King Tube?
There's a lot to be said for a technology that's still
going strong, virtually unchanged, after half a century. If you're in the market for a new
TV set, chances are what you'll buy will differ only in detail from what you could have
purchased in the early 1950s, when the North American NTSC color standard was adopted. And
even that was just a modification of the black-and-white system that first went into
action in the 1930s.
The heart of a conventional television is what is
informally called a "picture tube" and only slightly more accurately referred to
as a "cathode-ray tube" (CRT). In the past, sets that used such tubes were often
called "direct-view" televisions, but that's somewhat misleading these days as
there are displays that use other technologies that are also direct-view.
The CRT is one of the last widespread uses of vacuum tubes,
but it has lasted because it has been exceedingly difficult to produce an alternative
technology that performs as well. Some of the newer systems are getting close, however.
The tube consists of a large, bulbous front portion with a
relatively flat viewing surface, tapering toward the rear to a thin glass neck. In older
sets, there was often a prominent bulge on the back to accommodate the rear portion of
this component. At the back of the tube is a device that shoots a stream of electrons (the
cathode ray) toward the front of the tube, through the neck. This electron beam can be
deflected by magnets, so a series of electromagnets is mounted outside the neck to direct
the beam. A particular combination of voltages fed to the magnets can direct the beam to
any spot on the front surface of the tube. A "local oscillator" feeds a sequence
of signals to the magnets that make the beam sweep across the screen in a series of
scanning lines, left to right, top to bottom.
The inside of the tube's front surface is covered with
phosphors that glow when the electron beam hits them, in proportion to the strength of the
beam. As the beam sweeps across the screen, it virtually paints a picture, line by line,
frame by frame.
In color sets, the phosphors are clustered in groups of
dots in three colors: red, green, and blue. By varying the intensity of the beam as it
hits the dots, a full-color picture can be recreated. The wider the bandwidth of the
signal (i.e., the better it responds to high frequencies), the better the
horizontal resolution (i.e., the sharper the picture).
Traditionally, the front surface of a CRT has been curved.
This was partly to maintain focus as the electron beam swung through its arc, but it was
also necessary for tube strength. A CRT contains a vacuum, which puts immense pressure on
the outside of the tube; a curved surface helps it withstand the pressure, just as the
shape of an egg allows its thin shell to support a nesting bird.
Curved tubes led to various sorts of picture distortions,
however, so some means of creating a flat tube has been sought for years. In the past
decade, tubes did get flatter and squarer, but some curve remained. Recently, however,
advances in the use of tempered glass and computer analysis of stress patterns has allowed
most set manufacturers to create CRTs that are essentially (or completely) flat, without
unduly adding weight. Most other display technologies are inherently flat.
From the earliest days of television, people have wanted to
get a really big picture. In the beginning, you could buy big magnifying glasses that sat
in front of the tube, which created a somewhat larger image, but only for the person
sitting right in front of the set. A more sensible approach was simply to make bigger
tubes, and much progress has been made in this. But there is a limit to how big a CRT can
be; 40 inches seems to be the maximum.
The alternative is to project the picture onto a screen
using lenses. Until recently, virtually all projection TVs used specialized CRTs to
produce the image. The tubes used are much brighter than
a regular TV because the light must be spread over a relatively wide area. However,
because the only light-generating element in them are the phosphors themselves, brightness
is somewhat limited. That's offset to some extent by using multiple tubes, one for
each color.
For most users, the most practical sort of projection TV is
a rear projector, in which the tubes and lenses are contained in a large cabinet and the
images cast on a screen mounted on its front. One of the primary advantages of this system
is that the tubes, which must be carefully aligned so they combine together properly on
the screen, can be set at the factory. Because such sets do require a large enclosure,
however, there is a practical limit to how big they can be: about 60 inches, measured
diagonally.
For larger images, front projectors can be used. These
mostly use basically the same technology, but the projector and screen are separate. The
former can be quite modest in size, and mounted either on the floor or the ceiling. The
screen can be flat -- even a white wall will work -- but the images tend to be fairly
dark. To overcome that, curved screens are sometimes used, but that limits the viewing
angle.
