Additive Oscillator

True additive synthesis with up to 512 partials

What Is Additive Synthesis?

As the famous mathematician Jean Baptiste Joseph Fourier taught us, we can create any periodic signal by adding up sine waves of different frequencies at different amplitudes and phases. These sine waves are also called partials or harmonics. This Additive Oscillator module allows you to add up up to 512 partials to create the waveform you desire.

A conventional oscillator generates a waveform — a sine, square, or saw — as a single, fixed shape. Additive synthesis works differently: it builds sound from scratch by combining many individual sine waves, each at a different pitch (a harmonic) and with its own volume and phase. The first harmonic (the fundamental) defines the pitch you hear. The second harmonic vibrates at twice the frequency, the third at three times, and so on.

By controlling how much of each harmonic is present — its amplitude — you sculpt the timbre. A square wave, for example, contains only odd harmonics (1st, 3rd, 5th, …) each getting quieter as you go up. A sawtooth wave contains every harmonic. A pure sine wave has only the fundamental. With the Additive Oscillator, you control all of this directly, up to 512 harmonics at once.

Overview

The module is divided into the following sections, from top to bottom:

1. Harmonic editor — amplitude and phase sliders for up to 16 visible harmonics, with a scrollbar to navigate all 512
2. Waveform display — real-time preview of the current waveform shape
3. Tune / Pitch CV / Sync — pitch control
4. FM — frequency modulation input
5. Waveform select — preset waveform buttons and CPU mode selector
6. Output — level, soft clipping, and CV-controlled output gain
7. Phase Mod — per-harmonic phase modulation with harmonic delay
8. Amp Mod — per-harmonic amplitude modulation with harmonic delay
9. Odd/Even — attenuate even or odd harmonics
10. Damp — roll off high or low harmonics

Harmonic Editor

At the very top of the module are two rows of sliders and a scrollbar.

Amplitude Sliders (top row)

Each slider sets the amplitude of one harmonic, from 0 (silent) to 1 (full). The slider scale is skewed so that small values are easier to set precisely. Harmonics with an amplitude below −100 dB are skipped entirely in the computation.

Phase Sliders (bottom row)

Each slider sets the starting phase of the corresponding harmonic from 0 to 1 (representing 0° to 360°). Phase has no effect on timbre when you are listening to the oscillator in isolation, but it matters when harmonics interact with modulation or when the waveform is viewed in the display.

Scrollbar

Shows and scrolls through 16 harmonics at a time. Drag the scrollbar to navigate up to harmonic 512. The tooltip shows the current range (e.g. "Bin 17–32").

Right-click on any slider

Right-clicking an amplitude or phase slider shows a context menu: "Clear this bin and above" — instantly zeroes all sliders from that harmonic onwards. This operation is undoable (Ctrl+Z).

Auto-bandlimiting

The oscillator automatically limits how many harmonics it computes based on the current pitch. At higher pitches, fewer harmonics are needed before they would alias past 19 kHz, so the engine simply skips those. The small number display below the CPU buttons shows the current harmonic count in real time.

Waveform Display

The canvas in the middle of the module shows one complete cycle of the current waveform in real time. It updates every 100 ms. The yellow horizontal line marks the zero crossing.

Right-click or double-click the canvas to open a file dialog and load an audio file into the harmonic editor (see Audio File Import below).

Preset Waveforms

Six small buttons select the base waveform:

When a preset waveform is selected, the amplitude and phase sliders are ignored — the oscillator uses its built-in preset values instead. To go back to your slider settings, press C.

The Custom mode is also automatically selected whenever you load an audio file (see below).

Pitch and Tuning

PITCH CV input

Standard 1 V/Oct pitch CV input. Plug your keyboard, sequencer, or arpeggiator here.

OCT knob

Coarse tuning in integer octave steps. Range: −3 to +3 octaves.

FINE knob

Fine tuning. Range: approximately ±7 semitones.

SYNC input

Hard sync. When the input signal rises above 2.5 V (a rising gate edge), the oscillator's phase is immediately reset to zero. This allows you to synchronize the Additive Oscillator to another oscillator for classic hard-sync timbres.

FM — Frequency Modulation

FM input

Audio-rate or CV frequency modulation.

FM Amount knob

Controls how strongly the FM input modulates the pitch. Range: −1 to +1. Negative values invert the modulation polarity.

FM Mode button (V/Oct / V/kHz)

Selects the FM response type:

• V/Oct (button lit, default): Exponential FM. The FM input voltage is added directly to the pitch CV before the V/Oct conversion, so modulation depth scales with pitch. This is the standard FM used for vibrato and musical exponential FM synthesis.
• V/kHz: Linear FM. The FM input shifts the frequency directly in kHz. Modulation depth stays constant regardless of pitch. Use this for through-zero linear FM and carrier-to-sideband relationships that stay in tune across the keyboard.

Phase Modulation (PM)

Phase modulation changes the instantaneous phase of each harmonic based on an incoming audio signal, producing a result that sounds similar to FM but with more direct control over the harmonic content of the resulting spectrum.

