Pulse width modulation

A pulse wave jumps between a high level and a low level. Its pulse width, also called the duty cycle, is the share of each cycle it spends at the top. At 50% the wave is a square and sounds hollow. Narrow it and the sound gets thinner and more nasal, because the fundamental loses weight against the harmonics above it. Each width also leaves out its own set of harmonics: a 50% square drops every even harmonic, and a 25% pulse drops the 4th, 8th, 12th and every higher multiple of 4.

A fixed width gives one fixed tone. Modulating it means letting something move the width as you play, and the usual source is an LFO. As the width sweeps, harmonics keep fading in and out, and one oscillator starts to sound like two slightly out-of-tune ones beating against each other. That shimmer is a classic sound of analog string pads.

A PWM pad to try

1. Pick a pulse wave on one oscillator.
2. Route an LFO to pulse width with a slow rate, 0.3 to 1 Hz.
3. Set the amount so the width swings between about 50% and 15%.
4. Give the amp envelope an attack of about 500 ms, add some reverb, and hold C3, E3 and G3 (a C major chord).

Push the LFO rate up to 4 or 5 Hz and the pad turns fast and wobbly. Some synths also let an envelope move the width, so the tone changes over the life of each note; Roland’s Juno-60 (1982) can do this.

Two things are useful to know when you turn the width knob. A 25% pulse and a 75% pulse sound the same, since one is the other flipped upside down. Push it all the way to 0% or 100% and the wave flattens into a silent straight line, so many synths stop the width control short of either end.

On analog-style synths, look for a knob labeled PW or PWM in the oscillator section. Synth waveforms explained compares the pulse with the other shapes, and what an LFO does covers rate and depth.