Star Tech Journal

6
STAR*TECH JOURNAL/JUNE 1983
NEWS BITS/
HYBRIDFLATDISPLAYS
OTHER VIDEO GAME APPLICATIONS
HYBRID FLAT DISPLAYS
Data terminals, televisions and the like are
crying for a flat-panel display. The ubiquitous
cathode-ray tube (CRT) is simply too bulky.
But of the numerous flat-panel designs, none
can match the CRT's efficiency, low cost, high
resolution, and ability to display color - until
now, that is, according to engineers at Siemens
AG. By combining CRT technology with one
of the alternative approaches - plasma display
technology - they have developed a high-
resolution, flat-panel display loaded with
features - including the potential for low-cost
production.
A prototype of the display was built at
Siemens' Components Division (Munich, W.
Germany). It measures 14 inches diagonally,
and has an array of 448 by 720 picture
elements, or pixels. Each of these dots is
individually addressable, so the display can
handle graphics as easily as characters. Using
a matrix of 9-by-16 pixels, the panel can display
28 lines of 80 characters that are very pleasing
to the eye. It currently uses a green, mono-
chromatic display, but there is nothing barring
Siemens from extending the technology to full
color. The panel consumes only 20 watts, total.
To build the panel, Siemens begins with a
very shallow glass dish, the bottom of which is
coated with a conductive film. A thin control
plate and a perforated membrane called a
shield electrode are set into the dish, in that
order, and the assembly is covered with a
phosphor-coated glass sheet. The sheet is fused
to the rim of the dish, forming a 6-cm-thick flat
capsule that is subsequently evacuated. External
"driver" electronics are added to complete the
display.
The control plate has conductive electrodes
on both sides. On the side facing the back of the
display (the side facing the conductive bottom
of the tray), there are 448 horizontal conductors
- one for each row of dots on the screen. On
the front of the plate, there are 720 vertical
conductors - one for each column of dots on
the display. There is a hole in the plate at each
point where the horizontal and vertical con-
ductors intersect. In other words, the control
plate has 322,560 holes, one for each possible
Glass spacer frame
dot on the screen. The shield electrode, placed
between the control plate and the screen, is also
perforated with minute holes corresponding to
each dot. It ensures that the energy delivered to
the screen for each dot is consistent.
Like a conventional CRT, electrons are
used to excite the phosphor screen to produce
an image. But the means used to produce and
direct the electrons are very different. In a
conventional CRT, a gun at the back of the tube
generates a beam of electrons, and a deflection
mechanism forces the beam to "paint" an
image line by line on the phosphor-backed
screen.
In contrast, Siemens' display uses a high
voltage between the back electrode and the
horizontal row electrodes on the back of the
control plate to create an electron "gas" called
a plasma. Most of the gas remains confined to
this space, but small jets of the gas are allowed
to pass through the holes in the control plate
and shield electrode, and strike the screen.
As with a normal CRT, not all the dots are
lit at once. The screen is scanned at a particular
"refresh rate". In a CRT, it usually takes
1160th of a second for the beam to paint an
image on the back of the screen. Such a system
is said to use a 60-hertz ( cycles/second) refresh
period. The "persistence" of the phosphor
keeps each dot illuminated until the beam
returns to that point on the screen.
The flat panel is scanned by applying the
high voltage to the top horizontal electrode and
cycling through the column electrodes with a
second voltage to determine whether the dots
for that row will be on or off. After that row, the
next row is electrified, and the columns are
again cycled, and so on. When the bottom-right
comer of the display is reached, the process
starts all over again. It takes I/80th second for
this to happen in Siemens' display. Such an
80Hz refresh is incompatible with the standard
rate, which is one of the drawbacks of the
panel. Engineers are studying the problem, but
have found that slower speeds produce an
undesirable flicker.
The vertical column lines on the front of the
control plate not only determine whether a
Glass
trough
Small jets of an electron gas strike a phosphor-coated screen to produce an image on a
unique flat-panel display.
Continued on next page.