FM oscillator (6-operator)

The FM oscillator is a complete 6-operator FM synthesizer living in a single oscillator slot. Where classic FM synths hand you a fixed set of “algorithms” to choose between, Aconite gives you the raw wiring: a free matrix where you decide, continuously, which operators modulate which. Every one of those famous algorithms is just one particular pattern in that matrix, and you can dial in the ones the DX7 never had.

This is a different feature from the inter-oscillator FM & hard sync routings. That chapter is about wiring Aconite’s three main oscillators into one another. The FM oscillator here is a self-contained FM voice with its own six operators, entirely inside one of those three slots. You can even run it alongside the others: the FM oscillator on Osc 1, a Saw on Osc 2, a String on Osc 3.

Selecting the FM model

Pick FM from the oscillator’s WAVE selector, the same menu you use to choose Saw, Square, Wavetable, or any other model. Right-click the WAVE knob for the full type list and choose FM, or drag the waveform display to step onto it. The moment FM is selected, the FM editor takes over the oscillator’s model area: a full-width waveform scope across the top of the card (exactly like every other oscillator model), with the matrix on the left and the operator panel on the right below it. The scope draws a live render of the current FM patch, so it redraws as you edit the matrix, ratios, and envelopes.

A fresh FM oscillator is a single sine carrier and nothing else, so it starts out as a plain, clean sine. Everything below is how you build a richer tone up from there.

What an operator is

FM is built from operators. An operator is just a simple oscillator with its own envelope. What makes it FM is that operators can feed each other: when one operator is pointed at another, it does not add its sound to the mix, it bends the second operator’s pitch thousands of times a second, and that fast pitch-bending is what grows the bright, metallic, bell-like overtones FM is known for.

An operator plays one of two roles, and it can play both at once:

A carrier is an operator you actually hear. Its level in the OUT column sets how much it contributes to the final sound.
A modulator is an operator pointed at another one, shaping that operator’s tone rather than being heard directly.

The whole craft of FM is choosing which operators carry, which modulate, and how hard, and that is exactly what the matrix lets you draw.

The FM matrix

The FM MATRIX is a 6×6 grid plus one extra column, drawn as a grid of value-bearing circles (one per cell). Each circle’s fill tracks its amount, so the whole routing reads at a glance; empty cells stay greyed but always visible, so you can see every possible connection and drop a value into it. Read the grid as “modulator, then destination”:

Rows are modulators. Row 3 is operator 3 acting as a modulator.
Columns are destinations. Column 5 is operator 5 being modulated.
A cell where row 3 meets column 5 sets how hard operator 3 modulates operator 5. Drag a circle vertically to set its amount: up sends more, down to zero.

Two cells behave specially, and draw with a heavier ring so they stand apart:

The diagonal (row 1 / column 1, row 2 / column 2, and so on) is each operator’s self-feedback: the operator modulating itself. A touch of feedback thickens a sine toward a saw and adds brightness and edge; pushed up, it roughens the tone. It is the classic FM feedback that turns a lone operator gritty. Empty diagonal cells read as a dim fb label so you can always find them.
The OUT column, on the far right, is each operator’s carrier level: how much of that operator you hear in the final mix. An operator with any OUT level is a carrier; one with none is a pure modulator (or silent).

Because every cell is a free control, you are not locked to a menu of algorithms. Any classic FM topology (a stack, a pair of parallel operators, a tree) is just a particular set of cells raised, and you can morph between them by dragging cells up and down. Want two operators modulating one carrier, plus a third stacked on top? Set those three cells and you have it, no algorithm number required.

The operator panel

To the right of the matrix, the operator panel edits one operator at a time.

The 1–6 selector at the top chooses which operator you are editing. Click a number and the knobs, the waveform, and the drawable envelope below all re-target to that operator. Nothing you set on one operator disturbs the others.

