This module is a variation on the Serge Voltage Variable Q VCF module (VCFQ).
The VARIABLE Q VCF (VCFQ) is an excellent general-purpose VCF offering
simultaneous low-pass, high-pass, band-pass and notch (band-reject) outputs.
The resonance (Q) of this filter is dynamically variable by manual or
voltage control. The VCFQ has two signal inputs. One incorporates an
automatic gain control to prevent the filter from overloading at high Q
settings. The second input has a level control so that the percussive
effects of overloading the filter can be exploited. When a pulse is applied
to the Trigger input, the filter will ring, producing a damped waveform
similar to that produced by striking a resonant object. The nature of this
ringing is controlled by the Q and the filter frequency. Percussive effects
ranging from clicks to the sound of wood blocks and bell tones can be
produced and controlled. This ringing effect can be used in conjunction with
signals applied to either of the audio inputs to achieve highly controlled
complex tonal qualities.