Compact VCO

1. Tuning Knob Sets the VCO frequency when the Pitch input is at 0V. From ~8 Hz to ~8 kHz. Double-click for middle C.
2. Fine Knob ±1 semitone adjustment around tuning.
3. Hertz Knob ±5 Hz around tuning.
4. Shape Knob gradually morphs the waveform from a sawtooth to a square wave, with pulse width modulation in-between.
5. Sub FM Knob Adds sub-harmonics with a built-in FM algorithm.
6. Pitch Input Calibrated for 1V/octave pitch signals.
7. VCO Output Output signal.

Overview ⚓︎

Voltage-controlled oscillators are at the core of nearly every analog modular patch. As such, they can be complex beasts with plenty of knobs, faders, switches, inputs, outputs and all. This is great. We have one of those.

But sometimes, with too many possibilities, you don’t know where to start. Enters the Compact VCO: it has a pitch input, a nice tuning section, two timbre settings with full modulation support that put a massive array of tones at your fingertips, and a single output.

One nice touch is that the Shape and Sub FM knobs have folding modulation inputs, thereby ensuring uninterrupted modulation no matter what combination of knob position and modulation signal is encountered.

In Depth ⚓︎

Tuning ⚓︎

The tuning section is the same as the one found in the Classic VCO.

Shape ⚓︎

This describes the effect of the Shape knob when the Sub FM knob is fully counterclockwise.

Analog VCOs can usually produce different waveforms, with some kind of way to let the musician choose one of them—maybe a switch, or maybe discrete outputs for each waveform. They also often have a separate knob or modulation input to control the duty cycle (or pulse width) of the rectangular wave.

This VCO is different. It rolls all of the waveshaping duties into one knob.

• When fully counterclockwise, the output is a pure sawtooth. Play it low, play it loud.
• When fully clockwise, it is a rectangular pulse with a 50% duty cycle (a square wave).
• In-between, the waveform crossfades from a sawtooth to a rectangular pulse, and the duty cycle of the pulse changes. Modulating this knob with a LFO will produce a sound similar to the classic pulse width modulation effect.

Sub FM

Many traditional VCOs have sub-oscillator outputs. They are generally based around square or rectangular pulses at 1 or 2 octaves below the VCO’s frequency.

VCOs can also have inputs for frequency modulation (FM). Getting a stable, well tuned and reproducible tone out of FM on an analog modular synth requires at least two oscillators and a lot of patience.

The Compact VCO combines sub-oscillation and linear FM into an easy to use Sub FM knob.

• When fully counterclockwise, there’s no effect. The oscillator output is the pure shape selected with the Shape knob.
• As we start turning the knob clockwise, we introduce a sub-harmonic one octave below the oscillator frequency, and we hear a linear FM sweep effect.
• From the knob’s midpoint to its fully clockwise position, we introduce another sub-harmonic two octaves below the oscillator frequency while still enjoying a lovely FM sweep.

If you would rather have a FM sweep effect without the octave change, you can select a starting point at around ¼ or ¾ of the knob’s range. Then, you can apply a modulation deep enough for a sweep, but not so much that the sub-harmonic change. You can then adjust the oscillator’s root frequency to your liking with the Tuning setting.

If you want to know all the boring technical details on how the Sub FM feature works, see the Technical Notes—Sub FM section below.

Please note that high amounts of FM may introduce audible aliasing, especially when playing high notes at lower sampling rates. There’s a reason this knob is called Sub FM: it works great for basses, maybe not so much for soaring leads.

Technical Notes ⚓︎

Limits

The maximum oscillator frequency is limited to 13000 Hz, which is slightly above the frequency of the highest MIDI note. At a sampling rate of 44.1 kHz or 48 kHz, some aliasing can be heard in the upper range, especially when using the Sub FM feature.

Sub FM Implementation

Here is a thorough explanation how the Sub FM feature works.

• When fully counterclockwise, no FM is applied to the main oscillator.
• Turning the knob clockwise, we gradually introduce a sub-oscillator at ½ of the oscillator’s frequency. This sub-oscillator is used to apply linear FM to the main oscillator. This introduces a sub-harmonic one octave below the oscillator’s frequency.
• Turning the knob up clockwise up to its midpoint, the sub-oscillator’s amplitude is increased, which increases the amount of FM and has the side-effect of making the sub-harmonic louder.
• At midpoint, we reach the maximum depth of linear FM (this oscillator doesn’t do “through-zero FM”).
• From then on, a second sub-oscillator at ¾ of the main oscillator’s frequency is gradually mixed with the first sub-oscillator. Because of the magic of mathematics, this creates a second sub-harmonic two octaves below the main oscillator’s frequency. It also creates another prominent sub-harmonic at that sub-oscillator’s frequency.
• As the volume of the second sub-oscillator is increased, the volume of the first one is gradually lowered to remain within the limits of linear FM. Because there are two sub-oscillators at play and their relative volume changes quickly, there is more harmonic variation in the second half of the knob’s range.