codec, timing)
strf ... ; stream format (codec‑specific data)
LIST "movi" ; interleaved media data
00dc <size> <video frame>
01wb <size> <audio block>
…
idx1 ... ; optional index for fast seeking
- Chunk-ID (4 bytes) – f.eks.
avih,strh,movi. - Chunk-størrelse (4 bytes) – længden af de efterfølgende data (eksklusive ID- og størrelsesfelterne).
- Data – den faktiske nyttelast (headers, rå rammer osv.).
Fordi RIFF er udvidelig, kan du tilføje nye chunk-typer uden at ødelægge ældre parsere – et design der har holdt AVI i live i årtier.
FourCC – Kodec‑hvisker
Fire‑tegnskoder (FourCC) er limen, der fortæller en afspiller hvilken decoder der skal indlæses. Nogle almindelige, du vil se i AVI‑filer:
| FourCC | Codec (ffmpeg) | Typisk brug |
|---|---|---|
DIVX | mpeg4 (DivX) | Tidlig MPEG‑4 Part 2 |
XVID | mpeg4 (Xvid) | Open‑source MPEG‑4 |
MJPG | mjpeg | Motion‑JPEG (digitale videokameraer) |
H264 | h264 | H.264/AVC (sjælden men mulig) |
MP3 | mp3 | MP3-lydstrøm |
PCM | pcm_s16le | Ukomprimeret PCM-lyd |
FourCC findes i strh‑chunk (codec‑identifikator) og nogle gange i strf‑blokken (pixelformat, lydprøveformat).
Timing & synkronisering
AVI bruger et simpelt time‑base‑system:
- Global header (
avih):dwRate/dwScale→ billeder per sekund. - Per‑stream header (
strh): sin egendwRate/dwScalefor lyd- eller sekundære videostrømme.
Afspilleren multiplicerer ramme‑nummeret med scale/rate for at beregne en præsentationstidstempel (PTS). Hvis skalaerne ikke stemmer overens, vil du opleve den klassiske “audio drifts out of sync” fejl, der plager ældre værktøjer.
movi-blokken – Hvor mediet lever
Alle komprimerede rammer ligger i movi LIST. Hver ramme forudgåes af et chunk ID, der fortæller dig, om det er video (00dc) eller lyd (01wb). ID‑en indeholder også stream‑nummeret, så en fil med to lydspor vil have 01wb og 02wb.
Da data allerede er interleaved, kan en afspiller læse en videoramme, derefter den næste lydblok, og præsentere dem sammen uden at skulle søge langt frem. Det deterministiske skrive mønster gjorde AVI populær for tidlige optageenheder, der havde brug for lav‑latens disk‑skrivning.
Indeksering (idx1) – Spol frem, spol tilbage
Det valgfrie idx1 chunk er en tabel over offset‑ og størrelsesværdier for hver ramme i movi. Når den er til stede, er søgning en simpel tabelopslag. Hvis den mangler, skal afspillere scanne filen i realtid – hvilket kan forårsage en mærkbar “buffering” pause på store filer.
OpenDML (AVI 2.0) – Bryder 2 GB-barrieren
Den oprindelige RIFF-specifikation begrænser et chunks størrelsesfelt til en 32‑bit usigneret heltal → 2 GB maksimal filstørrelse. OpenDML (nogle gange kaldet AVI 2.0) introducerede:
AVIXLISTs – ekstra “udvidede” RIFF‑sektioner, der kan følge de første 2 GB.indxchunk – et 64‑bit‑kapabelt indeks.- Ekstra header‑felter for længere varigheder.
De fleste moderne værktøjer (ffmpeg, VLC) skifter automatisk til OpenDML, når outputtet overstiger 2 GB, men mange ældre afspillere hakker stadig på AVIX‑chunks, så du vil nogle gange se kompatibilitetshacks, der opdeler en lang optagelse i flere 2 GB AVI‑filer.
