Each STD module is a resistive branching network with RF filtering on each of the four channels. Any channel, A through D, may be either an input or an output. This permits combining stereo signals into mono inputs, splitting mono signals to multiple inputs, and even combining microphones with output shorting switches into a single amplifier input. The STD modules are available in 150 ohm, 600 ohm and 10 k ohm models. All inputs and outputs are balanced.