Discrete + Matrix 6.1 Surround
If five channels (plus one LFE channel) are good, then sixor sevenmust be better, right? That's apparently what the geniuses at Dolby Labs thought when they devised the Dolby Digital EX system. Actually, it started in the theaters (again), with a new 6.1-channel system first used in 1999. Since they did it in the theaters, they had to do it on DVD, and thus the 6.1-channel Dolby Digital EX was born. The only problem was that they didn't want this new format to obsolete the millions of Dolby Digital 5.1 DVDs out there; they needed the new format to be completely compatible with Dolby Digital 5.1 decoders. How, then, to add that extra rear channel that they were using in the theaters?
Not surprisingly, the Dolbyites turned to the tried and true, and devised a matrixing scheme to hold that sixth (and later seventh) channel of information. The new rear channel was matrixed into the left and right surround channels, just like in the good old days of Dolby Pro Logic, mixed in equal amounts so it could be extracted by an EX decoder. When they later decided to up the ante with separate right and left rears for a 7.1 system, the new seventh channel was mixed in out-of-phase. You get the idea.