Sweeping pulse width with an LFO animates a single oscillator into a thick, chorused timbre

A pulse wave’s harmonic content depends on its duty cycle (pulse width); a 50% square becomes a rectangular wave as the width shifts, a subtly different timbre. Pulse-width modulation (PWM) continuously sweeps this width with a low-frequency oscillator (LFO), so the waveform cycles between rectangular shapes and its spectrum is constantly in motion. On a single oscillator this yields a thick, shimmering, animated sound — closer in character to a chorus or mild flanger applied to the waveform itself than to the beating of two detuned oscillators. Slow PWM mimics the natural drift of several string players slightly out of tune, making it a vital tool for lush synthesized string pads; faster/deeper PWM gives the thick bass that became iconic in old-skool rave. The LFO rate sets the speed and the PWM depth (CV amount) sets how far the width swings. Where a synth lacks PWM, layering two slightly detuned oscillators approximates the effect.

Examples

VCV Rack: patch LFO OUT → VCO-1 PWM input, raise the PWM CV amount, set a moderate LFO rate; use as a bass oscillator for animated thickness. Compare with LFO → VCF FREQ for a ‘wub’ instead. A slow LFO on a string-machine patch gives a breathing chorus; two detuned saws stand in for PWM when unavailable.

Assessment

Explain the difference between a square and a rectangular wave, and why sweeping pulse width with an LFO sounds thicker than a static square. Explain why slow LFO-driven PWM suits string pads (which harmonics are continuously introduced/removed), and name a genre where PWM basslines are characteristic.