Doepfer A-122

Module A-122 (VCF 3) is a voltage-controlled low-pass filter, which filters out the higher parts of the sound spectrum, and lets lower frequencies pass through.
The cut-off frequency determines the point at which filtering takes effect. You can control this manually, or by voltage control (filter modulation, for instance by an LFO). Two CV inputs are available.

The cut-off slope is -24 dB/octave. The circuitry uses a Curtis chip, and is very similar to the classic Oberheim filter sound. Because of its different circuitry and controls, the A-122 has a considerably different sound to the A-120 'Moog-like' filter, especially in the way the resonance behaves.
Voltage controlled resonance: on the VCF 3, resonance can be controlled not just manually, but by voltages as well, right up to self-oscillation. In this case, the filter behaves like a sine wave oscillator.