[MPlayer-users] Using pan audio filter to create DPLII-compatible 5.1 to stereo downmix

[James Lee]

I'm trying to use mplayer to take a 5.1 audio file and create a Dolby
Pro Logic II compatible stereo file from it.  There doesn't seem to be
a built-in command to do this in mplayer which is independent of the
input codec, but I thought the pan audio filter might give me what I
need.

Getting a DPLII-compatible downmix involves matrixing the 6 channels
into stereo as follows (see
http://en.wikipedia.org/wiki/Dolby_Pro_Logic_II#Dolby_encoding_matrices):
# Dolby Pro Logic II Left Right Center Rear Left Rear Right
# Left Total 1.000 0.000 0.707 -0.8165 -0.5774
# Right Total 0.000 1.000 0.707 0.5774 0.8165

Now the problem is that the pan audio filter only supports positive
coefficients - but this needs negative ones because a phase shift is
being applied to the surround channels.  Is this something which can
be done in some other way in mplayer, or am I missing something here?
If not, does anyone know whether it would be hard to update the pan
filter to do this (I may have a look into doing it if not).

Thank you,
James