Modulating the pulse width of a square wave produces chorusing and Doppler-shift timbral effects

Pulse width modulation (PWM) continuously varies the duty cycle of a square (pulse) wave — the ratio of high to low voltage within each cycle. The pulse width is set by comparing the sawtooth carrier waveform against a threshold voltage. When the threshold is moved by a knob or control voltage, the pulse becomes narrower or wider, shifting the harmonic content. A slow LFO modulating the pulse width creates a subtle chorusing or detuning effect (because the waveform’s harmonic content oscillates). A faster or single-direction modulation (such as from an envelope) creates a Doppler-shift quality. At extreme settings (0% or 100% duty cycle), the wave disappears and goes silent; many modules restrict PWM to 10–90% to prevent cutout.

Examples

Classic sound: two oscillators slightly detuned + PWM on each = thick analogue pad. Modular patch: route LFO to pulse-width CV input for chorus effect. Envelope to PWM for a sweeping timbre change on attack.

Assessment

Explain the physical mechanism by which LFO-driven PWM creates a chorusing sound (hint: think about how the harmonic content of a pulse wave relates to its duty cycle). Explain why a PWM range of 0–100% can cause the sound to cut out.