Pro-Logic does not necessarily need something to be specifically encoded to work, but it does need at least a stereo signal to work. Most movies, even those on TV, were originally encoded in Dolby Surround of some sort and that encoding will carry over. The question, though, is whether your cable service is giving you stereo. Dolby Pro-Logic is 4 channels -- R, L, C, S -- encoded into two channels, so you need both channels to get proper decoding. Even if the material wasn't specifically encoded for Pro-Logic, decoding any stereo signal will work, you just may get some odd steering on occasion. But if you feed the decoder a mono signal, you aren't going to get any rear information, and probably won't have anything in the right and left speakers, as everything will collapse to the center. And remember that in Pro-Logic, the rears are mono and are not full-range, so even when working exactly as intended, you aren't going to get much more than ambiance.
