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While a number of high-definition television standards have been proposed or implemented on a limited basis, the current HDTV standards are defined in ITU-R BT.709 as 1080 active interlaced or progressive lines, or 720 progressive lines, using a 16:9 aspect ratio. The term "high-definition" can refer to the resolution specifications themselves, or more loosely to media capable of similar sharpness, such as photographic film. Currently 11% of American households have HDTV's. There are 8 HD broadcasting channels.

For example, the format 720p60 is 1280 720 pixels, progressive encoding with 60 frames per second (60 Hz). The format 1080i50 is 1920 1080 pixels, interlaced encoding with 50 fields (25 frames) per second. Often the frame or field rate is left out, indicating only the resolution and type of the frames or fields. Sometimes the rate is then to be inferred from the context, in which case it can usually be assumed to be either 50 or 60, except for 1080p which is often used to denote either 1080p24, 1080p25 or 1080p30 at present but will also denote 1080p50 and 1080p60 in the future.

A frame or field rate can also be specified without a resolution. For example 24p means 24 progressive frames per second and 50i means 25 interlaced frames per second, consisting of 50 interlaced fields per second.

HD programming and films will be presented in 16:9 widescreen format (although films created in even wider ratios will still display "letterbox" bars on the top and bottom of even 16:9 sets). Older films and programming that retain their 4:3 ratio display will be presented in a version of letterbox commonly called "pillar box", displaying bars on the right and left of 16:9 sets (rendering the term "fullscreen" a misnomer). Or, one can usually choose to zoom the image to fill the screen. While this is an advantage when it comes to playing 16:9 movies, it creates the same disadvantage when playing 4:3 television shows that standard televisions have playing 16:9 movies.

The colors will generally look more realistic, due to their greater bandwidth. The visual information is about 2-5 times more detailed overall. The gaps between scanning lines are smaller or invisible. Legacy TV content that was shot and preserved on 35 mm film can now be viewed at nearly the same resolution as that at which it was originally photographed. A good analogy for television quality is looking through a window. HDTV offers a degree of clarity that is much closer to this.

Two new pre-recorded disc formats support HDTV resolutions, namely HD DVD (supporting 720p, 1080i and 1080p) and Blu-ray (supporting up to 1080p). Most players for both systems are backward-compatible with DVDs. However, the two formats are not compatible with each other.

The increased clarity and detail make larger screen sizes more comfortable and pleasing to watch. Dolby Digital 5.1 sound is broadcast along with standard HDTV video signals, allowing full surround sound capabilities, while Blu-ray and HD DVDs will offer 7.1 Digital sound.

*Source: Wikipedia