WMV - Windows Media Video

WMV stands for Windows Media Video, a family of video compression formats Microsoft developed as the successor to its earlier Windows video work. It arrived when internet video was starting to work and Microsoft wanted a format that streamed well over a modem, played on every Windows machine and could be locked down by whoever was selling the video. The last version, WMV 9, was later submitted to a standards body and published as VC-1, which also appeared on Blu-ray discs. Today you meet WMV files mainly in older material: recordings made with Windows tools, training and corporate videos, camera footage from the era and downloads bought from a Windows Media shop.

How a WMV file is put together

There is a distinction worth knowing here, because it explains a lot of confusion. WMV is the codec, the compression applied to the picture. The container holding it is ASF, the Advanced Systems Format, which Microsoft designed for streaming. Files with video inside are given the .wmv extension, audio-only ones the .wma extension, and both are the same kind of file underneath.

ASF is built from objects, each identified by a long unique number rather than a readable name. A header at the front describes the file and every stream in it; a data object follows, containing the media chopped into fixed-size packets with timing information; index objects at the end let a player seek. Packetising the media is what made ASF good at streaming, because a server could send packets as they were needed and a player could recover from a lost one. The compression is lossy, and WMV was tuned for low bit rates, which is why old WMV clips often look soft when stretched onto a modern screen.

What metadata a WMV file carries

ASF has a proper metadata design, so a WMV can be quite talkative.

• The file properties object holds the creation date, the file size, the duration, the number of packets and the maximum bit rate.
• The content description object holds five classic fields: title, author, copyright, description and rating.
• An extended description object holds free-form name and value pairs. Microsoft's own names are prefixed and cover album title, genre, year, track number, publisher, provider, encoding settings and the name and version of the tool that made the file. Cover artwork can be embedded here too.
• Each stream has its own properties object naming the codec, the dimensions, the frame rate, the audio sample rate and the channel count.

There is no EXIF block in a WMV and no standard GPS field, so location is not normally part of the picture. The personal traces are the author name, the creation date and the encoding tool, which together say who made the file, when, and with what.

One extra field is worth understanding. ASF supports digital rights management, and a protected file carries an encryption object naming the licence server it wants to contact. If a report shows that object, the file is not simply a video: it needs a licence your computer may no longer be able to obtain, and no conversion tool can lawfully remove that. It is the usual reason an old purchased WMV refuses to play.

When WMV still makes sense

Almost never for new files. WMV made sense when Windows Media Player was the assumed player and bandwidth was scarce. Support outside Windows has always been patchy, phones and browsers generally will not touch it, and modern codecs deliver better quality per megabyte.

The alternative is MP4 with H.264, which plays on essentially everything and compresses better. Convert a WMV when you want to keep watching it or share it, keep the original if it is the only copy of something, and check for that encryption object first, because a protected file will not convert.