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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?

share|improve this question
I've tried the new version - 2.5.6, but still have the same sound sync problem. – Goto10 Jan 1 '12 at 21:51
Would Free AVI/MPEG/WMV/MP4/FLV Video Joiner be a good alternative to use? – Goto10 Jan 1 '12 at 21:54
How is it out of sync, sound first or video first? – jiggunjer Nov 5 '15 at 8:27

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.

share|improve this answer
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 '11 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 '11 at 23:42
Although I haven't tried mp4box myself, some comments on the GUI did mention sound sync issues. – Goto10 Jan 1 '12 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 '12 at 2:37

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