1 Tone
Strip the OPLL down to one operator and it does the simplest thing a synthesiser can: it reads a sine table around and around. To do that it needs a pitch, and the OPLL spells pitch as two numbers: a 9-bit F-Number and a 3-bit Block (the octave).
Drag the F-Number and Block below and watch the wave stretch. The Multiple control multiplies the operator's frequency by a fixed ratio, the first hint of how FM builds a timbre from one oscillator.
Why two numbers for one pitch?
The F-Number sets the frequency within an octave; the Block
shifts it by whole octaves, cheaply, as a bit-shift. It is the same
floating-point-ish trick the whole OPL family uses: a mantissa
(F) and an exponent (Block). Doubling the Block
doubles the frequency (one octave up) without touching the F-Number.