The Behringer 921A Oscillator Driver is a Eurorack-compatible oscillator-control module based on the authentic circuit designs from the Moog Modular System 55 and 35. The discrete circuitry of the Moog Modular System has a very distinctive sound and was used by artists like Keith Emerson, Stevie Wonder, Tangerine Dream, and many other pop, rock, and jazz musicians. The 921A Oscillator Driver features a frequency knob that can adjust between either octave or semitone frequencies and includes a rocker switch to select between the two types. The width of the pulse/square wave (PWM) is adjusted via a control knob that goes from 10 up to 90%. Additionally, there are two 3.5mm frequency inputs and two 3.5mm width inputs that allow you to patch in modulation or other control voltage signals. Two high-current outputs for frequency and pulse width are able to drive up to twelve 921B modules for vast flexibility and nuanced waveforms. A ribbon power cable and screws are included.