a lowpass with variable cutoff frequency and resonance.
RBJ has a cookbook on how to do this:
Here the cutoff frequency is called
w0, and the resonance is called Q:
LPF: H(s) = 1 / (s^2 + s/Q + 1)
b0 = (1 - cos(w0))/2
b1 = 1 - cos(w0)
b2 = (1 - cos(w0))/2
a0 = 1 + alpha
a1 = -2*cos(w0)
a2 = 1 - alpha
If you want to vary these parameters continuously while passing signal, you are not supposed to use biquads (because updating the coefficients causes glitches?) and you should use state-variable filters instead:
This is also a 2nd-order lowpass with variable Q.
Does anyone have insight into filter types commonly used to mimic the sound of traditional analog gear?
Generally any filter transformed to a digital IIR using the bilinear transform is an "analog-like" filter. Bessel is an exception because the important property of a Bessel filter is its group delay, which is not preserved by the bilinear transform. Bilinear transform attempts to preserve the frequency response primarily.