AW: four-pole state variable filters

[Haible Juergen]

	>And I think, tracking of the integrators is an issue,too. One could
	>adapt the capacitor values for each individual ota. If the otas are
	>driven with op-amp current mirrors, they should track.

Or built an expo converter with 4 outputs from a 5-transistor
array, like I did in my SSM2040 style filter. Avoids offset voltage
problems at low cutoff frequencies.

	>The switching problem you have mentioned is not so bad: only these
	>configurations make sense: lp->lp, hp->hp, bp->bp , no->no. I doubt
if
	>any other bp configurations make any audible difference. lp->hp
gives
	>a kind of bp, but this is allready there, so no need.

hp -> lp is the classic Yamaha CS series and Korg MS-20
configuration. Musically highly useful, but you want separate
control of the cutoff frequencies of both filters. In fact, if I were
to choose two most important configurations, then it would
be lp -> lp and hp -> lp.
Separate manual cutoff control, and common modulation
for both partial filters is a good compromise if you're
after a useful limit of parameters. You could apply the
modulation to the base of the "reference" transistor
in the quad expo converter described earlier, and the
manual cutoff control to the bases of the other transitors
in pairs.

JH.