Rene,
Quote:
...in Dolby Pro Logic II and IIx modes, one can even specify the degree of the blend. I'm not sure why this is not available in other modes.
When processing 2-channel material, the centre channel is derived by steering certain information out of the front L/R channels. It makes sense then that Dolby allows the end user to control the steering (i.e., how much is sent to the centre output).

With 5.1-channel material, there is no need for centre channel steering control. Think about it: 3 channels, 3 speakers; what's there to steer? Each front channel simply gets routed to its respective speaker.
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Sanjay