[MPlayer-users] Vorbis multichannel downmixing

[Lucas Clemente Vella]

I have been experimenting with 5.1 audio encoded in Vorbis, and I
found that MPlayer can't automatically downmix 5.1 Vorbis audio to
stereo, like in AC3 or DTS. It seems that it leaves this task to the
decoder, and since the decoder can't do the downmixing, MPlayer simply
discards all but the left and right channels. To workaround the
problem, I tried to use the pan filter with these settings:

-channels 6 -af pan=2:1:0:0:1:0.5:0:0:0.5:0.7:0.7:0:0

My first problem was to find out what channel order was being
delivered to the filter, the order was obviously different from what
Vorbis specify for 5.1 audio. After some trial and error, I found the
order. The remaining problem was to find what values to use. At first
I did a guess, but the final values were suggested in this forum
thread:
http://www.blender.org/forum/viewtopic.php?t=16557

These values worked fine for the movie Elephants Dream from that
thread, but when I played another movie in my TV via HDMI, whose audio
I converted from DTS to Vorbis, I get some strong noise in the parts
where LFE is strong (even if the pan filter, like above, is configured
to discard the LFE channel). There is no such problem when using the
default DTS dowmixing facility. So, do you know what may be causing
this strong noise effect? Should the fact that MPlayer can't
automatically downmix from 5.1 to stereo be considered a bug?

-- 
Lucas Clemente Vella
lvella at gmail.com.br