Yeah, the problem isn't the TV's scaling, it is adding blank information to the sides at the device level. The allshare adapter handshakes with a full 1080p signal that fills the 1:1 pixel mapping mode of my (Samsung) tv perfectly. My galaxy s4 through the same adapter sends along a properly formatted signal.
Since the native 2560x1600 resolution breaks down to a 16x10 ratio instead of the standard 1080p standard of 16x9, it can either add the bars, rescale the image [introducing a subtle geometric squash) or zoom the image and chop the top and bottom of the signal. Since zooming the image would result is lost information, and rescaling is pretty processor intensive I can see why they chose the route to add the sidebars.
My Samsung tv, which is only a few years old, doesn't have a zoom mode that doesn't also cut off some of the picture image, and since the black bars are part of the 1080p signal, it is technically displaying the picture correctly.
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