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The basics of digital TV and HDTV
In many ways, launching and promoting color TV to the American public in the 1960s was much simpler than today's challenge with HDTV. Although viewers obviously needed to buy a new set if they wanted to see programs "in living color," the FCC required color broadcasts to be backward compatible with existing black-and-white sets, so viewers could still watch the newer color broadcasts on their old sets. Not so with HDTV--if you want to watch a digital broadcast on your ordinary TV, you'll need to buy a set-top box that can convert the over-the-air digital signal into an analog signal for your TV. And we're not talking about anything fancy here, either--just standard, over-the-air broadcasting. (We'll dive into cable and satellite TV in a minute.)
Better TV
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DTV comes in a variety of flavors--or, more accurately, numbers. The most common DTV types are 480p, 720p, and 1,080i, which refer to the number of lines of horizontal resolution offered by the broadcaster. The letter designates whether that image is progressive scan or interlaced scan. The more lines of resolution, the more picture information you get and the higher the image quality will be.
To better understand what all of this means, allow us to give you a basic primer on the way traditional TV tubes work. If you look closely at your TV, you'll notice that the picture is made up of a bunch of horizontal lines--480 to be exact. A device at the rear of the picture tube shoots a beam of electrons at the phosphor-coated rear of the screen, painting a new picture 60 times per second but drawing only every other line. Every 1/30 of a second, the odd-numbered lines are drawn on the screen, and 1/60 of a second later, the even-numbered lines come in. So 30 times each second, the entire image is redrawn, but this happens in two interlaced pieces. Computer monitors use a progressive-scan method where the entire image is redrawn in one cycle, which makes for a smoother, more flicker-free image.
HDTV is a subset of DTV. Standard-definition broadcasts are typically 480i (which is the same as regular TV) or 480p, and the picture is the same square shape as that of most TVs. To qualify as HDTV, the picture needs to have a higher resolution than that, typically 720p or 1,080i. An HDTV picture is also projected wide-screen. The ratio of the width to the height is 16:9, giving it a shape similar to that of most theatrical motion-picture screens. Meanwhile, a typical TV's aspect ratio is 4:3.
More channels
In addition, because DTV transmissions can squeeze a lot more information into the 6MHz of broadcast bandwidth that a TV channel uses than the old analog system can, local stations have a lot more choices as to what they can do with that fat data pipe. Some of the possibilities include devoting the channel to ultrahigh-quality HDTV programming; simulcasting up to four different channels of standard-definition programming, as some PBS stations now do; broadcasting two or three channels of SD programming and leasing the additional bandwidth to wireless service providers such as pager companies; or a mix of the above.
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| • | The basics of digital TV and HDTV | • | The slow transition to digital |
| • | The new TV sets | • | Digital and high-definition shows |
