[MPlayer-users] -af surround-questions...

[Lars Grunewaldt]

hi there,

I'm just playing around with my onboard-card that supports multi-channel 
output (up to 6 channels).

It works fine, even if I don't have a 5.1 amplifier (I simply hooked up 
3 stereo-amplifiers to the soundcard... one could say this is overdoing 
things, but I like it - at least until I have money for a true serial 
5.1 amp).

First I tried some DVD's and AC3-5.1-Streams, working out of the box 
with -channels 4 or -channels 6.

Then I tried to use -af surround to find out what could be squelched out 
of 2channel-files like vcd's and stuff. Now I wondered...

1. whould the surround filter generate a center/LFE signal? Or, at 
least, a center signal (LFE is possible with -af sub=100:5, and it 
works, too)? Would be nice to have a center, but I'm not sure if the 
matrix encoding has a center signal? If not, a center should be "easily" 
to extract from the stereo signal? -channels 6 instead of -channels 4 
only produced silence center/LFE channels.
mplayer command line:
mplayer -ao alsa9:hw:0.1 -af surround,format=unsignedint -channels 4 vcd://

2. I had no chance to try it yet, but I'd like to use mplayer as a 
realtime software surround decoder, hooking up a signal from my TV 
through some sound-in (maybe of my bttv/hauppauge digital in, or through 
the mic input (what would be usable if I only use 4 channels for 
output). Is this possible?

3. What happens if there is no "real" matrix-encoded surround signal in 
the file? I tried some old mp3's which surely do not contain any 
surround signals, but still there was a signal on the rears. Of course I 
did not expect them to be silent tue to the way how the matrix encoding 
works, but there was a lot of stuff to hear. I tried a really long delay 
time (990ms) and it was clearly the rear-signal coming out of the rears 
(now much delayed). Could anyone explain how the matrix 
encoding/decoding works (I'm not that clear about it) and why this happens?

I hope anyone has some suggestions here, I find it quite interesting as 
it makes it possible to watch dvds and stuff with a more decent sound. A 
friend of mine has a dolby THX certified 5.1 sound setup he purchased 
about 1998, and Star Wars really kicks *ss on that thingy...

may the force be with you
  Lars

PS: well yes maybe playing 8 hours jedi knight 2 was too much, but...