- Mar 26, 2002
Does anything come out of the rear surround speakers? Or do you have to use PLII with a two channel source?
That's why, in general, it's best to decode "std" DD/DTS discs with a mode specifically meant to "expand" 5.1 sources to 6.1/7.1.
Actually, many people believe that using the "generic" 6.1/7.1 matrix decoders (such as Rotel XS or whatever your processor calls theirs) will provide better surround performance that Dolby EX, even when playing a Dolby EX disc. There are differences in the way the signal is distributed to the surround channels. The official Dolby EX decoder can put too much signal in the center back -- just like Pro Logic often collapsed the front sound stage into the center speaker. When this happens, you lose the big, diffuse surround field and most of the surround signal squawks at you from a single (essentially mono) center back speaker.
Here's why there is confusion. Virtually all receivers have had a 6.1/7.1 matrix decoder for the center back channel for a couple of generations now. They couldn't call these receivers Dolby Surround EX, because Dolby had done an exclusive deal with THX so only THX receivers could offer "Surrround EX" until very recently. So the receiver manufacturers just used their own decoders to handle Surround EX discs and to extract a center back signal from 5.1 discs. It's not like everyone doesn't know how matrix decoders work.
As it turns out, the generic decoders may work better than Dolby's version. These decoders are generally automatic -- if you have a center back speaker, they will come up with a signal to send to it and they will generally send equal volume signals to ALL of your surround speakers to preserve a diffuse soundfield. Think of the generic decoders as being like Pro Logic II in one of the "wide" modes where less signal is rerouted to the center speaker and more to the left/right speakers. Same deal, only now it's at the back of the room.
As a practical matter, most receivers are using their generic decoders for Surround EX discs even if they have a specific Surround EX decoder -- because most of the Surround EX discs released until recently didn't have the proper flag embedded in the bitstream to even turn on the Surround EX decoder, if available.
It is a 5.1 channel recording with a center back signal matrix-encoded
I don't buy this, never have never will! I have over 200 Discs most of which are 5.1 and perhaps a hand full are so called EX/ES matrix-ed mixes. Out of all my 5.1 discs I have only a few that the Surround channels collapses to the EX speaker. And of these most of these are 4.1 mixes recorded on a 5.1 channel format(such as the Abyss DVD). The 4.1 has a mono surround channel the will (so called) collapse to the EX channel when played in EX. The theory of a EX/ES channel actually recorded and matrix-ed into the surrounds is in my opinion a farce.
Prologic does not extract the Center channel out of a stereo inputs. It mearly places the Mono infase information from the Stereo input and routes it to the Center channel at the same time almost completely removes it from the Front L/R channels. If the Mono info becomes stronger in the right channel and fades from the Left channel the Prologic moves the sound from the Center channel to the right channel as the percentage of infase mono info moves. There is no mysterious Center channel encoded on a stereo mix or a Dolby Surround mix for that matter. That is why there are no Prologic mixes out there, Only Dolby Surround which has a Stereo Front L/R and a Matrix-ed mono Surround.
The EX/ES matrix-ed decoding simply takes the Infase Mono information that is encoded into the Surround 5.1 channels and places it into the EX/ES channel. Most all 5.1 mixes have this Mono info in order to blend the two surrounds together for certain scenes of a movie. I do feel that a sound engineer while mixing a EX/ES movie could work with that mono surround information to better the EX channel effectiveness.
Prologic is hardware not encoded software and in my opinion so is matrix-ed EX/ES. So I feel the EX/ES labeled discs are a farce just as the THX mastering discs are.
Now Im not saying matrix EX/ES is not good, I love my system set up to "decode" (if you will) the EX/ES channels on discs. But I have not even in one case found a matrix EX/ES encoded disc to be any Superior EX separation to most of the 5.1 discs I have.
I don't buy this, never have never will! I have over 200 Discs most of which are 5.1 and perhaps a hand full are so called EX/ES matrix-ed mixes. Out of all my 5.1 discs I have only a few that the Surround channels collapses to the EX speaker.
That is probably because the discs are being "decoded" by the "generic" center back processor, not the Dolby Surround EX decoder. In most receivers, the only way to activate the official Dolby Surround EX decoder is an embedded flag and most of the official Surround EX discs don't have the flag...so there ya go!
There are various flavors of matrix decoding. For example, Pro Logic and Pro Logic II use different algorithms and produce different distributions of sound across the front three channels. Same with the surround channels in a 6.1 system. The "generic" decoders behave more like the wide mode of Pro Logic II and keep more of the signal in the side surrounds.
You are quite correct that there is nothing technically different between a stereo recording and a Dolby Surround matrix stereo recording. However, most film masters for Dolby Surround soundtracks are actually mixed in four channels (front, center, right, surround). The mix is then fed into a Dolby Surround encoder which "encodes" the center channel by reducing the levels -3dB and recording it equally in the left and right stereo channels. The surround channel is encloded by shifting its phase by 90 degrees. Pretty simple stuff, really.
The decoder then behaves just like you describe: taking equal amplitude left and right signals and sending it to the center channel and taking the out of phase information and sending it to the surround channel. There is a difference between material mixed with a center channel and material mixed with conventional stereo monitors. The pans and placement of sounds in the mix will probably be better suited to Pro Logic decoding than a mix done on a two-speaker monitor set-up.
Dolby Surround has now been around long enough that we are on the third generation matrix decoder: first Dolby Surround, then Dolby Pro Logic, and now Pro Logic II. The source material has remained the same for 30 years, but the decoding algorithms have changed. Just like there are several "flavors" of center back channel decoding.
Interestingly enough, there is absolutely no difference between a disc labeled EX and one not labelled EX. Either the original film master had a center back channel signal fed to the Dolby encoder or it didn't. If it did, it will be on the DVD. If not, the center back decoder will just grab common signals between the left and right surrounds and send it to the center back speaker, adjusting levels for all surround channels depending on the specifics of that particular flavor of decoder.
What if they want a sound to come out of all three of the back channels?
They would simply encode the signal (sound) 90 degrees
out of phase in the left and right surround channels.
This will send the signal in equal amounts to all three
rear speakers when EX decoded.
This also applies to having a signal come out of all
four speakers in standard 2-channel Dolby Pro-Logic
decoding. This is sometimes called the "interior channel".