This is a response to MeBeJedi's post:
The PCM soundtrack on the LD's is not Pro-Logic II encoded! It can be Pro-Logic II decoded with the right equipment. Pro-Logic II was designed to create "5.1" sound from stereo sources, and didn't even exist until a few years ago, well after the last LD was ever pressed.
This is almost correct, but I want to clarify a couple of things.
The PCM soundtracks on the Laserdiscs are encoded with matrixed Dolby Surround information. Dolby Surround is a 3 channel (Left, Right, and a mono Surround track), 1980's version of surround sound. The "surround" track is encoded 90 degrees out of phase with the L and R signals with the intention that the decoder will place this info the in the rear speakers after a few ms of delay.
Dolby Pro-Logic was a (Decode-Only) way to better reproduce the surround environment by 1)extracting center channel info from the L and R signals to be put in a center speaker, and 2) "Logic Steering" a sound source (eg a TIE Fighter) into the corresponding L or R surround speakers for a slightly more realistic effect when going from front-to-back or back-to-front.
Dolby Pro-Logic II is just the current version of Pro-Logic, which takes those same matrix-encoded Dolby Surround stereo tracks, does the math (center channel extraction and more-advanced Logic Steering) and outputs 5.1 discreet channels of audio (L,C,R,LS,RS,LFE). As far as I know, PLII was not intended for Stereo sources, but rather matrix-encoded Dolby Surround sources.
I hope this clarifies rather than confuses!
-Joel