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Dolby Digital (DD) is the standard 5.1 descrete channel format
Digital Theater Systems (DTS) is a competing 5.1 descrete channel format. (I personally feel DTS sounds a little better than DD)
Both DD and DTS have new formats that add an additional descrete channel for a "back" surround speaker to supplement the two side surround channels. These formats are called DD EX and DTS ES respectively and are a 6.1 format. Many processors and receievers with 6.1 capability actually have outputs for two of these "back" speakers, but each speaker carries the same signal. It's important to note that in order to hear true DD EX or DTS ES, you need a DVD that has been formated with the necessary 6.1 soundtrack. Currently, there aren't many out there. My Gladiator DVD has a DTS ES soundtrack, but there aren't many titles out there yet with the extra descrete channel for the back speaker.
There is also something that is sometimes called Surround EX and sometimes called THX EX. This is regular 5.1, but the processor derives a matrixed "back" channel from the two descrete side surround channels. This is similar to the way good old Dolby Pro Logic derives the signal for the center channel from the two stereo channels for the main left and right speakers. Surround EX (or THX EX) will allow you to play any existing 5.1 DVD and enjoy improved surround sound through the additional back speaker (or speakers if your setup has two as mentioned above).
THX is not a sound format. Rather, it is a standard applied to equipment (and even source material) that meets a certain set of hardware and performance standards. Think of it like "USDA Approved." But don't think that gear isn't any good if it doesn't carry the THX logo. In fact, there's a ton of great gear out there that doesn't care to pay for the "privilege" of bearing the THX logo.
Are the new 6.1 formats to be taken seriously? Well, I don't think you have to have them to enjoy great sound, but it's one more thing you can add to improve your system just that much more. Good movie theaters have back speakers, but don't think that you have to have them. Just depends how far you want to take things in your own home. Hope this helps
An important footnote: Only THX licensees are allowed to use the term "EX" on their gear. Just one more point of confusion for the buying public. I, for one, get a kick out of how carefreely the manufacturers will will throw multiple formats/choices/upgrades/labels at the public, expecting them to comprehend it all, and seemingly oblivious to the mass confusion and resentment it initiates (especially from those who were convinced to spend large chunks of $$$ on the "state of the art" surround sound). It often appears that the state of the surround sound world closely resembles the weather in New England..."If you don't like the forecast, wait a minute".