FM & hard sync

Beyond simple mixing, the oscillators can interact. FM (frequency modulation) and hard sync are chosen as routings between the oscillators, with FM depth and a mode (exponential or through-zero), per-oscillator operator feedback for self-FM, and band-limited sync that stays clean.

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.

Hard sync

Hard sync forces Osc 1 to restart its waveform cycle every time a master oscillator completes one of its own cycles. This creates a characteristic sweeping, harmonically rich tone that was all over 1980s leads and remains a go-to for aggressive, cutting sounds.

Routing

The Sync Routing selector mirrors the FM routing structure. Osc 1 is always the slave, the oscillator whose cycle resets. Osc 2 and Osc 3 are the masters:

Routing	What it does
Off	No sync. Oscillators run freely.
2 → 1	Osc 2 resets Osc 1’s phase on each of Osc 2’s cycle completions.
3 → 2 → 1	Osc 3 resets Osc 2, which resets Osc 1, a sync chain.
2 → 1 ← 3	Both Osc 2 and Osc 3 independently reset Osc 1.

Making the sync sweep

The classic hard-sync sound comes from detuning Osc 1 (the slave) away from Osc 2 (the master). The master holds the played pitch; Osc 1’s natural frequency is tuned away (sharper, usually) using its Coarse and Detune controls. As Osc 1’s frequency climbs relative to the master, a sweep of harmonic notches and peaks moves through the spectrum.

The two most expressive techniques:

1. Static detune: set Osc 1 several semitones sharp of the master. This locks in a specific harmonic character and lets you play pitched lines where every note has the same aggressive, synced quality.

2. Swept detune: route an envelope or LFO from the modulation matrix to Osc 1’s pitch. The harmonic spectrum sweeps in real time, giving you the classic “rising sync scream” on attack or a slowly evolving sweep across held notes.

Tip

For the classic hard-sync lead, set Osc 2 routing to 2 → 1, put Osc 1 about a fifth sharp with Coarse, then connect a fast envelope to Osc 1 pitch (positive Amount, moderate Decay). The sync sweep brightens aggressively on each attack and settles back. Nudge Resonance up on the filter to emphasise the sweep further.

Why it stays clean

Aconite’s sync is band-limited: the reset edge is smoothed in a way that suppresses aliasing without dulling the characteristic sync snap. You can sweep it at any speed and through the full pitch range without the digital crackling that a naive sync implementation would introduce.

Combining FM and sync

FM and hard sync can be active simultaneously. This is a deep and unusual territory: the sync is reshaping Osc 1’s cycle while FM is simultaneously pushing its instantaneous pitch around. The combination produces timbres that neither technique reaches alone: sounds with sync’s harmonic focus and FM’s inharmonic complexity at once. Start with moderate amounts of each and use the modulation matrix to sweep them separately, listening to how each dimension changes independently.