[FFmpeg-user] Audio Stream Mapping issue

Tim Nicholson tim.nicholson at bbc.co.uk
Tue Nov 15 18:03:53 CET 2011

On 15/11/11 16:33, Clément Bœsch wrote:
> OK, let's quote this:
>      By default ffmpeg tries to pick the "best" stream of each type present
>      in input files and add them to each output file. For video, this means
>      the highest resolution, for audio the highest channel count. For
>      subtitle it's simply the first subtitle stream.
> "pick the best stream of each type [...] For video, [...], for audio
> [...]"
> I'm not sure where the confusion comes from…

I think the fact that, for audio a single channel, can be packaged 
either in its own stream, or be combined with others into a multi 
channel stream leads sometimes to thinking of one audio "item" as a 
single channel, and at other times as a single stream. The transition 
between these ways of thinking occurring subliminally and leading to 
confusion (That's my excuse anyway).

>> I am happy to contribute to better wording, once I understand what
>> the "correct" behaviour is.
> Quickly looking at ffmpeg.c (around L3934), the audio stream picking is
> what we already suggested: it loops against all stream, pick the one with
> the most number of channels (and thus *ignore* "score" equality) and add
> it.

With your help I think I have it sorted in my own mind, now. But I think 
the doc could be improved.

> So if you want to fix the documentation, you should IMO just reword in a
> sense to explicit the "only one pick per stream type". I'm not a native
> English speaker so I'm sure you will be able to propose a better wording
> than me; feel free to send a patch on ffmpeg-devel. If you don't, I'll
> take the time to of course.

Never tried submitting a patch so will haver to bone up on how to do it...


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