Resource Center: HDTV
Now that HDTVs are affordable, how do you want yours?
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Display Technologies CRT | Plasma | DLP | LCD | LCoS | Back to HDTV Overview CRTCathode-ray tube technology was used in the very first TVs, and after all these years, it’s still a common and inexpensive way to get a video image into your home. Direct-view CRTs are the familiar glass-fronted TV sets, which have evolved to include flat-glass and widescreen varieties. Rear- and front-projection CRTs use three tubes side by side (one each for red, green, and blue) to create an image that’s then sprayed onto a large screen. How It Works — A direct-view tube contains a “gun” consisting of a cathode that emits three distinct invisible electron beams and anodes that accelerate them. The beams are aimed at red, green, and blue phosphors set into the screen, which glow to produce the picture. (A metal grid on the tube’s inside face, called a shadow mask, helps keep the beams on target.) Combining the three primary colors in varying proportions produces the full color range. In the three-tube designs used in big-screen projection TVs, each tube fires at its own small screen coated with a single phosphor color. Lenses and mirrors focus the colored images onto the viewing screen. The tubes must be perfectly aligned (“converged”), or else distortions such as red or blue fringes around objects will appear. Pros
•Less expensive than competing technologies (except for front projection). •Reliable. Direct-view tubes last ten years or longer with little or no maintenance. •Very good picture quality, though the technology no longer has the commanding edge it once enjoyed over others with respect to reproduction of deep black and shadow detail. Cons •Big and heavy. •Direct-view screen sizes top out at 36 inches. •Susceptible to burn-in, and CRT projection sets are subject to convergence errors as well. They don’t do as well as the other technologies with ambient room light and produce images that usually look less sharp — although with careful adjustment a CRT projection set can look just as sharp as a flat-panel TV. Where It’s At — Having graced American living rooms for the past half-century, direct-view sets are fading fast in popularity, especially in the high-end market. Big-screen projection CRTs have largely been overtaken by lighter, shallower TVs based on other technologies. Plasma How It Works — Each pixel in a plasma display consists of three gas-filled sub-pixels (cells) coated with red, green, or blue phosphor. Electrodes above and below the cells (the top electrode layer is transparent) jolt them with varying amounts of voltage. This excites the gas in the cells to a plasma state, stimulating the phosphors to produce colored light. Pros
•Uniformly bright picture over a wide viewing angle — even in a brightly lit room. •Svelte design and large screen size. •Wide 16:9 aspect ratio screen and on nearly all current models enough pixels for HDTV resolution. Cons •Can be expensive, especially for screens larger than 50 inches. •On some sets, the black parts of the picture, such as dark shadows or letterbox bars on widescreen movies, look dark gray instead of black. (The best models now do pretty well, however.) •Subject to burn-in, where an image becomes permanently etched onto the screen. But this won’t happen unless you leave a bright stationary image on the screen for hours at a time. Recent models have burn-in prevention features, such as “pixel orbiters,” which exercise pixels by slowly shifting an image around the display. Where It’s At — Plasmas are available from many manufacturers, with 42-inch high-definition models available for as little as $1,200 and 50-inch displays starting at about $1,800. Expect to pay more for top performance, cutting edge features (such as 1080p resolution), or a really huge screen, however. DLP How It Works — High-def DLP TVs use a 16:9 chip, called a Digital Micromirror Device (DMD), packed with a million or more individually hinged and controlled “micromirrors” that pivot to reflect light from a lamp through a lens onto a screen. DLP sets fall mainly into two camps. Single-chip RPTVs and front projectors filter white light from the lamp through a rapidly spinning color wheel to produce color, while the more expensive ($15,000 and up) three-chip front projectors dedicate one chip each to red, green, and blue. (Some single-chip DLP rear-projectors now use high-intensity color LEDs instead of a white lamp, eliminating the need for a color wheel.) Pros
