
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.


Hope this helps.


Ciao,

--

Francois Dion

    '  _   _   _

 CISM (_) (_)  _) FM       Montreal , Canada       Email: CISM@ERE.UMontreal.CA

      (_)  / . _)             10000 Watts          Telephone no: (514) 343-7511

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