I think you mean Dolby Pro Logic and Pro Logic II. Both Pro Logic and Pro Logic II are designed to use a stereo signal (left and right channels only) to produce some form of surround sound (adding a center and surround speaker channels).

Pro Logic is the old surround processing that was based on a stereo analog signal into which a center channel and a surround channel (mono, with both left and right surrounds getting the same signal) were "matrixed" so that a Pro Logic decoder could extract them. When given a true stereo signal (i.e. a signal that was not originally intended for Pro Logic decoding), the Pro Logic decoders often had a hard time getting much out of the signal -- it would sometimes collapse entirely to the center channel, for example.

Pro Logic II is a new form of decoding intended to work with a stereo signal. It can still handle Pro Logic soundtracks, but it can also work well with purely stereo soundtracks. In many cases, Pro Logic II decoders include two modes -- one for movies, and one for music. Also, unlike the original Pro Logic, PLII produces a 5.1 channel output (discrete left and right surrounds and a subwoofer channel) and a full-range signal to all five channels (Pro Logic's surround signals did not cover the full 20Hz-20,000Hz frequency range, and I don't remember if the center did). Basically, Pro Logic II gets closer to the sound of Dolby Digital soundtracks than Pro Logic did. You might be interested in this link to Dolby's site .

Dolby Digital, by the way, is very different from these two. It is a digital format, with five discrete full-range channels (left, right, center, left surround, and right surround) and a ".1" low frequency effects ("LFE") channel. There is an extension of Dolby Digital called Dolby Digital EX that adds a surround back channel (one or two speakers at the back of the room) that is matrixed into the surrounds, much the way the Pro Logic matrixed in the center and surround channels.

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