[sdiy] SVF: 2-pole to 4-pole ?

[Neil Johnson]

Hi,

Ray Wilson wrote:
> All of the designs I have seen that make this claim actually involve 2
> two pole filters which are then able to be switched into a single 4 pole
> filter.

Mark wrote:
> although don lancaster refers to it in the active filter cookbook, i can
> not see how to transform a 2-pole state-variable filter (freq, Q and gain
> being tunable) into 4-pole.

A true 4-pole state-variable filter is fixed in its response, so you
cannot just switch it,  Ray is right about having two 2-pole filters
cascaded, and a switch to choose the filter.

If you want a "true" 4-pole filter, you'll be delving into s-plane/Laplace
algebra, which isn't too bad for the simple cases, like an audio filter.
I've been experimenting with a 3-pole filter (mainly 'cos I had 3 gain
cells to play with :-) and the maths is ok.

Neil

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