A VCO generates a periodic waveform whose pitch tracks an incoming control voltage

A Voltage-Controlled Oscillator (VCO) is the primary tonal sound source in a modular synth. It generates a continuously repeating waveform whose frequency is set by its tune knob and tracked to an incoming control voltage, usually at the 1V/octave standard. Its pitch input is just another CV jack, so anything that outputs CV — keyboard, sequencer, LFO, envelope — can play it. It sounds the instant its output is patched to an amplifier; no note-trigger is needed. Analog VCOs use analog circuitry (unlike a DCO), so they may drift slightly with temperature, giving a warm continuously-varying sound; digital oscillators add wavetable, FM, and physical-modelling voices. VCOs commonly output several geometric waves simultaneously from separate jacks, differing in harmonic content: sawtooth is harmonically richest, square has odd harmonics only, sine has none. The 1V/oct pitch response is exponential, so a linear input would break musical pitch tracking.

Examples

Sequencer pitch CV → VCO 1V/oct input, VCO saw → VCF → VCA: a sequenced monophonic voice. Patch an LFO into the 1V/oct input instead and the pitch wobbles (vibrato). Mixing two slightly detuned VCOs gives the classic analog ‘fatness.‘

Assessment

Explain what makes a VCO ‘voltage-controlled’ and name two CV sources you could patch to its pitch input. Name three waveforms it commonly outputs and one harmonic difference between sawtooth and square. What happens to pitch tracking if the pitch input is linear instead of exponential?