3. Hvor lever AVI i 2024?
| Trend | Hvad det betyder for AVI |
|---|---|
| Skift til MP4/MKV/ProRes | Nye produktioner foretrækker containere, der indfødt understøtter HDR, 10‑bit, VBR og rig metadata. AVI vælges sjældent til nyt indhold. |
| Ældre hardware | Gamle CCTV‑kameraer, camcordere fra begyndelsen af 2000‑erne og nogle spilmotorer udskriver stadig AVI. Du skal stadig indlæse disse filer i en moderne arbejdsproces. |
| Open‑source understøttelse | ffmpeg, libav, VLC, HandBrake og GStreamer holder AVI‑parsers i live, men de re‑encode ofte til MP4/MKV for distribution. |
| OpenDML-genopblussen | 4K‑overvågningsoptagelser, der kører i dagevis, overstiger hurtigt 2 GB. OpenDML (AVIX) er stadig den foretrukne løsning, selvom mange værktøjer håndterer det forkert. |
| Sikkerhedsforstærkning | Historiske AVI‑parsers accepterede fejlformede chunk‑størrelser, hvilket førte til CVE’er (f.eks. CVE‑2020‑13144). Windows foretrækker nu Media Foundation, som afviser mange ældre quirks og skubber udviklere mod sikrere pipelines. |
| Container‑agnostiske pipelines | Moderne medie‑rammeværk behandler AVI som blot et andet kilde‑element. Når dataene er demuxet til rå buffere, bliver containeren irrelevant for efterfølgende behandling. |
Kort sagt er AVI levende men aldrende – den overlever, hvor gammel hardware eller simpel interleaved lagring er påkrævet, men den er ikke førstevalg for nye projekter.
4. Praktisk: Arbejde med AVI i dag
A. Et kig på et minimalt header (hex)
52 49 46 46 00 00 00 00 41 56 49 20 ; "RIFF" + size + "AVI "
4C 49 53 54 20 00 00 00 68 64 72 6C ; LIST "hdrl"
...
RIFF‑magien (52 49 46 46) fortæller enhver parser “dette er en RIFF‑fil”. De næste fire bytes er den samlede filstørrelse (minus 8). Identifikatoren "AVI " låser filen i AVI‑familien.
B. Konvertering af Moderne MP4 → Ældre AVI
ffmpeg -i input.mp4 \
-c:v mpeg4 -q
```bash
ffmpeg -i input.mp4 \\ -c:v mpeg4 -qscale:v 5 \\ # MPEG‑4 Part 2 (DivX/Xvid kompatibel) -c:a mp3 -b:a 192k \\ # MP3-lyd (de fleste AVI‑afspillere forstår dette) -f avi output.avi
The command above forces MPEG‑4 Part 2 video (the codec most legacy AVI players recognize) and MP3 audio, then writes an AVI container. If you need OpenDML support for files larger than 2 GB, add the -movflags +faststart‑style flag that tells FFmpeg to use the extended AVIX chunks:
ffmpeg -i input.mp4 \\ -c:v mpeg4 -qscale:v 5 \\ -c:a mp3 -b:a 192k \\ -f avi -flags +global_header -movflags +faststart output.avi
Tip: Some older Windows Media Player versions still choke on the
AVIXextension. If you must stay under 2 GB, split the source into multiple AVIs using the-segment_timeand-f segmentmuxer.
