UHF No 70 (Net).indd - Ultra High Fidelity Magazine

Has the cathode ray
tube fi nally earned
the right to a comfy
retirement? In the
world of computers the answer
is pretty much yes. Except for
economy machines, or high
end machines for graphics
artists, new computers mostly
come with liquid crystal
displays. As the
price of LCDs
drops, its
v i c -
tory is
l i k e l y to be complete.
But video is another matter. Though
most computer users favor brightness
and (apparent) sharpness over all
else, owners of home theatre systems
are looking for much more. And new
technologies — some already here and
others on the horizon — will make the
home theatre experience much more like
watching a “real” movie.
The cathode ray tube is a diffi cult act
to follow, though.
The CRT: still alive?
It’s a vacuum tube, of course, one of
the very last ones to survive in massmarket
consumer products. It still works
very well because it is a mature technology.
The CRT has been refi ned to the
point where it has been able to fend off
a number of competitors. And fi rst in its
list of advantages is price.
Price is important, because TV sets
have become a commodity. What we
mean is that the choice is dictated far
more by price and (to a lesser extent)
features than by great technological
advantages. However, the CRT has more
than mere low cost to offer.
Future
Screens
The most important of these
is the range of brightness it can
offer. If it were an audio component,
we would call it dynamic
range. A CRT can be very bright,
but it must get very bright before
it overloads and treats all brightness
values the same. Of course the
tube itself is not the only factor
determining the range of
tones, but at its best it can
make more expensive
d isplay s
look washed
out.
A l o n g
with the wide brightness range comes a
vast range of colors, and it’s easy to see
why. If a display doesn’t wash out in the
bright scenes and doesn’t get murky in
dark scenes, it can present a wider gamut
of colors. That means a CRT-equipped
TV set has less need to “translate” a
color it can’t reproduce into one within
its range. We don’t want to overstate
this point, because no display can come
close to matching the range of colors
visible to the human eye…or even to
photographic fi lm.
If the CRT is so good, why would we
want to replace it?
Unfortunately the CRT also has
a long list of drawbacks. The tube is
large, and especially deep, it is fragile,
it is heavy, and — like other vacuum
tubes — it eats up energy. It is ill-suited
to TV sets bigger than 36 inches (measured
diagonally, about 91 cm). It is also
Can you buy the
perfect video screen?
Perhaps not yet…
ill-suited to the widescreen sets that are
now the norm in home theatre. Let’s see
why.
A CRT is a big glass bottle, with an
emitter of electrons in the “neck” at the
rear. The face is coated with colored
phosphor dots which glow when an electron
beam strikes them. A complex set
of magnetic control devices sweeps the
electron beam across the face, making
the appropriate dots glow to make up
the image.
The illustration shows an early CRT,
with a neck much longer than the width
of the screen. Those early CRTs were
round, to avoid light falloff in the corners,
and even many modern CRTs have
rather rounded corners. As manufacturers
began making larger screens (a 21”
tube used to be the “big screen” norm),
they were reluctant to increase the tube
depth in proportion. The short-neck
tube was born, and as tubes got even
wider, the necks got proportionately
shorter yet. The modern CRT is likely
to be something like this.
Notice that the electron beam going
to the extreme edge of the screen is
traveling a lot farther than the one
ULTRA HIGH FIDELITY Magazine 19
Cinema