FM Synthesis

One oscillator modulates another to make bells, electric pianos and metal.

Related

TimbreThe DX7 and FM synthesisModern

The question

With two oscillators, how can one engine produce electric piano, bells, metallic percussion and a fat bass? FM covers a wide range with few parts.

How it works

One oscillator is the carrier, the other the modulator pushing it. Each push adds sidebands at the carrier plus or minus modulator multiples. When the ratio is an integer, the sidebands land on harmonics and the sound has pitch. When the ratio is non-integer, they fall between harmonics and read as bells or metal. Depth is the index; a larger index means more sidebands and a brighter tone. An envelope on the index lets the attack start bright, then soften.

Settings and what you hear

The ratio sets the family; one to one leans woody, one to two hollow, non-integer bell-like. As the index rises, a sine grows sidebands, passing from electric piano to brass to noise. A short modulator envelope strikes metallically, a long one keeps the note bright. Feedback roughens the wave and suits bass and percussion.

What to listen for

Hear the electric piano first, noting the metallic head and the softer tail. Then hear the bells and their non-integer sidebands. Finally imagine raising the index and hear the sine move toward metal, without the pitch wobbling.

Common misconceptions

That FM means vibrato, when FM pushes frequency at audio rate; that FM is only for electric pianos, when it also makes bass; that additive is more direct, when FM reaches a similar spectrum with fewer oscillators.

Sources

• John Chowning, The Synthesis of Complex Audio Spectra by Means of Frequency Modulation — sideband theory checked, not quoted
• Gordon Reid, Sound On Sound Synth Secrets — FM ratios and index checked, not quoted

Used for fact-checking only; all prose is original.