Even Technics in the early days used I2C controls for E.Q, volume and front panel switches etc. I remember their control panel had a 16 way ribbon connector to their amp unit. Any analogue inputs were AtoD'd close to the panel and monitor outputs DAC'd with Sony chips The normal technique now is to keep audio design digital, process and filter it digitally, amplify it digitally class D and it never gets to be analogue except at the speakers, on a monitoring, headphone or output jack. Car radios have the added CANbus layer. Before I replaced my RC300, I used vcds to run the automatic radio system check. This went around each speaker pinging it with noise so you could find damaged speakers and it told you if the leads were broken. All done through V.Ws proprietary CANbus controller embedded inside. Since that also does the turn on/turn off and radio switch only function, that's why many have issues installing aftermarkets when they don't emulate all the V.W radio CAN commands. I've always got a Radio 56 and Gateway error with my clone and just put up with it. One day I might try fooling the system and try transplanting the RCD300 CAN controller. I haven't found a way to stop the error yet.