Featured on this page is a dual Voltage Controlled Amplifier. Simply put, this controls the loudness of a sound source. Loudness in its most basic form can be thought of like the volume of a stereo, but varying loudness is also a fundamental property of an instrument. For instance, plucking a violin string creates a very different sound than moving a bow across a violin string, even when the same note (frequency) is played.
Loudness is how our ears perceive the amplitude of a sound wave. In the synth world we generally think of waveforms as varying voltages over time, and how far a voltage moves from zero volts is its amplitude. An amplifier can be used to attenuate (decrease) or amplify (increase) a waveform’s amplitude. Being that this is a voltage controlled amplifier, we can use a control voltage source (sequencer, envelope, LFO) to adjust the amplitude or simply use the gain knob on the module. In musical (and Italian) terms this can create effects ranging from vibrato to pizzicato
What the VCA is actually doing is multiplying the two waves together. Some books will even refer to it as an analog multiplier.
(audio) x (control voltage) = (what we hear)
If you've got any algebra under your belt, you can think of audio and control voltage as variables. So if the control voltage is zero, we don't hear anything. And as the control voltage changes, so does what we hear. This can be seen and heard especially well if the control voltage changes much more slowly than the audio, as shown in the above video.
Amplitude variation is also used in radio communications, particularly AM (amplitude modulation) radio. In AM radio we use an audio signal (like the human voice) to vary the amplitude of a radio frequency signal (called a “carrier”). The radio frequency (RF) is modulated by audio at a transmitter site, which allows that audio to “take a ride” on the RF carrier signal. A receiver acts like a place for this amplitude modulated signal to “land”, where the audio is extracted from the AM signal through a process called demodulation. When we tune a station in on our car radio we are selecting the carrier frequency (e.g. 820kHz) and our radio demodulates it (mathematically dividing the AM signal by the carrier signal), extracting the information.