[FFmpeg-user] More on audio stream mapping

Clément Bœsch ubitux at gmail.com
Sat Dec 17 10:08:35 CET 2011

On Fri, Dec 16, 2011 at 12:11:50PM +0000, Tim Nicholson wrote:
> I have a file containing 1 video and 4 mono audio streams, and wish
> to create a file with 1 video and 2 stereo audio streams.
> Under the old API the extra audio stream was added after the output
> filename, but now it is supposed to be done via the -map and
> -map_channel commands.
> It therefore seems that the correct way to now achieve this is:-
> ffmpeg -i archive.gxf -c:v copy \
> -c:a libfaac -b:a 96k -ar 48k \
> -map 0:0 -map 0:1 -map 0:2 \
> -ac 2 -map_channel 0.1.0:0.1 -map_channel 0.2.0:0.1 \
> -ac 2 -map_channel 0.3.0:0.2 -map_channel 0.4.0:0.2 \
> out.mp4
> From my reading of the documentation the:-
> -map 0:0 -map 0:1 -map 0:2
> maps the video and first two audio streams to the output file, which
> is not the mapping I actually want but is the only way I can see to
> add the extra output audio stream. Then the:-
> -map_channel 0.1.0:0.1
> commands overwrite this mapping to put the audio channels where I want them.
> Adding streams by performing a mapping that is not wanted seems to
> me to be counter intuitive. Or have I misunderstood things?
> 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.

You can't unfortunately ATM have an output stream composed of multiple
input streams (internally an output stream is associated with only one
input stream); I though I added a note in the documentation, but my
wording is certainly approximative as ever.

But this might be possible through the filters when the merge filter will
be upstream (there is a pending patch on the mailing list). One could also
write some stream preprocessing code to merge the streams.

Anyway, you just hit a limitation...

Clément B.
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