FM without the maths
A modulator is a tone control for a carrier. That's the whole secret. Six rules I actually use, a table of where to start for each kind of sound, and four patches built step by step on Mellt.
The one idea
Every FM synth, from the famous one of the 1980s to Mellt, is made of oscillators called operators. Some you hear: those are carriers. Some you don't hear directly; they shake the pitch of a carrier thousands of times a second, and what you hear is the carrier getting brighter. Those are modulators. That's all FM is: a modulator is a tone control for a carrier.
If you've programmed an analog synth, you already know how to use it. On an analog synth, a filter cutoff sets how bright the note is and a filter envelope makes the brightness move. On an FM synth, the modulator's level is the cutoff and the modulator's envelope is the filter envelope. Hold that thought and three-quarters of FM is done. The rest is choosing what the modulator's pitch is relative to the carrier's, which is the ratio, and that decides whether the brightness is musical or metallic.
Six rules I actually use
- Start with two operators. One carrier, one modulator. Set every other operator's level to zero. Mellt opens on algorithm 2, where operator 2 modulates operator 1; leave it there. Get a sound you like from two, then add one more. Seven operators at once is how people decide FM is impossible.
- Modulator level is brightness. With the modulator at zero you hear a pure sine. Bring its level up and the sine grows harmonics. It's the only knob you need for the first ten minutes.
- Whole-number ratios are in tune; fractions are bells. Carrier at 1, modulator at 1, 2 or 3: the harmonics line up with the note, and you get brass, reeds, bass, organ. Modulator at 4.28 or 3.83 or 5.3: the harmonics don't line up, and you get bells, glass, metal. Ratio 0.5 on a carrier puts the note an octave down, which is the bass trick.
- Velocity goes to the modulator, not the carrier. Patch Velocity to the modulator's Level in the matrix, at around half depth. Now playing harder makes the note brighter, which is what every acoustic instrument does, and the patch starts to feel like one.
- Feedback at ratio 1 is a sawtooth. Every Mellt operator has feedback: its own hole on the routing board's diagonal, 0.00 to 1.00. A modulator at ratio 1 with feedback around 0.30 stops being a sine and starts being a buzz, and a buzz modulating a carrier is brass and bass. Past half way it turns to noise at 1x oversampling; at 2x and 4x it stays pitched for longer.
- The modulator's attack is the character. Instant attack, fast decay to nothing: a pluck, a tine, a mallet. An attack of a tenth of a second: a horn swelling. The carrier's envelope sets how long you hear the note. The modulator's envelope sets what kind of note it is.
Two more for Mellt specifically. Every operator reads a wavetable, not only a sine; swap the table only after the patch works on sines, because a bright table on a modulator gets harsh fast. And FM gets loud and sharp before it gets interesting, so turn things down, not up. The gain in each recipe below is there for a reason.
Where to start for the sound you want
| Sound | Algorithm | Ratios | The move |
|---|---|---|---|
| Electric piano | 4 · Pairs + Sine | 1 and 1, with a high one for the tine | Two carriers detuned a few cents; a modulator at ratio 9 to 13 with a 50 ms decay for the click. |
| Bell, glass | 3 · 3 + 2 + 2 Stacks | 1 and a fraction | No sustain anywhere. Long decays. Modulator decays a little faster than the carrier. |
| Brass | 1 · Stack or 2 | 1 and 1 | Feedback on the modulator, slow attacks on both, high sustain. |
| Bass | 2 · 4 + 3 Stacks | 0.5 and 2 | Mono, a little glide, feedback half way, modulator decays fast to a low sustain. |
| Organ | 8 · One Into Six or 12 · Additive | 1, 2, 3, 4… | Carriers at harmonic ratios are drawbars. Modulation barely needed. |
| Lead | 1 · Stack | 1 and 1 or 2 | Mono with glide, vibrato on the wheel, aftertouch to the modulator's level. |
Four patches, step by step
These came out of my preset lab: a tool that generates candidates from a recipe, renders each one through the real Mellt engine, loudness-matches them and throws out the harsh and the duplicate. These four survived. Level is a fader reading 0.00 to 1.00, so "30%" means 0.30 on it; times are in seconds. Operators you aren't using don't default to silent (a new Mellt opens with every operator at some level), so each recipe says which ones to set to 0. Everything else not mentioned stays at its default.
