How well does Dolby Pro Logic II work?

How does matrix upmixing (like Dolby Pro Logic II) work?

• How, exactly, does it map any 2 channel source to 6 channels? I assume it, of course, doesn't magically pull real channels out of thin air -- but it certainly adds to the spaciousness of fullness of many sounds. I've tried looking it up, but seriously, it seems like no sites out there really explain much about how it works (beyond mentioning it making 2 channels into 6... no sh!t, Sherlock...)

• Answer:

  Wikipedia should have a good explanation, but so do I. Step One: A) The left only audio goes to the left speaker, B) The right audio goes to the right speaker,(obvious so far I know) C) The audio that goes to the left AND right speakers is re-directed to the CENTER CHANNEL. So you get stereo sound(only left or only right) out of the sides, and then all of the mono in the center. That's the front 3 channels. From here things start to get complicated. The system seperates out sounds via "phase-shift". Kind of like switching speaker wires, to over-simplify. What this does is causes the "different" sounds of echos and such to be "thrown"(directed) to the rear channels. This gets you your 4th channel... in older surround, the back speakers were mono. Now a computer splits the sounds up, without using electrically guessed phase-shifts. There are logarithms used and the whole thing works better, but in a more complex way. But the basics from up above(left, center, right, echoes) still applies. And you can tell the reciever how much of each channel goes where, especially if you want the center coming out of the left and the right to some degree. But there's probably no theoretical limit to how many different channels you can "split" out of the main 2... although the seperate channels tend to make the whole thing sound worse as you get past the first 3 or 5(depending on who you ask).