[mythtv] Internal DVD Player multichannel support
lists at wildgooses.com
Tue May 2 22:03:21 UTC 2006
>>mode and be done with it. Maybe a keystroke in the different playback
>>switch between surround and "bypass" (stereo (front L+R), but with
>>to steer bass to the LFE channel) would be a good thing to do as well.
>might be difficult to change dynamically. leave this feature for a lot later
The way it's usually done is to consider 6 channel to be "normal" and
downmix from there.
So what I do in my case is hook up all 6 outputs to my 6 inputs on my
amp (conceptually at least) and then when it's a two channel signal all
that happens is that the rear speakers are silent.
I don't have any special upmixing which occurs when there is only stereo
to make it some kind of pseudo surround signal. I did play with some
matrixing idea to make this happen (do a phase difference and then play
it back with delay and attenuation from the surround channels), but it
didn't really float my boat...
So basically you just leave your amp in 6 channel mode all the time, no
need for any special processing
To downmix we simply steal the mplayer matrixing code... It's just a
case of splitting the signal to the speakers you haven't got, eg if no
center then half the center signal and patch it to the two front
speakers. No surround then just patch the surrounds to the front. No
sub then split the sub signal and send it to the front channels. To be
totally correct you should drop 3dB of attenuation if you merge two
channels together to avoid clipping
Bass management is slightly harder, but mplayer uses a fast high quality
solution. Essentially you generate a highpass/lowpass FIR filter and
convolve everything with this. Take the outputs to the respective
speakers. There is a fast convolver in lavc and from memory there is an
adequate if not optimal bit of code to generate filter coeffs in
mplayer, otherwise there is a slightly nice bit of code in brutefir (or
write our own... *shock*)
I did all of this in Jack + Brutefir though because I wanted to play
with optimising the filters... I have this mad idea that by using some
linear phase filters you can get better dispersion control from the
mains to the sub, hence run the sub higher. The effect is dramatic and
reduces the work the main speakers are doing and hence drops their
distortion dramatically - probably gives a nice boost for folks using
standard surround speakers in their setup...
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