Voltage Controlled Oscillator

A Voltage Controlled Oscillator (VCO) is the synth module that generates the pitch (frequency) of the notes we hear.will have both a coarse and fine frequency adjustment knobs.  That will allow the user to precisely "tune" this pitch source.  A VCO is akin to a single guitar string.  When the open string is plucked, it will generate a specific note.  The coarse and fine frequency adjustments on the VCO control panel are similar in function to the tuning knob on a guitar.  The VCO also has a 1V/OCT CV input.  That CV comes from the keyboard controller.  It is that CV that changes the pitch of the VCO much the same way as fretting a guitar string changes its pitch.

Like any oscillator, the frequency stays fixed unless something acts on it to change.  If the oscillator is set for a fairly low frequency, the corresponding note will be low on the musical scale.  Now suppose that the oscillator frequency could be precisely controlled with a voltage and nothing more.  What would that do for us?  Assuming the voltage was very stable, this voltage could be used to force the oscillator to exact note frequencies.  And so it does!  Differing, but specific voltages will cause the oscillator to function at frequencies that are musical notes according to the 1V/octave standard already explained.  The oscillator has a “main” control port called the 1V/OCT input.  The next question usually asked is "well what turns the oscillator on and off to make the notes?".  The simple answer is: nothing.  The oscillator is on all the time.  I'll explain that a bit later, let's just worry about voltage control for a bit longer.

A VCO normally just sits there outputting a variety of waveforms, at a frequency determined by the combination of the "coarse" and "fine tune" adjustments on the control panel.  The keyboard controller's CV output is added to the internal VCO control voltage to determine the final pitch output by the VCO.  Since the coarse and fine tune controls adjust the VCO pitch, in addition to the keyboard CV, one can make the VCO assume any frequency for any note.  Just because you press the A1 key on the keyboard and the keyboard controller outputs a very specific voltage for that key, nothing says the output pitch has to be the pitch of A1 (55.000Hz).  This flexibility allows for on the fly transposing of notes.  When multiple VCOs are used, each VCO will get the keyboard controller CV.  By adjusting the individual VCO frequencies, one can get very thick chords and note beatings.  With maybe eight VCOs running, one can get a "wall of sound" if so inclined.  That is the beauty of analog synthesis ... there is essentially no limits as to how many elements comprise the final sound.  (Well, your bank account balance usually is the limiting factor.  Analog synthesis modules are rather expensive.  A single VCO can easily cost in excess of $400.  Even a modest system will cost $5,000 or more.  Why then would anybody in their right mind have one of these things?  There are a great many digital synths out there that sell for a fraction of that price.  Are you nuts?  The answer is: the sound.  Digital synths can sound very good but going beyond the stock sounds is tedious in the extreme, has very limited capability, and the user interfaces for making changes, sucks.)

A VCO typically has a number of waveforms that are simultaneously available.  The different waveforms "sound" different because they are composed of differing harmonics that appear in differing strengths.  Your basic analog synthesizer creates sound by what is called "subtractive synthesis".  That is, you start out with waveforms that have lots of harmonics, and you create the sound you want by removing those elements of the sound that are unwanted.  It turns out that that approach is very good.  If you built up the harmonics in an additive way, a lot more equipment would likely be needed and the cost would be prohibitive.

A VCO has a variety of control inputs to alter the VCO waveform outputs.  Many of the changes to the VCO waveforms are to provide some sonic animation.  A steady-state pitch is very boring and sounds sterile.  Add a little vibrato and the sound is much more appealing.  By changing VCO output as a function of time, one can get very interesting sounds that are musically pleasing.  The ways in which a VCO output is changed over time is limitless.  Therefore, the sounds that can be produced are limitless.  THAT is why analog synths are so cool!

This verbiage only begins to scratch the surface of VCOs.  The more features a VCO offers, the more it will likely cost, but the more flexible it becomes.  One or more VCOs form the backbone of an analog synthesizer.  They create the pitch we hear and have a lot to do with the timbre of the sound produced.

Voltage Controlled Oscillator (VCO) Recap

A synth VCO generates musical pitches in response to a control voltage and has several input and outputs:

1V/OCT Input - A control voltage input that changes VCO frequency according to the applied voltage.  Negative or low amplitude positive voltages produce lower pitched notes.  More positive voltages produce higher pitched notes.  It is not uncommon for a VCO to be able to produce pitches from 1Hz to 30,000Hz with only voltage control.

FM Input - A control voltage sent to the FM input permits the VCO to be frequency modulated.  Vibrato is an example of frequency modulation.  There is nothing preventing the user from using the output pitch of one VCO to FM modulate a second (or third) VCO.  There is usually an FM level adjust panel control associated with this input, to adjust how much frequency modulation the VCO "sees".

PWM Input - This control voltage input changes the duty cycle of the pulse output. The full name is "Pulse Width Modulation", hence the shortened moniker "PWM".  This control input does not change the frequency of the VCO, it only changes the harmonic content of the pulse output by adjusting the duty cycle of the waveform.  Rectangular waveforms have varying harmonic content based on the "on" time of the wave with respect to the "off" time.  The ratio of "on" time to "off" time is the duty cycle and is usually expressed as a percentage of the "on" time.

Pulse Output - This output is a pitch source with a rectangular shaped waveform.  The duty cycle of this waveform can be adjust with a control voltage presented to the PWM input.  There is also a panel control to adjust the initial duty cycle when no PWM input is present.  By changing the duty cycle, the number and strength of harmonics is varied.  A 50% duty cycle pulse output sounds much like a clarinet.

Saw Output - This output is a pitch source with a waveform that is shaped like the tooth of a saw blade.  It is a linear ramp that quickly resets to zero, only to begin a linear ramp once again.  The "sawtooth" output is harmonically rich and is often used as the basis for creating many other sounds.  By itself, a sawtooth waveform sounds much like a bowed string on a violin.

Other Inputs & Outputs - Many additional inputs and outputs may be present on a VCO.  Their use will be documented in the user manual that accompanies the VCO.  As with many synthesizer modules, there are limitless variations possible. 

Note: Most pitch outputs of a VCO emit waveforms that measure about 10 volts in amplitude.  The most positive peak of the waveform will be about +5V while the most negative peak will be about -5V (for a total amplitude of 10V).  The reason for such "hot" signals is that one gets good sonic fidelity.  As a waveform passes through perhaps six or eight processing modules, each additional module degrades the signal somewhat.  That signal  degradation is cumulative.  But if you start out with a "hot" signal, you can degrade the signal much more before your ears will hear it.  After all, these "hot" signals are reduced in amplitude before recording.  When the amplitude is reduced for recording, the degradation is also reduced by the same amount ... it gets very small!