The A-121 is a voltage-controlled multi-mode filter with a cut-off slope of -12 dB / octave. Four simultaneous outputs are available, each with different characteristics: low-pass, band-pass, high-pass and notch (or band reject).
The cut-off frequency determines the point at which the filter starts to work. The frequency can be adjusted manually, or by voltage control (Filter modulation, for instance by an LFO or ADSR). Two CV inputs are available, whose control voltages are summed.
Resonance (Emphasis or Q ) can be adjusted manually, or by voltage control (voltage-controlled resonance / VCQ), right up to self-oscillation, in which case it will behave like a sine wave oscillator.