
		Midi And 4 Channel Surround Sound
		---------------------------------


I got some email today asking how to do Dolby surround midi files. I tought
it could be of interest to other people. Note: Only a ram wavetable midi
device can reproduce the surround channel, with this method, but the center
channel can be done on any midi playback device.

If you want something that comes out from the center channel, just put the
pan on the midi channel at 64. Coming from left? just put the panning to
the left (under 64). Coming from right? just put the panning to the right
(over 64).

For elements that have to be present on both sides i'd recommend dupli
cating the track and giving two opposite panning. This gives much better
results than panning to center and shouldn't come out the front channel.
Also, if you sampled something in stereo, then you can use the left signal
to create a patch for the left track and a separate patch for the right
track. This works very well.

Finally, for the surround channel, it's the same as stereo snce you use
two tracks. One for left, one for right. On the left you use the regular
instrument, and on the right, you use the *inverted* signal (a flip around
the X axis of the signal, as done by the invert function of most wave
editors). When i was asked the question, i hadn't tried this, but i just
did and it works. I use my own circuit to decode the surround channel (a
very simple design, found on archive.epas.utoronto.ca under the filename
surround.txt) and have not tried it on a real dolby surround or pro logic
unit, but it should work equally well.

Of course by using two voices per sounds, you use the GUS voices much more
rapidly, but the sound is much better, even when it's the same signal on
both channels because of the slight phasing between the left and right
channels. Plus, you get much more balance control.

So when you want something on the left, center or right channel, just use
mono panning. For the surround channel, on top of that, just add two voices
panned left and right, one playing the inverse of the other. Note, in mono,
this will disapear totally...

This should be very simple to understand by looking at this diagram:

P=panning, V=volume
                             CENTRE CHANNEL
                             P=64, V=presence
                              V        /|\
                              variation |
                                       \|/
 LEFT CHANNEL                                                   RIGHT CHANNEL
P=1, V=presence               P variation <-->                P=127, V=presence

                              Vleft=Vright /|\
                              variation     |
                                           \|/

                            P=1,normal patch;P=127,inverted patch,
                            Vleft=Vright=presence
                            SURROUND CHANNEL (either 1 or two speakers,
                                             this channel is mono)

For stereo instead of panned mono:
P=1, left signal patch;P=127, right signal patch, Vleft=left presence,
Vright=right presence. So for <--> variation, you use the two volumes instead
of P.

Francois Dion
