If you have a synth, odds are it has a CV, or “control voltage” input. In more modern synthesizers, like those from Dave Smith Instruments, digging into menus will allow you to assign control voltage inputs to just about anything: pitch, filter, LFO, etc.; in older models, there might just be one CV input that’s hard-wired to, say, oscillator pitch or filter position. Most people simply stick an expression pedal into the input and play away — which is what it’s designed to do! But, if you want to dive a bit deeper into how, and why, CV does what it does, this is the article for you.
However, let’s first look at this super easy application for utilizing the CV input on your synth via an expression pedal, courtesy of Grant Zubritsky, who besides playing and touring with bands like MS MR and Nick Murphy (f.k.a. Chet Faker), just launched his new course Synth Bass for Bass Players this week. Here’s Grant:
As you might have already guessed, control voltage is one of a few holdovers from the earliest days of synths. In modular synthesizers, which were some of the very first synths ever made, control voltage was the only way for different parts of the synth to talk to each other. This simple design solution — building a synth out of parts that all accept voltage on the same scale — meant that any part of the synth could be patched to any other part of the synth. Want to patch an audio-rate voltage (a.k.a.: pitch generated by an oscillator) into the rate control of an LFO for audio-rate modulation? Why not!
As modular synths emerged from their labs and into the musical landscape at large, three main control voltage standards emerged. And as was the case with his synths, Bob Moog’s standard of 0-5V proved to be exceedingly popular, in large part because of its simplicity and continued integration into his timeless line of synths. Using the principle that 1V = 1 octave, oscillators and other electronic parts could be easily tuned to simple voltages. (If you want to dig deeper into voltage standards, check out the differences between Buchla’s 0-15V paradigm, Korg’s 1Hz/octave, modules that use -5V/5V or the more contemporary 0-10V, and the aforementioned 0-5V systems!)
Alright, at this point, you are probably thinking: So what? Why all the fuss about these different systems? And what does this have to do with this keyboard sitting in front of me?
Well, the lineage of these systems lives on in modern analog (and digitally-controlled analog) gear. Know the expression inputs on pedals and the backs of keyboards? That’s often CV, controlled by some voltage-generating source like an expression pedal, which is often nothing more than a wah-wah-style foot control that generates a static voltage capable of being swept by your foot. And, so long as the CV systems match — that is, the expression input is listening for, say, 0-5V and the expression pedal is also outputting 0-5V — it’s easy enough to sweep through whichever parameter is assigned to the CV with your foot, leaving your hands free to play away.
Another question you might have is: Why are we using such an antiquated system for this? Don’t we have MIDI and other digital solutions? The short answer is yes, we have MIDI, and it will functionally work in this capacity. But, without diverting too far from the subject matter of this article, MIDI is divided into 128 possible steps, which means that the digital control information moves up or down 128 positions in a stair-step-like fashion. Voltage, however is completely continuous, offering a significantly higher resolution and nuance to controlling different parameters.
In the end, control voltage has a ton of applications beyond synthesizers and controlling parameters in pedals and other systems. For instance, analog mixing boards use control voltage (in combination with VCAs — variable-gain or voltage-controlled amplifiers) to determine where the fader is on each channel. House lights can use control voltage to change color and, in certain configurations, position and pattern. And the list as long as you can imagine, because the simplicity of a minimum and a maximum voltage means that anything you might want to control is simply a soldered connection away.