What Is FM?
Frequency Modulation synthesis works by using one oscillator — the modulator — to rapidly bend the pitch of another oscillator — the carrier. When the modulation is slow (below ~20 Hz), you hear pitch wobble, like vibrato. But when it runs at audio rate (above ~20 Hz), the ear can no longer track the individual pitch changes. Instead, it hears new frequencies — called sidebands — that appear around the carrier. The result is a completely different timbre, created entirely from sine waves.
Start with MOD OFF and play a note — you'll hear a pure sine wave. Then toggle MOD ON and listen to how the timbre changes. The modulator isn't audible directly; it's bending the carrier's pitch so fast it creates new harmonics. Raise the DEPTH knob to hear more sidebands appear.
The carrier alone is a pure sine wave — one frequency, no harmonics. Toggle MOD ON to hear what happens when the modulator starts bending its pitch.