I am trying to help my dad set up his Sennheiser RS 175 wireless headphones . He misplaced the adapter that goes with the RCA cable. But he had the Optical cable, so I used that. He is not getting a signal yet, so I am thinking we need to change the setting for audio out to PCM. Anyone know how to do that? I had DirecTV years ago, but don't know where to tell him to look. Help?