Below the selector sit the per-operator controls:

Ratio (coarse) sets the operator’s pitch as a multiple of the played note: at 1 it tracks the note in unison; at 2 it sits an octave up; at 3, 4, and higher whole numbers it lands on successive harmonics. Whole-number ratios keep the tone musical and in tune; the low 0.5 setting drops an operator an octave below for weight.
Fine nudges the ratio between whole numbers. Small amounts add shimmer and beating; larger amounts pull the operator inharmonic, toward clangs and bells.
Fixed Hz (the RATIO / FIXED toggle) locks an operator to an absolute frequency instead of tracking the keyboard. A fixed operator sounds at the same pitch on every note, which is how you build fixed formants, metallic clicks, and attack noises that stay put as you play up and down.
Level sets how strong the operator is. On a carrier this is loudness; on a modulator it is how hard it bends its destination, i.e. how bright and complex the result gets.
Waveform picks the operator’s own shape: Sine, Saw, Square, or Triangle. Sine is the classic FM operator and the most predictable; the others pack in extra harmonics before any modulation, for a rawer, brighter starting point.
Velocity sets how much your playing dynamics affect this operator. Turn it up on a modulator and playing harder makes the tone brighter and more complex, exactly the way a real instrument opens up when you dig in; leave it at zero and the operator ignores velocity.

The per-operator envelope

Under the knobs, each operator has its own drawable envelope, the same breakpoint editor used elsewhere in Aconite. Drag the nodes to shape how that operator rises and falls over the life of a note. The envelope does double duty: on a carrier it shapes loudness (the usual amp contour); on a modulator it shapes the brightness over time, because a modulator that fades away lets the tone start bright and settle warm.

The sustain marker (the ringed node) is the level the operator holds while you keep the key down; the segment after it is the release tail once you let go.
LOOP cycles a region of the envelope for as long as the note is held, for rhythmic, pulsing modulation.
SYNC locks that loop to your project tempo. Turn it on and a division picker appears (1/4, 1/8 triplet, and so on), so the modulation pulses in time with the track. Left off, the loop runs at the envelope’s own drawn speed.

Because every operator has an independent envelope, several of them can be evolving at once, in different directions and at different speeds, which is what gives a good FM patch its living, moving character rather than a fixed timbre.

The PITCH knobs

Below the WAVE selector, a small PITCH group holds the whole FM voice’s tuning: Oct, Coarse, and Fine. These shift the entire operator structure together, exactly like the tuning on any other oscillator: Oct by octaves, Coarse in semitones for intervals, Fine in cents for detuning against another oscillator. The per-operator Ratio and Fine controls set each operator relative to this base, so the PITCH knobs move the sound as a whole while the ratios keep their relationships intact.

Sweeping the whole structure: FM Depth

FM Depth is available as a destination in the modulation matrix (listed as Osc 1 / 2 / 3 FM Depth, filed under the oscillator group). It is a single control over the entire FM structure at once: feedback and every matrix edge together.

At its default it leaves the patch exactly as you built it.
Turned down to zero, every operator collapses to a pure tone with no sidebands: the brightness and clang vanish and you are left with just the carriers.
Turned up, the whole structure opens: more sidebands, more brightness, more complexity across the board.

Route an envelope or LFO to FM Depth and you sweep the FM character in real time without touching a single cell. A fast envelope on FM Depth gives the signature FM gesture: a bright, complex attack that mellows into a simpler body as the note settles.

A worked example: an FM bell

The fastest way to hear the matrix at work:

Set the oscillator’s WAVE to FM.
Select operator 2. Give it a high Ratio (say 3 or higher) so its overtones sit well above the fundamental.
In the matrix, drag the cell where row 2 meets column 1 up, so operator 2 modulates operator 1 (the carrier).
On operator 2’s envelope, draw a fast attack and a quick decay so its modulation is strong at the start and gone a moment later.

The result is a struck, metallic bell: bright and clangorous on the attack (while operator 2 is loud) and settling to a purer tone as its envelope decays. Raise operator 2’s Ratio for a more inharmonic bell; add a little self-feedback on the diagonal for extra bite; route FM Depth to an envelope to make the whole strike sweep.

Clean at the bright end

Bright, high-feedback FM patches (electric-piano tines, hard bells, anything with strong self-feedback or a high-ratio modulator) are exactly the sounds that make naive FM grainy and harsh. Aconite automatically oversamples those patches internally to suppress that aliasing, so an electric-piano voice comes out clean instead of gritty. There is nothing to set: it engages only for patches that can alias and stays out of the way for simple sine tones, so you get the clean result for free.