Different HDTV signals?

All of the above posts are correct. I just want to add some minor details. For HD channels only, such as channel 904 for HD-NBC rather regular channel 4 for NTSC NBC in my case, prime-time shows broadcast in HD would fill up the plasma screen. When it comes to commercials, those are not in HD. Therefore, they don't fill up the entire screen, and the black bars show up, so whenever the signals are not in HD, such as commercials or non-HD programs, the TV screen won't be fully filled.

Oh, lets add to that list; the many movies on the pay channels that aren't in oar. Some just weren't shot in widescree and some they just don't use the widescreen version / transfer.

I recently got a plasma display, and the aspect thing surely is a pain. My remote has a little red button to change aspect, and I expect it will wear out long before the plasma dies.

I agree with Darrylhifi. I have also noticed on FOX and ESPN, many times a standard definition telecast is broadcast on their HDTV channel without being reformatted with bars on the sides. The image ends up being flattened. The Indy car race from Miami last Sunday on ESPN-HD and reruns of "That 70s Show" on FOX-HD are good examples. Kudos to CBS, NBC, and HBO who have long since addressed this problem.

Yes ESPN looks terrible except when they are truly broadcasting a game with High Def Camera's . From what I understand Fox , even when say showing a football game in widescreen , is not true high def, however they are upgrading , and should be full blown high def capable this year. Your right ESPN actually uses widescreen even when not broadcasting in high def , its just a stretched standard def picture. But , all minor issues , that eventually will pass. Hi Def when done properly is great !