Technology and Equipment (continued)
Q. Are all HDTVs flat panels?
A. No--and not all flat panels are HDTVs. It is true, however, that many HDTVs are plasma or LCD panels and that almost all flat-panel sets other than small LCD models are now high-definition. Another big category of HDTVs consists of rear-projection sets (all of which are indeed HD nowadays), most using DLP or LCoS display technology, some LCD, and a handful of traditional cathode-ray tubes, or CRTs. You can also get front projectors based on the same technologies (though there are essentially no CRT-based models available), which project an image across the room onto a screen, as in a movie theater. A very small number of HDTVs using ordinary, if unusually shaped, picture tubes are still available in screen sizes up to 34 inches. The venerable CRT is poised to quit the HDTV field altogether very soon, though, and it is possible that CRT-based TVs of any type, HD or otherwise, will be virtually extinct by the end of the decade.
Q. So what are all these other display technologies--LCD, plasma, LCoS, DLP, and so forth?
A. Let's start with what they have in common. All use fixed-pixel arrays, meaning that instead of adapting the display to the signal (as in a multiscanning CRT monitor), they must adapt the signal to the display. For example, an HDTV native at 1366 x 768 will scale both 720p and 1080i signals to fit all 768 rows of its pixel grid. Some HDTVs also have options for purists to display only 720p without scaling, so there will be 48 "wasted" rows, half on the top and half on the bottom. Most manufacturers default to a full-screen view so consumers won't freak out when they see black bars on the top and bottom of their brand-new TVs.
And all are naturally progressive. Again, this contrasts with CRT-based displays, which can shift between interlaced and progressive modes according to the characteristics of the incoming video signal. (In practice, almost all CRT HDTVs employ just two scanning modes, typically 480p and 1080i, and covert all other video formats to one of those for display.) Finally, all except plasma use filters to obtain the red, green, and blue primary colors used to assemble a color TV picture; plasmas, like CRTs, use colored phosphors.
Q. Since almost all HDTV sets convert all video, including conventional standard-definition broadcast and DVD signals, to their native display resolution, why are HDTV broadcasts and discs necessary? Doesn't the TV make everything that comes into it high-definition?
A. No. It's like pouring a pint of milk into a quart container--the amount of milk doesn't magically double. If an HDTV set does a good job of scaling video to match its own display resolution, DVDs and clean standard-definition broadcasts will look very good, but the pictures will not contain any more actual detail than they would if viewed on a conventional TV. And any flaws in the picture will tend to be magnified, just as you would see when you enlarge a grainy photograph.
Q. So what are the differences among the various non-CRT display technologies?
A. That's a pretty involved story if you get into it, but here's a very abbreviated explanation.
LCDs have pixel cells filled with a liquid-crystal solution (LCD = liquid-crystal display) whose opacity changes according to the electrical current applied to it. Varying the opacity of the pixel cells according to the content of the video signal modulates the amount of light that passes through the panel from lamps behind it. In a flat-panel LCD TV, an array of red, green, and blue filters precisely aligned with the pixel cells provides color. LCD rear-projection TVs, on the other hand, use three small, precisely aligned panels, one for each primary color, bouncing the light that passes through them off a mirror and through a magnifying lens onto the inside of a translucent diffusion screen. And LCD front projectors send the light directly through a lens and out toward a reflective screen.
LCoS (liquid-crystal on silicon) works pretty much like LCD except that it is used only in projectors (front or rear) and places the liquid-crystal arrays on top of reflective surfaces, so that the light bounces off them instead of passing through. DLP (Digital Light Processing) projectors are configured similarly to LCoS models except that the imaging device is a semiconductor chip with an array of hundreds of thousands of tiny mirrors that flip back and forth at high speed under computer control to modulate the amount of light they reflect back.
Like an LCD panel, a plasma display deploys a pixel array across its entire viewing surface. But the pixel cells are filled with an inert gas, and the insides of their front surfaces are coated with phosphors that glow red, green, or blue when hit with ultraviolet light. And ultraviolet light is exactly what the gas emits when it is excited to a plasma state by an electrical charge.
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