1. A bell from two operators
Algorithm 3, 3 + 2 + 2 Stacks. Operators 1 and 2 only: set operators 3 to 7 to Level 0 first (on this algorithm 4 and 6 are carriers and would sound).
| Operator 1 (carrier) | Ratio 1.00, Level 100%. Envelope: Attack 0.001, Decay 3.4, Sustain 0, Release 1.5. |
|---|---|
| Operator 2 (modulator) | Ratio 4.28, Level 30%. Envelope: Attack 0.001, Decay 2.9, Sustain 0, Release 0.9. |
| Matrix | Velocity → Op 2 Level, +0.50. |
| Gain | −10 dB. |
Why it works: 4.28 isn't a whole number, so the partials fall between the harmonics of the note, and the ear hears metal. Sustain is zero on both, so the note can only die away, and the modulator dies slightly faster than the carrier, so the bell darkens as it rings, as a real one does. Try ratio 3.83, then 5.3, then 7: each is a different bell. Try 4.00 to hear it stop being one.
2. A bass with feedback
Algorithm 2, 4 + 3 Stacks. Operators 1 and 2 only: set operators 3 to 7 to Level 0 first. Voice mode Mono, Glide 0.03.
| Operator 1 (carrier) | Ratio 0.50, Level 100%. Attack 0.002, Decay 0.38, Sustain 80%, Release 0.16. |
|---|---|
| Operator 2 (modulator) | Ratio 2.00, Level 30%, Feedback 0.5. Attack 0.001, Decay 0.06, Sustain 27%, Release 0.16. |
| Matrix | Velocity → Op 2 Level, +0.20. |
| Gain | −6 dB. |
Why it works: the carrier at 0.5 puts the note an octave below the key, so a bass line played in a comfortable register comes out low. The modulator's fast decay to a low sustain is a filter envelope: a bright snap at the front of each note, then it settles. The feedback is what gives it an edge rather than a hum. Turn the feedback down to 0.2 for something rounder, or push the modulator's sustain up for a sound that stays open.
3. Brass that swells
Algorithm 2, 4 + 3 Stacks. Operators 1 and 2 only: set operators 3 to 7 to Level 0 first.
| Operator 1 (carrier) | Ratio 1.00, Level 100%. Attack 0.09, Decay 0.29, Sustain 87%, Release 0.14. |
|---|---|
| Operator 2 (modulator) | Ratio 1.00, Level 26%, Feedback 0.3. Attack 0.15, Decay 0.38, Sustain 85%, Release 0.14. |
| Matrix | Velocity → Op 2 Level, +0.45. |
| Gain | −10 dB. |
Why it works: both operators at ratio 1 keep everything harmonic, and the feedback turns the modulator into a saw, which is the raw material of every brass patch ever made. The modulator's attack is slower than the carrier's, so the note starts dull and opens up, the way a trumpet player's note blooms. For a section rather than a soloist, add operator 5 as a second carrier (algorithm 2 gives you 7 into 6 into 5) at ratio 1 with Fine set a few cents off, and copy operator 2's settings onto operator 6.
4. An electric piano with a tine
Algorithm 4, Pairs + Sine. Operators 1 to 4 on; set 5, 6 and 7 to Level 0 first.
| Operator 1 (carrier) | Ratio 1.00, Level 100%. Attack 0.003, Decay 2.2, Sustain 17%, Release 0.68. |
|---|---|
| Operator 2 (the tine) | Ratio 9.00, Level 41%. Attack 0.001, Decay 0.07, Sustain 0, Release 0.08. |
| Operator 3 (carrier) | Ratio 1.00, Fine −0.2, Level 96%. Attack 0.002, Decay 1.8, Sustain 16%, Release 0.68. |
| Operator 4 (modulator) | Ratio 2.00, Level 49%. Attack 0.001, Decay 0.38, Sustain 9%, Release 0.68. |
| Matrix | Velocity → Op 2 Level +0.53; Velocity → Op 4 Level +0.52. |
| Gain | −10 dB. |
Why it works: it's two of the bell patch side by side. Pair 2→1 is the hammer hitting the tine: a very high ratio with a decay of seventy milliseconds, so it's a click, not a tone. Pair 4→3 is the body of the note, a ratio-2 modulator that settles to a low sustain. The two carriers are very slightly detuned from each other, which is just enough to breathe. Both velocity pins matter: hit it harder and you get more click and more body, which is the whole feel of the instrument. Add the third pair, 6→5, at ratio 1 with a slow modulator decay for a warmer, more 1980s version.
What to do when it goes wrong
- Harsh. Modulator level is too high, or the modulator's table isn't a sine. Halve the level before touching anything else.
- Out of tune. A ratio isn't a whole number and you wanted it to be, or Fine is set on something. Bells are allowed to be out of tune. Basses aren't.
- Thin. Only one carrier. Add a second at ratio 1, detuned by a few cents, and copy the first's envelope onto it.
- Loud. It's FM. Turn the Gain down and leave it down.
- Lost. Set everything but operators 1 and 2 to zero and go back to the first rule.
Build one.
Mellt FM-1 is free for Apple Silicon Macs, as AU, VST3 and Standalone. Seven operators, twelve algorithms, and a matrix where Velocity to Op 2 Level is one pin.