How on earth did we electronically vary the amplitude of an audio signal before Digital Signal Processing (DSP)? In the digital domain varying the amplitude of an audio signal is simple. You simply multiply the sample value by a coefficient. For example, “0.5” , “0.01”, “0. 007”, etc… and the job is done. There are some issues with “zipper noise”, but basically, that’s it! However analogue is a different story, because we need to change a value in an analogue circuit. Often, the circuit needed to do this is non-linear and many creative approaches have been invented to do this. They are generally called “Voltage Controlled Amplifiers/Attenuators” or VCAs for short.