Oscillators

VCO

The voltage-controlled oscillator — the primary sound source in any subtractive synth voice.

What is a VCO?

A voltage-controlled oscillator — VCO for short — is the engine that makes a synthesizer make sound. At its core it's a circuit that produces a periodic waveform: a tone that repeats hundreds or thousands of times per second. The pitch you hear is just the rate of that repetition; the timbre, the difference between a violin and a clarinet, comes from the shape of one cycle.

What turns an oscillator into a *voltage-controlled* oscillator is the second word. The pitch isn't a knob you set and forget — it follows an incoming control voltage, in real time, sample by sample. Send 0V and the VCO sits at its base pitch. Send 1V and it jumps up exactly one octave. Send a sequencer's stepped output and it plays a melody. Send another oscillator running at audio rate and you get FM synthesis. The voltage *is* the music.

This is why every modular synthesizer in the world, from a Moog Modular in 1968 to a Eurorack rack assembled last week, agrees on the same standard: 1V/Oct — one volt per octave. A keyboard, a sequencer, a quantizer, an LFO — anything that outputs CV — can drive any VCO from any manufacturer and produce the same musical interval. The VCO is where that voltage becomes pitch.

Our VCO

Some VCOs output one waveform. Some pick between waveforms with a switch. Ours outputs all four simultaneously — sine, triangle, sawtooth, and square / pulse all live at once, so you can patch any of them, or mix several into a single voice. Combine the saw for body and the square for grit, or send the sine to one destination and the saw to another.

Under the hood, the sawtooth and pulse outputs use polyBLEP anti-aliasing. That's the difference between a VCO that stays clean at high pitches and one that develops a gritty digital buzz as harmonics fold over the Nyquist frequency. The triangle is built by integrating the polyBLEP square — alias-free at every frequency the engine can play.

The FM input has a per-input attenuverter and switches between exponential (musical-interval) and linear (frequency-space) modes. Sync input runs in either hard mode (full phase reset on every gate edge) or soft mode (phase reverses), each producing a distinct flavor of harmonic enrichment. ±2 octaves of coarse tuning, a ±100 cents of fine — it's a very classical VCO, kept honest.

In a patch

In a typical subtractive voice the VCO is the very first link in the chain: VCO → VCF → VCA → output. The oscillator generates a harmonically rich source, the filter carves it into a sound, the amplifier shapes it over time. Pick a sawtooth or square output, route it through a low-pass filter with envelope-controlled cutoff, and you have the architecture behind nearly every analog synth voice ever made.

Modulation routes are where it gets fun. Patch an LFO into the FM input for vibrato. Patch an envelope into the same input for pitch sweeps. Patch another VCO at audio rate into FM and you've crossed into FM synthesis — bells, gongs, metallic timbres. Run two VCOs slightly detuned into a mixer and you get the fat, beating sound that defines a lot of vintage leads. The VCO almost never plays alone.

Inputs

V/Oct (cv) — Pitch control input. 1V per octave standard. 0V = C4 (middle C). Connect a sequencer, quantizer, or keyboard CV here.
FM (cv) — Frequency modulation input. Modulates the oscillator pitch. Amount is scaled by the FM attenuverter knob. Use for vibrato (slow LFO), FM synthesis (audio-rate), or pitch sweeps.
PWM (cv) — Pulse width modulation input. Modulates the duty cycle of the square wave output. Amount is scaled by the PWM attenuverter. Produces a chorus-like thickening effect.
Sync (gate) — Oscillator sync input. Resets the oscillator phase on each rising edge. Hard sync produces harmonically rich timbres; soft sync is subtler and more musical.

Outputs

SIN (audio) — Sine wave output. The purest waveform with no harmonics. Good for sub-bass, FM carriers, or clean tones.
TRI (audio) — Triangle wave output. Contains only odd harmonics, softer than sawtooth. Good for flute-like sounds, bass, or gentle leads.
SAW (audio) — Sawtooth wave output. Contains all harmonics, bright and rich. The most common waveform for subtractive synthesis. Ideal for pads, leads, and brass-like tones.
SQR (audio) — Square/pulse wave output. Duty cycle controlled by PW knob and PWM input. At 50% it sounds hollow and clarinet-like. Narrow pulses create thin, nasal tones.

Controls

Coarse — Coarse tuning in semitones, +/-24 (two octaves up or down). Use to set the base pitch relative to C4.
Fine — Fine tuning in cents, +/-100 (one semitone). Use for subtle detuning against another oscillator to create a fatter sound.
FM Atten — FM attenuverter. Scales the FM input from -1 to +1. Negative values invert the modulation. At 0, no FM is applied regardless of the FM input.
PW — Pulse width. Sets the duty cycle of the square output from 5% to 95%. 50% is a true square wave. Only affects the SQR output.
PWM Atten — PWM attenuverter. Scales the PWM input from -1 to +1. Negative values invert the modulation.
Sync Mode — Sync mode toggle. HARD = phase resets to zero on sync pulse (dramatic, metallic harmonics). SOFT = phase direction reverses (subtler, more tonal).
FM Mode — FM mode toggle. EXP = exponential FM (default, pitch-space modulation - same musical interval regardless of base pitch). LIN = linear FM (frequency-space - preserves harmonic ratios, better for bell-like FM timbres).

Inspired by

A clean, classic East Coast VCO topology — 1V/Oct tracking, simultaneous sine/triangle/saw/square outputs, exponential or linear FM. The polyBLEP anti-aliasing is contemporary; the rest of the architecture is canonical.

Moog 901
Roland System-100M VCO
Doepfer A-110-1

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