What are FM and additive synthesis?
FM and additive synthesis both work directly with a sound’s harmonics. Additive synthesis builds a sound by adding together many sine waves, each a single harmonic, and FM synthesis creates rich harmonics by letting one oscillator modulate the frequency of another.
How additive synthesis works
Any sound can be described as a sum of sine waves at different frequencies, levels and phases, the idea at the heart of Fourier analysis. An additive synthesizer works with those components, called partials, directly. Set the level of each partial and how it changes over time, and you define a sound’s timbre from the ground up.
Partials at whole-number multiples of the fundamental are harmonics, and give a clear pitch. Partials between them, called inharmonic, give the metallic, bell-like character of struck objects. A realistic sound can need hundreds of partials, so additive instruments shape many at once, with spectral envelopes and filters that act on the partials themselves.
How FM synthesis works
In frequency modulation synthesis, one oscillator, the modulator, varies the frequency of another, the carrier, at audio rates. At those speeds the modulation produces new frequencies, called sidebands, spaced around the carrier. The ratio between the two oscillators’ frequencies decides where the sidebands fall. Whole-number ratios give harmonic tones, and other ratios give inharmonic, bell-like ones. The depth of modulation, called the modulation index, decides how many sidebands appear, so raising it brightens the sound.
FM synthesizers call their oscillators operators, each usually with an envelope of its own, and connect them in arrangements called algorithms, with operators modulating one another in chains, side by side or in feedback loops.
Spectral resynthesis
Spectral resynthesis brings analysis into the picture. It measures a recording’s partials and how they change over time, separates the noise and transients that don’t fit a harmonic model, and rebuilds the sound as synthesis you can edit, stretch and transform like any other.
What it sounds like
FM is known for electric pianos, bells, glassy pads, punchy basses and metallic percussion, and for timbres that change dramatically with how hard a note is played. Additive synthesis excels at organ-like tones, evolving spectra and precise sound design, where every harmonic can be shaped on its own.
Key terms
- Partial
- One sine-wave component of a sound, with its own frequency and level.
- Harmonic
- A partial at a whole-number multiple of the fundamental frequency.
- Fundamental
- The lowest frequency of a pitched sound, heard as its pitch.
- Operator
- An oscillator in an FM synthesizer, with its own envelope, that can act as a carrier or a modulator.
- Carrier and modulator
- In FM, the carrier is the oscillator you hear, and the modulator changes its frequency.
- Modulation index
- How strongly the modulator changes the carrier’s frequency, which sets how many sidebands appear.
- Resynthesis
- Analysing a recording and rebuilding it as synthesis.
In The Harmonic Pulsar
The Harmonic Pulsar is Xionia Audio’s harmonic synthesizer. It brings FM, additive synthesis, spectral resynthesis and cross-synthesis together in one instrument, so you can build a sound from its harmonics, or analyse a recording and rebuild it as editable synthesis. Its partial balance, FM depth, operator ratio, spectral envelope and morph can each have a Motion Wave of its own.