Screen Size
Screen size normally is measured diagonally, from bottom left corner to top right (or top left to bottom right), in inches. The only time this is ever tricky is when you’re comparing screens with different aspect ratios. Because a 16:9 screen, such as you’d typically find on an HDTV set, is wider relative to its height than a traditional, almost-square 4:3 screen, the same diagonal measurement can represent two very different display shapes. In particular, the height of a 16:9 widescreen display will be less than that of a 4:3 display with the same diagonal measurement.

Aspect Ratio: Widescreen vs. Standard
One of the first things you’ll notice when you walk into a store is that most HDTVs have screens that are almost twice as wide as they are tall — in other words, the screen’s aspect ratio is 16:9 instead of the squarish 4:3 you’re used to. This widescreen aspect ratio was adopted because it is a better match to the shape of the human visual field, is mathematically related in a simple way to the old 4:3 ratio, and is almost exactly the same as the ratio used for most movies made in the last 50 years. Besides movies, widescreen is also great for sports, delivering more of the action — a third more!

Resolution: What's HDTV and What Isn't
Historically, a TV’s vertical resolution has been given as the number of horizontal scan lines, or pixel rows, displayed from top to bottom of the screen, multiplied by 0.7 (the Kell factor, which adjusts for certain effects that reduce perceived resolution). Its horizontal resolution has been the maximum number of vertical lines, or pixel columns, discernible from left to right across a screen width equal to the screen height. (Measured this way, broadcast NTSC TV, with 480 picture-carrying scan lines per frame, has a resolution of approximately 335 lines, both horizontally and vertically, while DVD boosts the maximum horizontal resolution to 540 lines.) Since the advent of digital TV, however, standard practice has increasingly followed that of the computer industry, giving vertical resolution simply as the number of horizontal scan lines or pixel rows displayed and horizontal resolution as the number of discernible vertical lines or pixel columns across the entire screen width.

One of the two commonly used high-def signal formats is 720p (progressive-scan), which has 720 pixel rows and 1,280 pixel columns video per frame. The other is 1080i (interlaced), which has 1,080 pixel rows and 1,920 pixel columns per frame. Both use a 16:9 widescreen frame, and if you do the math you will discover that in both formats the number of pixel columns across a width equal to the screen height is the same as the number of pixel rows. The pixels are thus said to be square.

It would be nice if all HDTVs had resolutions matching one or both of the broadcast standards, but the specs can vary a lot — especially among two kinds of fixed-pixel displays, plasma and LCD. Some screens might match up perfectly with one of the high-def formats, but many have native resolutions such as 1,024 x 1,024 pixels or 768 x 1,366 pixels, so the TVs convert incoming video to match. These models are still considered HDTVs, which have come to be defined as any sets that can display at least 720 pixel rows or scan lines.

Color Temperature
In the video world, every color and shade is produced by different combinations of red, green, and blue light at different levels of intensity. Full black is a complete absence of light, and peak white is produced by an equal mix of colors at maximum intensity. The grayscale is the range of steps between full black and peak white. Ideally, the only thing that changes from one shade of gray to the next is the intensity of the light, not the color mix. For various reasons, however, most video displays find it hard to reproduce full black — CRTs do this best. Peak white also varies from the ideal, the variations being measured in terms of color temperature, which for a TV can range from 4,000 to 5,000 K (or kelvins), orangeish whites as from an ordinary incandescent light bulb, to well over 10,000 K, a bluish white produced by some high-intensity lamps. In 1953, the National Television System Committee (NTSC) decided that the correct shade of white for U.S. TVs is about 6,500 K, or the color of sunlight at noon on a clear day (at least under an unpolluted North American sky). The new ATSC digital TV system carries over the same color-temperature reference point.

Why does this matter? Since the largest part of a video signal consists of black-and-white information, any deviation from the white-level reference of 6,500 K — whether toward the red end of the spectrum or the blue end — will bias all images in the same way. More specifically, since the studio monitors used for color-correcting TV shows and DVD masters are calibrated to 6,500 K, your TV must be set to the same color temperature if you want it to accurately recreate the images.

Contrast Ratio
A technical spec that many manufacturers tout is contrast ratio, and it’s not unusual to see outrageously high claims, such as 10,000:1 or even higher. Unfortunately, marketing departments have glommed onto this spec, so the numbers should be taken with a grain of salt. These measurements are often not done uniformly or under “real world” viewing conditions. A properly calibrated CRT TV will yield a contrast ratio of 300:1 to 700:1, while some of the newer technologies, such as LCD and DLP (Digital Light Processing), can deliver up to 900:1. These figures are a lot lower than the hyped specs because the color temperature and white levels have been adjusted correctly and because they’re calculated using a checkerboard pattern rather than alternating white and dark screens.

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