Filter resonance boosts frequencies at the cutoff point; at maximum it causes self-oscillation

Resonance (also called Q or emphasis) is a parameter that boosts frequencies near the filter’s cutoff frequency by feeding the filter’s output back into its input. At low resonance, the filter simply attenuates frequencies beyond cutoff. As resonance increases, a peak forms at the cutoff — making the filter “ring” at that frequency. At maximum resonance, the filter self-oscillates and produces a pure sine tone even with no input signal; the cutoff CV then controls pitch of that tone (at 1V/oct if the filter tracks keyboard pitch). Resonance is central to the distinctive acid sound of the Roland TB-303, where high resonance and fast filter sweeps produce the characteristic squelch.

Examples

TB-303 acid bassline: high resonance, fast ADSR cutoff sweep, low cutoff offset → “wobbly” squelch sound. Self-oscillating VCF: all audio input removed, resonance maxed, CV from keyboard plays filter as a pure sine oscillator.

Assessment

Describe the tonal effect of slowly increasing filter resonance from 0 to maximum with the cutoff fixed. Explain why a self-oscillating filter can be played as a sine wave oscillator using a 1V/oct CV.