Convert DivX to MP4
DivX (and its close cousin Xvid) is the video compression that ruled downloaded video from the late 1990s through the mid-2000s. If you have .divx or .avi files from that era, they're probably DivX or Xvid inside. Both use an older codec called MPEG-4 Part 2, which technically fits inside an MP4 wrapper - but no modern player (Chrome, Safari, iOS, Android) actually decodes it there, so a plain wrapper swap would produce a file that only VLC could open. Wave decodes the DivX video and re-encodes it as modern H.264, so the resulting MP4 plays on every current device.
What actually changes
DivX started as a hacked build of Microsoft's MPEG-4 Part 2 encoder in 1999, went legitimate around 2001, and became the default codec for DVD rips through the 2000s. Under the FourCC labels DIV3, DIVX, DX50, and the near-identical Xvid (XVID), the actual bitstream is standard MPEG-4 Part 2 ASP.
MP4's spec accepts MPEG-4 Part 2 (it's in the same standards family - MPEG-4 vs MP4 untangles the naming), but every browser + mobile decoder skipped it in favour of H.264, so an mpeg4-in-mp4 output is essentially VLC-only. Wave decodes the DivX / Xvid frames with the on-device ffmpeg engine and re-encodes to H.264 via your browser's hardware-accelerated WebCodecs encoder. The audio track is converted to AAC alongside the video re-encode, so MP3, AC-3 and MP2 sources all come out as an MP4 that plays with sound in every browser. The whole pipeline runs on your device.
DivX's history is unusual: the original 1999 build (DivX ;-) 3.11 Alpha) was a hack that stripped DRM from Microsoft's early MPEG-4 encoder. The company DivXNetworks (later DivX, Inc.) went legitimate around 2001 with a from-scratch encoder, and by 2004 was selling DivX Certification stickers that appeared on hundreds of consumer DVD players, giving those devices the ability to play burned discs of MPEG-4 Part 2 video. That "DivX-Certified DVD player" era is why so much personal-video archive from the 2000s exists as DivX in AVI on burned CD-R and DVD-R discs.
The efficiency gap between DivX (MPEG-4 Part 2 ASP) and modern H.264 is about 2x - a DivX file at 1000 kbps looks like an H.264 file at 500 kbps. When Wave re-encodes to H.264, the visual quality is preserved and the file typically shrinks. Doing this once, from the DivX source, is much better than any workflow that keeps re-encoding through intermediaries; every re-encode adds a small amount of loss even if you target the same visual quality.
Three steps.
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01
Step 1
Drop the .divx or .avi file onto the card.
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Step 2
Wave inspects and shows Re-encode needed - the DivX video is transcoded to H.264 so the MP4 plays in browsers and on mobile.
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Step 3
Convert and save the MP4.
Answers, specifically.
Isn't DivX outdated?
For new encodes, yes - H.264 and HEVC beat MPEG-4 Part 2 on efficiency. The DivX files you already have are fine as source material; Wave just re-encodes their video to H.264 so the resulting MP4 plays in the browsers and phones that dropped MPEG-4 Part 2 playback years ago.
What's the difference between DivX and Xvid?
Codec bitstream: nothing meaningful - both are MPEG-4 Part 2 ASP. History: DivX was commercial (DivX, Inc.), Xvid was the open-source fork of an early DivX build. Files use different FourCCs (DIVX / DX50 vs XVID); Wave decodes and transcodes both to H.264 identically.
My iPhone still refuses the MP4. Why?
After Wave's conversion the video track is H.264, which iOS Safari, macOS Safari, Android and every desktop browser play natively. The old failure mode - an mpeg4-in-mp4 file that opened only in VLC - is exactly what Wave's transcode step is there to prevent.
What about the .div extension?
Same container. Some encoders wrote .div, others .divx, others plain .avi. Wave treats all three the same - the header bytes decide the container, not the extension.
Is DRM-protected DivX supported?
No. A small share of DivX files (mostly circa-2005 online rental content) carry DivX DRM. Wave can't decrypt them, and no browser-side tool can.
How does the output size compare to the source?
Typically smaller. DivX is roughly half as efficient as H.264 at the same visual quality, so a 700 MB DivX AVI often becomes a 400-500 MB H.264 MP4 at visually-identical quality. If you push the H.264 target quality higher (e.g. through a desktop encoder), you can get closer to matching size while still gaining compatibility.
Does the conversion take much longer than a simple remux?
Yes. Wave has to decode every frame and encode it again. With WebCodecs H.264 hardware acceleration, a 90-minute DVD-rip DivX at 720x480 typically finishes in 5-10 minutes on a modern laptop. On older hardware or in browsers without WebCodecs, expect 15-30 minutes. It is still faster than uploading to a cloud converter.
What about the DivX 3D format?
DivX 3D was a short-lived stereoscopic variant used mainly for burned discs in the early 2010s. It uses standard DivX bitstreams for left / right eyes wrapped in a proprietary container. Wave reads the video streams but the 3D signalling does not translate into a broadly-supported MP4 3D format; the output plays back as flat 2D.