Flat Panels Getting
Better
The notion of a skinny little television that could be hung
on the wall like a painting is probably as old as television itself. But as long as we
were stuck with cathode-ray tube technology for reproducing video images, the picture-thin
TV was bound to remain in the realm of science fiction.
In a conventional picture tube, an electron gun at the rear
emits a constant stream of electrons -- the cathode ray -- which varies in intensity
according to the video information carried on a broadcast signal or video recording. The
beam is deflected by magnets, so that it sweeps back and forth across the front of the
tube like the stream of water from your hose as you water your lawn. The inside of the
tube's front surface is covered with phosphors that glow when the beam hits them, so the
varying flow of electrons "paints" the picture point by point.
The original technology had it that, because the beam
swings through an arc, the screen must be curved. If some points on it were farther away
than others from the beam's source, it would be very difficult to maintain focus
throughout the image. The greater the distance between the origin and the screen -- the
deeper the tube -- the more gentle the curve could be. Even so, the corners would be
farthest away and most subject to distortion, so early tubes rounded the corners severely.
In recent years, sophisticated circuits have been developed
to adjust the electron beam "on the fly" to offset some of these inherent
characteristics, and today's top sets boast screens that are flat and have square corners,
without requiring enormous path lengths from electron gun to screen. There are limits to
how far this approach can be taken, and the current generation of cathode-ray television
sets are still boxy and heavy.
The desire for a panel TV is still with us, and from time
to time rumors emerge of a new technology that will deliver on the promise, usually with
the suggestion that it's being suppressed by sinister pro-cathode-ray forces who want to
foist their inferior system on a helpless public . . . like those purported to have buried
the technology for the super battery or runless pantyhose.
The reality isn't quite so dramatic. Technology that would
lead to such truly flat TV screens first appeared more than two decades ago, and is widely
available in a number of products. Its main drawback so far has been that, when it began
to approach a conventional picture tube in quality, it did so only at enormous cost.
Toshiba's B2 LCD Projector
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In fact, there are two such technologies,
each with its own strengths and weaknesses, each with its fans and detractors. The older
of the two is based on the liquid-crystal display (LCD), familiar from calculators and
digital watches. These are now fixtures in upscale camcorders, in which they are widely
used for color monitoring, and in laptop computers.
The LCD works on the principle of the polarization of
light. Anyone who has had a pair of polarized sunglasses has observed that if you put two
pairs together with the lenses at right angles they block the passage of light, but that
as you rotate them with regard to each other, more and more light is transmitted. An LCD
consists of two sheets of glass with the same polarizing filter as the sunglasses, mounted
at right angles to block the light. In between, however, is sealed a small amount of
liquid crystal, which has the property of "twisting" light as it passes through
when an electric voltage is applied to it. This interferes with the blockage and lets
through an amount of light proportional to the voltage.
An LCD can be compartmentalized into segments of numbers
and letters or into tiny picture elements -- pixels -- to make up a video image. In the
latter case, each pixel must be addressed individually in sequence, rather than scanned by
a continuous electron beam, but that means a perfectly flat, rectangular screen is easy to
achieve. As each pixel is turned on, an external backlight mounted behind the panel
becomes visible to a greater or lesser degree.
In its early days, the LCD television faced a number of
problems that mostly reduced it to the status of technical curiosity. One difficulty was
producing pixels small and numerous enough that they would be invisible individually and
the resolution would be adequately high. Microelectronic manufacturing techniques and
thin-film technology are gradually taming that aspect.
Pioneer's 50" PDP-505HD Plasma Monitor
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More serious was brightness. Since the
polarized panels only let some of the backlight through, and color filters reduce
transmission even further, the tradeoff has been dim pictures versus a bulky and
energy-hungry light source. That and the fact that LCD panels are inherently reflective
have made viewing angle a consideration as well; you don't have to go very far off to the
side before the picture washes out.
Still, a number of companies are addressing these problems
because they are determined that this technology will succeed, and the strides made in a
short time have been considerable.
But there's a rival: the gas plasma display. Like LCD,
plasma must divide the image into pixels and feed them a sequentially-switched signal to
create a picture. Other than that, its a very different matter.
Each pixel consists of a transparent electrode top and
bottom, separated by a space filled with electrically-responsive gas. When a voltage is
applied, ultraviolet light is produced by the gas, and it then strikes a sheet of glass
covered with phosphors. This functions exactly like the inside coating in a fluorescent
light tube and it glows brightly. Thus, instead of simply letting light from an external
source through, as an LCD does, the plasma display actually creates light.
Better than either, perhaps, is Texas Instruments' exotic
micromirror technology, but that's a topic to be addressed in a future column.
...Ian G. Masters
ian@mastersonaudio.com
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