This is a controller for a preamp. The preamp has 6 inputs and 2 outputs that are switched by relays. The relays are controlled by the shift register. The preamp also uses a digital volume control. (Not shown on this layout.) The the active input and output is indicated by LEDs as is the current volume level. Three additional LEDs indicate input / output selector status and mute. The input, output, and volume are controlled by rotary encoders.