FM synthesis uses one oscillator (the Modulator) to vary the frequency of another (the Carrier)

In frequency modulation two oscillators interact: the Carrier produces the audible tone, and the Modulator varies the Carrier’s frequency (rather than its amplitude, as in amplitude modulation). The block diagram is deceptively simple — the FM equation contains exactly the same terms as the AM equation, with one factor moved — yet the sonic result is dramatically different. Where AM generates only two side bands, FM produces a whole series. The two attributes of the Modulator that shape the output are its frequency (which sets where the side bands lie) and its amplitude (which sets how strong each side band is). Grasping this carrier/modulator split is the entry point to everything else in FM.

Examples

In a modular synth, patch VCO2’s output into VCO1’s linear-FM/CV input: VCO1 is the carrier you hear, VCO2 is the modulator. Raising VCO2’s frequency changes the timbre’s character; raising its amplitude thickens the spectrum.

Assessment

Draw a two-oscillator FM patch, labelling carrier and modulator and which signal feeds which input, and state how it differs from an AM patch using the same two oscillators.