Dolby EX is a matrixed format -- the surround back channel is encoded in the two surround channels, just like Pro Logic uses the left and right channels to carry the center and mono surround signals. There is not a discrete surround back signal in Dolby EX. While we're on the subject, DTS-ES Discrete does have a separate channel in the digital track, but DTS-ES Matrix (the more common version) uses matrixing to carry the surround back channel (the digital data is 5.1 discrete channels, just like Dolby EX).

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