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I have some mp4 video files with pretty poor video quality. I need to add a voiceover translation. I have seen some video software programs that could do the job but unfortunately most of these programs recompress also the video stream, so it becomes worse.

Is there any (preferably, free) software, which would allow me to remix only audio part of my mp4 files and leave the video stream as is, without recompression?

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up vote 6 down vote accepted

Yes, it's possible. What you need to do is demux and remux. Check out free tools like VirtualDub and Any Video Converter to split the original into separate streams, add what you want using Audacity or similar, then re-multiplex them together using a tool like AviMux. The video can remain unchanged.

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Thank you for the hints, "My MP4Box GUI" did the demux, then I mixed my voicevoer in Audacity and then again remuxed it with "My MP4Box GUI". – JustAMartin Jun 30 '12 at 20:46

Both Apple Logic can separate the audio from an .mp4 file for editing and then bounce a new audio track back to a video file without changing the video at all. Pro Tools can do similar but I can't remember if it does mp4 or not.

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Short answer is order to marry picture and sound, a new file needs to be created.

That being said, some software will allow you to edit the compression settings, and may have a setting to leave the video at "original quality" or "no compression" or "uncompressed". The latter may actually result in a larger file, as the file get decompressed and then re-saved with ALL the data intact instead of compressed.

Do some experimentation to see which option(s) work best with the software you have available. You can also download some free trial versions of software that are fully functional for 30 days before you get locked out. Most editing programs that allow tweaking of the video codec's options will work for you.

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You can remux video and audio without recompressing, for example see -vcodec copy options to ffmpeg. – koan Jun 30 '12 at 10:27
It's correct that a new file needs to be created, but that doesn't tell anything about recompression, which indeed can be avoided for the non-modified track. – leftaroundabout Jun 30 '12 at 19:32

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