Small audio change in a MP4 file

6 respostas [Última entrada]
GNUser
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Joined: 07/17/2013

Hello all,

So I have a couple mp4 files which have the audio set as only "C" channel, indicated by MediaInfo. It plays ok on VLC, but some older players I have don't play it.
I want to know if using MKVToolNix is possible to create a new file where audio channel is indicated as "L" or "R". Thank you very much.

GNUser
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Joined: 07/17/2013

Ok, so I have been getting some results with ffmpeg instead.

Using this:

ffmpeg -i input.mp4 -vcodec copy -acodec aac -ac 2 output.mp4

Gives me a file that plays well. However, this is reencoding the audio, which I would rather not do, simply "pan" the center audio to either right or left (probably right, since that is usually the earliest channel). I still haven't figured how to do that. Any help??
Thanks.

Magic Banana

I am a member!

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Joined: 07/24/2010

'-acodec copy' maybe? Once, on Trisquel 7, I had to downmix the stereo audio of a video to mono. The command I used was:
$ avconv -i stereo.webm -ac 1 -c:v copy mono.webm
ffmpeg has replace avconv (a fork) since then.

GNUser
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Joined: 07/17/2013

No, I tried that before but as long as you run -acodec copy it will not make any changes in the audio stream.
Still, I have discovered that even if I make a new encode with mono sound, it plays well too. So, I am using it now to make it go faster.
Thanks for the help anyway. I don't know who downvoted your post instead of replying, but I will upvote it, even if it is not the correct solution I think it can prove useful for other users.
Thanks!

aloniv

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Joined: 01/11/2011

https://trac.ffmpeg.org/wiki/AudioChannelManipulation

I think the second example using amerge doesn't reencode (it supposedly copies the mono stream into two streams and puts a copy for each channel) but I don't have a mono file to test currently.

GNUser
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Joined: 07/17/2013

Thank you for the help.
I had tried that, but in my experiments it was slower than re-encoding (maybe lossless, but still very slow).
Like I said above, in the meanwhile I discovered that re-encoding the original mono channel into a new mono channel works, and is twice as fast. I can't understand why, but it works at least.

GNUser
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Joined: 07/17/2013

Truth be said, it's just that I don't have a dedicated device to be playing these files in VLC, since VLC plays these great. But the device I need to play them does not so...

Anyway, thanks for the help, all is good now!