Hmmm, there seems to be some confusion here about the definition of HDTV.
Here's a long primer on American broadcast digital TV, which includes HDTV, and digital cable QAM. Perhaps others can provide the European and Asian standards.
The two standard over-the-air broadcast high definition TV (HDTV) picture formats are all required to be 16 units wide by 9 units high (16:9) instead of 4 units high by 3 units wide (4:3) as in standard definition TV (SDTV):
1.) A 720p picture is up to 1280 pixels wide by a minimum of 720 pixels high at 60 frames per second.
2.) A 1080i picture is up to 1920 pixels wide by a minimum of 1080 pixels high with 30 frames of 540 pixels by 1920 pixels every second, which are interlaced with the other 540 pixels by 1920 pixels in alternating frames.
NOTE: The HDTV standard has no requirement for how many pixels wide because the standard was set when CRT displays were still around and they only specified the number of vertical lines to be 720 or 1080. For example, many plasma HDTV sets only display 1024 pixels across by 720 pixels high. To maintain the 16:9 aspect ratio the 1024 pixels are spaced physically farther apart than the 720 vertical pixels. The TV set will take the 1280 or 1920 pixels that were broadcast and scale them to fit on the 1024 pixels on the display. Likewise, an LCD panel with only 1280 by 720 pixels must scale down a 1080i picture to fit. Properly done scaling will not stretch or squeeze the picture.
NOTE: Some HDTV sets have a 4:3 display, but they can legally display an HDTV picture provided they have at least 720 vertical lines or pixels. Sony sold a 4:3 CRT that would display 720 lines with black bars above and below to form a 16:9 HDTV picture.
The American broadcast networks ABC and FOX along with the cable/satellite channel ESPN use 720p HDTV because it works better with fast moving sports action. 720p requires less digital compression than 1080i to fit in the standard TV channel, which is limited to 20 megabits per second transfer rate. (The actual rate is 19-point something Mbps, but 20 Mbps is close enough for this discussion.)
The American broadcast networks CBS and NBC use 1080i. A 1080i picture requires more digital compression than 720p to fit into a standard 20 megabits per second TV channel because it has more pixel data.
However, most TV stations are squeezing more than one channel into their 20 Mbps TV channel. They are compressing the HDTV picture down to about 10 Mbps so that they can fit in a couple of extra standard definition channels. On most HDTV sets these extra channels show up as, for example, 16.1, 16.2, 16.3, etc. (some sets display a hyphen 16-2 instead of a dot) These are virtual channel numbers and not the actual physical broadcast channel number.
To make the switch to digital TV easier, the American FCC is requiring TV stations to broadcast a virtual channel number that is identical to their old analog channel number. HDTV sets built before 2003 are often not able to display or use virtual channel numbers. In the previous example, \"16\" is the current analog TV channel number for a TV station in my area. Several years ago, the FCC gave them the UHF physical channel 17 to broadcast the digital HDTV picture until 2009 when channel 16 will be turned off. I bet they will keep the virtual channel number 16.1 so that people can find channel \"16\" when analog broadcasts stop.
NOTE: Comcast cable is mapping all of these virtual channel numbers to a different cable channel on the DVR cable boxes. Channel 16.1 is mapped to channel 703 and 16.2 is mapped to channel 11 on my Comcast DVR, which probably confuses people further. However, if you use an unscrambled cable QAM tuner the 16.1 and 16.2 display correctly, at least with my Sony TV and Comcast cable setup. Your mileage may differ because the cable company can set virtual channel numbers to any number. For example, channel 10.2 is mapped to QAM channel 102.4031 on my Sony set.
It took me forever to figure QAM out because the cable company would not tell me what is the QAM number mapping - they only wanted to sell me an extra cable box for $6 a month to watch the free unscrambled QAM cable channels! Note that QAM is the digital format used by cable and it is different than the over-the-air digital broadcast standard. Unless the HDTV set has a QAM tuner, it will not be able to tune digital cable channels. Scrambled QAM cable channels require a cable card plugged in your TV set or a cable box to watch. This summer the FCC started requiring Comcast to support the cable card, but few TV sets support CableCard yet. But I dare you to call Comcast and try to get them to admit they support such a thing.
The American HDTV standard also specifies several 4:3 standard definition TV formats that can be broadcast as secondary channels. All of them are 480 lines or pixels high by a variable number of pixels wide. HDTV sets display them by scaling the picture to fit the display. To avoid stretching the picture, a 480p signal would be scaled up to either 960 by 720 pixels or 1440 by 1080 pixels depending on the display size. Black bars (on some sets white or gray) would appear to the left and right of the picture on a 16:9 display.
A Comcast cable engineer insisted to me that the cable company is merely passing on the same bit rate as broadcast by the over-the-air stations, however, the satellite companies are often compressing the HDTV signals even more so they can fit more channels on the satellite. I believe this based on watching both over-the-air vs. cable and satellite, but you can tell many of the cable TV HD channels are more compressed than the over-the-air channels because the picture will break up into big pixels whenever something moves across the screen. Fire and moving water seem to be affected by compression the most.
The Comcast ESPN and other 720p sports cable channels look very lightly compressed because the picture is perfect and motion artifacts don't show up, even during fast action.
Comcast appears to be doing more compression on cable channels such as A&E, HGTV, USA, TNT, etc. Many of these channels are broadcasting 4:3 aspect ratio shows by stretching them to fit the 16:9 picture and then calling them HD when they are not high definition! TNT has shown movies this way that I saw shown correctly in HD on other cable movie channels. Some are stretching the sides of the picture more than the center so that faces in the center look normal, but not at the edges. (Some sets call this non-linear stretching \"panoramic\") The problem is that no TV I am aware can undo a picture that was broadcast with panoramic stretch. My set can shrink a linearly stretched picture, but not undo a non-linearly stretched picture.
Another problem is the default Comcast DVR setup stretches all pictures to fit 16:9 unless you change it in the secret boot up screen used by the cable techs. An engineer told me they did this because the old plasma TV sets would suffer burn-in damage with letterboxed pictures. They idiot proofed it, but failed to provide the user with a simple way to change it. If you have an LCD of any age or a newer plasma set, burn-in is not an issue. My new Sony LCD HDTV also insisted on stretching the picture by default, but this was easy to change in the menus. I guess dumb people are upset by the black bars and don't understand picture aspect ratio, which generates too many support calls for these guys.
The American FCC forbids broadcasting 1080p over the air, but DVD players, etc. can output 1080p to TV's that can handle it (note 1080p has the same 1920 by 1080 resolution) The FCC forbids 1080p because when the standard was set it was believed that it would require too much digital compression to broadcast the double amount of data that 1080p requires. Few people can see the difference between 1080i and 1080p in a properly coded TV signal, except in very special cases such as when using still frame modes.
WHEW! No wonder people are confused.
Hockey has never looked so good. I regularly switch between HD and non HD game to remind myself how good HD is.