Originally Posted by Ratman
I'll ask again, why is more than 15ms required for ~5' distance?
Because they want the sound from the surrounds to arrive at least 15ms after the sound from the fronts, irrespective of the actual distance that the surround speakers are from you.
But that mandatory delay is based on old matrix decoding, where channel separation wasn't very good and dialogue used to leak into the surrounds. By delaying the surrounds, the dialogue from the front speakers arrived at your ears first, and your brain would ignore the same sound arriving later from the surrounds. This is called the Haas effect or presedence effect. The delay had to be between 10ms to 40ms for the effect to work.
With modern surround processing and discrete multi-channel, the delay should no longer be necessary. (See sivadselim's earlier post re matrix vs 5.1.) Shame that Theta continues to impose this delay. My old Lex allows the option
to delay the surrounds 1ms-15ms in PLII/PLIIx mode. But you can always turn it off (set it to 0ms).