CRTs still dominate high-resolution imaging

CRT.SEP–pm

CRTs still dominate high-resolution imaging

Low cost and high performance remain the hot selling points for CRT-based
monitors–ensuring their place at the top of the high-resolution display
market

BY PATRICK MANNION Associate Editor

Despite advances in flat-panel display technology, the CRT is still the
predominant high-resolution display medium on the market today. By all
accounts, it will remain so for many years to come. That has as much to do
with the CRT's own inherent characteristics as with the limitations of its
closest rivals, the much-acclaimed flat-panel display technologies. These
include liquid-crystal displays (LCDs) and emissive technologies like
electroluminescent and gas plasma displays. Active-matrix LCDs (AMLCDs),
for example, have been plagued by low yields and therefore high cost,
running into the $3,000 to $7,000 range, depending on size. Regardless of
the price premium, research continues because of the many advantages of
AMLCD technology. These advantages include low power consumption, small
footprint, and bright, dynamic, color images. The limited color
capabilities and relatively high power and voltage requirements of
emissive technologies have inhibited their acceptance on a large scale.
However, their high contrast, rugged construction, and wide viewing angle
have pushed them into niche markets in industry, military, and medical
equipment applications. Plasma screens can be very large (screens that
measure 1-m diagonal are available), and they are therefore valuable in
specialized CAM, CAD, and CAE systems. Unlike LCDs, however, these
displays are not conducive to portable computer designs. Neither, however,
is the CRT. The CRT is big. A typical 17-in. monitor, comprising the
basic elements shown in Fig. 1, reaches a depth of almost 18 in. and a
weight of up to 60 lb. The vacuum tube renders it useless for any
application that might subject it to persistent shock or vibration. The
trend toward flatter, squarer screens has resulted in still weaker tubes
because of alterations in geometry. What the CRT does have going for it is
low cost, high resolution, unlimited color capability, high brightness,
high contrast, and wide viewing angles. For applications where size and
power consumption do not factor in, the CRT is king.
Improving a mature technology like the CRT demands maximum investment
for what may seem like minimum gain. The eventual goal is to make the CRT
capable of displaying all the information the eye can perceive. To this
end, research has been concentrated on ways of improving resolution,
brightness, contrast, and screen size. Of these parameters, resolution is
by far the most important when it comes to evaluating performance.
Defined as the ability to delineate picture detail, resolution should not
be confused with addressability, which is a measure of the accuracy with
which the spot can be placed on the screen. Resolution depends on many
factors. These include spot size, line width, deflection effects (scan
velocity, defocusing, screen geometry), beam current, video amplifier
performance, optical properties of the phosphor, structure of the mask and
phosphor, and halation. A widely used metric that formulates the
resolution of a CRT is its modulation transfer function area (MTFA).
Although resolution may be the most important factor in improving the CRT,
brightness is a close second. The American National Standards Institute
(ANSI) specifies a minimum luminance value of 10.25 fL. Any value above
this will have little effect on the user's ability to discern images.
Unfortunately, high brightness requires high beam current, which produces
strong space-charge interactions. These interactions broaden the spot and
line width and so reduce resolution. The brightness is further reduced by
the use of a shadow mask in many color CRTs. This mask is needed for
accurate convergence, which is a measure of the ability of a color display
to maintain coincidence in the landing of the three beams on the screen.
Misconvergence results in the broadening of the line width of composite
beams and therefore also reduces resolution. It can also cause color
fringing effects at edges between dark and light areas. To nullify its
effect, convergence should be specified to be no larger than 1/3 of a
pixel width between any two colors in either orthogonal direction. While
helping to minimize misconvergence, the shadow mask absorbs a lot of the
electron energy emitted and also adversely affects the resolution. It has
been suggested that the CRT mask pitch should be smaller than 0.4 times the
5% dot size to avoid resolution degradation and uneven brightness (or
Moiré effects). The selection of a mask must be a compromise,
however, because of the higher cost of finer-pitch CRTs.
Another disadvantage of the shadow mask is the effect that the
bombardment of electrons has on its geometry. The mask heats up, expands,
