Voltage control lets any module parameter be driven by another module's output

The defining feature of analogue modular synthesis is voltage control: every important parameter (oscillator pitch, filter cutoff, amplifier gain) can be changed not just by turning a knob but by feeding in a control voltage from another module. This idea, commercialized by Robert Moog in the 1960s, makes modular systems maximally flexible — any module that outputs a voltage can, in principle, drive any parameter of any other module. The result is that distinctions between ‘sound sources’ and ‘modulation sources’ blur: an LFO can serve as audio; an audio oscillator can modulate a filter cutoff. Understanding voltage control is the conceptual prerequisite for understanding every patch.

Examples

An ADSR envelope generator outputs a rising-then-falling voltage that is patched into a VCF’s cutoff CV input, causing the filter to open and close with each note. The same envelope could simultaneously drive a VCA for amplitude shaping.

Assessment

Explain in your own words why a modular synth can produce sounds no fixed-architecture synth can, using the concept of voltage control. Then give two examples of a module being used in an unexpected role.