3 FM
The whole idea is this. Take the sine from stage 1 and, every sample, add a second operator's output to its phase. Now the carrier no longer reads its table at a steady pace: sometimes it races ahead, sometimes it lingers, and that warping turns a pure sine into a rich, harmonic timbre. That is phase modulation, the OPLL's kind of FM.
The phase wheel below is that idea, slowed down about 1800× so you can watch it happen. Everything on it is drawn twice, and the difference between the two is the whole effect: the pale hand turns at a perfectly steady rate, where the carrier would be if nothing touched it, while the bright hand is where the modulator has shoved it. The same pairing runs along the traces — the dashed pale wave is the plain sine that would have come out, the bright one is what actually does.
The controls are right underneath, because every one of them changes what the wheel is doing. Raise the modulator's Total Level for more depth, set a Multiple ratio (try 2:1 for a bell), and add feedback for the modulator's characteristic edge. The harmonic bars further down move as you go.
The whole mechanism, in three lines
Everything on this page is these three steps, run once per sample. Each operator keeps a phase — a position in the sine table, counted in cycles — and a fixed increment it adds every sample, which is what sets its pitch:
modPhase += modInc ·
mod = wave[modPhase] ·
out = wave[carPhase + depth·mod]
That bolded term is the entire idea: the modulator's output is
added to the carrier's phase before the table is read. Nothing
changes the carrier's increment, so its pitch never moves — only where in
the table it lands. A full-scale modulator shifts the carrier by about
four cycles, which is why a low Total Level warps the wave so
violently; the readout under the wheel prints that shift in turns, so
anything past 1.000 is the carrier being sent more than the
whole way round the table between one sample and the next.
Why the ratio makes the sound
When the modulator's frequency is a whole-number Multiple of the carrier's, the injected sidebands land exactly on harmonics and the result is pitched and musical: a 2:1 ratio gives a hollow, bell-like tone. Off-ratio settings push energy onto inharmonic partials and the voice turns metallic or clangy. Feedback routes the modulator into itself, an extra layer of warping that adds bite as you raise it — it has no trace of its own on the wheel, because it is already folded into the modulator's output, so the strip just names it.