A voltage-controlled filter at maximum resonance self-oscillates into a sine-wave oscillator

Voltage-controlled filters have a feedback path (‘resonance’ or ‘Q’) that, when increased to maximum, causes the filter to self-oscillate — it sustains a sine wave at its cutoff frequency without any input signal, analogous to audio feedback through a microphone-speaker loop. This makes the filter a secondary oscillator. The resulting waveform is typically a sine wave, which is difficult for standard VCO circuits to produce cleanly. The critical limitation is pitch tracking accuracy: most filters use less precise components than VCOs, so they may drift when tracking a keyboard or sequencer via CV. Filters intended for melodic self-oscillation label their frequency inputs ‘1v/oct’ rather than ‘freq’ to indicate pitch-accurate tracking.

Examples

Joranalogue Filter 8: ‘1v/oct’ input for accurate pitch tracking plus exponential and linear FM inputs. Standard filters: self-oscillate but drift in pitch, useful for noise/texture rather than melody. Playing a self-oscillating filter through a sequencer requires a 1v/oct-rated filter.

Assessment

Explain in two sentences why a self-oscillating filter produces a sine wave rather than the filter’s other waveforms. Explain why the label ‘1v/oct’ on a filter’s CV input matters for melodic use of self-oscillation.