Voltage Controlled Electro-Optical Mixer

A simple and inexpensive device is described which allows selecting and mixing audio channels by control voltage. The circuit takes advantage of the low self-noise of analog photocouplers. The slow response time of the phosphour is used to integrate the pulse width modulated LED drive. Channel crossfading is accomplished by adding a "dither" signal to the comparator string, producing apparent linear crossfades between audio channels.

Schematic diagram: