What is High-Def TV?

Posted By BrokenClaw on March 6, 2008

High-definition television (often shortened to high-def or abbreviated HDTV) refers to the increased level of detail and clarity which is possible on the latest generation of television displays.

Of course, terms like high definition are always relative to the previous technology, just as high fidelity audio in the 1950s was better than its predecessor, but not as good as its successor, digital audio.

All television pictures, as well as the computer monitor you’re looking at now, are comprised of a horizontal and vertical grid of dots. The horizontal rows are called scan lines. Everyone knows that if you get real close to a large standard television, you can see the individual dots of color (which aren’t always round or square), and the picture becomes unrecognizable. The same is true of a photograph in a newspaper, when you use a magnifying glass — the dots get bigger but the picture gets fuzzier.

The number of rows and columns defines the resolution of the display. On a computer monitor, the dots of color are called picture elements, or pixels. Since the physical size of the display doesn’t change, the more pixels you have, the sharper the image. On your computer monitor, the native resolution is pre-set by the manufacturer, but it’s usually possible to change the resolution in the Display Settings. Nevertheless, unless you are running some special program that requires a specific setting, there is usually no reason for you to change your monitor’s resolution.

Computer monitors have always had higher resolution than televisions, because computer monitors were designed for displaying text. On a television, it’s difficult to read text that is more than about 40 or 50 characters across the screen, such as news and sports tickers, but computer monitors can display very tiny, but legible, text with 200 or more characters across the screen.

On standard televisions, you have no control over the resolution. In fact, it’s necessary for commercial TV broadcasts that all TVs have the same resolution. Another hidden characteristic of television displays is the way that the picture is actually drawn on the screen. On standard TVs, the picture is drawn in two steps. On the first pass, every other scan line is drawn, then on the second pass, the other lines are drawn. This process is called interlaced scanning.

The shape of a television picture is described as a ratio of the width to the height, called the aspect ratio. On standard televisions and television broadcasts, the aspect ratio is 4:3. In other words the picture is one-third wider than it is tall. High-def TVs usually come in a wide screen format, primarily to accommodate movies, with an aspect ratio of 16:9. In other words the screen is almost twice as wide as it is tall.

What characterizes HDTV is the increased number of lines of resolution. The actual pixel dimensions of high-def TVs vary from manufacturer to manufacturer and model to model, but they all identify themselves in terms of the horizontal lines. The first level of HDTV is 720 lines of resolution. A step up is 1080 lines of resolution.

The other defining characteristic among high-def TVs is the method of scanning. As noted above, interlaced scanning has been a normal part of television technology for some time. The opposite of interlaced scanning is progressive scanning. In this process, the whole picture is drawn line by line from top to bottom in one pass. Progressive scan gives a better quality picture with less flicker on fast moving objects.

HDTV also carries better audio. Standard television broadcasts have either mono or stereo sound, but HDTV has the capability of more audio channels up to Dolby 5.1 surround sound.

All high-def TVs can display regular analog TV programming, using the video software built into the TV, as well as standard digital television. However, for high-def programs they require a digital high-def signal, either over-the-air from a TV station, through a cable or satellite provider, or from a digital high-def source from a Blu-ray disc, gaming console, or computer video. In other words, not all digital TV is high-def, but all high-def TV requires a digital television.

When comparing the specifications of high-def TVs, you will see them listed as 720p (720 lines of resolution with progressive scan), 1080i (1080 lines with interlaced scan), and 1080p (1080 lines with progressive scan). Displays with 720p usually have the capability of 1080i. However, 1080p is now the top of the line, and you will often see 1080p displays advertised as true HDTV or full HDTV.


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