AVS Special Member
Join Date: Aug 2001
Location: Lawrenceville, NJ
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First off, Prologic is very old and obsolete today. It has been replaced by Dolby PL II for 5.1 and Dolby PL IIx for 6.1 and 7.1 speaker setups. These newer processing algorithms work much, much better than the old Prologic.
The 2 channel material does NOT need to be encoded. PL II and PL IIx can simply work with a stereo source. I'm not even going to try to explain how it's done since I don't know most of the details, but certainly using the stereo information such as common (mono) content, the phantom sound stage created by the stereo signal, the phasing of L/R content etc..., you can imaging that it's possible to do a good job with today's powerful DSPs.
Note also, that the "Music" (vs Movie) flavour of Dolby PL II and IIx allows you to customize the width and depth of the surround sound field for 2 channel recordings.