This module typically has a pair of oscillators behind one panel that is prewired where one oscillator modulates the other’s frequency (known as Frequency Modulation or FM synthesis); some also allow you to quickly switch them so that the first modulates the amplitude of the second, or some other variation. They may also have waveshapers built in. They are based on a popular module created by Buchla, which is a standard of the “West Coast” approach to synthesis.
