FM synthesis creates complex, evolving sounds from the simplest building blocks: sine waves modulating each other. It is the sound of the 1980s - electric pianos, bells, slap basses, glassy pads - and of a great deal of modern bass music, where FM produces aggressive, metallic, growling textures that subtractive synthesis cannot.
A short history
FM synthesis was discovered by John Chowning at Stanford University in the late 1960s. Stanford licensed the technology to Yamaha, and in 1983 Yamaha released the DX7 - not the first FM synth, but by far the most successful. It appeared on countless records of the decade.
The DX7 had a reputation for being fearsomely difficult to program, and many units were sold with their factory presets still intact. Modern FM synths - Native Instruments' FM8, Ableton's Operator, Logic's Retro Synth, Arturia's DX7 V and the free Dexed - make FM far more approachable.
How FM works
In FM, one oscillator - the modulator - changes the frequency of another - the carrier. You hear the carrier; you do not hear the modulator directly, only its effect.
You have already met slow frequency modulation: an LFO modulating pitch creates vibrato. Speed that modulator up into the audio range - above roughly 20 Hz - and something remarkable happens. Instead of a wobble, you hear new harmonics, called sidebands, appearing around the carrier. From two simple sine waves, FM can create rich, complex spectra.
Ratio
The ratio between the modulator's and carrier's frequencies decides which harmonics appear.
- Whole-number ratios (1:1, 2:1, 3:1) produce harmonic spectra - tones with a clear pitch, like conventional instruments.
- Non-integer ratios (1:1.41, 3.5:1) produce inharmonic spectra - metallic, bell-like, clangorous sounds.
Modulation index
The modulation index (often just "amount" or "level") is how strongly the modulator affects the carrier. Low amounts add a few gentle harmonics; high amounts add many, making the sound brighter and harsher - FM's equivalent of opening a filter.
Putting an envelope on the modulator's level makes the brightness change over time. A fast decay on the modulator gives a bright attack that mellows - the classic FM electric piano and bell.
Operators and algorithms
In FM synths, each oscillator with its own envelope is called an operator. The DX7 had six. An algorithm is the arrangement that decides which operators modulate which, and which are heard as carriers.
- Stacks - operators in a chain, each modulating the next - create very complex sounds.
- Parallel carriers - several carriers heard together - work like additive synthesis.
- Feedback - an operator modulating itself - adds noise and buzz, turning a sine into something closer to a sawtooth.
Modern FM synths often show algorithms as diagrams, and some - like FM8 - let you route any operator to any other freely.
FM and filters
Many modern FM synths add filters, effects and other waveforms to the original sine-only design, combining FM's harmonic complexity with subtractive shaping. FM-generated tones fed into a filter are a cornerstone of modern bass design.
Four FM recipes
- Bell - carrier and modulator at a non-integer ratio (try 1:3.5). Modulator envelope with an instant attack and a long decay. Carrier with a long release.
- Electric piano - 1:1 ratio, modulator level moderate, with a fast decay on the modulator. A second pair at a higher ratio and quicker decay adds the "tine" attack.
- Bass - 1:1 or 1:2 ratio, modulator level set fairly high, with a short envelope on the modulator for a plucked attack. Add a little feedback for grit.
- Percussion - non-integer ratios, very short envelopes and a pitch envelope on the carrier produce metallic hits, toms and synthetic snares.
Experiment
FM rewards experimentation more than theory. Small changes to ratio and index can transform a sound completely. Make a habit of saving anything interesting.
Do this now
- In an FM synth, set up a single carrier and modulator, both sine waves, at a 1:1 ratio. Raise the modulation amount slowly and listen to the harmonics appear.
- Change the ratio to 2:1, then 3:1, then 1:1.41. Describe each sound.
- Put a fast-decaying envelope on the modulator level and build a bell.
- Build an FM bass and run it through a low-pass filter.