Dolby Digital EX
Dolby Digital EX takes the Dolby Digital 5.1-channel setup one step further with an additional center surround channel (reproduced through one or two speakers) for extra dimensional detail and an enveloping surround sound effect. Feature films originally released in Dolby Digital Surround EX (the cinema version) carry the encoded extra surround channel in their subsequent DVD releases, as well as onto 5.1-channel digital satellite and TVasts. [Source: Dolby Technologies]
Image: Example of a home theater surround sound system using Dolby 5.1 Surround set-up.