As the proud owner of a new 46" HDTV, I have some questions. During some HD shows, commercials are displayed in a 4:3 ratio while other commercials come in at a 16:9 ratio but do not fill up the entire 46 inches. And sometimes during an HD newscast, a story has the main part of the video in a 4:3 ratio but the surrounding areas are multi-colored with vertical letters NNN HD. So why the different types of display? I assume its because an HD channel doesnt always broadcast everything in HD, but why the 2 types of ratios?
It depends on what format they were originally created and broadcast in. However, you have the option of controlling the aspect ratio that you view, in order to fill your screen (though some may cause distortion). I'd suggest reading the manual that came with yours, to find out your aspect ratio settings/controls.
Rick, As Mike points out, it's just the way certain things were shot. Most commercials are NOT in HD (yet). So when I'm watching a show that is in HD such as Saturday Night Live, the commercials are often Letterboxed (black bars down the sides).
When I'm watching the news, it may be in HD, but the field reports are often shot on lower end equipment in 4:3 format and then shown in a 16:9 frame with animated bars on both sides (instead of just black bars).
As far as shows that appear to be in widescreen, but not filling your screen, they were most likely shot as widescreen video, just not HD. For example, just about any DV camera out there now has a widescreen mode (I shoot most of my events in this mode), but that's not necessarily HD. So it's a smaller frame.
As the cost of shooting in HD continues to decline, more and more content will be shot in it. For now, you'll just either have to live with it, or use the controls on your TV to make everything fill the screen (which I hate, and don't do).