e-dogg: Dolby 5.1EX is an encoding method for including what represents a rear center channel in the otherwise discrete rear surround channels. Thus a 5.1 signal actually carries the information that enables a 6.1 (or 7.1 since DD 7.1 at this point sends the same information to both rear center speakers).

JasonJ: It is my understanding (which could easily be wrong) that when the 5.1 signal is converted to 6.1, the source material that is extracted for the rear center(s) is then subtracted from the rear surrounds. Consequently this represents a 100% loss of that signal. Furthermore the volume level of the rear surrounds is reduced by 3 DB. Consequently, the option of playing back an DD 5.1EX in 6.1 mode without a rear center channel speaker(s) can result in a significant amount of signal loss depending upon what was being sent to that channel and will result in a overall reduction in rear presence due to the 3db volume drop.

All: I hope some of the discussion here points to why a Outlaw directed FAQ is highly desirable. They more than any of us should have the correct answers.