Convert WMV to MP4

A format built for one platform

WMV — Windows Media Video — was Microsoft's video format through the late nineties and 2000s, and it was very good at what it was designed for: streaming over slow connections to Windows machines running Windows Media Player. It was the default for corporate training material, for e-learning, for Windows Movie Maker exports, and for a great deal of downloadable video.

Its strength was also its limit. WMV depends on Microsoft's own codecs, and Apple, Google and the browser vendors never adopted them. The result is a large body of video that plays perfectly on one operating system and nowhere else, and that becomes harder to open every year as even Windows moves on.

DRM is the other complication. Windows Media carried a rights-management system used for purchased and rented content, and a protected file cannot be converted by anything without its license. That is not a limitation of this tool; it is the point of the protection.

How it works

The Windows Media video and WMA audio are decoded and written into an MP4 container as H.264 and AAC. Both halves need re-encoding — the audio is as platform-specific as the video, which is why a converted file gains sound on a Mac that had none before.

Because much WMV was encoded for early-2000s bandwidth, the result is frequently smaller than the original while looking the same or better. H.264 is simply more efficient than the codecs of that era, so the same picture costs fewer bytes.

What to check afterwards

Play the first few seconds and the last few. WMV files produced by older encoders occasionally have imprecise duration information, and the end of a long recording is where that shows up. If the original was a Movie Maker export, also check that any titles or captions came across as part of the picture — they usually did, because Movie Maker burned them into the frames rather than storing them as a separate track.

Why so much of this exists

The volume of WMV in the world is out of proportion to how little it is used today, and the reason is that it was never really a consumer choice. It was the default. Windows Movie Maker exported it, Windows Media Encoder produced it, screen-capture software wrote it, and organizations standardized on it because every desktop they owned could play it without installing anything.

Formats that arrive by default rather than by decision are the ones that leave the largest archives behind, because nobody was keeping track of what they were choosing. XPS documents have exactly the same history on the document side of Windows, for exactly the same reason.