Quote Originally Posted by GaToy
THX EX
THX EX is the name of a new Dolby Digital sound format, using 6.1 channels. The format adds an additional surround center channel to the current 5.1 setup to create a more engrossing surround field that is compatible to the front field in terms of spatial integration. Unlike sound formats like DTS,THX EX does not necessarily require additional equipment to listen to the audio track. It is an extension and as such backwards compatible with the current Dolby Digital standard. If you do not have a THX EX decoder in your home theater setup, the playback will be automatically converted for 5.1 playback.

~C.C.~
http://hometheater.about.com/library.../aa061400a.htm
That was true until about two years ago. With the original round of 6.1 receivers, only the THX certified models were allowed to use the "EX" trademark to describe the back surround channel decoding. The theatrical format was Dolby Digital EX, while the consumer version was called THX Surround EX. Dolby and Lucasfilm jointly developed the EX spec, so THX partly owned the trademark. Non-THX receivers could still do the back surround decoding to the correct specs, they just couldn't call it EX. Rather, they called it 6.1 Matrix or some other linguistic variation. It was about two years ago that the EX trademark was opened to all receiver makers. It's no different from THX Surround EX, only the name is different.