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Contemporary Keyboard

July 1978
Principles Of Voltage Control, Part I
Bob Moog

This column begins a series on the basics of voltage control, the technical principle underlying the design of most synthesizers. Voltage control can be discussed in either technical or musical language. I hope to help you understand how these two languages are related. I believe you should have this sort of understanding to make musical progress beyond merely imitating Ian McRockstar's latest licks on your synthesizer.
Music is the art of shaping and controlling sound. Like the light source of a movie projector, musical sound is merely the carrier of an artistic message, not the message itself. The artist determines the musical message by the way he or she shapes and articulates the sounds, just as the filmmaker shapes light with patterns on film.
Technically, voltage control is the use of signal generating and modifying circuits, each of whose operating characteristics (parameters) is directly related to the magnitude of one or more control voltages. Coupled with such voltage-controlled circuits are a multiplicity of sources of changing voltage that the musician applies to the voltage-controlled circuits in order to produce the desired sound changes. In principle, any circuit parameter can be voltage-controlled. The technical feasibility of voltage control has increased with the advance of the state of electronic technology. When I began working with synthesizer circuits fifteen years ago, high quality, reasonably priced silicon transistors were just becoming available. Today several manufacturers make complete voltage-controlled circuits as single chips. These new devices generally exceed the performance of the earlier circuits while greatly reducing size, power requirements, and cost.
Musically, voltage control allows the musician to impart a wide variety of sound changes to synthesizer tones rapidly, accurately, and conveniently. Voltage control affords the flexibility of combining, proportioning, and processing musical gestures before they reach the instrument's sound-producing path. In this sense, a musician deals more directly with the content of his musical message when he or she plays a synthesizer, whereas traditional acoustic instrument players (such as guitar players) deal directly with the carrier of the musical message. Thus, as we shall see, voltage control is a new musical resource with potential for implementing more complex musical gestures than are possible with most acoustic instruments. At the same time, voltage control demands more understanding and reasoning on the part of the musician to yield its musical promise.
The voltage-controlled amplifier is perhaps the most basic and easy to understand of electronic music circuits. I will use the VCA to define and discuss several concepts of voltage control, especially as they are implemented in synthesizers. I will use some simple block diagram representations and even a couple of equations. Don't be put off by the equations! I will be using them just to clarify the basic concepts.
First what is an amplifier? You could say either "a device for strengthening a signal" or "a device that introduces hum, noise, and distortion," depending on how upbeat you feel. Let's forget about hum, noise, and distortion for a while. I've discussed them at length in previous columns. Let's just say that an amplifier is a circuit with an input and an output, in which the output voltage changes in exact proportion to the input. Here is a block representation of an ordinary amplifier. ej is the input voltage, e0 is the output

An engineer would write the following equation describing the operation of this ideal amplifier: Eo=Gej

The equation simply says that, to find out what the output voltage of the amplifier is, you multiply the input voltage by the amplifier's gain. ej, can be any voltage within the range of the circuit, but G is considered to be a fixed number that depends only on the way the circuit is built.
Now suppose that you want to change the gain of the amplifier. For instance, suppose this were a power amplifier driving a speaker, and you wanted to adjust the loudness. You would use a volume control to cut down the level of the input signal. The block diagram would then look like this:

The output of the volume control (e2) goes directly to the amplifier input. The volume control has a gain too, but it is less than one. That is, the volume control can only cut down the signal amplitude. That is what the term "attenuator" means. The equation for the composite gain of the amplifier and attenuator is eo = vG ej

That is, to get the composite gain of the amplifier with the volume control, you multiply the gain of the volume control, v (less than one), by the gain of the amplifier, C (usually greater than one). In principle, you can string as many signal processors together as you like. The composite gain of the string is calculated by multiplying the gains of the individual signals processors together.
A voltage-controlled amplifier is an amplifier whose gain is determined not by the way the circuit is built, nor by the setting of an attenuator, but by the magnitude of a control voltage that is applied to it. Here is a block diagram of a voltage-controlled amplifier:

ec is called the control voltage. The mathematical shorthand tells us that the gain C is not a fixed number, but somehow depends on the magnitude of voltage ec. Suppose, for instance, that the gain were directly proportional to ec in the following manner: when ec = 0, then the gain is 0; when ec is 2 volts, the gain is 1; when ec = 4 volts, the gain is 2, and so on. The equation that relates C to ec is then G=1/2ec
If we draw a graph that shows what the gain is for any value of ec, here is what we have:
The graph shows the control characteristic of this voltage-controlled amplifier. The straight line on the graph tells us what the amplifier's gain is for any control voltage up to +5 volts. We say that this amplifier has a linear control characteristic, because the graphic representation is a straight line.
The equation that relates ej (the signal input), ec (the control input) and eo (the output) is simply Eo= (ej ec)/2
Thus, to find out what the output voltage of a VCA with a linear control characteristic is, you simply multiply the signal input voltage by the control voltage. Such a VCA is a form of analog multiplier, a device that computes the arithmetic product of two voltages. All of this is a bit heavy on technical terms and equations. Next month's column will show how this relates to musical uses of the VCA. Then we'll go on to other voltage-controlled circuits.