Originally Posted by KMO
The problem with using EX all the time is that it will collapse the surround field.
Only if 100% of the information in the L/R surround channels is correlated mono.
What if the surround at a point is actually mono?
Then those specific sounds will be heard from behind you. Any surround informaton that isn't mono will remain at your sides. This is consistent with how those sounds would have phantom imaged when using only 2 surround speakers.
It will collapse to the rear speakers, and you'll get nothing from the sides; quite a different effect from the imaging if it were coming from both sides.
5.1-speaker set-ups tend to have the surrounds slightly rearward of the listening area, causing dual-mono content to image behind the listener, which is where EX decoding places them.
The same thing happens up front: play a mono CD using 2 speakers and everything will phantom image in the centre of the soundstage, which is where a matrix decoder would place those same sounds.
In an EX encoded film, phase differences between left and right surround are used to ensure that those channels aren't collapsed into the rear unwantedly.
No, the small phase shift is to allow the mixing engineer the choice of placing mono content behind the listener or at the sides. Correlated mono sounds go to the rear speakers, decorrelated mono sounds remain at the sides.
Since stereo information isn't extracted by the EX decoder anyway (whether on an EX encoded soundtrack or not), it always stays at the sides and doesn't collapse to the rear. So it is perfectly fine to use EX decoding on regular 5.1 material.