[sdiy] 4-pole SVF theory

[Tom Wiltshire]

Hi All,

Can anyone point me at some good resources for state variable filter theory? I've spent a long time playing with the classic 2-pole, 12dB oct SVF filter, and I think I get that now. So I want to move along. How do I convert it into a 4-pole filter? (without just sticking a second one on the end of the first one!)
Presumably there are good reasons why you don't see lots of 4-pole SVF filters around, otherwise I'd have thought it'd be the killer synth filter  - LP, HP, BP outputs, independent Q control - cool, no? So what's wrong with it?

Pleez, help me edukate meself!

Thanks,
Tom