FM stereo in the analog world is two signals. L+R (aka mono) and L-R When L-R is inverted and processed against the L+R you will get two signals, L is one, R is the other. AM stereo worked the same way, but I don't know if any stations still broadcast that way. It was popular with very few stations in the mid 80s and then seemed to die off. I personally have been in the booth of radio stations where the turntable is a stereo one, and they only have one channel connected to their mono mixer. They couldn't comprehend that stereo isn't just two channels of the same signal. I blame the move toward narrow stereo mixes for that. If they had grown up listening to Queen or early Beatles tracks, they'd understand what wide stereo is all about. Dave