digital-audio-input selector
As the name indicates, the selector isintended to choose one of up toeight digital audio signal inputs,which it does with the aid of a multiplexer.The multiplexer, IC6 is controlledby preset up/down counter IC2. Thecounter is set with DIP switch S3(note that the MSB switch is notused in this application).The various inputs are selectedwith press-keys S1 and S2.
GatesIC1d and IC1e, in conjunction withnetworks R1-C1 and R3-C2, provideeffective debouncing of the keys.Resistor R5 and capacitor C3ensure that when the power isswitched on, the counter is set.If fewer than eight inputs areneeded, the number can be reducedto four by resetting jumper J1 so thatpin 9 of IC6 is linked to a fixed level.The non-used inputs of the multiplexer,pins 1, 2, 4, and 5, must bestrapped to earth.Which of the inputs is selected isindicated by one of four or eightLEDs that are controlled by 3-to-8decoder IC3 at the outputs pf IC2. Iffour inputs are used, D5–D8 must beomitted.Since the digital-audio- input circuitsare identical, only one is shown(in dashed lines at the top left-handside of the diagram). Each has anoptical input (IC5) and a coaxialinput (K1). It needs only one inverter(here IC4a); the others (IC4b–IC4e)are strapped to earth.The output of the selector alsohas an optical output (IC7) and acoaxial output (K2).The current drawn by the selectordepends primarily on the numberof optical modules (each of whichdraws 20–25 mA).