A four-operator FM keyboard with no filter. Four sine-wave operators, six ways of wiring them, an envelope on each, and velocity that changes the tone as well as the volume. From those it makes electric pianos, bells, brass, basses, pads and a wooden log drum.
The idea
An FM operator is an oscillator with an envelope, and nothing else. There is no cutoff, no resonance and no tone control anywhere in this instrument, and it still makes metal, glass, wood and breath. What decides which of those you get is the wiring. An operator either reaches the output, and you hear it, or it bends the pitch of another operator, and you hear what it does to that one. The algorithm chart at the centre of the face shows that wiring, because it is the one control whose effect you cannot guess.
Two choices make patches easy to play. Modulation depth scales with the note, so a patch keeps its timbre from the bottom of the keyboard to the top. And velocity drives modulation, not just loudness: playing harder adds overtones, so a patch feels played rather than triggered.
Controls
- Algorithm picks one of six wirings. Stack runs all four operators in series and is the densest. Twin is two independent pairs, the shape of the classic FM electric piano. Fan is three modulators onto one carrier and stays clean further up. Split is one modulator into two carriers that share an envelope. Pair is two modulators onto one carrier with a bare sine beside it, the bell recipe. Additive has no modulation at all. A new algorithm applies from the next note.
- Each operator has a Ratio, its pitch as a multiple of the played note. Whole numbers keep the sound pitched; the √2 family (0.71, 1.41, 2.83, 3.53) lands between the harmonics, which is how FM makes bells. The last five entries are fixed frequencies that ignore the keyboard, so they stay put as you play, like the click of a mallet.
- Each operator has a Level and an envelope. Operators 1 and 2 have a full attack, decay, sustain and release; operators 3 and 4 have attack and decay only and always die away. A modulator that fades and leaves a pure sine behind is the signature FM gesture: the note starts as metal and ends as glass.
- Feedback lets operator 4 modulate itself, from a pure sine at zero to a bright, sawtooth-like tone at the top. It is calculated rather than looped, so it cannot run away.
- Vel → Index sets how much velocity opens the modulators. At zero the tone stays the same however hard you play.
- Brightness scales all the modulation in a patch at once, to dull or push a preset without editing four levels.
- Level sets the output volume.
The voice settings choose between eight-voice polyphony, monophonic and mono legato playing, with note priority and a glide of up to two seconds.
Getting started
FM Keys is an instrument played with MIDI notes, from a keyboard controller or an on-screen keyboard. Start with E-Piano Glass and then E-Piano Bark, the same electric piano played softly and hard. Then try Bright Bells, Warm Bass, Brassy, Airy Pad, Pluck Attack, Log Drum, Split Bell or Pure Tones, which turns all modulation off and shows what the four operators are on their own.
