[sdiy] Pole Dancing
don at till.com
Tue Jan 24 20:21:47 CET 2023
On Jan 24, 2023, at 1:25 AM, Richie Burnett <rburnett at richieburnett.co.uk> wrote:
Ah, good point; I forgot to mention that case.
To plot a State Variable Filter place two poles at the origin. That's a SVF running open loop. Then add negative s^0 feedback to tune it. Without damping it's an oscillator. So then add some negative s^1 feedback to set the Q.
To plot a Sallen-Key style filter, place two poles on the left real axis, and add positive s^1 feedback to set the Q.
Adding negative s^0 feedback raises the value of the s^0 coefficient, which has the effect of lowering the middle coefficients if you rescale the polynomial, thus raising the Q.
Adding positive s^1 feedback lowers the value of the s^1 coefficient directly, raising the Q.
Donald Tillman, Palo Alto, California
More information about the Synth-diy