and gives a doming effect. A recent cure for this is the flat tension
mask, pioneered by Zenith Electronics. This holds the mask under constant
tension, and so inhibits any tendency to warp or otherwise distort.
The use of the mask exemplifies the overall attention given by
manufacturers to ergonomic details. Looking at a poorly displayed image,
especially one that is under-refreshed, with the resulting flicker, can
give users a headache. The flicker can be avoided by increasing the
refresh rate (a refresh rate of 70 Hz virtually eliminates flicker), but
this requires more sophisticated drive circuitry. Increasing phosphor
persistence can also reduce flicker. However, like in frequency
adjustment, there is an optimum point. Clinton Electronics Corp.,
Rockford, IL, a manufacturer of CRT tubes, has conducted extensive
research into methods of predicting the likelihood of flicker occurring
under given refresh and reviewing conditions. One method uses Fourier
analysis in a relationship called the “ripple ratio.” The method uses
harmonic analysis of a periodic light source to characterize human
susceptibility to perceived flicker produced by that source. However, Roger
Pearson, a Research and Development Engineer at Clinton, warns that many
designers applying harmonic analysis to CRT-phosphor persistence
characteristics are not aware of the variety of phosphor characteristics.
Designers assign an exponential decay curve which is inaccurate because
many phosphors are a combination of two or more component phosphors. Each
component may have its own decay characteristic. Terms used to describe
phosphors include short, long, medium-long, and medium-short, depending on
the particular blend of phosphors. The terms refer to the time it takes
the light emitted from the phosphor to decay by 10% after the electron
stimulation is removed.
The source of the electrons, the electron gun, is also the object of
much research. A typical gun is formed from a nickel cylinder on top of
which is sprayed an oxide powder with a low work function. This gun
readily emits electrons when heat is applied. The problem is that the
oxide tends to sputter away quickly, and it has a limited life and current
capability. To counteract this, Thomas Electronics, Wayne, NJ, is hoping
to incorporate dispenser cathodes into its line of custom CRT displays. A
dispenser cathode uses the same oxide, except it is impregnated into a
host matrix metal like tungsten, which is then machined and polished. This
gives a very smooth, uniform, emitting surface, and therefore a better
quality beam. The lifespan of the cathode is improved as it is no longer a
surface spray of powder, but rather an actual reservoir of oxide in a cake
format. The dispensers can be driven at much higher current densities.
Combined with a cleaner beam, the higher current densities allow for
better resolution.
Although dispenser cathodes have been around for a number of years,
their high price has relegated them to the realm of CRT projection
systems. However, according to Thomas' Doug Ketchum, their long-life
characteristic is making their use in regular CRTs more and more inviting.
At the moment, Thomas Electronics makes custom CRTs. Using primarily
Toshiba, and secondarily Matsushita and Philips tubes, Thomas adds
contrast enhancement filters, custom deflection coils, magnetic shielding,
and field cancellation. A typical selection of products can be seen in
Fig. 2. The largest of these, the 21-in. 21M206P, is a high-resolution,
magnetic deflection, electrostatic focus, cathode ray tube. The tube uses
P4 phosphor with paper-white luminescence and medium-short persistence.
The face plate has a light transmission of 52%. The maximum accelerator
and focus voltages are 25,000 and 8,000 Vdc, respectively. The peak
heater-cathode voltage, cold or hot, is +/-175 Vdc. When analyzing
high-resolution displays for performance, two popular buzzwords are
dynamic focus and dynamic astigmatism correction. Dynamic focus
counteracts the effect of moving the beam from the center of the screen.
The further it moves away, the further it goes from the optimum focal
point, causing the dot to get bigger. Astigmatism describes the
distortion of an image as it moves into the corner of the screen. The trend
toward flatter screens has reduced the naturally compensating effect of
the screen's curvature. Both dynamic focus and astigmatism correction
fall into the general category of nonlinearity correction. Errors in
linearity are inevitable and are intrinsic to the physics of magnetic
deflection and CRT geometry. One of the most obvious effects of
nonlinearity is the pin-cushion effect caused by horizontal and vertical
lines that are not straight. The key to correcting this lies in the yoke.
Most in-line color CRTs include deflection yokes that are corrected for
top and bottom pin-cushion effects within the 2% limits imposed by ANSI.

