A format from the Windows 3.1 era
AVI (Audio Video Interleave) was introduced by Microsoft in 1992. For about fifteen years it was the default for PC video: camcorder captures, webcam recordings, DivX and Xvid films shared online, digital camera clips and the output of early editing software. Hard drives full of family footage from the 2000s are often entirely AVI.
The container shows its age. It has no proper support for B-frames, variable frame rates or modern streaming, its timing model is fragile, and files over 2 GB needed a later extension called OpenDML. Most phones, browsers and smart TV apps no longer open it, and some editors import only the audio. Converting to MP4 gives the same footage a container that modern devices and websites understand.
What is usually inside an AVI
The extension says little about the actual video. Before converting it helps to know what you probably have:
- DivX or Xvid (MPEG-4 Part 2). Typical of films and TV episodes downloaded between 2002 and 2010.
- Motion JPEG. Used by many digital cameras and webcams until around 2012. Each frame is a separate JPEG, so files are large: 30 seconds of 640×480 can exceed 50 MB.
- DV. From MiniDV camcorders captured over FireWire. About 13 GB per hour at 720×576 or 720×480, interlaced.
- Uncompressed or lossless video. From capture cards and some screen recorders. Gigantic files.
- H.264. Less common, from some later cameras, dashcams and security DVRs.
Two ways the conversion can run
Browser: fast path first
In the browser the tool first reads which codecs the AVI contains. H.264 video is copied into the MP4 without touching it, which takes seconds and loses nothing. AAC audio is copied too; MP3, PCM and other audio is converted to AAC at 128 kbit/s.
Every other video codec — DivX, Xvid, Motion JPEG, DV, uncompressed — is re-encoded: H.264 at CRF 23 with the ultrafast preset and standard 8-bit 4:2:0 colour. That runs at roughly real time on a laptop and gives a file that plays on phones, browsers and TVs. If a copy fails, for example because of a damaged index, the tool falls back to the same re-encode.
Server: always H.264
Server processing always re-encodes the video to H.264 with the slower medium preset at CRF 23, so the result plays on any device regardless of what the AVI contained, and is usually smaller than the browser's re-encode. Free use is one server job a day for files up to 500 MB; Premium is unlimited and accepts up to 4 GB.
Size expectations
How big the MP4 ends up depends almost entirely on the source:
- DV and uncompressed AVI shrink dramatically when re-encoded, often to a tenth of the original size or less. A 13 GB hour of DV typically lands in the low gigabytes.
- Motion JPEG shrinks just as much.
- DivX and Xvid films are already compressed. Re-encoded to H.264 at CRF 23, the size stays similar and can grow slightly with the browser's fast preset.
- H.264 AVI copied into MP4 stays nearly the same size.
If you need the file smaller still, compress video applies a stronger setting afterwards.
Problems specific to old footage
- Interlacing. DV and many camcorder and TV captures are interlaced. On a modern screen fast motion shows thin horizontal comb lines. The converter keeps the fields as they are and does not deinterlace, so the lines remain in the MP4.
- Black bars and odd aspect ratios. 720×576 DV is meant to be shown at 4:3 or 16:9 with non-square pixels. Players that respect the aspect flag show it correctly; a few stretch it. If that happens, set the display aspect in your editor or player.
- Audio out of sync. Broken AVI indexes from interrupted recordings can cause drift. If a copied file drifts, the full re-encode on the server rebuilds the timing and is worth a try.
- Large archives. Browser conversion holds the whole file in memory. For a batch of multi-gigabyte DV captures, the server option or a desktop machine with plenty of RAM is more reliable.
The original AVI is never changed. Keep it until you have checked the MP4 plays where you need it.