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Remux MKV to MP4 or MOV on a Mac

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Remuxing means taking the encoded video out of one container and putting it into another without decoding it. The picture that comes out was never decoded and rebuilt. No quality is lost, because nothing is re-compressed. You are changing the box, not the contents.

It is also fast in a way transcoding never is. There is no encoder running, so the job finishes about as quickly as your disk can copy the file.

Why you would want to

.mkv is a good container and almost nothing in Apple’s world will open it. Final Cut Pro will not import it. Compressor does not recognize it. QuickTime will not play it. Meanwhile the video inside that MKV is usually H.264 or HEVC, which every one of those applications has supported for years.

So the picture was never the problem. The wrapper was. Remuxing to MOV or MP4 solves it without touching a single frame.

MOV or MP4?

Both are the same underlying format, and both are natively supported on a Mac.

  • MOV is Apple’s variant, and what Final Cut Pro and Compressor prefer. Choose it for editing.
  • MP4 is the portable one. Choose it for playback, sharing, and anything going near a browser or a phone.

If your video is MPEG-2, as most film DVDs are, MOV is the safer choice. MP4 can carry MPEG-2, but tools disagree about how, and MOV avoids the argument.

What a plain remux still gets wrong

This is the part most guides leave out, and it is why “just use ffmpeg -c copy” produces a file that opens but misbehaves.

The frame rate reads wrong. MKV stores timestamps in whole milliseconds, and 23.976 fps does not divide into whole milliseconds. Frames end up 41 and 42 ms apart, and Final Cut averages that to about 23.81 fps. A stream copy copies those timestamps straight across, so the new file has the same problem.

The audio may be silent. DTS-HD Master Audio and TrueHD are common on disc backups and Apple’s media stack decodes neither, so the track exists and produces no sound. A remux alone does not change that.

Surround can land in the wrong speakers. MOV needs a named channel layout. Write six channels without naming them and Apple guesses, which is how dialogue ends up somewhere other than the center.

Timecode disappears. MKV has nowhere to store a SMPTE timecode track. If your workflow needs one, a remux has to create it.

What ClipReady does

ClipReady is a Mac app that does the remux and then fixes those four things specifically:

  • the picture is copied across untouched: H.264, HEVC, MPEG-2 and ProRes;
  • every timestamp is snapped back onto the exact frame grid;
  • audio Apple cannot read is converted to 24-bit PCM, and audio it can read is copied as it is;
  • the channel layout is written by name and checked against Apple’s own decoder;
  • a real tmcd timecode track is written at the output rate.

Every track shows you what will happen to it before the job starts: Copied, Converted, or Stays behind, with the reason. When the job finishes, the file is loaded back through AVFoundation and VideoToolbox and checked; if a check fails, nothing is saved.

Containers in: MKV, MXF, AVCHD (.MTS), M2T and TS, MP4, MOV, M4V. Out: MOV or MP4.