FM synthesis

Explain how FM synthesis makes its sound. Name the carrier and the modulator, and say what the modulator's Level, its Ratio and an envelope on its Level each do to the tone.

The synth is beside you under For reference. Start each line with its opening words.

Tick each point your lines make.

• The carrier is the oscillator you hear: it sets the note's pitch.
• The modulator is not heard itself: its output changes (modulates) the carrier's frequency.
• A slow modulator is heard as vibrato: the pitch wobbles up and down.
• At audio rate the wobble is too fast to hear as one: new frequencies (sidebands) appear around the carrier's, heard as a new tone.
• More modulator Level (the FM amount, the modulation index): more and louder new harmonics, a brighter tone, with no filter.
• Whole-number ratios (1 : 1, 2 : 1) put the new harmonics on the harmonic series: in tune, musical.
• 1 : 1 gives every harmonic, as a sawtooth has; 2 : 1 gives odd harmonics only, hollow, as a square wave has.
• Other ratios (such as 1.41 : 1) put them between the harmonics: inharmonic, clangy, metallic, bell-like.
• An envelope on the modulator's Level moves the brightness: bright at the start, duller as it dies away, as in FM electric pianos and bells.
• FM made its name in 1980s digital synths (the Yamaha DX7, 1983).
• Every line: name the control, say what it does to the sound, and why.