replacing (and syncing) an audio track of an MP4 video file

[rotchild]

Hey there. Well, I played this rock show with my band, and a friend recorded it with a camera and sent me the mp4 file of one of the songs on the set. Just for the record, the song is incomplete (it's missing a little bit of the beginning. And one of the guitar players recorded the set with a ZOOM Audio recorder and sent me the mp3 (he splitted the recording into several mp3 files, including the song that my other friend filmed). Needless to say, the audio from the ZOOM recorder is much better than the audio from the camera.

What I wanna do is to replace the audio from the mp4 video file (recorded with the camera) with the mp3 audio recorded with the ZOOM device. The problem is that I will also have to edit the mp3 file to cut out the beginning of the song, and then sync it with the video file.

What is the easiest way to do it? Can somebody help?

[pippas]

I'm sure there are lots of ways of doing this, I can only describe how I would do it - using free tools. Whether that's the easiest way?....

• Take your video clip, and extract the audio track from it by dragging and dropping the file into Pazera
Export that as a 48KHz .wav file

• Take your mp3 audio track and drag and drop that into Pazera. Export that as a 48KHz .wav file

• Open the track extracted from your video in Audacity

• Add the converted mp3 track in Audacity as well, onto a new track

• Use the 'Time shift' tool to line up this second track with the required part of the imported video audio track above.
DO NOT MOVE THE POSITION OF THE FIRST TRACK.
You may need to magnify the timeline to line up the start points of the 2 audio tracks exactly. The sync will depend on how well you do this.

• Reduce the volume to zero of the required part of the top (video audio) track by editing that track in Audacity

• Export this new mix as an .m4a file from Audacity (Follow the ffmpeg instructions in Audacity if necessary)

• Open MP4BoxGUI and select 'Demux'

• Drag and drop your original video clip into MP4BoxGUI and select the video track by ticking the box. Press Demux

• In Mp4BoxGUI select 'Mux'

• Drag and drop the .h264 video only file you have just created from the step above.

• Drag and drop the .m4a file you created in Audacity

• Press 'Mux'

That will give you your original video clip with the audio from the mp3 file. As long as you have lined up the tracks accurately in Audacity, it should be in sync.

As I say, there may be other ways, but whether they are easier?.....they certainly can't be any cheaper!

[rotchild]

Thanks a lot pippas! I will only have time to do this on sunday. But I'll try your method and report back. I know how to "mess" with audacity, so it must be straight forward... Thanks again!

[rotchild]

Ok, I have synced the new audio with the original one (the original audio from the video), to almost perfection... Both now are now exactly 1:53 (1 minute and 53 seconds) The problem is when muxing (the new .m4a audio file, with the .h264 video only file). After I "mux it" using My MP4Box GUI, the edited audio track is OK (still 1:53) , but the video is longer !!! It's clocking at 2:15 (2 minutes and 15 seconds). And it has nothing to do with me syncing it wrongly, and the original audio file ran at a 2:07...

What am I doing wrong? Please help...

[pippas]

Are both tracks the same length if you simply de-mux the clip, and open the original audio in Audacity... Then, without any editing, export that original audio from Audacity as an m4a and mux that with the .264 video?

That will tell you if it's Audacity that's causing the problem or not.

Two things I would mention....

I usually export my edited audio from the audio editor as a .wav file (I usually use Audition rather than Audacity) and then convert that into m4a using either FAAC or NeroAAC (both free)

Never had any problems with sync doing that.

Mp4Box does not convert anything, so the video must be as the original - it can only be the edited audio that's wrong.

Don't forget to use the video audio track as the 'master' length. So the added 'new' audio track must be shorter so that it's the original audio track - even if it's now mostly silence - that determines the length and position of the 're-muxed' audio track.

[rotchild]

I synced with the time shift tool as you instructed, it wasn't that.

I used an alternative muxer/demuxer (mp4muxer) and was having the same exact problem. The new mp4 was muxing with 2:15 of total time, instead of 1:53. For whatever reason, it only worked when I manually adjusted the framerate to 29.970... Not even sure if it was that, but it worked. Thanks a lot again!!

[pippas]

Glad you've managed to sort it out.

I've always left the fps box in MP4box set as 'no change' and haven't had any problems. Mind you, I'm in PAL Land so it's always 25 or 50 fps... whether that makes any difference, I'm not sure?....