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Contemporary Keyboard

August 1978
Principles Of Voltage Control, Part II
Bob Moog

In last month's column I explained that the gain of a voltage-controlled amplifier (VCA) depends on the magnitude of one or more control voltages. By selecting and shaping the control voltages, the musician determines the envelope or contour of whatever signal happens to be passing through the amplifier's signal path. A VCA with a linear control characteristic (mode) is one in which the graph of the amplifier's gain versus the control voltage magnitude is a straight line. A linear mode VCA is actually an analog multiplier: the voltage at the output is the arithmetic product of the signal and control input voltages.
To illustrate the operation of a linear mode VCA, let's say we feed a steady audio signal into the VCA's signal input, and a slow triangular voltage contour into the VCA's control input. Here are graphs of the signal input, control input, and output.
The VCA Output 25 the audio signal input, shaped (multiplied) by the control signal. Note that the control signal itself does not appear at the output, except as the envelope of the shaped output. The rising portion of the envelope is called the attack and the falling portion is called the decay. If you were to feed the VCA output to an amplifier-speaker, you would hear a short puff of sound.
Many modular VCAs have more than one signal input and/or control input. A typical modular linear mode VCA, whose block diagram is shown below, has two signal inputs, two control inputs, and one source of fixed control voltage which is adjustable by panel attenuator P2.

The "signal mixer" combines signal inputs #1 and #2, just as any audio mixer would. The control mixer combines the outputs of attenuators P1 and P2 and the unattenuated control input #2 to produce a control voltage sum that determines the VCA gain. These mixers are simple circuits that are a small portion of the total VCA.
The musical effect of shaping the amplitude of a sound with a modular VCA depends almost entirely on the musician's choice of control signals and the settings of P1 and P2. Let's go through some basic amplitude-shaping functions to see how they are implemented with one or more VCAs.

Pedal Volume Control
The simplest pedal volume control is simply an audio attenuator built into the pedal mechanism, which does not involve a VCA at all. To achieve pedal control of a VCA gain, you must connect a battery to the input of a pedal attenuator and use the output of the pedal as the VCA control signal. Pedal controllers for synthesizers often contain a built-in battery or other voltage source. The block diagram of this setup is shown below. There are several advantages to shaping volume by pedal control of VCA gain, rather than by direct pedal attenuation of the audio. First, long audio cables down to the pedal and back up to your console are avoided. Second, noise in the pedal attenuator mechanism can be essentially eliminated by capacitor C, which smooths out possible "glitches" as the pedal is manipulated. (You certainly can't have a capacitor like this in a pedal that handles audio directly, since the capacitor would completely suppress the audio's higher frequencies.) And third, you can tailor the response of the pedal by adjusting P1 and P2 on the VCA. P2 determines the minimum gain of the VCA when the pedal controller is all the way off, while P1 determines the amount that the VCA's gain increases when the pedal is full on.

Envelope Shaping
An envelope generator is a circuit that produces a voltage that rises and then falls according to a predetermined pattern, each time it receives an initiating trigger. Also called a contour or transient generator, the envelope generator does not process audio, but serves only as a control signal source. I'll discuss envelope generators in detail in a future column.
Generally, the envelope generator output is applied directly to an unattenuated VCA control input, and Fixed Control Voltage attenuator P2 is turned off. A steady audio signal is applied to one of the signal inputs. With this setup, the envelope of the controlled audio output is a replica of the control signal produced by the envelope generator. For special applications, some fixed control voltage may be added by turning up P2. This opens up the VCA even in the absence of an envelope control voltage, and enables the envelope generator to partially articulate (accent) the tone, instead of fully articulating it.

Amplitude Modulation
If a VCA control signal repeats regularly, the controlled audio output will be amplitude-modulated. Suppose, for instance, that the control signal is the sine output of a low frequency oscillator, as shown below:
Now, most linear VCAs will not respond to negative control voltage sums, so the VCA gain looks like this:
and the controlled audio out then looks like this:
This output consists of short bursts of sound alternating with silence. The envelope of the sound is a distorted form of the original control signal. To produce a smoother amplitude modulation, turn up P2 until the output of the control signal mixer (and therefore the VCA gain) looks like this: and the controlled audio output looks like this:
Thus, the Fixed Control Voltage attenuator P2 is a handle on the amount and character of amplitude modulation.