How is Surround EX encoded onto a film soundtrack?
A discrete rear soundtrack is created for a film, and is then matrix-encoded into the existing discrete surround left and right channels. This is done by dividing equally the surround back channel signal and adding it to the left and right surround channels. This is the same method the center channel information is encoded in a Dolby Pro-Logic track. Dolby also specifies a phase shift in the left and right surround channel signals of +/- 45 degrees (rear information I assume added after the phase shift.
