FM & operator feedback

Beyond simple mixing, the oscillators can interact. FM (frequency modulation) is chosen as a routing between the oscillators, with FM depth and a mode (exponential or through-zero), plus per-oscillator operator feedback for self-FM. For oscillator sync — hard/soft sync, the routing topologies, and single-osc Sync Ratio — see Oscillator sync.

Frequency modulation (FM)

FM lets one oscillator modulate the pitch of another at audio rate. At low depths it fattens the tone with beating and sidebands; at high depths it generates complex, clangorous harmonic structures, the same mechanism behind classic FM synthesizers, with all three of Aconite’s oscillators available as operators.

Routing

A single FM Routing selector wires the oscillators together. Osc 1 is always the carrier: it is what you hear as the output.

Routing	What it does
No FM	Oscillators run independently.
2 → 1	Osc 2 modulates the pitch of Osc 1. Classic two-operator FM.
3 → 2 → 1	Osc 3 modulates Osc 2, which in turn modulates Osc 1, a three-operator chain.
2 → 1 ← 3	Both Osc 2 and Osc 3 modulate Osc 1 independently, adding their FM contributions together.

FM Depth

FM Depth controls how much pitch modulation is applied to the carrier. At low values you get subtle enrichment: added overtones and a slightly richer, more complex timbre. Push it further and sideband pairs multiply, the sound brightens dramatically, and you start to hear the inharmonic clang that makes FM famous. Very high depths produce dense, metallic spectra.

Tip

Modulating FM Depth with an envelope from the modulation matrix is one of the fastest ways to create a bright-then-warm decay arc. Map a fast attack/slow decay envelope to FM Depth and the attack snaps bright while the tail settles back.

FM Mode: Exp vs Linear

FM Mode switches between two mathematical relationships between modulator and carrier pitch:

• Exp (exponential): modulation is applied in a way that preserves musical intervals. When Osc 2 rises a fifth, the effect on Osc 1’s timbre is musically consistent regardless of Osc 2’s pitch. This is the most intuitive mode and works well for harmonic enrichment and vibrato-adjacent effects at low depths.

• Linear (through-zero): modulation is applied linearly in frequency. At high depths the carrier is swept through zero cycles per second, which generates the classic through-zero FM timbres: bell-like symmetry, cancellation nulls, and tones with a strong even-harmonic component. More unpredictable, more extreme, and the sound of classic digital FM synthesis.

Linear mode opens up territory that Exp cannot reach. If Exp FM sounds warm and organic, Linear sounds metallic and electric.

Operator feedback (self-FM)

Each oscillator has its own FB control that feeds a small amount of its own output back into its own pitch input. This is the classic DX7 operator feedback trick, and it works the same way here:

• On a Sine wave, low FB values add a gentle odd-harmonic brightness; higher values morph progressively from pure sine toward sawtooth as the sine folds back on itself. The feedback path is per-oscillator and separate from the cross-oscillator FM routing.
• On other waveforms (saw, square, triangle) feedback adds edge and harmonic grit on top of the existing waveform character. The effect is subtler than on a sine because there is already harmonic content, but it thickens and hardens the tone.

Note

Operator feedback is an oscillator’s self-modulation; it is distinct from the FM Routing controls, which wire one oscillator into another. Both can be active at once, and combining them (for example, Osc 2 with high FB feeding into Osc 1) gives access to dense, complex timbres.

Oscillator sync

Hard and soft oscillator sync — the routing topologies (2 → 1 / 3 → 2 → 1 / 2 → 1 ← 3), the dial-able Amount, Hard/Soft mode, Reset Phase, and Sync Ratio (a full sync sound from a single oscillator, with no second oscillator spent) — has its own page, with the SYNC tab, the live sync view, and the mod destinations: see Oscillator sync.

Sync and FM can be active at once. This is deep, unusual territory: sync reshapes the oscillator’s cycle while FM pushes its instantaneous pitch around, for timbres with sync’s harmonic focus and FM’s inharmonic complexity together. Start with moderate amounts of each and use the modulation matrix to sweep them separately, listening to how each dimension changes on its own.