[FFmpeg-user] Audio Stream Mapping issue

Tim Nicholson tim.nicholson at bbc.co.uk
Tue Nov 15 17:23:36 CET 2011

On 15/11/11 16:08, Clément Bœsch wrote:
> On Tue, Nov 15, 2011 at 03:27:48PM +0000, Tim Nicholson wrote:
>> On 15/11/11 10:19, Clément Bœsch wrote:
>>> On Tue, Nov 15, 2011 at 02:05:35AM -0800, hughht5 wrote:
>>>> I would use the -map_channel option, however the documentation says it is
>>>> limited at the moment:
>>>> (From http://ffmpeg.org/ffmpeg.html)
>>>> Note that "-map_channel" is currently limited to the scope of one input for
>>>> each output; you can’t for example use it to pick multiple input audio files
>>>> and mix them into one single output.
>>> AFAIU, Tim only has one file so it doesn't apply here.
>> Quite so.
>>> Tim, I'm not sure to understand what you really want from your 4 stereo
>>> audio streams. I can't tell if you want to pick only some (then -map) or
>>> actually merge the channels of each streams into one single audio stream
>>> (then what you need is indeed the -map_channel feature).
>> All I actually wanted to do was rewrap mxf->mov with no other
>> changes. (They were in fact 4 mono streams mxf does not support
>> stereo streams afaik)
> My bad, I misread it was mono.
>> ffmpeg -i in.mxf -map 0 -vcodec copy -acodec copy tim2.mov
>> or more succinctly:-
>> ffmpeg -i in.mxf -map 0 -c copy tim2.mov
>> performs this.
>> However according to the documentation the -map 0 *should* be
>> unnecessary as the default *should* be "for audio the highest
>> channel count" wheeras it seems to be the first audio stream only.
> The documenation sounds quite obvious to me; the selection pick "the
> best", and in case of score equality, only the first one is picked. Maybe
> it should indeed be explicited, or the behaviour changed.

In my experience, of much documentation writing and editing of others 
write ups, something that is abundantly clear to the writer can be 
completely misinterpreted by end users approaching things from a 
different starting point, it seems to be the way of things. That is why 
I first rasied the issue here to try and get a consensus of what the 
expected behaviour should be, and then try and understand how the 
documentation can be read to mean that...

I am not sure where you get the 'pick "the best"' from the 
documentation, it only says this for video, not audio!

I am happy to contribute to better wording, once I understand what the 
"correct" behaviour is.

> Sorry for the first misunderstanding, I'll do some checks and send a patch
> for this in a while if no one does.



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