PM input

The modulation source. Any audio or CV signal.

PM Amount knob

Controls how strongly the PM input shifts the phase. At full positive or negative, a 5 V input produces a full 360° phase shift.

PM Delay knob (0–512 samples/harmonic)

This is the unique feature of the Additive Oscillator's PM section. Rather than applying the same phase shift to every harmonic simultaneously, the delay creates a cascade: each harmonic receives the PM signal from a different point in the past.

At 0 delay, all harmonics are modulated identically. As you increase the delay, each harmonic reads its PM value from progressively further back in time. The result is a smeared, waterfall-like sweep of phase modulation across the harmonic spectrum — instead of an instant timbral change, the modulation ripples through the harmonics over time.

PM Direction switch (L2H / H2L)

Sets which direction the delay cascade runs:

• L2H (Low to High): the fundamental receives the most recent PM value; higher harmonics receive progressively older values.
• H2L (High to Low): the highest harmonic receives the most recent PM value; the fundamental receives the oldest.

Amplitude Modulation (AM)

Unlike conventional AM (which modulates the overall output level), this section modulates the amplitude of each harmonic individually, using the same delay mechanism as the PM section.

AM input

The modulation source.

AM Amount knob

Controls the depth of amplitude modulation. Range: −1 to +1. Negative values invert the modulation (harmonics that would be boosted are attenuated and vice versa).

AM Mode button (EXP / LIN)

Selects the AM response:

• EXP (button lit, default): Exponential AM. The input voltage shifts each harmonic's amplitude exponentially (in dB). This gives a more musical, perceptually uniform response. A positive input boosts harmonics; a negative input cuts them.
• LIN: Linear AM. The input voltage is added directly to the harmonic amplitude. This can produce ring-modulation-like effects at high depths.

AM Delay knob (0–512 samples/harmonic)

Same cascade delay concept as PM Delay. Each harmonic receives its AM value from a different moment in time, creating a temporal sweep of amplitude modulation across the spectrum.

AM Direction switch (L2H / H2L)

Same as PM Direction — controls whether the delay cascade runs from low harmonics to high, or high to low.

Odd/Even Harmonic Control

This section lets you gradually fade between odd-harmonic content (square, triangle-like timbres) and even-harmonic content (hollow, flute-like timbres).

Odd/Even knob (−1 to +1)

• Positive values: Attenuates every N-th harmonic. With N=2, this silences harmonics 2, 4, 6, 8… pushing the spectrum toward odd-only content (square wave character).
• Negative values: Attenuates every harmonic that is not a multiple of N. With N=2, this silences harmonics 1, 3, 5, 7… pushing the spectrum toward even-only content.
• Center (0): No attenuation.

N knob

Sets which harmonic multiple the Odd/Even knob targets. Default is 2 (every second harmonic). Set to 3 to target every 3rd harmonic, and so on. The tooltip displays the ordinal name of the harmonic being targeted (e.g. "2nd harmonic"). Maximum value adjusts with CPU mode: 128 (LOW), 256 (MED), 512 (HIGH).

Odd Input

CV modulation of the Odd/Even parameter.

Odd Amount knob

Controls how strongly the Odd CV input modulates the Odd/Even position.

Damp — Harmonic Filter

Damp rolls off the upper or lower part of the harmonic spectrum, acting as a spectral filter applied directly to the harmonic amplitudes. Unlike a conventional audio filter, it does not filter the mixed output signal — it attenuates individual harmonics before mixing.

Damp knob (−1 to +1)

• Positive values: High-pass damp. The cutoff point moves from the fundamental toward the highest harmonic as the knob increases. Harmonics below the cutoff are attenuated; moving toward +1 progressively removes lower harmonics, making the sound thinner and brighter.
• Negative values: Low-pass damp. The cutoff point moves from the highest harmonic toward the fundamental as the knob decreases. Harmonics above the cutoff are attenuated; moving toward −1 progressively rolls off upper harmonics, making the sound warmer and more sine-like.
• Center (0): No dampening — all harmonics pass at their full amplitude.

Steepness knob (0–120 dB/Oct)

Controls how steeply the damp filter rolls off on either side of the cutoff. At 0, there is essentially no roll-off at all. Higher values create a sharper, more knife-edge cutoff. Default is 12 dB/Oct.

Damp Mode button (EXP / LIN)

Sets how the cutoff point moves as the Damp knob is adjusted:

• EXP (default): The cutoff point moves exponentially through the harmonic range. This feels more natural when sweeping, as each position corresponds to a perceptually equal step.
• LIN: The cutoff point moves linearly. Sweeping from 0 to +1 divides the harmonic range in equal-harmonic steps.

KB Track button (KBT)

When enabled (lit, default), the cutoff point scales with the number of active harmonics at the current pitch. Since the oscillator automatically limits harmonics at higher pitches, this means the damp filter tracks the keyboard: the cutoff sweeps proportionally to the harmonic content available at each note.

