FM Synthesis

Video

Subtractive synthesis involves using oscillators to generate harmonic content and filters to change the tone of this content. FM synthesis uses oscillators to generate the harmonic content like subtractive synthesis, but instead of using filters, it changes the tone by modulating the original oscillator (the carrier) with one or more other oscillators (modulators).

• All oscillators produce only sine waves.

Effect

Interaction Of Modulator And Carrier

• If the modulator is set to a whole number ratio of the carrier's pitch (1:2, 4:1 etc), then it adds harmonic overtones.
• At other ratios, inharmonic overtones are added and the sound becomes dissonant.

However if the modulator pitch is set slightly away from a whole number ratio (it is slightly detuned), the effect can add character to the sound and make it less sterile.

Note: The Yammaha DX7 (Propellerheads' PX7) group each oscillator with its envelope controls and call them operators.

Carrier

• Pitch will change the base pitch, and effect any modulators by altering the ratio relationship.
• Volume/Level Out effects the volume of the generated sound.

Modulator

The two settings for the modulator are:

• Pitch - Effects the harmonics added to the carrier.
• Volume/Level Out - Effects the quantity of harmonics added to the sound.

A sound is built by using one or more oscillators to add harmonics to the carrier, each providing a different amount and quantity of harmonics.

Different ratios of between modulator and carrier frequencies produce different harmonics (Mod:Carrier).

• 1:1 Sawtooth
• 2:1 Square
• 2:3.5 Bell (Inharmonic)

Algorithms

The algorithms dictate the way the different operators are configured - which are acting as carriers, which are acting as modulators and which operators they modulate.

Some algorithms include operators feeding back to themselves, which (given the 1:1 ratio) will create a sawtooth wave.

Algorithm Types

There are four main types of combination, though some algorithms are hybids.

• Stacked are comprised of one or more stacks containing a carrier modulated by one or more modulators where the modulators are in series. The stacks don't interact, meaning modulators from one stack don't effect modulators or carriers from another.
• Branch are comprised of one or more carriers, each modulated by multiple modulators where the modulators are in parallel, effecting the carrier at the same time.
• Root are comprised of multiple carriers, effected by a single modulator.
• Carrier Only are comprised of only carriers. Carriers might feedback into themselves.

Detune

Each oscillator has a detune control. By detuning multiple oscillators, a wider, chorus effect is achieved.

Envelopes

• The envelope and the output level for each operator effect the same characteristics.
• Both control modulation levels
• Both control volume levels
• For a Modulator, control its frequency (like VCF).
• For a Carrier, control its amplitude (like VCA).