Like regular TV tubes, CRT-based projectors use a
continuously varying spot, whose resolution is determined by the bandwidth of the system.
The alternative technologies break the picture into individual picture elements (pixels)
that are switched on and off in sequence. Resolution is determined by the number of pixels
employed in any device (plus, of course, the limitations of the source material).
The most common is the liquid-crystal display (LCD). In its
most elementary form, this is a reflective device used in the displays of many
calculators, cell phones, and the like, mostly to communicate alphanumeric information.
The manipulation of light polarity that makes the elements of an LCD reflective can also
make them more or less opaque. Placed in front of a backlit panel, the LCD elements can
allow light to pass through or not (or partly).
An active matrix LCD television display contains thousands
of individual pixels, arranged in rows that duplicate the scanning lines of a regular
television, and color clusters that duplicate the phosphor dots in a CRT. Instead of an
electron beam sweeping back and forth, however, tiny transistors (one for each pixel) vary
the opacity of each element in sequence, creating the picture as it selectively lets the
light from behind through.
The direct-view version of this technology is familiar in
laptop computers, camcorder monitors and the like. While such displays have a lot of value
when it comes to compactness and convenience, their image traditionally has been somewhat
coarse compared to a CRT, thanks to the limited number of pixels used. Newer screens are
much improved in this respect, and large LCD screens may well become a contender for the
much-sought-after "flat screen that can hang on a wall like a picture."
LCDs are also used for projection sets, both the front- and
rear-projection sort. They have an inherent advantage in that, unlike CRTs, they are not
light sources, rather they modify the light from an external lamp, like a photographic
slide projector. On the other hand, LCDs are relatively poor transmitters of light -- much
of it gets reflected back to the bulb -- and simply cranking up the wattage will ultimate
destroy the panel.
Still, the first LCD front projectors were far more
flexible than their CRT equivalents because they didn't need the critical alignment of
multiple tubes. Like a slide projector, an LCD unit could simply be aimed at a screen and
focused with a simple twist of the lens. The image could even be zoomed to fit the screen.
Early LCD projectors had low resolution, however, with
easily visible individual pixels. That is largely being dealt with by refining
manufacturing techniques to make much smaller elements, which is an absolute must in these
early days of digital television, especially the high-definition variety.
The dimness of the original LCD projectors, which used a
single panel, has been largely cured by the use of multiple panels for the three colors.
Unlike CRT front projectors, however, these are aligned in the projectors by means of
mirrors or prisms so that optically the three are in the same virtual plane, and can be
focused and zoomed as if they formed a single panel.
The main rival to the direct-view LCD is the plasma display
panel (PDP). Unlike an LCD, which either reflects or transmits light, each pixel in a PDP
is an actual light source, rather like a miniature fluorescent light. This not only saves
on energy consumption, but PDPs are free from the reflection problems that can limit the
viewing angle of an LCD display
Many of the early plasma screens rendered colors
unrealistically, and there was often a problem with brightness, but both these have
improved dramatically in recent models. The PDP's thin, flat profile, plus its extremely
wide viewing angle, make it a good candidate for on-the-wall TV, although current models
may be a bit too heavy for lots of walls.
The champ when it comes to brightness is the digital light
processing (DLP) system developed by Texas Instruments, and now turning up on the high-end
projection models of a wide range of manufacturers. This system uses microscopic mirrors
to reflect (or not) an external light on a screen; each pixel contains a mirror that
actually moves in response to the incoming signal.
At its best, a DLP picture can rival a movie projector in
brightness, and -- as is true with all pixel-based systems -- resolution is only limited
by the number of pixels provided and the quality of the source material.
The choice of technology is very wide when it comes to
television displays, and some of these will undoubtedly fall by the wayside. It's
interesting to note how similar the various systems now look when viewed side by side.
And it's remarkable that, given all the new technology, how
well the old-technology CRT stands up. There's definitely lots of life left in that
system.
...Ian G. Masters
ian@mastersonaudio.com
|
