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

Tim Nicholson tim.nicholson at bbc.co.uk
Fri Jan 13 10:27:47 CET 2012

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 

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?


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