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THX Surround EX™ (Dolby Digital EX)

THX Surround EXTHX Surround EX, also known as Dolby Digital EX, refers to the Dolby Digital version of the new "6.1"-channel surround sound format that extends the 5.1-channel surround sound format with one (or two) additional speaker(s) located in the back of the audience (i.e., back surround channel).  THX Surround EX is actually a 5.1-channel format with the back surround audio channel matrixed into those of the right and left surround.  The back surround channel is matrixed in similar fashion as the front center channel is matrixed into the front right and left channels in the Dolby Surround Pro-Logic format.  As such, the back surround channel is not discrete and therefore THX Surround EX is not a true "6.1" format.

What about the extended surround "7.1" format?  Well, "7.1" is not a true surround sound format.  Rather it refers to home theater equipment manufacturers' proprietary implementation of two back surround channels using the same back surround audio signal that was discussed above.  Usually, the audio signal fed to the two back surround channels are identical, but some manufacturers may choose to mix some information from the left and right surrounds.  Again, these are proprietary implementations that vary across manufacturers and are not industry standards.  The use of two back surround speakers fills in the sound better from the rear of the audience than single back surround speaker implementations.

To take advantage of THX Surround EX, you must have both: 
(1) a DVD player and use one of the digital audio outputs (e.g., "Toslink" optical or coaxial).
(2) a THX Surround EX decoder, usually a function of the receiver or pre-amplifier.  Look for the  "THX Surround EX" 
      words or logo on the faceplate.  Receivers featuring THX Surround EX decoding are still relatively expensive, 
      particularly those that comes with six or seven channels of amplification for the full frequency channels.

DTS Extended Surround Matrix is a competing extended surround matrix format, while DTS Extended Surround Discrete 6.1 is the only true discrete 6.1 format.

Also see: Surround Sound Formats page for a full explanation, Dolby Digital, DTS Extended Surround, DTS
Additional Information and References: Lucasfilm THX Surround EX page, Dolby Digital home page

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