[FFmpeg-user] Audio channel mapping of stereo channel without re-encode

[Carl Eugen Hoyos]

Claudiu Rad-Lohanel <jazzman <at> misalpina.net> writes:

> >> From the source audio I would like to take 
> >> either only the left or the right channel
> >> into my outputs. While only transcoding the
> >> video and copy the audio.

> > This cannot work.

> Indeed, you are right. But from what I know so 
> far, this is because the level of data access 
> that we have now is either at the stream level 
> (audio, video, subtitles, etc) after demuxing, 
> either somehow at the uncompressed frame level 
> after decoding.

No.
If this were true, the relevant filters might 
even work as suggested by you: After all, we 
usually aim to be as fast as possible.
But the problem is that stereo (in real codecs, 
I am of course not speaking about pcm audio) is 
encoded in a way that makes it impossible to 
extract one channel without decoding.

Carl Eugen