looks like Andrew Robinson managing editor over at hometheaterequipment.com had a few things to say about the S790 and its 4K upscaling capability : Post 9 and 12
Sony Ships 4K Blu-ray Player (BDP-S790 Including UltraViolet™)
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It actually doesn't matter for the player is still a 1080p player at heart and as for the 4K projector, well, it does it's own upscaling, making the player's upscaling largely irrelevant. There's actually no way to test Sony's 4K upscaling claims as it pertains to the Blu-ray player for even if left on your HDTV isn't going to look any different. I couldn't even feed the upscaled 4K image to my JVC e-Shift fauxK projector for even it can only accept a 1080p signal before upscaling it internally to QFHD.
It's a stupid feature on a stupid player that is half cocked and nothing more than an interim product aimed at cashing in on the uninformed before the player you really want, and will NEED, is released."
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Why bother with 4K upscaling? 1080p upscaling was one thing when we all had 1080p HDTVs but no one has 4K displays. Furthermore, when we do have 4K displays why would you make a pit stop at buying a 4K upscaling Blu-ray player when real 4K players are right around the corner? I have to believe that early 4K displays will have to upscale, which again, makes the Sony player silly. Now, are the other features it offers worthwhile? Maybe, I can't comment on that, but the feature they're hoping will attract people, 4K, that's unnecessary.
I must say I'm not impressed with how 4K is being handled and rolled out thus far. It's really disheartening to me because I'm a huge proponent of the technology."