Here is where things may start to get confusing for the consumer. All HDTV is digital, but not all Digital TV is HDTV. As stated previously, in the answer to question #1, the same bandwidth for digital TV broadcasting can either used to supply a video signal (or several) and other services, or can be used to transmit a single HDTV signal.
Although there are technically 18 different standards for digital TV broadcasting (all Digital TV tuners are required to decode all 18 standards), the practical application of DTV has come down to 3 standards. These standards are: 480p, 720p, and 1080i.
480p
If you have a progressive scan DVD player and TV, you are familiar with 480p (480 lines of resolution, scanned progressively). 480p is similar to the same resolution of standard broadcast TV (and is referred to as SDTV or Standard Definition Television), but the image is scanned progressively, rather than in alternate fields. 480p does provide an excellent picture (especially on smaller 20-27" screens). It is much more film-like than standard cable or even standard DVD output, but it only provides half the potential video quality of an HDTV picture, therefore its effectiveness is lost on larger screen sets.
Although 480p is part of the approved DTV broadcasting scheme, it is not HDTV. This standard was included as one of the DTV broadcasting standards to provide broadcasters the option of providing multiple channels of programming in the same bandwidth as a single HDTV signal. In other words, 480p is just more of what we already have with only a slight increase in image quality.
720p
720p (720 lines of resolution scanned progressively) is also a digital TV format, but it is also considered as one of the HDTV standards. As such, ABC and FOX use 720p as their HDTV broadcasting standard. Not only does 720p provide a very smooth, film-like image due to its progressive scan formula, but image detail is at least 30% sharper than 480p. As a result, 720p provides an acceptable image upgrade that is visible on both medium (32"- 37") size screens as well as larger screen sets. Also, even though 720p is considered high-definition, it takes up less bandwidth than 1080i, which is covered next.
1080i
1080i (1,080 lines of resolution scanned in alternate fields consisting of 540 lines each) is the most commonly used HDTV format, and has been adopted by PBS, NBC, CBS, and CW (as well as satellite programmers HDNet, TNT, Showtime, HBO, and other pay services) as their HDTV broadcast standard. Although there is still a debate as to whether it is that much better than 720p in the actual perception of the viewer, technically, 1080i provides the most detailed image of all the 18 approved DTV broadcast standards. On the one hand the visual impact of 1080i is lost on smaller screen sets (below 32").
However, the two drawbacks to 1080i are:
1. It takes up the most bandwidth of all the DTV broadcast formats.
2. It is an interlaced signal, which means that the displayed image is made up of lines that are scanned alternately instead of progressively as in 480p and 720p.
3. 1080i cannot be displayed in its native form on an LCD, Plasma, or DLP television, so those types of sets need to convert the 1080i signal to either 720p or 1080p in order to display the image on the TV screen.
In other words if you have a 1080p HDTV, a Flat Panel or DLP TV will deinterlace the 1080i signal and display it as a 1080p image. This essentially removes any visible scan lines present in the interlaced 1080i image, resulting in very smooth edges. By the same token, if you have a 720p HDTV, your TV will deinterlace and downscale the 1080i image to 720p for screen display.
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