FM Operator (OP)

A single DX-style FM operator: one oscillator that can modulate, be modulated, or both. Chain several together — patching one's output into another's FM input — to build the classic stacked "algorithms" of FM synthesis, from glassy bells to metallic basses.

How it works

Each operator is an oscillator whose phase is bent by whatever arrives at its FM input. In PM mode it behaves like a DX7 — the modulator offsets the lookup phase; in FM mode the modulator nudges the running frequency instead. Mod Ix sets how deeply that input bends the tone, which is what turns a clean wave into a rich, sideband-laden timbre. With nothing patched to FM, an operator is simply a plain oscillator.

Controls

  • Ratio — frequency as a multiple of the base pitch (0.25–32); whole numbers stay harmonic, in-between values give clangorous, inharmonic tones.
  • Wave — the source waveform: Sin, Saw, Sqr, Tri, Abs (rectified sine), or Hlf (half-wave sine).
  • Level — output amplitude.
  • Mod Ix — modulation index: how hard the FM input and feedback bend the tone (0–10).
  • Detune — fine pitch offset in cents (±100); small amounts thicken or beat operators against each other.
  • Fdbk — self-feedback; the operator modulates itself, sweeping a sine smoothly toward a bright, sawtooth-like buzz.
  • Phase — starting point of the waveform cycle, and the point Sync resets to.
  • KeyScl — key scaling: eases modulation depth back as pitch rises, keeping high notes clean instead of harsh.
  • Mode — PM (DX7-style phase modulation) or FM (true frequency modulation).
  • Freq — Ratio multiplies the base pitch by the Ratio knob; Fixed ignores the Ratio knob and holds the base pitch instead. Either way the Freq input still tracks 1V/oct.

Inputs & outputs

  • FM — audio-rate modulation in; the signal that bends this operator. Patch another operator's Out here to build a stack.
  • Freq — 1V/oct pitch.
  • Sync — a trigger hard-resets the phase to the Phase offset.
  • Lvl — level CV; unpatched is full volume, positive boosts toward 2×, negative attenuates toward silence.
  • Out — the operator's audio.

Tips

  • Two operators is all you need to start: make one a modulator (often a high Ratio), patch its Out into the other's FM, and ride Mod Ix for the timbre sweep.
  • A touch of Fdbk on the final operator adds grit and brightness without a second modulator.
  • Integer Ratio values keep things tonal; nudge Detune or use fractional ratios for bells, mallets, and metal.