My apartment has a simple door intercom system which uses a hybrid two-wires line to carry both the receiving and transmitting audio signals for each floor unit. The wires are connected to each other in parallel. Wires other than Audio are the Power (Vcc), Ground, and a Video line which isn't relevant to the question.
My goal is to build a circuit which could feed a normal audio line (not a microphone) to the system, and extract another normal audio line out from it. The problem is that the idle voltage of the intercom audio wire seems to be an odd 14.6V which I can't interface. (I tested and it isn't the intercom power.)
I don't know how to get a normal audio signal out (which could be connected to a recorder or a sound card) and how to feed the line with audio from an mp3 player, or any regular AUX jack.
I need a simple, modern circuit to incorporate into my modern project, with these requirements in mind. Also, feedback/echo elimimation is preferable, but not a must.
From my understanding, a normal audio output has a typical maximum peak-to-peak voltage of 3 volts.
I googled quite a lot of keywords, but none seemed appropriate. Please provide an understandable explanation on how would I go with designing such circuit.