You can use upsample with DD 5.1 and I believe DTS as well, and you'll still get all 5.1 channels. The catch is that there's not enough CPU horsepower left to overlay additional processing modes, so if you have a 7.1 speaker setup you can't use Upsample and Pro Logic IIx together with a Dolby 5.1 source. As sportsound says, it's a slick little undocumented feature - and one that I'd probably use a lot with a 5.1 setup.