5. Common Pitfalls & How to Fix Them
| Symptom | Likely Cause | Fix |
|---|---|---|
| Audio drifts out of sync after a few minutes | Mismatched dwRate/dwScale between avih and strh (or a VBR audio stream) | Re‑encode audio to a constant‑bitrate format (e.g., MP3 128 kbps) or use -vsync 2 in FFmpeg to force frame‑accurate timestamps. |
| “Cannot play this video” on Windows Media Player | Missing or corrupt idx1 index, or OpenDML (AVIX) chunks not recognized | Run ffmpeg -i broken.avi -c copy -map 0 -f avi repaired.avi to rebuild the index; or use aviindex (part of mplayer) to generate a fresh idx1. |
| File size capped at 2 GB despite long footage | Encoder used classic AVI (no OpenDML) | Add -use\_open\_dml 1 (FFmpeg) or -format avi2 (VirtualDub) to enable OpenDML extensions. |
| Green or corrupted frames | Incompatible FourCC (e.g., H.264 in an AVI without proper headers) | Stick to codecs known to work in AVI (mpeg4, msmpeg4v2, MJPG, XVID). If you must store H.264, use the h264 FourCC and ensure the strf chunk contains the SPS/PPS extradata. |
| Playback stalls on network streams | AVI’s lack of robust streaming metadata (no moov atom) | Wrap the AVI in a streaming protocol (e.g., RTSP) that handles byte‑range requests, or convert to MP4/MKV for smoother streaming. |
6. Debugging Tools You Should Keep Handy
| Tool | Platform | What It Does |
|---|---|---|
| ffprobe / ffmpeg | Cross‑platform | Dumps every chunk, FourCC, timestamps, and can rebuild indexes (-c copy). |
| MediaInfo | Windows/macOS/Linux | Human‑readable summary of streams, codecs, and container flags. |
| VirtualDub | Windows | Classic AVI editor; can rebuild headers, add OpenDML, and preview frame‑by‑frame. |
| GSpot (legacy) | Windows | Identifies obscure FourCCs and suggests appropriate codecs. |
| aviindex (part of MPlayer) | Linux/macOS | Generates a fresh idx1 chunk for broken files. |
| Hex editors (HxD, Bless) | Any | Directly inspect RIFF headers when you suspect malformed chunk sizes. |
A typical workflow when an AVI refuses to play:
- Inspect with
ffprobe -show_format -show_streams file.avi. - Check the index:
ffmpeg -i file.avi -c copy -f avi -y temp.avi(FFmpeg will rebuild it automatically). - Validate FourCCs:
mediainfo file.avi. If you see an unknown codec, consider re‑encoding that stream. - Repair with VirtualDub → “File → Re‑open as AVI (OpenDML)”. Save a fresh copy.
7. When (and When Not) to Use AVI
Good Use‑Cases
| Scenario | Why AVI Works |
|---|---|
| Legacy camcorder ingest | The device outputs native AVI; transcoding adds unnecessary quality loss. |
| Simple interleaved capture | Low‑latency write to disk without needing complex container features. |
| Compatibility with old Windows‑only software | Some industrial automation tools only understand AVI. |
| Archiving raw, uncompressed video | AVI can hold PCM audio and uncompressed RGB24 video without extra overhead. |
Bad Use‑Cases
| Scenario | Why AVI Fails |
|---|---|
| HDR or 10‑bit color | No standard way to store those pixel formats; you’d need a custom FourCC that most players ignore. |
| Variable‑bit‑rate streaming | Lack of a proper moov‑like atom makes adaptive bitrate impossible. |
| Rich metadata (chapters, subtitles, tags) | AVI’s chunk model doesn’t define standard containers for subtitles or extensive tags. |
| Cross‑platform mobile distribution | Modern mobile players expect MP4/MKV; AVI may not be hardware‑accelerated. |
If you’re starting a new project, treat AVI as a fallback for legacy pipelines, not a primary delivery format.
8. Future Outlook – Will AVI Ever Make a Comeback?
The short answer: unlikely. The industry has coalesced around ISO‑BMFF‑based containers (MP4, MOV, HEVC‑MP4, etc.) because they support:
- Extensible metadata (ISO‑UserData, UUID boxes).
- Fragmented streaming (moof/mdat) for adaptive bitrate.
- Native HDR/10‑bit/12‑bit video definitions.
AVI’s design, while elegant for its time, is fundamentally limited by its 32‑bit size fields and its reliance on external FourCC‑driven codecs. Even though OpenDML extended the size limit, it never gained widespread adoption beyond niche surveillance and archival tools.
