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Old 01-11-2017, 12:04 PM   #16
earlevel
Human being with feelings
 
Join Date: Dec 2015
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Quote:
Originally Posted by Nowhk View Post
I see. So in fact which kind of filter I've (uhm, in fact, Martin) did? An "hybrid" one pole second order filter?
A one-pole filter can only be first order (two-pole second order, etc.). I think you're seeing the delay elements with feedback in Martin's filter and thinking that they are two one-pole filters with feedback. They aren't—they are integrators.

Here, I found a block diagram of an analog state variable filter:

http://www.electronics-tutorials.ws/...102.gif?x98918

See how it's summing bandpass and lowpass feedback? Here's the basic digital version, from my site:

http://www.earlevel.com/main/2003/03...riable-filter/

Note that the lowpass feedback is fixed at -1, and bandpass feedback controls the peaking (resonance).

Anyway, I haven't seen your modified filter, maybe it's the same as Martin's, maybe not—I'm just responding to your comments.

This state variable (generally attributed to Hal Chamberlin, from his book Musical Applications of Microprocessors) works pretty well, but only to about a sixth of the sample rate. I think I mentioned Andy Simper's trapezoidal-integrated version that is far better, and only slightly more complicated. If you really want a synth filter, start with that.
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