FM Synthesis

Everything so far has been subtractive: start with a bright wave, carve it down. FM synthesis (frequency modulation) builds sound the opposite way. It creates harmonics out of thin air, using nothing but two sine waves. It is also, gloriously, the sound of the 1980s, and it is hiding inside more modern bass music than most producers realise.

Carriers and modulators: how FM actually works

Take one sine wave you can hear (the carrier) and wiggle its pitch with a second sine wave you never hear directly (the modulator). Wiggle slowly, a few times per second, and you get vibrato: the LFO lesson in disguise. But keep speeding the modulator up. At some point the wobble crosses into audio rate, hundreds or thousands of cycles per second, and your ear stops hearing motion and starts hearing tone. The pitch wiggle becomes new frequencies, called sidebands, stacked above and below the carrier.

In FM language, the oscillators are called operators, and two numbers define everything you hear:

• Ratio: the modulator speed relative to the note. Whole numbers (1, 2, 3) lock the new sidebands onto the harmonic series, so the result sounds musical and pitched. In-between ratios (3.51, 5.79) scatter them into inharmonic places, which is exactly what bells and gongs do in real life.
• Amount (the modulation index): how hard the modulator shakes the carrier. Zero is a pure sine. A little adds warmth. A lot conjures a whole spectrum of harmonics from nowhere. In this lab the amount also fades over each note, which is why every preset has that struck, ringing quality.

Why the ratio decides the character

Here is the mental model: FM places sidebands at the carrier frequency plus and minus multiples of the modulator frequency. When the ratio is a whole number, every one of those sidebands lands exactly on a harmonic of the note, so your ear hears one coherent, brighter tone. Nudge the ratio off the integers and the sidebands land between the harmonics. Your ear can no longer fuse them into a single pitch, and the sound turns clangorous and metallic. That is not a flaw. Real bells have inharmonic partials for the same physical reason, which is why FM does bells better than any other method.

From a Stanford lab to every 80s ballad

FM synthesis was discovered, almost by accident, by composer John Chowning at Stanford in 1967 while experimenting with extreme vibrato. He published the technique in 1973 and Stanford licensed it to Yamaha, who spent a decade turning it into silicon. The result was the DX7 in 1983: one of the first affordable, mass-produced digital synths and still one of the best selling synthesizers ever made. Its glassy electric piano, bass and bell presets are all over a decade of pop. If you have heard an emotional 80s ballad, you have heard a DX7.

FM also had a second, stranger life in video games. The Sega Genesis and countless arcade boards used Yamaha FM chips, so an entire generation of game music is pure frequency modulation. When people say something sounds "chiptune but expensive", they usually mean FM.

FM vs subtractive: when to reach for it

Subtractive synthesis is warm, thick and forgiving: rich source, gentle sculpting. FM is precise, glassy and percussive: it excels at sounds with a hard, detailed attack (keys, bells, plucks, mallets) and at aggressive digital textures. It is also famously touchy, since small ratio changes cause big timbre changes. Modern synths refuse to pick a side: Ableton Operator is a full FM synth, and wavetable synths like Serum and Vital include FM between their oscillators, so you can add a pinch of clang to an otherwise subtractive patch.

Today you will hear FM in lofi and R&B keys (that soft DX7 e-piano is everywhere), in the growling bass of drum and bass and neurofunk, in hyperpop bells, and in interface and notification sounds, which love its clean, precise transients. Now prove your ear can spot it:

Two operators vs six

This lab is a two-operator FM synth: one modulator, one carrier, which is enough for bells, keys and growls. The DX7 ran six operators, patchable into 32 different arrangements called algorithms: modulators modulating modulators, parallel stacks of carriers, feedback loops. That is where FM earned its reputation for being deep and slightly terrifying. But every algorithm decomposes into the pair you just learned. Master ratio and amount on two operators and the six-operator monsters become legible: they are just this lesson, stacked.