This genre of Dolby Surround Sound, Dolby EX adds a rear center channel to the existing left and right rear channels. (I think it is also referred to as Dolby 6.1). I received the following explanation of the format direct from Dolby Labs:
As to your compatibility question, the Dolby Digital Surround EX format, which adds a third surround channel, is fully compatible with 5.1. The extra surround information is not on a separate, discrete channel, but rather is matrix encoded onto the left and right surround channels of an otherwise standard 5.1 soundtrack. You can at your option decode the extra channel using Surround EX equipment, or not. If you choose not to, no information is lost because it's all there on the standard two surrounds. Currently, Surround EX decoding is available exclusively on THX-certified home theater components.
Best regards,
Info @ dolby
As to your compatibility question, the Dolby Digital Surround EX format, which adds a third surround channel, is fully compatible with 5.1. The extra surround information is not on a separate, discrete channel, but rather is matrix encoded onto the left and right surround channels of an otherwise standard 5.1 soundtrack. You can at your option decode the extra channel using Surround EX equipment, or not. If you choose not to, no information is lost because it's all there on the standard two surrounds. Currently, Surround EX decoding is available exclusively on THX-certified home theater components.
Best regards,
Info @ dolby