[FFmpeg-user] Using amerge to convert dual mono to stereo

Tim Nicholson tim.nicholson at bbc.co.uk
Fri Jan 13 11:46:31 CET 2012

On 13/01/12 09:27, Tim Nicholson wrote:
> On 13/01/12 07:59, Tim Nicholson wrote:
>> On 12/01/12 18:01, Nicolas George wrote:
>>> Le tridi 23 nivôse, an CCXX, Tim Nicholson a écrit :
>>>> I am trying to use the amerge filter to convert dual mono to stereo,
>>>> but get a warning from amerge:-
>>>> "[amerge @ 0x1933a80] Inputs overlap: output layout will be
>>>> meaningless"
>>>> and sure enough what I get is stream 1 on both channels of the
>>>> stereo output with no stream 2 at all....
>>> I[...]
>> Well what I wanted was the first mono stream to be left and the second
>> mono stream to be right. What I got was the first mono stream on both
>> channels.
> Running again (with simpler parameters) and looking at the console
> output when I ran it I notice the following:-
> [...]
> [amovie @ 0x1791e80] seek_point:0 format_name:(null)
> file_name:glitz4-DV.mov stream_index:1
> [amovie @ 0x17f8fc0] seek_point:0 format_name:(null)
> file_name:glitz4-DV.mov stream_index:2
> [amerge @ 0x17f9a60] Inputs overlap: output layout will be meaningless
> This is presumably because they are both flagged as "mono" rather than
> "left" and "right"
> [abuffersink @ 0x1791e00] auto-inserting filter 'auto-inserted aconvert
> 0' between the filter 'Parsed_amerge_2' and the filter 'out'
> [amerge @ 0x17f9a60] in1:mono + in2:mono -> out:stereo
> That is what I would expect. based on the docs saying:-
> "If the channel layouts of the inputs are not disjoint, the output will
> have all the channels of the first input then all the channels of the
> second input, in that order", and what I want
> However the next line is:-
> [aconvert @ 0x17ef2e0] fmt:s32 cl:stereo planar:0 -> fmt:s16 cl:mono
> planar:0
> This last line suggests that aconvert is being invoked automatically and
> turning the stereo back into mono which is not what is wanted. The
> question is why, and how to avoid it?

...and to answer my own question, manually add an "aconvert" thus turning:-

ffmpeg -i in.mov -f lavfi -i "
amovie=in.mov:si=0  [a1];
amovie=in.mov:si=1  [a2];
[a1][a2] amerge" -vf " scale=720:576 "
-c:v dvvideo -pix_fmt yuv420p -c:a pcm_s16le -ar 48k -ac 2 out.mov


ffmpeg -i in.mov -f lavfi -i "
amovie=in.mov:si=0 , aconvert=s16:mono:planar [a1];
amovie=in.mov:si=1 , aconvert=s16:mono:planar [a2];
[a1][a2] amerge" -vf " scale=720:576 "
-c:v dvvideo -pix_fmt yuv420p -c:a pcm_s16le -ar 48k -ac 2 out.mov


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