Here's a description of the "non-discrete" rear center channel. Matrixed? In a
manner. Processed as discrete channel. Yup.

"The new sound format provides filmmakers with greater flexibility. With
conventional surround technology, the sound field in the back of the theatre
can be perceived as coming from a general area to the left or right of the
auditorium. With Dolby Digital Surround EX, back surround information is
reproduced by the theatre's rear speakers, while left and right surround
information is reproduced by the side speakers. The addition of a new surround
channel with this use of speakers allows a sound designer to create true
"fly-over" and "fly-around" effects that are smoother and much more accurately
placed, either directly behind or directly beside the audience."

5.1 systems are long outdated. Haven't had one since laser discs.

RB