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Flat-Panel TV's: Plasma and LCD

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.

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 décor-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 display / Flat-panel display
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"!

Now that you know the difference between flat-panel TVs and CRT TVs, we'll look at the main differences between plasma TVs and LCD TVs.

How a plasma TV panel works
A plasma TV is sometimes called an "emissive" display - the panel is actually self-lighting. The display consists of two transparent glass panels with a thin layer of pixels sandwiched in between. Each pixel is composed of three gas-filled cells or sub-pixels (one each for red, green and blue). A grid of tiny electrodes applies an electric current to the individual cells, causing the gas (a mix of neon and xenon) in the cells to ionize. This ionized gas (plasma) emits high-frequency UV rays, which stimulate the cells' phosphors, causing them to glow the desired color.

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Each individual plasma cell is switched on and off by its own electrode. An HDTV-capable plasma TV can have up to 3 million of these cells!

Because a plasma panel is illuminated at the sub-pixel level, images are extremely accurate, and the panel's light output is both high and consistent across the entire screen area. Plasma TVs also provide very wide horizontal and vertical viewing angles, especially when compared to conventional rear-projection TVs. Picture quality looks sharp and bright from virtually anywhere in the room. Because plasma TV screens do use a phosphor coating (like direct-view and projection CRT TVs), the potential for image burn-in exists, so it's important to follow the manufacturer's recommendations on day-to-day use.

Until recently, all flat-panel TVs were progressive displays - at any given moment all of the pixels are illuminated. But plasma TVs based on innovative AliS technology (Alternate Lighting of Surfaces) are proving that a non-progressive picture can look outstanding. Typical plasma panels have a strip of electrodes for each horizontal row of plasma cells, while ALiS panels share an electrode strip between two rows of cells. At any given instant only half the panel's pixels are turned on. It's somewhat similar to interlaced-scanning on a CRT-based TV - in fact, ALiS technology was developed as a way to make a simpler, lower-cost plasma panel capable of displaying interlaced HDTV signals (1080i).

ALis Panel / Progressive Panel
Two HDTV-capable plasma panel designs. The ALiS panel (left) is a simpler design that uses a "trough" structure instead of the conventional "cell" structure, while also streamlining the drive electronics (fewer electrodes). Bottom line: Both panel designs can deliver outstanding picture quality!

Because there is less screen "real estate" taken up by electrode strips, there's more illuminated area, and as a result, ALiS panels provide a seamlessly clear, bright image. Other ALiS benefits include high-efficiency - they require only half the voltage of conventional plasma drive systems - and extended panel life.

How an LCD TV panel works
An LCD TV is sometimes referred to as a "transmissive" display - light isn't created by the liquid crystals themselves; a light source (bulb) behind the panel shines light through the display. A white diffusion panel behind the LCD redirects and scatters the light evenly to ensure a uniform image.

The display consists of two polarizing transparent panels and a liquid crystal solution sandwiched in between. The screen's front layer of glass is etched on the inside surface in a grid pattern to form a template for the layer of liquid crystals. Liquid crystals are rod-shaped molecules that bend light in response to an electric current - the crystals align so that light cannot pass through them. Each crystal acts like a shutter, either allowing light to pass through or blocking the light. The pattern of transparent and dark crystals forms the image. It's the same display technology behind your digital watch but way more sophisticated.

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The multi-layered structure of an active-matrix LCD panel. Because they use red, green and blue color filters in place of phosphor dots, LCD panels are completely immune to image burn-in.

LCD TVs use the most advanced type of LCD, known as an "active-matrix" LCD. This design is based on thin film transistors (TFT) - basically, tiny switching transistors and capacitors that are arranged in a matrix on a glass substrate. Their job is to rapidly switch the LCD's pixels on and off. In a color TV's LCD, each color pixel is created by three sub-pixels with red, green and blue color filters.

One of the biggest challenges for LCD TV manufacturers has been speeding up the "pixel response" time (how fast an individual pixel's color can change without blurring) to ensure that fast-moving objects don't exhibit "motion lag" or ghosting. It's especially critical for larger-screen LCD TVs where much of the viewing will be DVD movies and/or HDTV.

An important difference between plasma and LCD technology is that an LCD screen doesn't have a coating of phosphor dots (colors are created through the use of filters). That means you'll never have to worry about image burn-in, which is great news, especially for anyone planning to connect a PC or video game system. LCD TVs are extremely energy-efficient, typically consuming 60% less power than comparably-sized tube-type direct-view TVs!

 

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