Video
Converting MKV to MP4: what is kept and what is lost (audio tracks, subtitles)
MKV, or Matroska, is the favourite container for films, series and game recordings: it can carry several audio tracks, several subtitle languages and chapters in a single file. That richness comes at a price: many TVs, browsers and apps play it badly, or not at all. Converting it to MP4 solves the compatibility problem, but you need to know exactly what the conversion keeps. We tested it on a file built for the occasion.
Why MKV causes problems
MKV is an open and very flexible format, popular for archiving. But it was never adopted as a standard by the major manufacturers: some smart TVs ignore it, iPhones and iPads do not open it with their default apps, most social networks and messaging apps reject it, and not all browsers play it.
MP4, on the other hand, plays everywhere. Converting means changing container, and often codec: to guarantee compatibility, FileXvert always re-encodes the video to H.264 and the sound to AAC, the combination every device recognises.
Our test
To find out what survives the conversion, we built a 30-second MKV representative of a film file: an H.264 video track in 1080p at 60 frames per second, a French audio track in AAC, an English audio track in Opus and a French subtitle track in SRT format. The file weighs 85.9 MB. We then converted it to MP4 with exactly FileXvert's settings, with no size reduction.
| Element | In the MKV | In the resulting MP4 |
|---|---|---|
| Video | H.264 1080p 60 fps | H.264 1080p 60 fps, SSIM 0.976 |
| Audio, track 1 | French, AAC | French, AAC 128 kbps |
| Audio, track 2 | English, Opus | Missing |
| Subtitles | French, SRT | Missing |
| Size | 85.9 MB | 64.0 MB |
The picture is kept with high fidelity (an SSIM of 0.976 remains hard to tell from the original in normal playback), and the file is 25% lighter. However, only the first audio track is carried over, and embedded subtitles are not. This is the default behaviour of FFmpeg, the encoder FileXvert uses: it keeps one video track and one audio track, the one it considers the main one.
What this means for your files
For a personal video, a screen recording or a clip with a single audio track, this limit changes nothing: everything that matters is kept. For a film or series in a multilingual version, however, the conversion produces a single-language version, in the language of the first track, without subtitles.
- Check which audio track comes first in your MKV: that is the one that will be kept. Most players, such as VLC, show the track order in the Audio menu.
- If you need the subtitles, keep them separately: many MKV files come with a separate .srt file, which players can load alongside an MP4.
- Never delete the original MKV: it is the file that contains every language and every subtitle track.
Size and conversion time
Since the video is re-encoded, the conversion is real work for your processor: every frame is decoded and then recompressed. For a full two-hour film in 1080p, that takes a long time, and the file is far beyond the free plan's 100 MB. FileXvert is best suited to excerpts, clips and videos a few minutes long; to convert a whole video library, installed software such as HandBrake will be more comfortable, and lets you choose exactly which tracks to keep.
If the resulting MP4 is too heavy, the reduction levels apply as for any video: balanced divides the size by about three, and smallest size drops to 720p.
Do you really need to convert?
Before converting, ask yourself whether the problem is the file or the player. On a computer, a free player such as VLC plays MKV without any trouble, with all its tracks and subtitles. On a TV, a streaming stick or a recent media player often supports it. Converting to MP4 is mostly justified when the file has to be shared, published, sent to someone whose equipment you do not know, or played on a device that refuses MKV.