Resolution vs. application The resolution requirements from a CRT
increase in proportion to the image-quality level required by different
applications. For high-performance workstations and CAD/CAM monitors, the
resolution required is at least 1,280 x 1,024 pixels. This is just above
the regular, commodity monitor resolutions of 1,024 x 768. Typical of
this high resolution is the 20-in. ViewSonic 8 monitor (see Fig. 3), from
ViewSonic, Santa Fe Springs, CA. The ViewSonic 8 has a 1,280 x 1,024-pixel
resolution, along with up-front controls, anti-glare and anti-static
screen, and MPR II compliance. The MPR II standard originates from Sweden
and defines the maximum amount of magnetic radiation that may be emitted
from a monitor. The need for such protection is still questionable.
However, there has been extensive sensationalist media coverage of the
possibility of side effects due to prolonged exposure to a CRT monitor.
This alone is enough to scare prospective buyers into making protection
from low-frequency electrical (MPR I) and magnetic radiation a high
priority. Another feature of the ViewSonic 8 is the ability to scan any
horizontal frequency in the range of 30 to 64 kHz. This ability is
popularly known as multisyncing, a phrase coined by NEC (see box, “The
scanning of multiple frequencies”). The ViewSonic 8 has 16 preset modes.
Eight of the modes are preset at the factory (although they can be
re-programmed by the user). The remaining eight modes are user defined.
The display uses an Invar shadow mask, and a degausser control erases the
magnetic field. The monitor has a video bandwidth of 110 MHz, is
compatible with IBM, Macintosh II, or Sun systems. It is available now for
$2,399, with quantity discounts. Different companies have coined
different terms to describe the same ability to work with any scanning
frequency. Some call the ability to store preset frequencies autosizing.
However, true autosizing means the monitor can maintain a constant
resolution no matter what the graphics adapter dictates. Although this is
technologically feasible, it is an extremely expensive option and is not
considered cost effective by either vendors or users.
Along the same vein of adaptability, CTX International, Inc., Walnut,
CA, has its version of multisyncing, which it calls autoscanning, in the
CPS1760 (see Fig. 4). This is a member of CTX's ProSeries line of monitors
for high-end graphics and CAD/CAM applications. The CPS1760's 17-in.
screen has a resolution of 1,280 x 1,024. It uses an NEC-manufactured
flat-square tube with a medium-short persistence phosphor. The dot pitch
is 0.28 mm, the video bandwidth is 100 MHz, and the horizontal and
vertical scanning frequencies are 30 to 65 kHz and 50 to 90 Hz,
respectively. The CPS1760 sells for $1,399 and is available now. Still
another implementation of microprocessor-controlled frequency scanning and
storage comes in the form of digital memory sizing (DMS) from Electrohome,
Kitchener, Ontario, Canada. Incorporated into the ECM 2000 color monitor
(see Fig. 5), the DMS feature allows the monitor to store up to 32
different signals. These are in addition to the eight standard signals
preset at the factory. The horizontal and vertical scanning ranges are 15
to 38 kHz and 45 to 120 Hz, respectively. Although the screen is large
(20 in.), a resolution of 1,024 x 768 keeps the ECM 2000 series at the
lower end of the CAD/CAM and desktop-publishing markets for which the
monitor is geared. The monitor is UL, FCC, and CSA approved and has a
maximum video bandwidth of 40 MHz. The ECM 2000 costs $3,195 and is
available now.

