Q: If a HDTV is limited to automatically displaying 720p what will be the picture results if the signal come at 1080 off air or cable?
A: Curt, when an image of one size is displayed on a screen with a different native resolution (number of pixels), then the image has to be scaled. If the image is smaller — a 720p image on a 1080p set — then the expanded parts of the picture have to be “invented”. The computer that is the HDTV’s controlling circuitry actually identifies features such as lines and objects in the image, and does its best to guess what dots to add to the expanded image in order to make it look good. It’s a tricky thing, and if done badly, the image quality can suffer. (This is what the “upconverting” DVD players do.)
If the image is larger than the screen — a 1080p image on a 720p set, as in your question — then the controller has to decide which part of the image to throw away in order to get it to fit on the screen. Again, it needs to preserve lines and the edges of objects, so this too is a complex process. And done badly, it’s even more likely to result in visible flaws in the image.
While many people may not be able to see the difference between a 1080p image and one scaled down to 720p, my personal preference is to not throw away any information for an image. That’s why — everything else being equal — I prefer 1080p as the native resolution for an HDTV. Whether you get a 720p or 1080p set, however, it’s very important to get one that scales well.