‘Frequency Modulation’ (yes, the same as that thing you listen to on the radio) synthesis was made popular by Yamaha in the early 1980s with their line of DX synthesizers, which were instrumental in both the downfall of classic analogue synths, and giving keyboard players worldwide a polyphonic-palette of groundbreaking new sounds to use. Dedicated FM synthesizers are digital in nature due to the instability of analogue VCOs, and the nature of FM synthesis techniques makes it very easy to create un-pitched and metallic tones, rather than standard subtractive sounds.
FM synthesis is based on two key things – a ‘modulator’ oscillator, and a ‘carrier’ oscillator. These oscillators usually both use a sine waveform, and from this the modulator oscillator works just like an LFO – because it modulates the frequency/pitch of the carrier oscillator. You can try this yourself on a normal subtractive synthesizer, by setting up a sine wave oscillator and an LFO, and using the LFO to modulate the pitch of the oscillator – as you increase the rate of the LFO, the sound becomes non-harmonic. Note that in FM synthesis, the word ‘oscillator’ is often replaced with the term ‘operator’. As you change the modulation of the carrier operator, the frequency of the carrier will constantly move up and down depending on how the modulator is set up (e.g. it’s depth and rate), and in doing this different harmonics are created (called ‘sidebands’), because these harmonics surround the carrier frequency depending on how it is modulated.
Because of the somewhat lifeless sound of the operators, FM synthesizers tend to include somewhere around 4-8 operators on a synth to spice things up. These extra operators can be routed in all sorts of different and interesting ways, called ‘algorithms’. For example, with the addition of an extra modulator operator, we can arrange the operators so that they go ‘’modulator 1’ & ‘modulator 2’ go into the carrier’, or ‘modulator 1 goes into modulator 2, which goes into the carrier’ – this being more complicated and creating a new waveform. Therefore, using many operators can produce unique and lifelike sounds unachievable with other types of sound synthesis.