A voltage-controlled oscillator's frequency is set by an applied control voltage, not just a manual knob

A voltage-controlled oscillator (VCO) generates a periodic waveform whose frequency can be set either internally (a knob) or by an externally applied control voltage. This is the hinge of modular synthesis: because pitch is a voltage, it can be driven by a keyboard, a sequencer, a random source, an envelope, or another oscillator (for FM) — anything that outputs CV. The same principle extends to waveshape and modulation: the Buchla 158, for instance, sweeps continuously from sine to sawtooth and accepts wideband frequency modulation. Voltage control is what lets the whole patch modulate the oscillator automatically rather than a hand turning a dial.

Examples

Model 158: frequencies ‘may be controlled internally or with externally applied control voltages’; ‘waveshape is continuously adjustable from sine to sawtooth; oscillators may be wideband frequency modulated.‘

Assessment

Explain why representing oscillator pitch as a control voltage lets a sequencer or envelope drive it. Give one modulation source you would patch to a VCO’s frequency input and what it does.