That said, software preservation will keep AVI parsers alive for decades. Projects like FFmpeg, GStreamer, and VLC will continue to support the format, ensuring that the massive archive of 1990s‑2000s footage remains accessible. In a world where “digital archaeology” is becoming a real discipline, knowing how to read and repair AVI files is still a valuable skill.
9. Quick Reference Cheat‑Sheet
| Item | Command / Setting | Explanation |
|---|---|---|
| Create classic AVI | ffmpeg -i src -c:v mpeg4 -qscale:v 5 -c:a mp3 -b:a 192k -f avi out.avi | Simple, widely compatible. |
| Enable OpenDML | ffmpeg -i src -c:v mpeg4 -qscale:v 5 -c:a mp3 -b:a 192k -f avi -use_open_dml 1 out.avi | Allows >2 GB files. |
| Re‑index broken AVI | ffmpeg -i broken.avi -c copy -f avi repaired.avi | Rewrites idx1. |
| Split >2 GB into chunks | ffmpeg -i long.avi -c copy -map 0 -segment_time 1800 -f segment part_%03d.avi | 30‑minute segments stay under the limit. |
| Inspect header | ffprobe -show_format -show_streams file.avi | Dumps all RIFF chunks and stream info. |
| Add a subtitle track (non‑standard) | ffmpeg -i video.avi -i subs.srt -c copy -metadata:s:s:0 language=eng out.avi | Works only with players that read the txt stream; not universally supported. |
Keep this table bookmarked; it covers 80 % of everyday AVI tasks.
10. Best Practices for Archiving AVI Files
Even though AVI is a legacy container, many institutions still have petabytes of it sitting on tape or in cold‑storage. Treating those assets with a disciplined workflow will save you headaches down the line.
| Practice | Why It Matters | How to Implement |
|---|---|---|
| Validate on ingest | Corrupt headers or missing indexes can go unnoticed until playback. | Run ffprobe -v error -show_format -show_streams file.avi immediately after copying. Log any non‑zero exit codes. |
| Generate a checksum | Guarantees bit‑exact preservation across media migrations. | Use SHA‑256 (sha256sum file.avi > file.avi.sha256). Store the checksum alongside the file in your catalog. |
| Create a modern proxy | Most downstream workflows (editing, streaming) expect MP4/MKV. | Encode a low‑bitrate MP4 proxy (ffmpeg -i file.avi -c:v libx264 -crf 23 -c:a aac -b:a 128k proxy.mp4). Keep the proxy in the same directory with a clear naming convention (*_proxy.mp4). |
| Document FourCCs and codec versions | Some FourCCs map to multiple codec implementations (e.g., DIVX could be DivX 5, 6, or 7). | Extract the codec private data (ffprobe -show_private_data) and store it in a side‑car JSON file (file.avi.codec.json). |
| Migrate to OpenDML for large files | Files >2 GB will become unreadable on older players. | When transcoding, always pass -use_open_dml 1. If you’re only copying, use aviindex to rebuild an OpenDML‑compatible index. |
| Store metadata in a side‑car | AVI has no standard for extensive tags (e.g., creator, location). | Use XMP side‑car files (file.avi.xmp) or embed a small INFO LIST chunk manually if you need minimal in‑container metadata. |
| Regularly test playback | Bit‑rot can affect codecs as well as containers. | Schedule a quarterly job that runs a headless player (e.g., ffplay -autoexit -frames 10 file.avi) and reports any failures. |
By applying these steps at the moment of acquisition, you avoid costly “repair‑the‑archive” projects later.
11. Frequently Asked Questions (FAQ)
Q1: Can I store H.265/HEVC video inside an AVI file?
Short answer: Technically yes, if you supply the correct FourCC (HEVC or HVC1) and include the SPS/PPS NAL units in the strf chunk. In practice, very few players support it, and many will treat the stream as unknown. For reliable playback, stick to MPEG‑4 Part 2 or Motion‑JPEG.
