[sdiy] 3 Pole state-variable filters

[jhaible]

> In the style of Matrix-12/Xpander: BP2+AP, LBP and HBP
>
> That is, you can either balance a zero with a pole and avoid slopes,
> or you use it on either side of the passband to create a 12dB/Oct-6
> dB/Oct division between sides. In the allpass case you have 6dB/Oct
> slopes on both sides.

So you either spend one real pole (not compensated by a zero) for
an extra LPF or HPF (assymmetrical slopes), or you're compensating
the real pole with a zero to make an all pass, resulting in a
non-minimal-phase BPF filter. Ok, no need to choose the zero
equal to the negative real pole, so there would be other ways to preserve
symmetrical slopes than adding a pure all pass. Remaining question is
can I get anything different in amplitude response than from a 2pole
BPF, or is there just an extra degree of freedom for phase ?

JH.