[sdiy] 4-pole SVF theory

[David G Dixon]

> If you look at current single Chip SVF's implemented with 4 
> pole capability, late CEM, late SSM and Roland, they are ALL 
> 2pole x 2 serial. And those of more discrete design JP6 and 
> others that i cant remember the name of are also 2 pole x2 
> serial. I bet the designer of these had a good reason for 
> this, particularly the CEM since they where designed for CD 
> applications.

Most of the filter textbooks I've looked at always generate multipole
filters by putting 2-pole filters in series, and tacking on a 1-pole if an
odd number is desired.  This would certainly be the easiest way to go about
it.  Of course, if you were using, say, a 2164 as the VC resistor, then you
could control both filters with the same control voltage.

What I'm not sure about is what you'd actually get from all of the output
taps.  For that I'd need to crunch the transfer functions (based on current
balances).  There might also be some clever things you could do with
feedback across the two stages.