[FFmpeg-user] aresample=matrix_encoding=dplii output is still 5.1 ??

[pehache]

Hi,

I used to apply a Dolby Pro Logic II encoding this way :

"-af aresample=matrix_encoding=dplii -ac 2"

But I learned that "-ac 2" does a full downmix of the 5.1 stream, which 
is not what I want : after the DPL II resampling one simply want to 
extract the 2 (encoded) front channels and that's it...

The other question is: why the output of the DPL II encoding is still a 
5.1 stream, and not just a 2.0 stream ?

I'm a bit lost... and will appreciate any explanation.