•Though it still can’t yet reproduce dark scenes quite as well as the best CRTs can, DLP can now come very close. DLP sets typically achieve deeper, more realistic shadows and blacks than LCD models and are comparable to plasmas and LCoS displays in this characteristic. •Because the DMD mirrors are so close together, DLP sets normally don’t have a problem with the “screen door” effect (a faint image of the pixel grid) that was especially noticeable with early LCD projectors. •No danger of screen burn-in. •DLP front projectors can cost much less than comparable CRT models, while DLP rear-projection TVs now challenge CRT models in price and are much slimmer and lighter. Cons •DLP rear-projection TVs are somewhat more expensive than the same-size CRT sets. •A small percentage of people occasionally see “rainbows” in images produced by single-chip DLP projectors with color wheels: fleeting separations of the three primary colors into narrow streaks at the edges of moving objects. Incidence of this effect has been dramatically reduced as projectors have moved from the three- and four-segment color wheels of early designs to faster-spinning six-, seven-, and eight-segment wheels, however, and it is completely eliminated in models that use LED or laser light sources instead of conventional white lamps. Where It’s At — Many manufacturers offer DLP RPTVs, with prices starting at about $1,500. And don’t rule out DLP front projectors. Basic models start at around $700, while HDTV-ready models go from about $1,000 up. LCD How It Works — A matrix of thin-film transistors (TFTs) supplies voltage to liquid crystal-filled cells sandwiched between two sheets of glass. As with plasma panels, a trio of red, green, and blue cells make up one pixel. When hit with an electrical charge, the crystals “untwist,” allowing light generated by a lamp behind the screen (for flat-panel TVs) or a lamp shining through a small LCD chip (for projection TVs) to filter through. The higher the voltage, the more a cell “opens up.” Pros
•Direct-view models are only a few inches deep, and their pictures hold up well in bright light. •Using the same wattage lamp, a typical LCD projector will create a brighter image than a DLP model — but most DLP projectors produce enough light for just about any viewing. Cons •Of the fixed-pixel technologies, LCD has the most trouble with blacks. Some light always passes through (the cells are never completely opaque), so the best black is usually a very dark gray. Flat-panel LCDs are significantly better in this respect than they were a few years ago, however. •Because of how light goes through LCD cells, flat-panel displays usually have narrower viewing angles than plasma TVs. •Low-resolution LCDs show pixelation and screen-door effects on big screens. Front projectors with XGA (1,024 x 768) or higher resolution have a reduced screen-door effect. •Flat-panel LCDs may sometimes blur fast motion. The latest models have greatly reduced this tendency, however. Where It’s At — Prices have tumbled, with 42-inch rear-projection models available for as little as $1,500 and 42-inch flat panels for even less. LCD front projectors are priced competitively with DLP models but are less numerous. LCoS How It Works — LCoS borrows from both LCD and DLP technology. As with LCD, each pixel in an LCoS display has liquid crystals that untwist to transmit light, and — as the name indicates — these crystals are applied to a silicon chip instead of sandwiched between glass. As in DLP, light is reflected off the chip toward a screen. LCoS front projectors and RPTVs can be designed with either a single chip, using a color wheel or prisms to separate the light, or three chips, one for each primary color. Pro
•LCoS rear-projection TVs are high-resolution displays (typically 1080p) with tight pixel spacing, yielding an extremely detailed yet smooth image, especially on a very large screen. Cons •Can be expensive. •Struggles to reproduce deep blacks, though performance is getting distinctly better. Where It’s At — LCoS has had a somewhat tortured history, with most companies who have adopted it bailing out soon thereafter (apparently because the chips are very hard to manufacture in quantity). Still, the technology’s resolution capability makes it a real contender among fixed-pixel displays, especially for exceptionally large screens. JVC, which has championed LCoS for years with its high-end D-ILA front projectors, now has some RPTV models, and Sony has made a substantial commitment with its SXRD projectors. |
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