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Dolby Digital EX - why does it need a digital flag?

post #1 of 4
Thread Starter 
Hi,

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 When Pro-Logic decoding superseded Dolby Surround, no changes to the encoding technology were required, as if sound, such as dialogue, was needed to come from "centre-front", then it'd just been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.

Many thanks!
Cazalinghau
post #2 of 4
Thread Starter 
Having thought about this, I guess another use for it is for older films which have a monaural or nearly monaural surround track (I read in an older thread that Out of Africa is like this). In this case, if you have EX decoding "always on" (i.e. not enabled via a flag) then all the surround signal of such films would only come out of the back speaker, giving you a poor surround experience. I guess the user could always manually turn EX-decoding off, but having the flag allows you to keep EX on continually, although you'd use its successor technology Pro Logic IIx instead these days.

So I wouldn't be surprised if Pro Logic IIx spreads a monaural surround signal equally across all 4 surround channels when the flag is not present, but to only the two surround-back speakers when the flag is on. Can anyone confirm this?
post #3 of 4
Quote:
Originally Posted by Cazalinghau View Post

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.
Makes sense that it would be encoded like a centre channel since it is a centre surround channel. And just like Dolby Surround soundtracks were compatible with 2-speaker playback (the centre channel would phantom image between the 2 main speakers), EX soundtracks are compatible with 5.1-speaker layouts (the surround-back channel will phantom image between the 2 surround speakers).
Quote:
Originally Posted by Cazalinghau View Post

if this is the case, why is the digital flag necessary?
It's not necessary, just there for convenience (to have your receiver automatically switch to EX decoding mode). In the old Pro Logic days, you had to manually switch your receiver to Pro Logic decoding. The EX flag was an attempt to automate that task. However, content creators were inconsistent about using the flag, so it never became a big deal either way.
Quote:
Originally Posted by Cazalinghau View Post

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.
No such thing as "Dolby Digital IIx". Perhaps you mean Dolby Pro-Logic IIx? If so, then it will behave like EX decoding when certain sounds are intended to be heard directly behind you (i.e., it will send the same signal to both rear speakers). But it has the advantage of stereo rears, so if a sound is intended to move around the surround field, PLIIx can send independent signals to the rear speakers for smoother pans. EX can't do that.
post #4 of 4
Thread Starter 
Hey sdurani,

Thanks for the response. Yes, I did mean Pro Logic IIx! My Yamaha receiver has an 'auto' setting that I think detect the flag and will switch to EX (I think) if the flag is detected, but it seems to me better to set it to always decode the surrounds using Pro Logic IIx as a don't want mono sound from the back.
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