FM synthesis makes sound by using one oscillator to bend the pitch of another so fast that the wobble turns into new harmonics. Get two ideas, the carrier and the modulator, and the rest of FM starts to fall into place. It is the method behind electric pianos, bells and those bright, glassy tones that are hard to reach any other way.
The ideas here apply to any FM synth, in any DAW, or a browser instrument. Names and layouts differ, but the carrier, the modulator, the ratio and the modulation index mean the same thing wherever you meet them.
FM synthesis, short for frequency modulation, makes sound by using one oscillator to change the frequency of another very fast. The oscillator you actually hear is called the carrier, and its base frequency is the note you play. A second oscillator, the modulator, pushes the carrier's pitch up and down. When that pushing happens slowly you hear vibrato, a gentle wobble. When it happens fast enough to be inside the range of hearing, at audio rate, the wobble stops sounding like movement and starts sounding like new frequencies. Those extra frequencies are the whole point of FM.
This is a different way of thinking from the more common subtractive approach, where you start with a bright, harmonically rich wave and filter parts of it away. If you are still getting a feel for how a synth is built, our guide on what a synthesizer is walks through the oscillator, filter and envelope chain that most synths share. FM sits alongside that as another method for the first stage, how the raw sound is generated.
Two oscillators, one job. The carrier is the voice you hear and it sets the pitch. The modulator is silent on its own; all it does is shake the carrier's frequency. Speed and depth of that shake decide everything about the tone. Almost every control on an FM synth is really adjusting how the modulator affects the carrier.
FM makes sound by adding new frequencies, called sidebands, around the carrier. When the modulator bends the carrier at audio rate, it does not just move the pitch; it creates a cluster of extra tones above and below the carrier's frequency. Those sidebands are the harmonics you hear. Two things decide where they land and how loud they are: the ratio between the two oscillators, and how hard the modulator pushes.
The ratio of the carrier's frequency to the modulator's frequency sets the harmonic character of the sound. Simple whole-number ratios place the sidebands at neat, musical intervals, so you get a clear pitch and a harmonic tone. Ratios that are not whole numbers scatter the sidebands at odd, unrelated positions, so the sound turns inharmonic, clangorous and metallic, the kind of ring you hear in a bell. This one choice does more to shape an FM patch than anything else.
| Carrier to modulator ratio | What you hear | Often used for |
|---|---|---|
| 1:1 | A full harmonic series, close to a sawtooth. Warm and rich, with a clear pitch. | Basses, brass, organ and full-bodied tones |
| 1:2 | Mostly the odd harmonics, so it sounds hollow and reedy, a little like a square wave. | Clarinet-like and woody tones |
| Other whole numbers, such as 2:1 or 3:2 | Still harmonic and musical, but with a different balance of harmonics and its own colour. | Electric pianos, plucks, tuned percussion |
| Not a whole number, such as 1:1.41 | Inharmonic and clangorous, a metallic or bell-like ring where the pitch is harder to place. | Bells, chimes, gongs, mallets, glassy tones |
The other main control is the modulation index, often just labelled the amount or the depth. It sets how far the modulator pushes the carrier, and that decides how many sidebands appear and how strong they are. Keep the index low and the sound stays close to a pure sine, with almost no extra harmonics. Turn it up and more sidebands appear, so the tone gets brighter and busier. In everyday terms the modulation index is FM's brightness control, doing a job much like the filter cutoff does in a subtractive synth.
Real FM synths give you more than one pair of oscillators to play with. Each oscillator is called an operator, and an operator can act as a carrier you hear or as a modulator that shapes another. The fixed wiring that says which operators modulate which, and which ones reach your ears, is called an algorithm. Stacking operators, so that a modulator is itself being modulated, is how FM reaches its more complex and evolving tones, though it also makes the results harder to predict.
Most instruments sold as FM synths, including the well-known ones, do not use true frequency modulation at all. They use a very close relative called phase modulation, which is easier to keep stable and in tune in a digital design. The two are closely related and sound essentially the same in normal use, so the name FM has stuck as the everyday label. It is worth knowing if you read deeper, but it changes nothing about how you actually use one.
FM is good for bright, harmonically complex sounds that subtractive synthesis struggles to reach, especially anything metallic, glassy or bell-like. Because it builds harmonics rather than filtering them out, it can produce sharp, ringing overtones and clean high detail that a filter cannot easily create. These are the sounds FM is best known for:
The instrument that made all of this famous is the Yamaha DX7, released in 1983 and one of the best selling synthesizers ever made. Its electric pianos, bells and basses run all through the pop, R&B and film music of that era, and those sounds are still a shorthand for what FM does well. FM can produce warm and simple tones too, but its real advantage is the bright, inharmonic territory that is awkward to filter your way toward.
Subtractive synthesis starts with a harmonically rich waveform and takes frequencies away with a filter; FM starts simple and builds harmonics up by modulation. They reach a finished sound from opposite directions. Subtractive carves down from a full wave, while FM adds detail onto something as plain as a sine.
That difference shows up in how the two feel to program. Subtractive is fairly intuitive, because you can hear a filter open and close and watch the tone follow. FM is less direct: its main controls are the ratio and the modulation index, and a small change to a ratio can jump the sound from musical to metallic in one step. This is why FM has a reputation for being harder to dial in by ear, and why many people learn it by starting from presets and nudging them. The oscillator, filter and envelope path that most synths are built on is the subtractive one, covered in our guide on what a synthesizer is.
FM and subtractive are not rivals you have to choose between. Plenty of modern synths do both, running an FM section into a filter so you can build harmonics up and then carve them back down. If you are new to synthesis, the subtractive path is the gentler place to start, because you can hear each control at work. FM rewards you later, once you are used to listening for what one small change does.
FM clicks into place faster once you have felt how any synth reacts when you move a control while a sound is playing. Reading the theory gets you the map; hearing a change happen is what makes it stick.
ARGISH is a self-playing synthesizer built for exactly that kind of listening. It is not an FM synth, but because it plays itself, you can set a mood and a key, build a progression on the chord wheel, and turn a knob to hear what changes in real time, which is the same listening habit FM rewards. It runs in your browser, or as a plugin in your DAW: an Audio Unit, a VST3, an AAX for Pro Tools and a standalone app.
You can hear it free in your browser first, no install and no account, then decide.
FM itself is best explored in a dedicated FM instrument. ARGISH is here to build the listening habit, not to stand in for one.
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