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Standard or High Definition: It’s All in the Pixel Count

Most of us don’t think much about the images we look at on a TV, flat-panel display, or on a laptop or desktop computer screen; that is, we don’t think about what comprises the image–because our eyes and brain merge all the elements of color, brightness and contrast into a “picture” we immediately see. The process is similar, but not exactly analogous, to what happens when we watch a movie. We don’t sit there and dissect the 24 still pictures presented sequentially every second (in the case of a TV program, 30 frames per second). Our visual system and brain merge the rapidly moving images into motion, and if the TV show or movie is any good, we’re happy.

What is a Pixel?

But if you move up really close to an electronic image display and use a magnifying glass, you can see the tiny individual squares of red, green, and blue “pixels” (short for “picture elements” in modern video-speak) that comprise the image. The pixels are actually square or rectangular in new technologies like LCD (liquid-crystal display), plasma panels, and in DLP (digital light processing) or LCD front and rear projection sets. And the smaller the pixels, and the more of them there are distributed vertically and horizontally across the face of the screen, the greater the “resolution” or detail we will see in the image. (On the old but familiar CRT picture tube TV, the picture elements are groups of round red, green and blue phosphor dots that glow when the electron beam in the tube strikes them.)

How Many Pixels?

LCD, plasma, and DLP devices are all called “fixed-pixel” displays because the panel has a predefined and fixed number of pixels in its display format. Knowing the number of pixels in each direction (horizontally and vertically) will tell you how sharp an image it will produce, as well as whether it will display a true High Definition TV image or only Standard Definition, both of which are part of the new digital TV standard. The pixel count will also affect the cost. Lower resolution costs less; higher resolution costs more, sometimes a lot more. And don’t make the mistake of thinking that just because a TV is labeled “digital” means that it is capable of producing a high-definition picture. Within the new digital TV set of standards, there are three levels of resolution permitted: Standard (SDTV), Enhanced (EDTV), and High Definition (HDTV).

Going Native

If you are considering purchasing a larger screen and/or a High Definition TV (HDTV), there is a phrase describing potential image clarity that you must understand. That term is “native resolution,” and it refers to the maximum degree of clarity that one of the new digital TV formats is capable of displaying. Native resolution of a fixed-pixel display is defined as the total number of horizontal pixels across each scanning line by the total count of vertical lines stacked top to bottom. For example, a Standard Definition fixed-pixel display would have 704 pixels across each of 480 scanning lines (704 x 480) and that would represent a squarish 4:3 aspect ratio screen shape, the image shape in which virtually all TV programs were photographed until the advent of HDTV, which, as part of the HDTV standard, requires a “widescreen” aspect ratio of 16:9, similar to that of most commercial movies.

Standard (Analog) TV Definition

As our TV system gradually converts from analog TV transmission and display to an all-digital system, a process that will continue until 2006, Digital TV allows for three standards of “definition” or clarity, two of which encompass High Definition TV. Standard Definition, as explained above, is equivalent to 480 interlaced horizontal lines (480i) stacked from the top to the bottom of the screen. The “interlaced” lines mean that the image (a frame) is made up or alternating fields of 240 lines that are scanned across the screen every 1/60 th of a second. As the fields combine or “interlace” on the screen, a full frame of TV in Standard Definition is presented.

DVD Resolution

Stored on a DVD is an MPEG digital bit stream representing the video. Older DVD players would output only 480i images from a DVD, because most older analog TV sets would only work with a 480i input. However, even inexpensive new DVD players now have internal circuits that will output the DVD’s video in either 480i or 480 progressively scanned lines, known as “480p.” This is called Enhanced Definition, and gives a smoother more film-like look, with no visible scanning lines. Any fixed-pixel display will display this standard, so when you see a display described as having a native resolution of “800 x 600” pixels, you know that it has enough resolution to extract virtually full clarity from a DVD player that outputs a 480p, progressively scanned image. That resolution is not high enough to display full HDTV, but it’s nearly enough to capture every line of a wide-screen DVD, which calls for 852 x 480 resolution. Put another way, the resolution would measure 852 pixels across each of 480 horizontal lines scanned sequentially from top to bottom. Typically this is the resolution of the least expensive plasma and LCD thin-panel displays, as well as inexpensive DLP projectors that use the 800 x 600 DLP chips.

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DVD Quality is not HDTV

In practice, most of these displays will look quite detailed with DVD playback and even HDTV signals that are “scaled” or down-converted to fit the display’s 852 x 480 native resolution. However, such a display or projector will not let you view true High Definition signals in their original resolution.

HDTV Resolutions

To take full advantage of HDTV’s ability to render spectacular clarity and detail, you must find an LCD, plasma, or DLP device that has a native resolution of either 1,280 x 720 pixels (720 lines progressively scanned with a widescreen 16:9 aspect ratio) or “1080i” (1920 x 1080), which represents a 16:9 widescreen image with 1920 pixels across each of 1080 interlaced scan lines. These are the only two High Definition formats defined by the HDTV standard. All network broadcasters use one or the other for their HD programs. For instance, ABC and Fox broadcast in 720p, while CBS, NBC, and PBS use 1080i. Likewise, cable and satellite networks will use one or the other: HBO, HDNet, DiscoveryHD, and Showtime use 1080i, whereas ESPN uses 720p. Broadcasters choose one or the other for different reasons. Progressive scanning (720p) produces a smoother, more film-like look, but a 1080i image actually contains greater detail. Though it has fewer lines, the native progressive scan format (720p) eliminates motion artifacts that originate in interlacing. For subject matter that contains a lot of rapid motion–Monday Night Football, basketball or hockey games, for example–720p will produce a clearer, more stable picture than 1080i. Alternatively, for subject matter that has very little motion, 1080i is capable of rendering more picture detail. And because 720p has the highest data bandwidth and horizontal scan rate, it usually means that 720p programming is converted or “scaled” to 1080i for transmission (it occupies less digital “space” than 720p).

Making Every Pixel Count

When you choose an HD display, it must be able to receive and display both of these formats, either natively or by converting (scaling) the incoming HD signal to the display’s native resolution. For example, a 1280 x 720 fixed-pixel LCD or plasma panel or DLP projector will have an internal scaler that will convert every incoming video signal so that it “fits” its 1280 x 720 native resolution. How well the internal scaler or converter does this may vary from one brand of set to another. And many outboard HDTV digital cable boxes and satellite tuners can be set to output their signals to exactly match your HD display’s native resolution. Sometimes your HD set’s internal scaler may do a better job than the outboard cable box or receiver at converting, say, 1080i to 720p, or 480i to 720p. In other cases, there may be little or no difference. You can find out with a bit of experimentation, but that is a subject for another newsletter. You can get a quick fix on this by viewing a display in a store with a variety of different input signals, including analog cable and regular broadcast TV, as well as DVDs and High-Def signals. Of course, it’s difficult to do that because retailers love to showcase their new sets with HD programming so the image has the greatest impact. But if you ask, most stores will switch to a local cable or broadcast feed that will give you some idea of what you’ll see viewing regular analog TV signals. And be prepared to become an HD snob. Once tasted, HD images are very seductive, and it’s hard to go back to viewing “old” 4:3 non-HD signals. But each year brings more and more HD programming, so your care in understanding and choosing the right fixed-pixel display now will assure you of beautiful HD images in the future as more and more High-Def programming becomes available.

by Alan Lofft (bio), Axiom Audio (reprinted with permission)

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