First post - I've been trying to google an answer for this, but can't find it! Hopefully someone can help. :-)
My understanding of Dolby Digital EX is that it is a 6.1 channel technology that uses the matrix method to encode an extra back-centre channel into the SL and SR channels. There's also a digital flag that specifies when EX encoding is used which is used by the processor to know when to extract this extra channel. So far, so peachy.
I thought I'd understood it, and was expecting the centre-back channel to matrixed in as an out-of-phase signal in the two surround channels, in a similar way to the surround channel is matrixed into the L & R channels in vanilla Dolby Surround. But this does not seem to be the case: it seems be encoded as an in-phase monaural signal across both surround channel - i.e. like the centre channel in Dolby Pro-Logic.
if this is the case, why is the digital flag necessary? When Pro-Logic decoding superseded Dolby Surround, no changes to the encoding technology were required, as if sound, such as dialogue, was needed to come from "centre-front", then it'd just been encoded in phase into both the L and R channels anyway. Similarly in vanilla Dolby Digital 5.1, if a sound was required to be directly behind you, it would just be encoded in equal measure into both surround channels.
The only purpose I can think of for this flag, is that if the sound engineer wants to encode a sound into both surround channels, but *not* the surround-back channel, he can encode the signal *out* of phase, and then the EX decoder will "leave the signal" in SL and SR.
I'd be very interested to know if Dolby Digital IIx does anything differently based on the presence or absence of this flag. Of course, unlike EX, IIx is able to support 7.1 channels with separate signals out of back-left and back-right.