Read data from stereo jack

I recently connected a Logitech Harmony to Arduino using infrared signals. But since infrared is barely reliable I was thinking about another approach:

The Harmony has stereo jack plugs to connect external infrared emitters. What would I have to do if I wanted to connect a cable from the stereo jack plug to the Arduino directly? What has to be done in terms of power conversion and would it be possible to "read" the signal?

know which, if not both, signal lines out are carrying the desired signal, what is the signal level, maybe what the line impedance is, and what is the signal protocol, as a start.

depends upon the first answer,

depends upon the first and second answer, but most likely, yes, with proper signal conditioning and knowledge of the protocol.