four-pole state variable filters

[Brian Towles]

> Is this just a mistake ? The usual SVF has 2 poles.  You can get any
> even order response by cascading such  2-poles.

no mistake ... i realize casading two 2-pole filters could work.  however, i
see some drawbacks to this method:

1. the filters might have to be "tuned" together

2. to get all the LP,HP,BP,notch functions, a selector switch would have to
be used to select the correct output from the first stage to use in the
second stage.  with a single, 4-pole filter, all the functions could be
output at the same time.

3. you can't change any of the filter coefficients independently in a casaded
design - i really don't know if this is good or bad?

here's a more general question, related to 3:

- in a standard 2-pole SVF design, changing the resonance of the filter
changes the coefficient of s ( right? ).  what does the resonance do in a
4-pole filter? does it just change the coefficient of s^3? what about s^2 and
s?

any ideas?

brian