Surround sound decoding is getting out of hand. I've been in the computer industry for a decade so I understand that there are exponential advances in technology, but even with that in perspective, we have to question the road this industry is on. Confusion abounds. Arguments arise on forums over which is the best technology.
The "latest and greatest" is Dolby's Pro Logic IIx, first demonstrated for us at CEDIA in September, 2003, and which is now available on many products. But before we get to that, let's take a stroll down memory lane and talk about how surround sound has changed over the years.
Matrix Decoding (a quick refresher)
Matrix Decoder or Matrix Surround Decoder are the most generic terms available for the principle behind all of the surround sound technologies based on a two-channel carrier.
In such systems, all sounds are contained within two channels, customarily referred to as "Left Total" and "Right Total". Using a variety of technologies, the system delivers (at least) four channels at playback: Left, Center, Right, and "Surround".
For complete details on how this actually works, please see our Article on Dolby Pro Logic / Pro Logic II. Basically, sounds that are common to both original channels and in phase are delivered to the center channel, and sounds that are common to both original channels but out of phase are delivered to the surround channel.
While Dolby's Cinema Matrix system, Dolby Stereo, and the consumer equivalent, Dolby Pro Logic, are not the only Matrix Surround Sound systems in the world, but they are arguably the best as evidenced by their commercial success. Nothing else comes close. For that reason alone, I will regard Pro Logic as the baseline.
Discrete Decoding (a quick refresher)
Whereas any Matrix decoder derives multiple channels from two, when we talk about a "Discrete" Surround Sound System, each and every channel is unique at the source. There is a carrier channel for every playback channel. When we say "5.1", we mean 6 channels: 5 Main Channels (Left, Center, Right, Left Surround, Right Surround) and 1 LFE (Low Frequency Effects) channel. We could also have "4.0" (where we have Left, Center, Right, and Surround), or we could have "6.1", or any conceivable combination thereof, technology permitting. The point is, whenever we say "Discrete", or use the X.X designation, we are talking about separate carrier channels for every playback channel.
A Summary of Available Formats
With those clarifications in mind, lets take a look at what is on the table today, in the order they came to market
Should You Specifically Shop for a Receiver that has Pro Logic IIx?
The answer depends.
First, if your system consists of five Main speakers, in terms of Matrix decoders you need go no further than Dolby Pro Logic II. Everything else is a stab at making use of center surround/rear speakers. If you are staying with 5.1 and don't have Pro Logic II, but like to listen to music, you may want to consider trading up to hardware that has Pro Logic II or Pro Logic IIx. It would be the difference between listening to music in two-channel versus surround, while also benefiting all Dolby Stereo movie material you may have (VHS, Laserdisc, older movies on DVD, and even most TV material).
If you are into THX and have an Ultra2 SSP or Receiver, I don't think there would be much point to rushing out and replacing your hardware just to get Pro Logic IIx. Virtually all Ultra2 units include Pro Logic II for music, while the Ultra2 modes in principal have much the same goal as Pro Logic IIx (when applied to a 5.1 discrete soundtrack). One could argue that Dolby's take on it works/sounds better or vice versa, but in practice, having heard both, I don't think we are apt to appreciate any such difference on a daily basis.
While inevitably all SSPs and Receivers will in time have Pro Logic IIx, the only consumers who might replace their hardware to get it would be those who have an obsession with using all their speakers all the time, or those who have to have the latest and greatest at any cost.
Which Matrix Decoder Should You Use?
I don't want to sound rash here, but my answer is: Dolby's (whatever flavor you have). If you want the nitty-gritty, please revisit the extensive listening tests Stacey Spears did in comparing Dolby Pro Logic II to DTS Neo:6, and even the less mainstream Logic7, and Circle Surround. I concur with his summary that Dolby works best in most circumstances. 'Nuff said.
The confusion, this multiple choice game of selecting a surround sound scheme when trying to enjoying a movie or CD, is our own fault. We as consumers bought into EX just because it was so "cool", then we got upset that there were only (at the time), like, five DVDs to play. The industry responded, in spades, with a cornucopia of options for using those extra speakers, and Dolby Pro Logic IIx is the latest of those.
Though I've already made a case for keeping the gear you've got, looking forward, I advocate Pro Logic IIx as the next baseline for a couple reasons: One, it conforms to the speaker layout that our experiences have found works best: seven main channels (plus subwoofer of course). From our first EX trials, we've advocated using two speakers in the rear (even though the center surround in EX is just one channel). A single center surround/rear speaker results in a very narrow sound field regardless of what you're pumping through it. So 7.1 makes sense.
More importantly though, a 7.1 system equipped with Dolby Pro Logic IIx as a base takes the guesswork out of what surround decode scheme you should use: Your only choices are "Music" or "Movie", and that is easy to make. Pro Logic IIx will deliver seven main channels for you, regardless of source, without any concern for whether it will sound "right" or "natural". And it will sound right and natural. Leave it at that.