[sdiy] 3 Pole state-variable filters

[Neil Johnson]

Don and all,

> > LPF or HPF).  Or is there a way to "distribute" 3 poles equally
> > on both slopes of a BPF function ?

>       s^2 + s
> -----------------
> s^3 + s^2 + s + 1

Yep, that's the one!  Just tap off the two intermediate points (s and s^2)
and sum them to produce the BP output, which also gets fed back via the Q
gain cell to vary the resonance.

I does sound quite nice too (imagine a filter half-way between a 2-pole
and a 4-pole :-)

Now all I need to do is get it all onto a 2"x2" PCB.

I had thought about a 4-pole filter, but that would have needed an extra
VCA for the Q gain (e.g. 3080, 3280, etc).  Its going to be a well-packed
PCB anyway, so maybe I'll reserve that idea for the surface-mount version!

Ho hum, on with the show...

Neil

--
Neil Johnson :: Computer Laboratory :: University of Cambridge ::
http://www.njohnson.co.uk          http://www.cl.cam.ac.uk/~nej22
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