If you are into electronic music or music production you are most likely using VCAs. With or without knowing. They can be found in for instance analog synthesizers, effect devices, analog mixing desks, guitar pedals. If the abbreviation isn’t yet familiar to you, it will be by the end of this post as I will have used it 22 times by then.
In its simplest form, a VCA is nothing more than an amplifier with an input and an output for signals to be amplified, and a Control Voltage (CV) input that allows you to control the amount of amplification. With a high CV you get lots of amplification, with a low cv you get little amplification. You could almost say it’s a volume knob that gets controlled by voltage rather than by hand. Another way to look at VCAs is as a multiplier:
output = CV x input
With the exception that in regular VCAs this doesn’t work for negative CVs. If that’s what you want to do, you need a balanced modulator (a.k.a. ringmodulator).
In the case of VCAs in analog synthesizers, there are a three peculiarities worth knowing:
‘A’ for amplifier
While the A stands for amplifier, most synthesizer VCAs are actually voltage controlled attenuators. This means that when they are fed with the lowest possible CV, they attenuate the input signal to (almost) 0 and put out (nearly) nothing. When the highest possible CV is fed in, they do not attenuate at all. When they don’t, most VCAs pass the input signal to the output at ‘unity gain’: the level of the output remains identical to the signal input.
Lineair and exponential
I’ve been writing about the lowest and highest possible CV so far, but what happens to the attenuation in between these values? Well, that depends… At least you can expect the amount of attenuation to follow the CV smoothly from one value to the next, because of the nature of analog electronics (continuous, so no discrete steps, like in the digital world). In one category of VCAs, the attenuation follows the CV linearly. This means that when the CV doubles the ‘amplification’ also doubles (or actually the attenuation halves). This is called a ‘linear response’ VCA, and when amplification is drawn in a graph as a function of CV you will see a straight line.
The other category is called ‘exponential response’ VCAs. These respond exponentially to incoming CVs. You can think of it this way: when you imagine a CV evenly and gradually rising from 0 to 5V, the exponential response makes the VCA’s output rise faster and faster as the CV gets closer to 5V. In a graph it looks like a curve (see figure above). Physically, this exponential response bears important similarities to the way we perceive loudness of sounds, meaning that it is very useful when synthesizing the loudness contours of a sound.
A VCA is not just a VCA
While amplifying or attenuating signals seems like a trivial task, an analog VCA often does a little more. Design choices, component selection and component limitations all impose a subtle character on the sound fed through the VCA. This character often becomes more prominent with higher signal levels, when the VCA is pushed into saturation.
Now you know the basics of VCAs. If you feel like experimenting with VCAs a bit, don’t forget they can do so much more than merely control the loudness of a sound (hint: how about using them to control the depth of filter envelopes with your keyboard velocity, or to gradually fade in vibrato with an envelope after a note is held).