ZX Audio InfoSearch

Synthesis Types

Hear and visualize additive, subtractive, FM, AM, wavetable and granular synthesis with an interactive sound workbench.

Field guide / DSP

beginner · 4 min read · Updated

Synthesis means making sound. A waveform describes how a signal moves over time; its spectrum describes the frequencies inside it. Different synthesis methods give you different ways to build or reshape that signal.

SOUND LAB / 01

Six ways.
A world of sound.

Choose a method.
Shape it. See it. Hear it.

FIG 01 / FM ANATOMYINDEX 2.50 · RATIO 2:1

The modulator pushes the carrier’s phase

Follow the orange control cable: it never reaches the speaker. It plugs into the carrier’s phase input and pushes the carrier’s phase wheel ahead and behind. The blue output is the height of that wheel’s dot, traced over time.

FM synthesis anatomyA 220 hertz modulator oscillator feeds an index control. The scaled signal enters the 110 hertz carrier's phase input and pushes its phase wheel ahead and behind. Only the carrier's output is cabled to the output speaker.MODULATOROSC 2 · CONTROL220 Hz · ×2OUTCARRIEROSC 1 · AUDIO110 HzPHASE INOUTOUTPUTININDEX β05INOUT MODULATOR 220 Hz sine · control only +1−1 INDEX β 2.50 β × modulator = phase push +5 rad−5 rad ±2.50 rad CARRIER 110 Hz sine · phase wheel OUTPUT the only signal you hear +1−1 control signalphase pushaudioFM synthesis anatomyA 220 hertz modulator oscillator feeds an index control. The scaled signal enters the 110 hertz carrier's phase input and pushes its phase wheel ahead and behind. Only the carrier's output is cabled to the output speaker.MODULATOROSC 2 · CONTROL220 Hz · ×2OUTINDEX β05INOUTCARRIEROSC 1 · AUDIO110 HzPHASE INOUTOUTPUTIN MODULATOR 220 Hz sine · control only +1−1 INDEX β 2.50 β × modulator = phase push +5 rad−5 rad ±2.50 rad CARRIER 110 Hz sine OUTPUT the only signal you hear +1−1 controlphase pushaudio
Control (moves the carrier’s phase)Audio (what you hear)Pure carrier, index 0

Index 2.50: the push swings ±2.50 rad, twice per carrier cycle. The dot races ahead, stalls and briefly turns backwards, so the output folds into a more complex wave with more sidebands.

Try this: change the modulation index. The modulator changes the carrier; it is not mixed into the audio output.

About this model

A sine carrier is phase-modulated by a sine at twice its frequency. At a constant index this has the spectrum of sinusoidal FM; raising the index spreads energy into sidebands.

Six ways to make a sound

Additive — build it from ingredients. Add sine waves together. A harmonic is a whole-number multiple of a fundamental frequency: 110, 220, 330 Hz and so on. Change their levels to change the tone. Try moving from one partial to twelve: a smooth sine becomes a brighter, buzzy wave. Real additive synths can animate hundreds of partials, including inharmonic ones.

Subtractive — start rich, remove some. Begin with a harmonically rich oscillator, then filter it. Lower a low-pass filter’s cutoff and the high frequencies fade first. This demonstration filters a finite saw-like harmonic series; a complete subtractive synth also has envelopes, resonance and modulation.

FM — let one oscillator bend another. A modulator changes a carrier’s instantaneous frequency, creating sidebands around it. Modulation depth controls how much spectral complexity appears; the frequency ratio determines whether that complexity feels harmonic or metallic. Here the ratio is fixed at 2:1. The model uses sinusoidal phase modulation, which gives the same steady-state spectrum as sinusoidal FM at a fixed modulation index. Not every FM sound is bell-like.

AM — change its level at audio rate. Multiply a carrier by a changing amplitude. Slow modulation sounds like tremolo; fast modulation creates sidebands. This example uses a 440 Hz carrier and 110 Hz modulator, so the new frequencies are 330 and 550 Hz. Ordinary AM retains the carrier; ring modulation uses a bipolar multiplier and can remove it. The two are related but different.

Wavetable — move between stored waves. Read a repeating waveform from a table. Scan through a collection of waves and the harmonic balance changes. Here you interpolate between a sine and a finite saw-like wave. Production wavetables often contain many frames and use band-limited playback to avoid aliasing.

Granular — build it from tiny windows of sound. Play short overlapping grains, each with a smooth volume window. Change their duration, density, position or pitch to turn a recording into a cloud of texture. This deliberately periodic example windows a sine into repeating grains; hear the new frequencies as the windows shorten. It demonstrates windowing, not the full variety of sampled, random granular textures.

Read the instrument

Each method opens on an animated anatomy of its patch. Blue cables carry audio and orange cables carry control. Each card is attached to the part it explains, and the output card builds up the sound you hear one cycle at a time. Pause the motion or drag the moment slider to hold any instant, then move the method’s slider and watch the cards change.

  • Additive: twelve sine partials, each on its own fader, are added at every instant.
  • Subtractive: the filter’s response curve sits over the saw’s harmonics. Harmonics above the cutoff fade.
  • FM: the modulator pushes the carrier’s phase wheel ahead and behind.
  • AM: the modulator sets a VCA’s gain, so the output stays inside the orange envelope.
  • Wavetable: one read head moves through two stored cycles, and their weights set the blend.
  • Granular: windows cut grains from a source, and overlapping grains are summed.

In FM, the modulator feeds the carrier’s modulation input. Only the carrier reaches the output. In AM, the modulator controls a gain stage in the carrier’s audio path. Additive instead mixes the oscillators’ audio outputs together. Those connections are the central distinction between the techniques.

For FM, the carrier is the oscillator making the sound and the modulator changes its timing. This example shows a 110 Hz carrier and a 220 Hz modulator. The phase-push trace is the displacement in radians used by this model, not a second audio wave being mixed in. At zero index the push is zero and the output is the unmodulated carrier, sitting exactly on the grey ghost. AM instead multiplies its carrier by a nonnegative gain envelope.

The cable routing follows the clear operator-routing approach in Cymatics’ FM synthesis introduction. The diagrams and interactive model here are our own.

Some methods put little or no energy at 110 Hz. AM, for example, starts at its fourth harmonic, 440 Hz. A repeating cycle does not guarantee that you hear the repetition rate as the strongest pitch.

The sound and diagrams use the same generated cycle. Playback repeats it at 110 cycles per second with a soft attack and release. Peak limiting keeps samples within range; it does not match perceived loudness, so keep your volume comfortable as you switch methods.

What to try next

  • Hear additive, then wavetable. Both can approach a saw-like sound here, through different controls.
  • Close the subtractive filter, then add partials in additive. Both change brightness, starting from opposite ends.
  • Hear FM, then AM. FM redistributes energy across many sidebands; this simple AM example adds just two.
  • Shorten the grains. A shorter window spreads energy across more frequencies — the same time/frequency tradeoff behind spectral analysis.

Synthesis methods can be combined. A wavetable oscillator can feed a subtractive filter, an FM oscillator can become a grain source, and almost any parameter can follow an envelope. These names describe techniques, not mutually exclusive kinds of instrument.