An LFO is a sub-audio oscillator used as a control source to cyclically modulate another module's parameter

A low-frequency oscillator (LFO) generates a periodic waveform below the audio range (typically under 20 Hz) — too slow to be heard as a pitch, but fast enough to produce perceptible cyclical change. Rather than feeding the audio output, its output is routed as a control-voltage source to a parameter of another module, giving time-varying movement without hand-turning a knob. Where you route it names the effect: to an oscillator’s (VCO’s) pitch input it produces vibrato; to an amplifier’s (VCA’s) level input, tremolo; to a filter’s (VCF’s) cutoff, a sweep or wobble. Two controls shape it: rate sets how fast the parameter wobbles, and depth (often set by an attenuator) sets how far. The waveform sets the shape — triangle (smooth), sawtooth (ramp that resets sharply), square (abrupt flipping). This one-oscillator-drives-another-module’s-input routing is the fundamental modulation idea in modular synthesis, and LFOs can be clock-synced for rhythmic modulation.

Examples

LFO triangle at 4 Hz → VCO FM/pitch input = vibrato. Same LFO → VCA CV input = tremolo. LFO → filter cutoff = a slow sweep (sawtooth resets sharply). Faster rate = faster wobble; larger depth = wider swing.

Assessment

Given an LFO, a VCO, a VCA, and a filter, describe the cable connection that produces (a) vibrato and (b) tremolo, and state which module’s parameter the LFO controls in each. Then explain what a 2 Hz LFO on filter cutoff sounds like, and the difference between vibrato and tremolo in terms of the target parameter.