How Is HD Television Broadcast?
In many ways, HD television broadcasts in the same manner as earlier forms of television. The TV station sends out a signal into the airwaves, which is picked up by a receiver on top of the television. Cable stations deliver the signal through their cable networks, while satellite TV services beam it directly into homes via an orbiting satellite. Because of the enhanced clarity of HDTV, the specifics differ a bit from older TV broadcasts.
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Digital Broadcasting
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HDTV requires digital broadcasting, which differs from the older analog broadcasting TV stations used. The signal goes out as a mathematical code---a long string of 1s and 0s---which contains all the audio and video information for the show. That uses much less bandwidth than analog signals, which allows the digital signal to carry much more information, and thus a sharper, more clear picture.
Sub-Channels
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Digital TV stations also use MPEG-2 compression, which further shrinks the signal and allows broadcasters to divide into multiple channels called sub-channels. Some of those sub-channels are sent in standard definition, equivalent to those of older TVs. Other sub-channels can be sent out in high definition, creating a sharper, clearer version of the same show.
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Digital Tuner
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Once the signal goes out, the TV needs to pick up the signal and interpret it as sound and images. Televisions require a digital tuner in order to pick up digital signals. Most HDTVs possess a digital tuner by default, though older TVs require a digital converter in order to function.
Image Density
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When the HD signal appears on the screen, it needs a screen of sufficient pixel density in order to display the image as crisply as possible. Every TV has hundreds of thousands of pixels on its screen---tiny dots that constantly shift colors to produce the image. Older TVs have 480 horizontal lines of pixels on their screen (referred to as standard definition). For an HDTV to work, it needs 1080 horizontal pixel lines to function: more than twice as many as the older standard definition TVs. Note that the resulting clarity has nothing to do with the physical size of the TV. A 20-inch HDTV still proves a sharper image than a 40-inch standard definition TV.
Combinations
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For HDTV to function, all of the components need to be in place. The TV station needs to broadcast a high definition digital signal and the TV has to be capable of receiving digital signals, as well as possess the screen density to display an HD image. If the station doesn't send out the show in high definition, it won't look as sharp on your TV, even if it's an HDTV. Conversely, if you don't have an HDTV, high-definition signals will look exactly the same as standard-definition signals.
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References
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