Dolby Digital Surround EX
Dolby Digital Surround EX home theater systems include rear surround speakers that are made possible by an additional surround channel. This digital Surround EX channel drives the center rear surround speaker. This setup is also referred to as 6.1 in industry parlance (6 speakers or audio channels; and 1 subwoofer).
The rear Surround EX channel does not have its own discrete channel. Instead, its signal is matrixed (intermixed) with the left and right surround channels, just like Dolby Pro Logic has the center speaker information encoded in the left and right front channels.
When a Dolby Digital EX program is decoded, the three surround outputs are all derived from a 3-channel Pro Logic process — none of them are strictly discrete anymore. It is also the case that many conventional 5.1 movies decode nicely with Dolby Digital EX processing, thus keeping the added back speakers active. Newer processors offer Pro Logic IIx technology, thereby deriving four separate surround outputs instead of three, making much more effective use of 7.1-speaker systems for all movie and music programs, whether Surround EX encoded or not.
The first consumer devices with Surround EX were based on licenses from Lucasfilm THX, so you’ll also see it called THX Surround EX in stores and online when you’re shopping. THX isn’t a surround-sound format itself but rather a certification and testing program for home (and movie) theater equipment and movies. So equipment such as A/V receivers and speakers may be THX certified, as may DVDs themselves. THX’s main mission is to create a set of standards for surround-sound playback and then certify the equipment or DVDs that meet those standards.
Many newer DVDs are encoded for Dolby Digital EX and have that extra channel of surround information onboard. Also, if you're playing a regular Dolby Digital 5.1-channel DVD, a THX Ultra 2 decoder can simulate 6.1- or 7.1-channel surround by processing the audio information in the regular surround channels and sending the information to your rear surround speaker.
If you’re playing a movie (say on a DVD) that has been encoded using Dolby Digital EX, and you have only a regular Dolby Digital decoder in your A/V receiver, you won’t have any problems. You still get the full 5.1 surround sound you expect, with nothing missing. You just won’t get certain sounds re-routed to the extra surround speakers behind you.