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Given no surround channel info, what does the Dolby encoder do to an LCR mix to create an LtRt that would be different than a straight stereo mix. In the mix the center information should be down 3db when things are paned to center to keep them equal power, and everything else seems to be on the decoder side. I'll have to reread the Dolby stuff to figure out what the "all pass" encoder stage does
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I tried to answer these questions, I'm sorry I was not clear. The all pass filter applies a frequency-dependent time delay such that for any given frequency the amount of phase shift is the same. This is not like a standard delay circuit, which delays all frequencies by the same amount of time. Because it can potentially "smear" transients, you might think it would be very audible but I find it to be an extremely sublte effect, not worth considering. Electrically, though, it has a very important role for the decoder hardware. This is because a different amount of "all pass" phase shift is applied to all surround signals. The value of this technique is that you are able to send the same signal simultaneously to front and surround speakers. Even though you are sending an identical signal to both, the use of differnt all pass filters on front and surround channels make them look electrically different from each other, so that the decoder detects the difference and routes them to their proper speakers. It was a clever choice by Dolby.
To answer your specific question about your LCR mix, it is the presence of the all pass filters that the encoder has that your simple stereo mix does not. For LCR, and no surround, the all pass filters do nothing useful so there is effectively NO DIFFERENCE with or without the encoder, as long as you have -3db center pans. PT stereo panners are not, but I believe the surround panners are. Somebody correct me if I am wrong.
While this may seem like good news to you, it does not mean you should be mixing without hearing the mix being decoded! The decoder will put information into the surrounds whether you want it there or not. The decoder tends to make stereo sound MUCH less wide, subtle and not so subtle panning will most likely be lost, and there are the inevitable steering problems, where dialog will get pulled away from the center into surrounds by all manner of synth pads and reverb tails. You cannot always predict what will happen. While the actual balance of elements within your mix will probably not sound much different, their positioning may become distracting. Get Dolby hardware, or at least a plugin package (I hear Circle Surround is okay too, but I've never tried it). As long as you are getting a decoder you might as well get an encoder and do it right, and no longer fear the surround speakers. It will be a good investment.