The VCDO was a wonderful module, capable of both great grungy digital gratings and silky smooth metallic drones. Often referred to mistakenly as a DCO the beauty of the 80-21 was the fact that as it was Voltage controlled it phased with its partners to produce a rich and full bodied sound. The best effects (in my mind that is) were those produced by stepping through the wave tables using a clock or sequenced gate.