Q2: Why does Windows Media Player sometimes show a black screen but still plays audio?
Explanation: The player has successfully opened the audio stream but failed to locate a usable video decoder for the FourCC. This can happen when the FourCC is custom or when the required codec isn’t installed. Installing a codec pack (e.g., K-Lite) or re‑encoding the video to a known FourCC resolves the issue.
Q3: Is there any way to embed subtitles directly into an AVI file?
Answer: AVI does not define a standard subtitle stream. Some tools cheat by adding a “txt” stream (FourCC txt ) that contains plain‑text subtitles, but only a handful of players (e.g., VirtualDub with a plugin) will render them. The recommended approach is to keep subtitles in a separate .srt or .ass file, or to re‑mux into a container that officially supports subtitles (MP4, MKV).
Q4: My video shows a “frame rate mismatch” warning in MediaInfo. What should I do?
Solution: Verify that the dwRate/dwScale values in both the global avih and per‑stream strh headers are consistent. If they differ, re‑mux with FFmpeg using -video_track_timescale to force a uniform time base:
ffmpeg -i broken.avi -c copy -video_track_timescale 1000 fixed.avi
Q5: Does AVI support multiple audio languages?
Yes, but with caveats. You can add several audio streams, each with its own strh/strf pair and a distinct stream number (01wb, 02wb, …). However, there is no standardized way to label the language; you must rely on external metadata (e.g., an accompanying .xml file) or embed a custom INFO chunk.
Q6: How can I extract raw frames from an AVI without re‑encoding?
Command:
ffmpeg -i source.avi -c:v copy -f image2 frame_%05d.bmp
Replace bmp with png or tiff if you prefer lossless image formats. The -c:v copy flag tells FFmpeg to dump the compressed frames as‑is; if the codec is MJPEG, the output will already be JPEG images.
Q7: Are there any security concerns when opening AVI files from untrusted sources?
Yes. Malformed chunk sizes can trigger buffer overflows in legacy parsers (e.g., older DirectShow filters). Always open unknown AVIs in a sandboxed environment or use a modern library like FFmpeg that performs strict bounds checking. Updating Windows Media Foundation and disabling legacy DirectShow filters further mitigates risk.
12. TL;DR Opsummering (for de Utålmodige)
- AVI = RIFF‑based, interleaved container introduced with Windows 95.
- FourCC tells the player which codec to use; common ones are
DIVX,XVID,MJPG,H264,MP3. - Timing is driven by
dwRate/dwScalein the global and stream headers. moviholds the actual media;idx1(optional) speeds up seeking.- OpenDML (
AVIX) lifts the 2 GB limit but isn’t universally supported. - Use cases today: legacy camcorder ingest, simple interleaved capture, archival of raw PCM video.
- Avoid for new projects: no HDR, 10‑bit, VBR, subtitles, or rich metadata.
- Toolbox:
ffprobe,ffmpeg, MediaInfo, VirtualDub, aviindex, hex editors. - Best practice: validate, checksum, generate modern proxies, and migrate large files to OpenDML.
13. Afsluttende tanker
AVI’s simplicity is both its strength and its Achilles’ heel. It gave early PC users a straightforward way to store synchronized audio‑video pairs, and that design philosophy—interleaved chunks, a clear header layout, and extensible FourCC identifiers—still influences modern containers. While the industry has moved on to more feature‑rich formats, the sheer volume of legacy footage means AVI will remain a “must‑know” for anyone working in video preservation, forensic analysis, or any field that must bridge the past with today’s workflows.
If you ever find yourself staring at a dusty .avi on a hard drive from the late‑90s, you now have the conceptual map, the command‑line recipes, and the troubleshooting checklist to bring that footage back to life—whether you choose to keep it in its original container or transcode it into a modern, HDR‑ready format.
Happy demuxing!