FM Synthesis

Frequency Modulation (FM) synthesis produces complex timbres by using one oscillator to modulate the frequency (or phase) of another. Popularized by the Yamaha DX-7, FM synthesis excels at bell-like tones, electric pianos, basses, and metallic textures.

Basic Concept

In FM synthesis, a modulator oscillator’s output modulates the phase of a carrier oscillator:

• Carrier — The oscillator you hear. Its base frequency determines the pitch.
• Modulator — The oscillator that modulates the carrier. Its frequency and amplitude shape the timbre.

Phase Modulation vs. Frequency Modulation

SynthEdit’s oscillator uses phase modulation (PM) rather than true frequency modulation. Phase modulation produces the same results as FM synthesis but is simpler to control — the pitch stays stable regardless of modulation depth. This is the same approach used by the Yamaha DX-7.

Building an FM Patch

Simple Two-Operator FM

The smallest useful FM patch chains a modulator into a carrier’s Phase Mod input, with a Level Adj in between as the depth/brightness control — current oscillators (Oscillator HD, Oscillator (naive)) don’t expose a dedicated depth pin of their own, so you scale the modulator’s own signal instead. This is the same approach the “Phase Mod (2-operator FM)” entry on the Audio Demos page is built with:

1. Insert two Oscillator HD modules
2. Connect the modulator’s Audio Out to a Level Adj module’s Input 1
3. Connect the Level Adj’s Output to the carrier’s Phase Mod input
4. Add a Slider and connect its output — labelled Slider — to the Level Adj’s Input 2 (this is the timbre control — Level Adj multiplies the two, so more level here means more phase modulation reaches the carrier, and more harmonics)
5. Connect the carrier’s Audio Out to a Sound Out module
6. (Optional) connect a MIDI-CV 2 module’s Pitch output to both oscillators’ Pitch inputs so the patch tracks your keyboard and the modulator:carrier ratio stays locked to the note instead of drifting
7. (Optional) replace the slider with an ADSR envelope for a time-varying timbre, separate from the envelope driving the VCA — high modulation on the attack that fades produces classic FM percussion and bell sounds

The classic “Oscillator” (Old category) works differently. It has its own built-in PM Depth pin, so it doesn’t need the Level Adj step — wire the modulator straight into it. It’s kept only so older projects still load, so the Level Adj approach above is the one to reach for in a new patch.

Adjusting the Sound

• Modulator frequency controls the harmonic spacing — try detuning by octaves or specific ratios (2:1, 3:2, 4:1)
• Modulation depth controls brightness — more modulation adds more harmonics
• Envelopes on modulation depth create evolving timbres — high modulation on attack that fades produces percussive, bell-like sounds

Operator Configurations

FM synthesizers typically chain multiple oscillators (called “operators”) in various arrangements called algorithms. SynthEdit doesn’t lock you into a fixed algorithm — you wire whatever topology you want.

Stack — modulator → modulator → carrier

Each modulator modulates the next operator down the chain. The deeper the stack, the more complex the spectrum.

Parallel — multiple modulators feeding one carrier

Two (or more) modulators both drive the same carrier’s Phase Mod input. SynthEdit sums the modulator signals automatically, so you can route several Audio Outs into a single Phase Mod input. The result is an additive-like richness on top of the carrier.

Branch

Combine stack and parallel paths — for example, one operator modulating two carriers, or a stack whose output is then summed in parallel with a second modulator. There’s no fixed structure; you have complete freedom to design your own operator topologies.

Tips

• Use sine waves for the cleanest FM tones (as in classic DX-7 patches)
• Sawtooth or pulse modulators create harsher, more aggressive timbres
• Small frequency ratios (1:1, 2:1, 3:1) produce harmonic tones; non-integer ratios (1:1.41) produce inharmonic, bell-like sounds
• Modulate the modulation depth with an ADSR envelope for time-varying timbres