Voltage Controlled Amplifier (VCA) Explained

What Is a VCA?

A VCA is a processor that can alter the amplitude of a signal proportional to the control voltage applied to its amplitude modulation control input. Simply, it is just an amplifier whose output you can control with a control signal.

Inputs Explained

A voltage-controlled amplifier (VCA) has two kinds of inputs:  

• Carrier (Signal Input): This is the input where a bipolar signal comes in. Subsequently, it is the main signal, and all the alterations are made to this signal.

• Modulator (Control Input): A Unipolar positive signal usually comes in through this input, which alters the bipolar signal coming from the signal input.    

How Does It Work?

A VCA amplifier usually accepts a bipolar signal at its carrier or signal input and a uni-polar positive signal at its modulator or control input. Furthermore, the output signal is the instantaneous product of both signals. It is the multiplication of both the amplitudes at each instant in time. A lot of VCA's have gain knobs available for setting to different values. A carrier signal can only pass through a VCA if either a positive bias is provided or a positive signal comes in through the control input. So when it is biased to 0, and no modulator signal comes in, no signal will be present at the output. For this reason, a uni-polar positive signal is usually provided to the control input. In addition, if the sound needs to collapse frequently for specific effects, a bipolar signal is also sent.  

Uses of VCA's

• Tremolo: Tremolo is a smooth and slow repeating change in loudness. VCA is very good for achieving this effect.
  For example, while the VCA is biased to a positive value, put an audio signal through the signal input. Pass a low-frequency bipolar signal like a sine wave through the control input. The resulting signal will have slow repeating changes in loudness, which will be your tremolo effect.  

• Envelope Shaping: When we hit a note on a guitar, there is an attack and then a slow decay. The same effect can be achieved on electronic instruments like synthesizers to make them sound more accurate and give emotion. The name of this process is envelope shaping.   To do this, we need an envelope generator and a VCA. The envelope generator creates an envelope that changes over time. It generally has 4 parameters: Attack, Decay, Sustain, and Decay. Basically, an audio signal is passed through the signal input of the VCA. At the same time, the Control input gets the signal from the envelope generator. The resulting sound has a volume envelope.

• Amplitude Modulation: If the frequency of the modulator is kept low, then it only produces periodic volume changes to the audio signal. This is called the tremolo effect. If the frequency of the modulator goes into the audible range, a strange thing happens. The modulation becomes so fast that the original audio signal's shape changes, resulting in a different sound with a different pitch. VCA is pretty good for achieving amplitude modulation.

• Volume Control: VCA can control the volume of anything that produces control voltage. You just need to pass the audio signal through the signal input and connect the control input to your foot pedal, mod wheel, etc. They can then control the volume of the resulting sound.