[FFmpeg-devel] [PATCH] doc: document amerge filter as an alternative for the -map_channel limitation.

[Nicolas George]

Le quartidi 24 nivôse, an CCXX, Tim Nicholson a écrit :
> Did not work for me because as my source was pcm_s24le the following took place:-
> 
> [amerge @ 0x17f9a60] in1:mono + in2:mono -> out:stereo
> [aconvert @ 0x17ef2e0] fmt:s32 cl:stereo planar:0 -> fmt:s16 cl:mono planar:0
> 
> i.e. the automatically inserted aconvert turned the stereo back into mono.

This is not related to amerge, you can get the same result with:

./ffmpeg_g -f lavfi -i 'amovie=t24.flac' -f wav - > /dev/null

(where t24.flac is a s24 FLAC file):

[abuffersink @ 0x2c4f340] auto-inserting filter 'auto-inserted aconvert 0' between the filter 'Parsed_amovie_0' and the filter 'out'
[aconvert @ 0x2ce81a0] fmt:s32 cl:stereo planar:0 -> fmt:s16 cl:mono planar:0

The problem is the combination of two misfeatures:

- The lavfi device only supports S16LE in the output audio pads, this is
  hardcoded in the source.

- The aconvert filter does not make any effort to avoid conversions when it
  can, and thus will probably always downmix to mono if the next filter
  accepts it.

The limitation of lavfi is quite easy to remove.

The problem of aconvert is more difficult. In the meantime, it can be worked
around using the aformat filter.

Regards,

-- 
  Nicolas George
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