Converting 6 channel audio to 2 channel audio in a MKV file
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and remux. I've found ffmpeg to be a little wonky with audio. i.e. If I use ffmpeg to trim / extract a particular time segment of audio, it'll differ from other more accurate applications. It'll do it, and it'll be close. But when I was trying to use ffmpeg to extract an element to sync with an external audio source, I had real issues with lining up that external audio +/- 1/1000th of second with ffmpeg. With sox, it's spot on everytime, shift the cut points and it stays linear. Shift it with ffmpeg and your sync point changes. Fractions of a second, but still annoying(+/- 0.050 aka 1/2 of 1/10th of a second but noticeable drumming at 60p in HD). </rant>
ffmpeg does decent with audio ONLY.
ffmpeg does decent with video ONLY.
ffmpeg is not that decent when doing both video and audio at the same time. IMHO
Basically -acodec copy and/or -vcodec copy the original content, then handle individually until the last possible moment.
Well, for a/v sync issues make sure to use the '-async' option with ffmpeg (with the sample rate of the audio as the parameter), that should prevent any kind of a/v desync. I've tested it, and I don't notice any a/v desync if you use this option.
Using this command line:
ffmpeg -i file.mkv -vcodec copy -acodec libmp3lame -ab 160k -ar 48000 -async 48000 -ac 2 output.mkv
I get the error:
Resampling with input channels greater than 2 unsupported.
Can not resample 6 channels @ 48000 Hz to 2 channels @ 48000 Hz
This works fine:
mencoder input.mkv -oac mp3lame -lameopts q=2 -ovc copy -o output.mkv
but the issue is I need to use ffmpeg because it is what the program is using to transcode.
I know this is an old post, but hopfully someone will see activity.