Guide 6 of 9 · Every Try it box opens the synth with its steps pinned on the rack

FM & distortion: changing the timbre

The filters guide was all about taking away: start with a bright sound and carve it darker. This guide goes the other way. Every technique here adds new harmonics that weren't in the sound before, which is how you get bells, metallic clangs, growls, fuzz and digital grit from plain old waves.

Modulation, but fast

In the modulation guide, an LFO moved a knob a few times a second, and you heard it as movement: a sweep, a wobble, vibrato. So what happens if the modulation gets faster? Ten times a second, a hundred, a thousand?

Somewhere around 20–30 times a second, your ear stops hearing movement and starts hearing a new tone. The wobble is too fast to follow, so it blends into the sound itself and becomes part of its colour. That's audio-rate modulation, and it's one of the most powerful ideas in synthesis.

FM: one oscillator bending another

FM (frequency modulation) means using one oscillator to wiggle the pitch of another, very fast. The one you hear is the carrier. The one doing the wiggling is the modulator. Neither is special: they're both ordinary VCOs, patched modulator OUT → carrier FM.

Two knobs matter most:

How hard: the carrier's FM Amt. A little gives a gentle sheen; a lot gives a bright, complex tone full of new harmonics.
The ratio between the two pitches. When the modulator sits at a simple multiple of the carrier (the same note, an octave up, two octaves up), the new harmonics line up musically and you get clear, bell-like and electric-piano tones. Knock the ratio slightly off and the harmonics stop lining up: clangy, metallic, inharmonic sounds, like gongs and hit metal.

And here's the trick that makes FM sound alive: put a VCA and envelope on the modulator. Then the FM amount changes over each note, so the sound starts bright and complex and mellows as it rings, exactly like a struck bell.

Try itMake a bell

Two sine VCOs, two VCAs and an envelope, with no cables. The left VCO is the modulator, set an octave up; the right one is the carrier, the one you'll hear. Build a plain voice first, then bend it:

Patch the right VCO's OUT into the VCA under the ADSR, that VCA's OUT into the Output's MONO, and the same OUT into the Scope's A so you can watch the wave.
Patch the Keyboard's V/OCT into the V/OCT of both VCOs, its GATE into the ADSR's GATE, and the ADSR's OUT into that VCA's CV. Play A–K: a plain, soft sine.
Now the left VCO: patch its OUT into the VCA under it, and that VCA's OUT into the right VCO's FM. Still plain: that VCA is shut.
Patch the ADSR's OUT into the CV of the VCA under the left VCO too. Play: a bell. Each note starts bright and mellows as the envelope closes the FM.

Then play with it:

Set the left VCO's Oct to +2 for a brighter, glassier bell.
Nudge the left VCO's Fine: clangy, inharmonic metal.
Turn the right VCO's FM Amt up and down to hear how much brightness it adds.

⏻ Build lesson 12 in KablesThe modules are waiting, unpatched. These steps come with you.

Ring modulation

FM changes the carrier's pitch. What if the fast signal changes its volume instead? Patch an oscillator into a VCA's CV and the VCA multiplies the two signals together. This is ring modulation: it produces new tones at the sum and difference of the two frequencies, and often loses the original notes altogether. Bells, robots, and the Daleks.

In the bell patch above, try patching the left VCO's OUT straight into the CV of the VCA under the ADSR.

Drive: pushing a signal too hard

Turn a signal up too far and it can't get any louder; the tops of the wave get squashed flat. That squashing is distortion, and it adds harmonics. A smooth wave goes in; something edgier comes out. The Drive module has three ways of doing it:

SOFT rounds the peaks gently: warm saturation, like a tape or tube pushed a little.
HARD chops them off: fuzz.
FOLD does something stranger. When the wave hits the ceiling, it folds back down on itself instead of flattening. Push harder and it folds again, and again. Each fold adds more harmonics.

That last one is special: with folding, how hard you hit it changes the tone, so turning the input level up and down sweeps the brightness, a bit like a filter in reverse. This style is often called "west coast" synthesis, after the Buchla instruments from California, where it started.

Bitcrushing: digital on purpose

Digital audio is a long list of numbers, each one a snapshot of the sound. A Bitcrusher makes that list deliberately worse, like an old games console or a cheap sampler:

Bits sets how many volume steps each number can have. Fewer steps means gritty, buzzy noise. 16 bits is clean; 3–4 is wrecked; 1 is pure square-wave grime.
Rate holds each number for several samples in a row, which creates metallic, robotic tones that weren't in the original. This is called aliasing.

Glitch: breaking time

Distortion bends the shape of a wave. Glitch bends time. It remembers the last few seconds of whatever goes in, and now and then replays bits of it: stutters, reverses, octave jumps, tape stops. Because it listens to the clock, every glitch lands on the beat. Wrong, but in time.

Taking away, adding in

Filters and these techniques work best together. A classic move is to add harmonics with FM or drive, then shape the result with a filter and an envelope. Make it rich, then sculpt it.

What to remember

Modulation faster than about 20–30 times a second stops sounding like movement and becomes tone.
FM: a modulator oscillator wiggles a carrier's pitch. Simple pitch ratios sound musical and bell-like; odd ratios sound metallic.
An envelope on the FM amount makes notes bright at the start and mellow as they ring.
Ring modulation: an audio signal into a VCA's CV multiplies two sounds into new ones.
Drive squashes or folds waves to add harmonics; with folding, louder input means brighter tone.
Bitcrushers and glitch effects add digital grit and break time, on purpose.