The GMSN! Pure VCF is used to filter two incoming audio signals. This can be done manually using the cutoff knob or automatically using control voltages. There is also a feedback knob controlling the resonance. This causes an amplification of the signal at the cutoff frequency, giving the classic squelchy acid sounds. The feedback can also make the filter self oscillate providing a very clean sine wave output, with temperature compensation. This sine wave can be calibrated to 1 volt per octave.