At its core, the Digitone (and Digitone Keys) is a four operator Frequency Modulation (FM) synth in the style of the classic 80s implementations. However, unlike the early FM synths, the Digitone use its FM engine more like a complex tone generator than a complete synthesizer voice (although it does have this capability too). The Digitone signal path is more similar to a typical subtractive synth than a classic FM voice.

The idea of this design is to harness the raw, and often complex, soundscapes of FM synthesis and use a more well-known and approachable subtractive method for the overall sound shaping.
FM is a synthesis method where you add harmonics to the timbre by using modulation or layering to shape the sound. You add harmonics by modulating one oscillator’s pitch with the output of another oscillator. FM works similarly to how you would apply vibrato via an LFO. At slower modulation rates, it is simply vibrato, but when the modulating frequency reaches audio rate, the vibrato effect becomes a part of the sound and turns into a timbral effect instead.

Read more about the separate parts of the Digitone FM syntheses