Rear-Projection HDTV
Television was definitely due for an
upgrade. The standard TV system that we watch every day was actually developed in the
1930s, and although it has been enhanced in some ways over the years -- color, stereo
sound, and better techniques for squeezing the last bit of quality out of the signal -- if
I still had the set my parents bought 50 years ago to watch Queen Elizabeth's coronation,
it would function perfectly today.
But that's all changing. Several years ago, government
regulators in the United States adopted a standard for digital video transmission, one
configuration of which is high-definition television (HDTV). In theory, all American TV
stations will have switched to digital by the beginning of 2007, but there is some doubt
as to whether or not that deadline will be met, although several hundred stations are
already on the air. Whether the deadline is met or not, an all-digital system is coming in
the US, and when it does, analog TV will cease to exist as an over-the-air medium.
In Canada, there is no such government-driven switchover,
but digital TV is inevitable anyway. Already, there are cable and satellite services
carrying a slate of HD signals, including the American networks, a movie channel, several
pay-per-view channels, and a handful of Canadian commercial HD stations. There is no doubt
that high-definition TV is here to stay.
With it has blossomed a whole range of sets capable of
displaying the high-quality images. While small direct-view HD sets do exist, the action
is in large-screen projection units, which better suit the spectacular images HDTV is
capable of.
I recently had a look at this new generation of display
devices. At first, it was my intention to look only at smaller sets -- 40" to
46", diagonally -- which would fit in modest spaces, but it became apparent that many
manufacturers are starting with larger sets, and will introduce smaller ones later. So, in
the end, I examined a couple of small sets (40" and 43"), a couple of medium
sets (46" and 47"), and a couple of larger units (50" and 51"). That's
by no means the maximum: HD sets are available as large as 65", but I decided to hold
off on those for now.
Most of the sets use regular CRT technology (see below for
explanation), but we included one LCD unit and one that uses the newest technology,
Digital Light Processing (DLP). The brands we chose didn't include every source of such
sets, but are nonetheless representative of the market: one Euro-American company (RCA),
one Korean (Samsung), and four Japanese (Hitachi, Panasonic, Sony, and Toshiba).
I found that all six monitors in the sample were terrific
television sets, but there were differences among them. For instance, I tended to find
that the pixel-based technologies -- LCD and DLP -- yielded sharper images and needed less
attention in the setup phase. Maybe that's understandable given the price differential:
these were definitely the most expensive sets in the group.
Is the 50" LCD Sony really worth more than twice as
much as the 51" CRT Toshiba? That depends on your budget and your degree of purism,
probably, but it's also true that you are definitely not going to be burned by picking the
more conventional (and cheaper) set.
I also found that, in my viewing circumstances, the smaller
sets tended to look a bit sharper, all else being equal. That, however, is a matter of my
particular space, which, while not tiny, is more suited to the more modest screen sizes.
The larger sets I was able to place at a greater distance, and thus compensate for their
greater size, but I found I preferred to watch a closer screen. In other rooms, however,
the results would be different.
The investigation did, however, point to the absolute
necessity of a careful setup procedure, as most of the sets -- the CRT ones -- were
invariably too bright when first turned on. I expected that to be the case, but most
viewers probably believe that the way a set is adjusted at the factory is ideal. Wrong!
In the end, my reactions had less to do with the particular
monitors I watched than with high-definition television itself. Signals vary, but the good
ones are spectacular. As someone who has watched television in Canada literally since its
beginning (and earlier, as signals slopped over the border from the US), and who has been
subjected to demos of prototype HD systems for something like a quarter of a century, I
can pronounce the present system to be something of a miracle. And all these monitors do
it proud.
The ABCs of rear projection
There's a limit to how large a conventional television can
be made. The practical maximum seems to be 38", diagonal, for a standard 4:3 screen.
One or two 40" tubes have appeared, but that size is more economically handled by
rear-projection television (RPTV), and in fact RP sets do begin at 40" and range
upwards to about 65".
Rear projection (and front projection as well) is all about
generating a small video image and using lenses to blow it up to fill a big screen. The RP
sets available are usually categorized by the technology used to generate that image.
CRT
Cathode-Ray Tube. This is the same technology that is used
in conventional television sets, in which an electron beam (formerly known as a cathode
ray) sweeps across a phosphor-coated surface causing the phosphors to glow in proportion
to the strength of the beam, thus "painting" an image. In RP sets, three such
tubes are used, for each of three colors (red, blue, green), to create a full-color image.
It's important that the output of the three tubes line up exactly, which is called
"convergence," and CRT-based RP sets usually provide a way to check convergence
and correct faults.
All the light in the final projected picture comes from the
glowing phosphors, which traditionally has resulted in somewhat dark images. Recent
improvements have brightened things considerably, however, bringing CRT more into line
with the levels we expect from direct-view televisions.
LCD
Liquid-Crystal Display. Originally used as a low-power
display in things like calculators, LCDs are now widespread in laptop computer monitors
(and many desktops as well), camcorder viewfinders and the like. Liquid crystals have the
property of "twisting" the polarity of light passing through them. If a light
passes through a crystal and then through a fixed polarized filter, the light will be
partially blocked, depending on the current fed to the crystal.
LCD video panels are divided into picture elements, or
"pixels." Instead of the continuously varying electron beam sweeping across a
phosphor-covered screen, each horizontal line is divided up into discrete pixels that are
switched on and off in sequence, their brightness proportionate to the power applied. RP
sets use three LCD panels, for the three above-mentioned colors, with a powerful light
shone through them from behind. Their outputs are combined to be focused on the screen
through a lens.
Early LCD images were characterized by visibility of
individual pixels, and by the difficulty of creating a convincing black. The technology --
especially with the coming of high-definition television -- has virtually eliminated both
problems.
DLP
Digital Light Processing. Developed by Texas Instruments,
DLP is also a technique whereby the image is divided into individual pixels. In the DLP
chip, each pixel is represented by a tiny mirror that actually moves to reflect light from
an external source through a lens or away from it. At any moment, a pixel is fully on or
off, but it can flicker on and off as many as 5000 times a second, the percentage of time
it is actually reflecting determining how bright that pixel will be on the screen.
Theoretically, because it is reflective, there is no limit to how bright a DLP image can
be.
Professional DLP projectors use three chips for red, blue,
and green. Today's generation of consumer DLP units mostly use a single chip that reflects
light through a spinning wheel on which are mounted filters that correspond to the colors.
This is reminiscent of the original color TV system developed at CBS by Peter Goldmark in
the early 1950s. That succumbed to an all-electronic system developed by rival RCA, but
was revived in the early days of the Moon Walk program as an efficient way to send color
pictures from space.
...Ian G. Masters
ian@mastersonaudio.com
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