Pulse Width Modulation and Serial Filtering

Two techniques that cost nothing and fix the most common complaint about subtractive synthesis: that it sounds static.

Pulse Width Modulation

A square wave is on for half its cycle and off for the other half — a 50% duty cycle. Pulse width is what happens when you make that uneven: on for 30%, off for 70%. The waveform is still a square, but the harmonics change completely.

Set the width by hand and you get a different tone. Set an LFO modulating the width and you get movement — a slow, breathing thickness that sounds like two detuned oscillators but costs one. It is the sound of a string pad, and of about half the bass patches ever made.

Sync the LFO to a note value and that breathing becomes rhythmic instead. Same control, different musical job.

Serial filtering

One filter shapes the sound one way. Two filters in series shape it in ways a single filter cannot reach — the output of the first feeds the input of the second, and each one keeps working on what the last one left.

In Analog, send Filter 1 100% to Filter 2. Now put a high-pass on one and a low-pass on the other and you have a band you can move from both ends independently, with separate resonance on each. A single band-pass filter cannot do that.

Add Drive into the chain and the two filters stop being tidy: filter, then saturate, then filter again, and the second filter is now shaping harmonics the saturation created rather than ones the oscillator made.

Do this now

1. Load Analog. Oscillator 1 to a square wave, everything else off.
2. Set the pulse width off centre. Listen — thinner, more nasal.
3. Assign LFO 1 to pulse width, rate around 0.3 Hz, a small amount. Hold a chord. It should now sound like more than one synth.
4. Turn on Filter 2 and route Filter 1 into it. Low-pass on 1, high-pass on 2.
5. Move both cutoffs while the chord holds. Squeeze the band until it is almost a whistle, then open it back up.
6. Add Drive between them and do it again.

Two controls neither of which is exciting on its own, and between them most of what makes a subtractive patch sound alive.