Moog Ladder Filter Generalizations Based on State Variable Filters
We propose a new style of continuous-time filter design composed
of a cascade of 2nd-order state variable filters (SVFs) and a global
feedback path. This family of filters is parameterized by the SVF
cutoff frequencies and resonances, as well as the global feedback
amount. For the case of two identical SVFs in cascade and a specific value of the SVF resonance, the proposed design reduces to
the well-known Moog ladder filter. For another resonance value,
it approximates the Octave CAT filter. The resonance parameter
can be used to create new filters as well. We study the pole loci
and transfer functions of the SVF building block and entire filter.
We focus in particular on the effect of the proposed parameterization on important aspects of the filter’s response, including the
passband gain and cutoff frequency error. We also present the first
in-depth study of the Octave CAT filter circuit.