If you don’t know what control voltage (CV) is, then this is just the article for you. I promise that it won’t get too heavy on the science, engineering, or math. Control voltage is used with synthesizers, keyboards, Eurorack modules, and effects pedals. Those who already use CV will probably find this article full of oversimplifications, but it will provide a basic understanding of what CV is and some of the many ways you can use it.
What Does That Knob Actually Do?
The very first thing to understand is that, if you’ve ever turned a knob on a device (musical or not), you’ve already used control voltages to get a desired result. A knob (also called an “attenuator,” a “variable resistor,” or a “potentiometer,” which is commonly shortened to “pot”) is rarely the actual thing creating the desired result. Instead, turning the knob adjusts the resistance, which passes a certain amount of voltage to the component you’re actually changing. Usually, that voltage sweeps from 0 volts to whatever maximum voltage the component is expecting (such as from 0 volts to +5 volts, 0 volts to +10 volts, or sometimes it might sweep from negative to positive, such as from -5 volts to +5 volts).
If you looked inside the device (just in your mind — don’t actually do it) and connected a wire with +5 volts going through it right to the output of the knob, thus bypassing the knob (potentiometer), it would make your device do the exact same thing as if you had turned up the knob all the way to let +5 volts through it. When the knob is all the way up, it doesn’t impede* the flow of the voltage.
*This is the root of the word “impedance,” which you may have heard before.
What makes control voltages so interesting and powerful is that they allow you to experiment with lots of CV sources (the things that send various voltages) going to lots of CV destinations (the things that change based on how much voltage they’re currently getting). Here are some basic examples of CV uses:
Keyboard Note Creating a Specific Pitch
For this example, let’s presume that you have an analog oscillator (or VCO, which stands for “voltage-controlled oscillator”) whose pitch is being determined by the voltage coming from the keyboard. On a 61-note keyboard (five octaves, C to C), the lowest note sends 0 volts and the highest note sends +5 volts. Every octave jump is one more volt than the previous octave. So, the lowest C is 0 volts, the next C is +1 volt, the third C is +2 volts, all the way up to the highest C which sends +5 volts. This system is called “one volt per octave.” All of the other keys in between have specific voltages from 0 volts to +5 volts.
Conversely, if you patch the keyboard’s control voltage into an inverter (a module that flips a control source’s voltage upside down) and then into the VCO, you could make a keyboard that plays high notes on the low keys and low notes on the high keys. It’s a great way to break out of your standard playing techniques.
Keyboard Note Creating a Specific Filter Cutoff Frequency
If you take the voltage from that keyboard and send it to the cutoff of an analog filter (VCF, or “voltage-controlled filter”), then the filter will open up more as you play higher notes on the keyboard. This tends to make the filter sound more natural up and down the keyboard.
There are literally countless examples of things you can do with control voltages. But the absolute-best way to learn is to simply connect the “CV Out” jack of a source to a “CV In” destination and listen to what it does. Put an attenuator (knob) between the two and see what happens when you send more or less of the voltage source to the destination.