[FFmpeg-user] More on audio stream mapping

Nicolas George nicolas.george at normalesup.org
Thu Dec 22 11:57:22 CET 2011

Le sextidi 26 frimaire, an CCXX, Tim Nicholson a écrit :
> It also appears that you cannot use -c:a copy if using -map_channel
> as the copy copies the stream as is, and not the channel.

That one is true, and is not an arbitrary limitation of ffmpeg, it is an
inherent limitation of the audio encoding system: a lot of codecs encode
channels together, not separately. This is quite normal, as channels are
usually very similar, and compression is all about taking advantage of

For the same reason, you can not crop or scale a video without decoding and
reencoding it.

It is, of course, possible to do these things with some codecs. With the
uncompressed ones, of course, and there are other examples: you can crop
JPEG without decoding it, as long as the left and top dimensions are
multiple of 16. But these examples are very specific, and there is no
generic way to do anything.


  Nicolas George
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