Voltage-Controlled Oscillator (VCO)

Voltage-Controlled Oscillator (VCO)

So far we’ve talked about what synthesis is and various methods used to generate waves.  In the “Building Blocks” section we’re going to look at all the different parts of a synthesizer and how they work together to create sounds.

When I talk about “parts”, I’m not referring to the actual capacitors, resistors and other components that make up a synthesizer.  Instead, I’m talking about the logical sections that each do their part to generate or modify a signal.  In some synths, all these parts are combined together into one unit.  Others have them broken up into physically separate modules that need to be patched together (which is why they call them “modular” synthesizers).

 

The Voltage-Controlled Oscillator

To get any sound, first we need a source.  The most common source is called a Voltage-Controlled Oscillator, or VCO for short.  These modules produce waves that range from simple to complex, depending on the model.   Some have a single output and a switch that lets you choose between waveforms, and some have multiple jacks that let you access several different waves simultaneously.

VCOs also have a Control Voltage (CV) input.  The pitch of the wave produced by the VCO goes up and down in response to the CV it is supplied with (hence the name Voltage-Controlled Oscillator).  Control voltages can come from just about anywhere.  You can connect a keyboard with a CV out to your oscillator and play notes, or you can get your CV from a module that spits out random voltages and get random pitches.  That’s part of the fun of modular synthesizers; you aren’t limited to a certain way of doing things.  Just start patching stuff together and see what it sounds like.

It is (sometimes) important for your oscillators to be in tune, so they must change pitch by a known amount given a certain change in voltage.  To facilitate this, there are several different control voltage “standards” around.  Two of the most popular are Volts per Octave and Hertz per Volt.

  • Volts per Octave: Each volt equals 1 octave.  If 2V=A2, 3V=A3, 4V=A4, etc.  This scheme was popularized by Moog in the ’60s.  Today it is probably the most popular scheme, and is used in most modern modular synths.
  • Hertz per Volt: Each octave equals a doubling or halving of the voltage.  If 2V=A2, 4V=A3, 8V=A4, etc.  This scheme is mostly found in Yamaha and Korg synths.

Many VCOs have other parameters that are voltage-controllable, too, in addition to having knobs to control them.

Let’s take a look at an actual VCO.  This is the VCO-J for the Mattson Mini Modular.

Voltage Controlled OscillatorLet’s break down what each of the inputs, outputs and controls does.

Outputs:

  • Square Output: Outputs a square wave.
  • Square -1 Output: Outputs a square wave 1 octave lower than the square output.
  • Square -2 Output: Outputs a square wave 2 octaves lower than the square output.
  • Triangle Output: Outputs a triangle wave.
  • Saw Output: Outputs a sawtooth (ramp) wave.
  • Saw +1 Output: Outputs a sawtooth (ramp) wave 1 octave higher than the saw output.
  • Lin Out: Linear CV output.  Basically just mirrors the CV input so you can daisy-chain several oscillators together.

Inputs:

  • Mod In: Mixes this voltage with the main CV input, allowing you to use a second source to modulate the pitch of the waves.
  • 1V/O In: CV input that affects oscillator pitch.  Each volt raises/lowers the pitch by 1 octave.

Controls:

  • Mod Amt Knob: Controls the amount that the mod input affects the pitch of the oscillator.
  • Master Octave Switch: Raise or lower the pitch of all the outputs by 1 octave.
  • Tune Knob:  Raise or lower the pitch of the oscillator until it is outputting the desired note for a given control voltage.
  • Fine Tune Knob:  Like tune, but more subtle.  Once you get it close with the tune knob, walk it in with this one.
  • V/O Adjust Screw: Use a screwdriver to adjust the scaling of the oscillator so that it responds correctly to a 1V/O input.

Here are some additional controls, inputs and outputs you might find on other oscillators:

  • Hard/Strong Sync: This input allows you to keep an oscillator (slave) in sync with another one (master).  Connect the output of the master oscillator (some VCOs provide a pulse out for this purpose) into the sync input on the slave VCO and it will be forced to restart its cycle whenever the master does.  This works fairly well if the oscillators are pretty much in tune with each other, but can create different timbers if the frequencies are different and the slave wave gets restarted mid-cycle.
  • Soft/Weak Sync:  Similar to the hard sync input, soft sync can refer to a few different sync methods.  Some VCOs use this input to restart the wave cycle, but only if the wave is above a certain threshold.  Others use it to reverse the direction of the wave.
  • FM:  A voltage in this input increases or decreases the frequency of the oscillator (like the mod input above).
  • Duty Cycle:  Use this control or input to change the amount of time a square wave is high vs. low.  A normal square wave is high and low for equal amounts of time, but you can also adjust it so that it has a short high pulse followed by a long low gap, or it could be high for most of the cycle and only drop low for a short time.

Duty Cycle

  • Pulse Out: Outputs a short high pulse on every cycle of the oscillator.  Essentially a square wave with a low duty cycle.  This is output is handy for connecting to the sync input of another oscillator.
  • Wave Shape ControlWave Shape: Some oscillators give you continuously-variable control over the shape of the waveforms they produce.  As you turn the knob (or increase the control voltage), the wave morphs from triangle to saw to square to pulse (or however it’s set up).
  • Exp Out: Many VCOs have exponential amplifiers in them.  This allows a linear voltage (like 1V/octave) to control pitch exponentially (each octave is twice the frequency of the one below it).  Some oscillators provide you with access to their exponential amplifier’s output.

 

 

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