[FFmpeg-user] map_channel with H264/AAC input question

Nicolas George nicolas.george at normalesup.org
Wed Feb 29 09:40:45 CET 2012

Le primidi 11 ventôse, an CCXX, Robert Reinhardt a écrit :
> I have an H264/AAC video with an audio program that has two channels
> (left/right), and I'd like to separate these channels into discrete
> program tracks (left channel as mono to one track, right channel as mono
> to another track). I was hoping the map_channel option would work without
> needing to reencode the audio, but I can only seem to get this type of
> channel mapping by using avfilter / pan option, which reencodes the audio
> to pcm_s16le first, and then encode back to AAC.

AAC encodes stereo in a coupled way: the left and right channels have a lot
of similarities, not taking advantage of them for compression would be a
pure waste.

So there can be no simple way of separating them without transcoding.

And even if there were, ffmpeg currently does not have the infrastructure to
do that kind of bitstream manipulation, let alone the exact filter you need.


  Nicolas George
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