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I am trying to join (merge) two or more .mp4 files together, without re-encoding.

Here is what I did:

  1. Started Avidemux 2.5.5.
  2. With File->Open, selected Input1.mp4.
  3. I received this message - "H.264 detected. If the file is using B-frames as reference it can lead to a crash or stuttering. Avidemux can use another mode which is safe but YOU WILL LOOSE SOME FRAME ACCURACY. Do you want to use that mode?". I chose "No".
  4. With File->Append, selected Input2.mp4.
  5. I received the same "H.264 detected" message again and chose "No".
  6. Selected the Format to MP4 (from AVI).
  7. Saved the output file (called Output.mp4) with File->Save->Save Video.

Unfortunately, when I play the Output.mp4 video in VLC, the sound is out of sync with the second video. How can I correct this?

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  • I've tried the new version - 2.5.6, but still have the same sound sync problem.
    – Goto10
    Jan 1, 2012 at 21:51
  • Would Free AVI/MPEG/WMV/MP4/FLV Video Joiner be a good alternative to use?
    – Goto10
    Jan 1, 2012 at 21:54
  • How is it out of sync, sound first or video first?
    – jiggunjer
    Nov 5, 2015 at 8:27

2 Answers 2

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I had horribly garbled audio when trying to combine 5 different clips into one. They were all different audio and video formats and, no matter what I tried, it turned into a garbled mess when trying to combine more than a couple clips at once.

The solution was to convert each and every clip individually to PCM raw audio with the same frequency sampling (44.1k) and remixed to stereo sound. Then I could easily combine the clips with raw audio and everything worked perfectly. I then re-encoded the combination to vorbis and it was still in sync.

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I have had much better results joining multiple video files using mkvmerge from the MKVToolNix tools.

Just download and install the windows installer from here. Then run the mkvmerge GUI, add the first file, and then use the append button to add the additional files. This does give you an mkv file instead of an mp4 file, but VLC should be able to play that without any trouble. Also, there is no re-encoding going on, only re-muxing.

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  • Is there any similar free software that works with MP4 files? Although VLC does a great job at playing lots of different formats, I'd prefer to stick with MP4 for consistency.
    – Goto10
    Dec 30, 2011 at 12:01
  • You're in luck, it looks like mp4box supports concatenating mp4 files. I have not really used mp4box, so I can't say how well it work. Let me know if it works for you.
    – cmorse
    Dec 30, 2011 at 23:42
  • Although I haven't tried mp4box myself, some comments on the GUI did mention sound sync issues.
    – Goto10
    Jan 1, 2012 at 21:49
  • I saw that too. But, there are a large number of variables when it comes to concatenating media files together (codec used for the audio/video, multiple tracks, subtitles, etc.). The comment I saw that mentions sync issues was using the trimming feature, not concatenation. The only way to be sure is to try it for yourself.
    – cmorse
    Jan 2, 2012 at 2:37

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