Classic 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. Pulse Width Knob Adjusts rectangular output duty cycle from 5% to 95%. Double-click for 50%.
5. FM Depth Knob Controls the amount of linear frequency modulation from the signal connected to the FM input.
6. FM Input Audio-rate frequency modulation input.
7. Hard Sync Input Resets the VCO every time the input signal crosses a threshold slightly above 0V. For best results, connect to the sawtooth output of another VCO.
8. Pitch Input Calibrated for 1V/octave pitch signals.
9. VCO Outputs The VCO can produce 5 different waveforms, each with their own output. They are:
   • Sub-oscillator: square wave at half the VCO frequency
   • Pure sine wave
   • Triangular
   • Sawtooth
   • Rectangular: duty cycle set by Pulse Width knob

Overview ⚓︎

This is a classic VCO for subtractive synthesis. Inspired by many traditional oscillator designs, it provides individual outputs for sawtooth, triangular, rectangular and sine waveforms, as well as an additional sub-octave square wave oscillator.

It features hard sync and linear FM inputs for increased sonic possibilities.

In Depth ⚓︎

Tuning ⚓︎

Tuning is affected by the main Tuning knob, the Fine and Hertz knobs, their respective modulation inputs, and the Pitch input.

The Pitch input works at the 1V/octave standard, and is meant to be patched to a pitch signal from a keyboard, sequencer or quantizer. You can achieve the same effect with the Tuning knob’s modulation inputs with the attenuverter set to 100%.

When a fine frequency adjustment is required, use the Fine knob. Its whole range covers a single semitone, from -100 cents to +100 cents, centered at 0 cents. The associated modulation input can be useful to add a vibrato effect to the oscillator since it is calibrated to match the restricted range of the knob.

The Hertz knob is another fine-tune control that changes the frequency by a fixed amount of hertz. When using two VCOs at the same pitch, it can be interesting to slightly detune them with the Hertz knob. The effect will be more pronounced in the lower register where a few hertz can make a big difference in pitch, and less in the upper register.

While adjusting the Tuning knob, three values are displayed:

• Oscillator frequency, in hertz.
• Guitar-style tuner that shows the note that is the closest to the current frequency. The > and < symbols on either side of the note give an idea how close the frequency is to the note. A note is well tuned when it looks like this: >C4<.
• Ratio between the current frequency and the pitch signal on the Pitch input. At 1:1, the oscillator will play the pitch corresponding to the voltage on the Pitch input. This can be useful for tuning FM patches.

These values are computed from the current position of the Tuning, Fine and Hertz knobs, and don’t take modulation inputs into account.

Linear FM ⚓︎

This oscillator supports so-called through-zero linear frequency modulation, enabling it to create timbres reminiscent of popular synthesizers, sound cards and game consoles from the 1980s and early 1990s. Connect an audio-rate signal to the FM Input jack, such as the Sine output from another Classic VCO module, and adjust the FM Depth knob to control the amount of frequency modulation.

In FM synthesis, the VCO signal being modulated is called the carrier, and the source connected into the FM Input jack is called the modulator.

The cleanest sounds will be achieved with simple ratios between the frequency of the modulator and the carrier (such as 1:1, 2:1 or 3:1), and when the carrier is a sine wave or a triangular wave.

Using the sawtooth or rectangular output for the carrier is hit-or-miss, and using the rectangular output for the modulator will change the pitch of the carrier unless the Pulse Width knob is at 50%.

Hard Sync ⚓︎

Hard sync consists in creating new timbres by resetting the VCO to the start of its cycle every time the Hard Sync input voltage crosses a threshold slightly above 0V. The VCO will then follow the fundamental frequency of the signal connected to the Hard Sync input, and its tuning will affect the timbre of the sound.

The typical hard sync configuration uses two oscillators: a master and a slave. The master signal should be connected into the slave’s Sync input. Both VCOs should have the same Pitch input. Setting the slave’s Tuning knob above the master’s value and modulating it with a LFO or an envelope will result in the classic hard sync sound coming out of the slave’s output.

For best results, the Hard Sync input should be patched to the sawtooth output of another Classic VCO module to minimize digital aliasing artifacts.

In a complex patch with multiple VCOs with FM or ring modulation, the Hard Sync input can also be patched to a Keyboard Trig output to restart all VCOs to the same phase when a new note is played so that it always sounds the same.

Pulse Width ⚓︎

The Pulse Width knob controls the duty cycle of the rectangular wave output and has no effect on other outputs. When centered (default double-click setting), the output will be a pure square wave (50% duty cycle). When fully counterclockwise, the duty cycle will be 5%. When fully clockwise, the duty cycle will be 95%.

Modulating the pulse width with a LFO can produce the classic pulse width modulation effect. The Pulse Width knob has folding modulation inputs, guaranteeing an audible effect for any modulation input no matter what the knob settings are.

Outputs ⚓︎

The module provides separate outputs for all waveforms it produces. Each output is represented by a picture of the waveform. From top to bottom, they are:

• Sub (square wave one octave below the VCO frequency)
• Sine
• Triangular
• Sawtooth
• Rectangular

If you want a mix of different waveforms, you can connect these 5 outputs to a Mix 5 module’s 5 inputs.

It can be even more fun to use a Polarizing Mixer and experiment with phase-cancellation effects by using a positive sawtooth and negative square.

Technical Notes ⚓︎

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 hard sync or FM features.