Reader Question: 1080i Signal on a 720p HDTV

Q: I’ve noticed that some HDTVs are listed as 720p, but their specs says that they supports 1080i. Why wouldn’t those TVs be listed as 1080i? With these types of TVs, would a 1080p signal come through as 1080i or 720? If an upscaling dvd player is used on those TVs, would it upscale to a 1080i or 720p image?
J Neubauer

A: This is an excellent question, and I can see why you and many others find this so confusing.

There are two specifications at work here. The first is the physical resolution of the display. In the case of a 720p panel, this is typically a matrix of 1,280 by 720 pixels. These are the physical dots that make up the image that you see.

Image signals come in a wide range of resolutions, from the (roughly) 640 by 480 standard definition image, to 1080p (which is a matrix of 1,920 by 1,080 points of information). If the display has more or fewer physical dots than there is information in the signal, the display has to scale the image to fit the screen. If there are fewer points in the signal, then these get expanded so that the are represented by more than one physical pixel. If there are more points, then some of this information has to be “boiled down” so that the picture fits; in the process, some of the detail gets thrown away.

So you can display 1080i and 1080p signals on a 720p display, but you won’t see all the information that is contained in the original signal.

As for upscaling DVDs, it depends on the model, but some let you select which output resolution you want. Note that your HDTV will also do the upscaling; the reason that the upscaling DVD players exist is that they often have better a better scaling processor than those found in some HDTVs.