When disabled, the cutoff is calculated against the fixed maximum harmonic count (as set by the CPU mode), regardless of the current pitch. This can create a more consistent timbral character across the keyboard.

Damp Input

CV modulation of the Damp position.

Damp Amount knob

Controls how strongly the Damp CV input modulates the Damp knob position.

Output Section

Output knob (0–200%)

Master output level. At 100% (default, ~2 o'clock), the oscillator outputs at its nominal level. The output can be boosted up to 200% for driving downstream gain stages.

Soft Clip knob (0–100%)

Applies a gentle, analog-style soft-clipping waveshaper to the output. At 0% (fully left), the signal passes through unaffected. As you increase the knob, the output is progressively blended with a polynomial soft-clip curve that rounds off peaks smoothly, adding gentle harmonic saturation. At 100%, the output is fully soft-clipped, with a maximum output of approximately 3.5 V.

The red CLIP LED blinks briefly whenever the output signal exceeds ±5 V, regardless of the soft clip setting.

Output Mod input + Output Mod Amount knob

A CV input that modulates the output level. The Amount knob controls the depth and polarity (−1 to +1). The modulated output is clamped between 0% and 200% gain.

Output jack

The audio output. The signal is produced at the nominal Voltage Modular level (±5 V max before soft clip).

CPU Mode

Three mutually exclusive buttons control how many harmonics the oscillator computes:

The small number display below these buttons shows how many harmonics are actually being computed at the current pitch. At high pitches, this is always less than the selected maximum because harmonics above 19 kHz are suppressed automatically.

Switching CPU modes adjusts the scrollbar range, the N knob range, and the oscillator's harmonic limit immediately. The amp and phase slider values above the new maximum are preserved but inactive.

Audio File Import

The Additive Oscillator can analyze any audio file and translate its spectrum into harmonic amplitudes and phases, letting you resynthresize the timbral character of any sound.

To load a file: right-click or double-click the waveform display canvas, or drag and drop an audio file onto the module. Supported formats: OGG, MP3, WAV, AIF, AIFF.

What happens:

1. If the file is stereo, you are asked whether to use the left channel, right channel, or their average (sum to mono).
2. Leading and trailing silence is trimmed.
3. DC offset is removed.
4. If the file is longer than 1024 samples, the module automatically extracts a single cycle: it uses autocorrelation to detect the fundamental period, then isolates one clean cycle starting at a zero crossing. If no clear period is found, it searches for the nearest zero crossing to 1024 samples from the start. As a last resort, the full trimmed file is used.
5. A Fast Fourier Transform extracts 512 amplitude and phase bins.
6. The amplitudes are normalized so the loudest bin is at full scale.
7. The results are loaded into the amp and phase sliders.
8. Custom mode is automatically selected.

The load operation is fully undoable (Ctrl+Z restores the previous slider state).

Tip: Single-cycle waveforms give the most predictable results. Longer recordings of pitched instruments and synth tones also work well — the automatic cycle detection will find and extract one representative cycle. Inharmonic sounds (cymbals, hi-hats, noise) will produce spectra with energy spread across many high harmonics, which is physically accurate but may not track pitch as expected.

Quick Recipes

Classic square → sine morphing

Start with the Square preset. Slowly turn the Damp knob counter-clockwise from center. As you go toward −1 the upper harmonics roll off and the sound gradually approaches a pure sine.

Plucked string character

Load a custom spectrum (or use Saw), then patch an envelope into the Damp CV input (Damp Amount to taste). The envelope sweeps the damp filter closed over time, mimicking the natural decay of upper harmonics in plucked strings.

Metallic / inharmonic tones

Set N to any prime number (3, 5, 7, 11…) and set the Odd/Even knob to a positive value. Every N-th harmonic is attenuated, breaking the simple whole-number harmonic series and creating cluster-like inharmonic timbres.

PM waterfall

Patch a slow LFO into the PM input and set the PM Delay knob to around 100–200 samples. The phase shift cascades through the harmonics in a slow ripple, creating animated, living timbres that evolve in ways no conventional filter or LFO can produce.

AM shimmer

Patch a high-frequency oscillator (or audio signal) into the AM input with AM Mode set to EXP. Increase AM Amount and the AM Delay knob simultaneously. The amplitude of each harmonic rises and falls at a slightly different time, creating a shimmering, chorus-like texture that acts within the harmonic domain itself.

Spectral resynthesis layering

Load two different audio files into two instances of the Additive Oscillator. Route both to a mixer, tune them together, and use Damp on each to split the spectrum — one instance covers the low harmonics (Damp negative), the other covers the high harmonics (Damp positive). The result combines both timbral characteristics in a single layered voice.

Connections at a Glance

If you have any module specific questions, please visit the Monkey Business Audio forum. For questions about Voltage Modular in general, please visit the Cherry Audio forum. If you don't want to ask your question in the forum you can send me a pm or an email.