Remuxing: MKV to MP4 without re-encoding
MKV (Matroska) is a flexible container that can hold almost any codec and many tracks. Most MKVs from OBS, cameras and downloads carry H.264 or HEVC video, which MP4 also accepts, so the picture is copied into an MP4 box without decoding. A multi-gigabyte file takes seconds, and quality is identical.
Only video in other codecs (VP9, AV1, VP8, MPEG-4) is re-encoded to H.264, which takes minutes and depends on your computer's speed. The card tells you which path your file will take before you start.
What happens to the sound
- AAC and MP3: copied.
- FLAC, Opus, Vorbis and PCM: converted to AAC (Opus in Firefox), since many MP4 players skip them.
- Dolby Digital (AC-3/E-AC-3) and DTS: copied unchanged; TVs, VLC and Apple players play them, but browsers can't, so the preview here may be silent.
Only the main sound track is kept. If your MKV has several languages or a commentary track, the card shows how many tracks will be dropped.
Recording in OBS? Save time next time
OBS recommends recording to MKV because a crash can't corrupt it, then remuxing to MP4. This page does exactly that remux in the browser. OBS's own File › Remux Recordings does the same on the recording PC.
Subtitles and chapters
Subtitle tracks and chapters stay behind; MP4 players handle them inconsistently. Keep the MKV if you need them, or extract subtitles with a desktop tool such as MKVToolNix.