The limitations of color While resolutions of 1,280 x 1,024 may be
sufficient for many CAD/CAM, CAE, and desktop-publishing applications, it
is not adequate for the medical industry. Here, the ability to discern
even the smallest detail can be a life-saving capability. According to Bob
Valentine of Display Technologies, Inc., Elgin, IL, color monitors are
limited in resolution to below 1,600 x 1,200. This is because of the
limitations of the shadow mask, which must be of an extremely fine pitch
to be able to resolve the three beams. Beyond this resolution, monochrome
CRTs are dominant. This is partly because only a single beam is involved.
The beam can be focused down the center of the lens, taking full advantage
of the best part of the lens. The resolution can therefore be increased
because of the improved beam quality. Display Technologies has taken
advantage of the high-resolution and high-luminance capability of
monochrome CRTs by introducing the Eagle (see Fig. 6). This is a 19- or
21-in. 1,600 x 1,280-pixel monochrome monitor with a 76-Hz refresh rate.
The Eagle's resolution fits in at the cutoff point for color monitors and
has a horizontal scan rate range of 80 to 105 kHz. The vertical scanning
frequency range is 60 to 76 Hz, and the video bandwidth is 265 MPPS.
Multiple agency approvals have been obtained. The monitor is completed to
customer requirements, and pricing varies accordingly. The QES-1000
series of monitors from Dynamic Displays, Eau Claire, WI, are also in the
1,600 x 1,280-pixel-resolution group. The horizontal and vertical scanning
rates, however, are not as high at 15.75 to 85 kHz and 30 to 120 Hz,
respectively. The video bandwidth is 150 MHz, the size stability is +/-1%
and the brightness is 60 fL. Screen sizes range from 7 to 23 in. The
monitors may be in a portrait or landscape orientation in a standard or
custom chassis. Image Systems Corp., Hopkins, MN, also has a portrait
monitor, the M21PMAX (see Fig. 7), with a similar resolution of 1,024 x 1,
664. However, it does not stop there. Other monitors available from the
company go as far as 2,048 x 1,536 with bandwidths of 200 MHz. Image
Systems sees the need for resolutions as high as 2,048 x 2,048 with
bandwidths up to 350 MHz. The targeted markets are medical imaging,
simulation, engineering, and other technical industries. To achieve this
level of performance, the company is working on new tube, yoke, and
video-amp technology. Meanwhile, the M21PMAX has a 21-in. screen and
horizontal and vertical scanning frequencies of 48 to 108 kHz and 60 to 80
Hz, respectively. The screen brightness is 65 fL. The monitor is
compatible with the PC, PS/2, and Macintosh computers, as well as Sun,
Apollo, Digital Equipment, and Silicon Graphics workstations, among
others. The M21PMAX is priced at $2,500 and is available now. The
medical-imaging market may be relatively small, but its importance negates
any attempt to scrimp on cost. Because of this, manufacturers like AEG
(Basking Ridge, NJ) and Tektronix (Beaverton, OR) have no qualms about
bringing the technology to the edge, confident that buyers will not balk
at the expense. The GMA 202 from Tektronix is an example of this (see Fig.
8). The 19-in. monitor has over 4 million pixels (compared to 2 million
for 1,280 x 1,600 screens) and a screen brightness level of 400 fL. It
weighs in at a hefty $5,175. Besides medical applications, the monitor is
designed for picture archival and communications (PACS), document
retrieval, and electronic publishing markets.

An alternative to the shadow mask To overcome the limitations of the
shadow mask for color CRTs, Sony Corp., Graphics Display Div., San Diego,
CA, has successfully marketed its Trinitron color technology. The
Trinitron tube has three distinct advantages over shadow masked tubes (see
Fig. 9). The first advantage is its use of a one-gun, three-beam electron
gun. This allows all three beams to be focused through the center of a
single lens. The monitor's beam quality is similar to that of a monochrome
monitor. The second advantage is in its use of a grille instead of a
mask. The grille allows for high beam transparency with vertically
uninterrupted stripes. Apart from improving the beam's current/dot-size
ratio (see Fig. 10), the grille also eliminates the doming effect found in
shadow-mask tubes. The third advantage lies in the geometry of the tube
itself. The cylindrical surface (as opposed to the spherical shape of
shadow masked tubes) means that the displayed image is distortion-free
from any angle and gives less ambient light reflection. Sony has used the
Trinitron's advantages in the DDM2801C. This 20 x 20-in. 2,048-dot x 2,
048-line color monitor was designed for high-resolution applications where
color is of vital importance. These applications include air-traffic
control, medical and scientific imaging, CAD/CAE, and electronic prepress.
The monitor has a video bandwidth of 60 to 300 MHz and a brightness of 23
fL. However, at a cost and weight of $36,000 and 238 lb, respectively, the
DDM-2801C has a limited market. The monitor is available in industrial
quantities within 5 months ARO. Although the CRT is not ideally suited to
military applications, efforts to ruggedize the device have seen some
success. Codar Technology, Longmont, CO, has made such an effort in
producing the Eagle series of ruggedized 16- and 19-in. monitors. The
16-in. version users a Zenith FTM tube and has a resolution of 1,280 x 1,
024. The displays are ruggedized from the ground up and conform to
MIL-STD-810 for shock absorption. The monitor can withstand 20 g for 11 ms
operating, and 30 g nonoperating. The design is sealed and conduction
cooled with a modular approach to the electronics to help reduce the MTTR.
Other features are auto-syncing, built-in test, and compatibility with
most workstations–including those from Sun and Digital Equipment. The
16-in. version of the Eagle is less than $10,000 and is available 90 days
ARO.

