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FM Synthesis

Carrier/modulator ratio/index: metallic vs tonal.

What is FM synthesis?

Frequency Modulation (FM) synthesis uses one oscillator (the modulator) to rapidly change the frequency of another oscillator (the carrier). When the modulator runs at audio rates - hundreds or thousands of times per second - it generates entirely new harmonics that neither oscillator could produce alone.

The result is a vast range of timbres from a very simple setup: just two oscillators and a few parameters.

Think of it like this: vibrato is a slow wobble in pitch that you can hear going up and down. FM synthesis is what happens when you speed that wobble up so fast it stops sounding like wobble and starts creating entirely new tones.

How it works: carrier + modulator

A clean carrier sine wave has a fixed frequency. A modulator sine wave pushes the carrier's frequency up and down. When the modulator is fast enough, the result is a complex waveform with new harmonics - the signature sound of FM.

Modulation depth: from subtle to extreme

The modulation depth (called "index" on the machine) controls how strongly the modulator affects the carrier. At low depth, the wave barely changes. At high depth, it becomes wildly complex and metallic.

A brief history

John Chowning discovered this technique at Stanford University in 1967 while experimenting with extreme vibrato. When he pushed the vibrato speed into the audible range, the tone suddenly transformed into something complex and bell-like.

Yamaha licensed the patent and released the DX7 in 1983 - the best-selling synthesiser ever made. FM synthesis defined the sound of an entire decade.

Try the machine

The machine below gives you a single carrier-modulator pair with feedback and vibrato. Two oscillators, one filter, and a handful of parameters - that is all you need to reach from warm electric piano to screaming metallic noise.

Machine 19

Volume 80%

Carrier 261.6 Hz

Ratio 1.00:1

Index 1.8

Tone 50%

Mod Wave sine

Brightness 5%

Attack 90 ms

Release 560 ms

Vib Depth 12 c

Vib Rate 2.2 Hz

Presets

The controls explained

Ratio - the most important control. It sets the frequency relationship between the modulator and the carrier.

Integer ratios (1:1, 2:1, 3:1) produce harmonic overtones - tonal and musical, like a clarinet or organ.
Non-integer ratios (1.41:1, 2.76:1, 3.51:1) produce inharmonic partials - clashing, shimmering frequencies like bells, gongs, and metallic percussion.

This single parameter is why FM synthesis excels at both warm keys and clanging metal.

Index - controls how strongly the modulator affects the carrier's frequency.

Low values: subtle shimmer, gentle warmth added to the carrier's pure sine tone.
Medium values: new sidebands appear, the sound grows brighter and more complex.
High values: aggressively metallic and buzzing.

Think of it as a brightness knob with far more range than any filter.

Feedback - routes the modulator's output back into its own frequency input.

Small amounts: thickens the modulator, adding grit and presence.
High values: pushes toward chaotic, noise-like behaviour - breathy textures, cymbal sounds, distorted bass.
Extreme: the modulator effectively becomes a noise source.

Other controls

Carrier sets the fundamental pitch. Tone applies a low-pass filter after the FM stage, letting you tame the brightest harmonics without reducing the index.

Attack and Release shape how the sound fades in and out. Vib Depth and Vib Rate add a slow pitch wobble to the carrier - the musical vibrato that Chowning originally started with before he accidentally invented FM.

Classic FM sounds are everywhere: slap bass patches, bright brass stabs, 80s pop keyboard parts, metallic bell hits in early hip-hop, and nearly every electric piano patch from 1983 to 1990. The DX7 shipped over 200,000 units. If you have heard recorded music from the 1980s, you have heard FM synthesis.

Getting started

FM synthesis rewards precision. Small changes to ratio and index produce large timbral shifts, which makes it both powerful and famously difficult to program by ear.

The trick: start with integer ratios and a low index, then increase the index gradually until you hear the timbre you want. Once you have a feel for how ratio and index interact, the entire space opens up.

Try this: set Ratio to exactly 1:1 and Index low. You will hear a warm, slightly nasal tone. Now change the Ratio to around 3.51:1 without touching anything else. The sound transforms into a bell. That single parameter shift - from integer to non-integer ratio - is the core of FM synthesis.

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