Different HDTV signals?

When I tune into HDTV movie channels, the picture fills the entire screen on my 51" widescreen rptv. However, when I put on some of the HD major network channels, like CBS, NBC and Fox and some others, there are bars on the sides of the screen regardless of the picture setting. I'm using cable to receive the signal. Are there two (or more) different HDTV signals being broadcast?
All HDTV is 16:9 widescreen. It sounds like you're getting a non HDTV digital feed from the cable. Also, remember that not all network shows are broadcast HDTV.
Yes The above is correct, when the program is broadcast in standard definition, you will see bars all around your screen, when they are broadcasting in High def, you will view it in widescreen. Perfectly normal. If you are not getting any widescreen broadcasts during primetime your cable company is not transmiting them in the High Def format. If some of the time on the networks you are getting widescreen , thats because some shows like NYPD Blue are produced in high def, while a show like Friends is not.
Also many cable companies with high Def capability broadcast the networks in High def on different channels on your cable box , For example CBS is channel 2 in NY/NJ but if I want to view it in high def on my system it is channel 183.
real high definition programming is in wide screen 16:9 aspect ratio anything other isn't really actually high definition its being converted from something else to sort of high definition.
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 !
something I really don't like is for example sometimes ER is shown in letterbox high definition format this really sucks I have a 16:9 screen why am I forced to watch this in letterbox screen having to look at black bars along the top and bottom above the pictures. It should be the full 16:9 screen. Another problem like on Showtime or HBO they will show a movie or boxing in high definition but only 4:3 aspect ratio I want it 16:9 or I don't even bother watching it would rather watch the standard definition in 16:9

The remote control for my plasma has a button for the aspect as well. I used such feature a lot when I was using the S-video cable for regular TV watching while the component video input was reserved for the DVD. Now that I have the Yamaha AVR as my video switcher, which does all the video up conversion, I use only the component video cable for every thing. Thus, the problem does not exist any more. With the component video input, I can never adjust the aspect of the screen. What video input on the plasma that you use?
If you are watching Boxing on HBO or Showtime and it is in a 4:3 Format, it is not in high definition.
Lej1447...I am using S-Video interface. My plasma is not HD, but the picture, using SVideo is sharp enough that my eyes can't distinguish pixels from normal viewing distance. The wire run from the Sony HD Direct TV receiver to the plasma is about 20 feet, so I have not spent the money to try component interface. I will probably go to DVI some day for the Direct TV receiver.