Four channels, each with input -> attenuverter control -> output. Each input is normaled to +5v if there is no jack in the input. Each output channel (after the attenuverting stage) is also sent to a summing mixer, and to an analogue logic or, a.k.a. the peaks part from peaks and troughs.
If an individual output chnanel is patched, it's removed from the summing mixer and the peaks output.