Plasma TV & LCD TV 101 - The Basic Facts

TVs are changing in major ways, both inside and out, and the most exciting trend is the shift to ultra-thin, lightweight flat-panel TVs. With many models measuring 4 inches deep or less, the dream of hang-on-the-wall TVs is finally a reality. By completely redefining the way we think about TVs, flat-panel technology is probably the most revolutionary innovation in the history of TV — even more significant than the switch from black and white to color, or from analog signals to digital HDTV.

CRT images are typically slightly out of focus at the screen's edges because the electron gun's beam is at a greater angle. A flat-panel TV's perfectly flat grid of pixels can produce images that are very accurate and consistent, from corner to corner.

Flat-panel TVs create bright, crisp images without using the traditional CRT (cathode-ray tube) or "picture tube." These super-slim TVs use either plasma or LCD (Liquid Crystal Display) panels. In the past few years, LCD and plasma technologies have improved dramatically in picture resolution, brightness, and contrast ratio. Now even finicky videophiles are replacing their large, heavy CRT-based TVs with d้cor-friendly plasma or LCD panels. And the best news is that flat-panel screens are getting bigger, better, and cheaper!

Scan lines are out, pixels are in!

The CRT technology at the heart of typical tube-type direct-view and rear-projection TVs has been around for over 100 years. After decades of development and fine-tuning, tube TVs deliver excellent picture quality at relatively affordable prices — today a 30" widescreen tube TV costs much less than the same size plasma or LCD screen. Another CRT advantage is multiple resolutions. The electron gun can change its scanning rate, so, for example, an HDTV-ready TV can scan at one rate for high-definition signals (like 1080i), and another for progressive-scan DVD (480p).

Although their picture and price are appealing, CRT-based TVs have cabinets that are unavoidably heavy and deep. The weight is due to the picture tube — a sealed glass vacuum tube (and the popular flat-glass picture tubes like those in Sony's Wega TVs are especially heavy). The depth is due to the scanning electron gun that creates images by firing a beam of electrons in horizontal passes across the phosphor dot-coated front of the tube. (Phosphors emit red, green or blue light when hit by electrons.) Another CRT disadvantage is susceptibility to distortion caused by a nearby magnet, like the large magnets in non-video-shielded loudspeakers.

The alternative to a conventional CRT-based TV is a space-efficient flat-panel TV based on plasma or LCD technology. Flat-panel screen technology is fundamentally different from traditional CRT technology. Neither LCD nor plasma TVs uses an electron gun or picture tube, yet they create sharp, bright images. In fact, plasma and LCD screens offer brightness and wide viewing angles that are comparable to direct-view CRT TVs (their viewing angles are much wider than those for "big-screen" rear-projection CRT models).

CRT images are typically slightly out of focus at the screen's edges because the electron gun's beam is at a greater angle. A flat-panel TV's perfectly flat grid of pixels can produce images that are very accurate and consistent, from corner to corner.

Flat-panel TVs create images by using a perfectly flat, fixed grid of square or rectangular pixels (short for "picture elements"). You may hear plasma and LCD TVs referred to as "fixed-pixel" displays. Flat-panel TVs only have a single resolution, which is called the "native resolution" of the panel. This number is a pixel count: generally given as the number of horizontal pixels by the number of vertical pixels, such as 640 x 480 or 1024 x 768. The various video signals we watch (480i, 480p, 720p, 1080i) are converted to match the panel's native resolution by a "scaler" that's either built in or a separate component.

Flat-panel TVs can create images that are extremely accurate because the screen's light and color information are controlled digitally at the pixel level! In fact, every pixel in a flat-panel screen actually has three sub-pixels: one each for red, green and blue. Each individual sub-pixel is switched on and off by its own electrode. By carefully controlling the voltage applied, the intensity of each sub-pixel can range over 256 shades. Combining the sub-pixels produces a possible palette of over 16.7 million colors (256 shades of red x 256 shades of green x 256 shades of blue) for each pixel!

This incredible level of precision, along with a totally flat screen, allows flat-panel TVs to have perfect image focus and geometry from top to bottom, side to side, and corner to corner. CRT images are typically slightly out of focus at the edges because the electron gun's beam is at a greater angle. And rear-projection TVs sometimes can have convergence problems where the red, green, and blue CRTs become misaligned so that the picture isn't as sharp as it should be.

One other advantage — especially for home theater fans — is that flat-panel TVs aren't affected by the magnets of nearby unshielded speakers. You can't have picture tube distortion if there is no "tube"!

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