I hate to be picky about this, but I would like to clear up a few points.

The 5.1 playback format derives from cinema playback systems. These systems
have multiple channels ranged across the front to lend spatial stability to
the signal, regardless of where the listener is seated in the theater. It is
true that in a 4 channel LCRS system such as Dolby Stereo, the center
channel gets primarily dialogue, but that is not necessarily set in stone. 

True, one can rely on 2 speakers to place the dialogue in a center phantom
channel. But, in that situation, if a listener is sitting on the left side
of the theater (for example), he will hear the dialogue coming from the left
speaker - a distracting experience. This is what the center channel are
intended to do - plant the dialogue, or any other center-critical material,
firmly in the middle of the screen. The additional front channels in 7 or 8
channel theatrical systems are just extensions of this concept.

As for panning, you should use some "pan law" to reduce the level of signal
as it pans through the center, not just a reciprocal gain change from one
channel to the other. Use a straight line and you get an apparent increase
of level in the middle, relative to when the signal is only in the left or
right channels. I think this is what RD means by constant-power panning.
There are various pan laws, but most are 3, 4 or 6 dB. In Csound you can do
this with a table of a negative going half-sine, scaled so that the minimum
value is -3 dB (or whatever your pan law is) relative to each end. Or
something more bizarre, if you prefer.

5.1 is a format for delivery to home sound systems. It is digitally encoded
into some delivery format, such as DVD (whatever it ends up like), so I'm
not sure why you would want to limit yourself to that. 

As for a derived center channel, you would probably only want to sun the 2
front channels, not the rear ones, into the center. The level of this
channel would then need to be reduced by some amount, to prevent the center
channel form dominating the soundstage. 6 dB seems right in theory, but in
practice, you might want to determine the appropriate attenuation
empirically.

Joy sticks - yes. Go to any film dubbing theater and you'll see at least
one, usually several, in the mixing console.

Hope this helps, and is not insultingly elementary.

-David.

Richard Dobson & tolve wrote (among other things):
> 
> 5.1 originated as a Cinema surround format, and the centre channel is
> typically used for dialogue (localized), rather than for 
> music. I don't think
> in itself it would do much to remove 'hole-in-the-middle' 
> effects - plain
> stereo constant-power panning can do that already.
> 
> tolve wrote:
> > 
> enhanced part is the extra speaker located at middle front 
> -quite useful in large venues to avoid the "hole in the 
> middle" sound effect and for more precise perception of 
> position of sounds. ok if you prefer to call this pentaud or 
> something. just want to make it clear that we're not adding 
> another dimension.

> > desired method:
> > pull 4 discrete tracks (mono soundfiles) into csound 
> individually using diskin. these would then be used to 
> generate automatically the fifth (center) track (mono soundfile).
> > 
> > Important: the positioning of the sounds and the manner in 
> which they pan should not be altered by the addition of this 
> extra track (speakers). the result would be, in the case of 
> 5.1, five separate aiff files.
> > 
> > to further clarify: the levels of the four discrete tracks 
> would be adjusted accordingly and the new fifth file would be 
> created in such a way as to maintain all positioning of 
> sounds in the quad image as they were in the original quad mix.
> > 
> > Incidentally, if anyone is feeling additionally ambitious, 
> 7.1 adds to the above additional enhancements: 2 speakers, 
> one on each side midway between the front and rear speakers