Prognosis Despite the attempts by Codar to make the CRT-based monitors
as rugged as possible for the military market, the fragile CRT itself is
still the backbone of the system. Flat-panel technologies still have the
edge in this respect. However, an upcoming technology called “Microtips”
(Electronic Products, Dec. 1991, p. 17) could save the CRT by changing its
format into a flat-panel technology also. The technology involves using
tiny cathodes to emit electrons that strike phosphor on a screen. The idea
is the same as regular CRTs except the emitters are extremely small,
require very small voltages, and do not require any heating. Already,
6-in. monochrome CRTs have appeared measuring only 2 mm thick. According
to Carlo Infante, Senior Consulting Engineer at Digital Equipment Corp.,
Maynard, MA, the prospects for the technology are numerous and exciting.
Although Digital is already strong in the CRT display arena, it has joined
a consortium of other companies to investigate the Microtips technology
further. Called the MCC, the consortium is based in Austin, TX, and
comprises numerous industry-leading display and microelectronics
manufacturers. These include IBM, Hewlett-Packard, Compaq, and Apple.
Regardless of whether or not “Microtips” takes off, it will be some time
before any technology arrives to jeopardize the market for the
time-honored CRT–as we know it.

Box:

The scanning of multiple frequencies

Multiscanning was originally implemented in 1982 by Systems Research
Laboratories (SRL), Dayton, OH, with the introduction of the first
multiple-line-rate monitor. SRL has since updated this original with the
2143 All-Sync, 19-in. ruggedized color monitor. This is capable of
displaying signals from standard RS-170 (15 kHz) to high-performance
workstations (66 kHz). Monitors with this feature use a microprocessor to
recognize the incoming video signal. The microprocessor measures the
horizontal and vertical timing characteristics of the signal and compares
them to those of a selected frequency range in a stored database. If the
measured frequency is within the selected frequency's tolerance range,
then no change is performed on the monitor. When the input frequency
shifts outside the present frequency window, then the microprocessor
starts a search for a match of the new frequency. The list of saved
frequencies is scanned for a match of the horizontal and vertical
frequency. When a match is obtained, the alignment data for this new line
rate are loaded into digital-to-analog converters and sent directly to a
digital control port. The deflection coils are then driven according to
the new data.

CAPTIONS:

Fig. 1. To maintain the competitive edge over flat-panel technology, each
subsection of a CRT monitor remains the object of intensive research and
development.

Fig. 2. To the bare-bones tube, Thomas Electronics adds contrast
enhancement filters, custom deflection coils, magnetic shielding, and
field cancellation.

Fig. 3. Along with such ergonomic features as up-front controls, MPR II
protection, and anti-glare screen, the ViewSonic 8 has 16 preset modes for
its multisyncing feature.

Fig. 4. The CPS1760 is a member of CTX's ProSeries line of monitors for
high-end graphics and CAD/CAM applications.

Fig. 5. Electrohome's implementation of microprocessor-controlled
frequency scanning and storage comes in the form of digital memory sizing
in its ECM 2000 color monitor.

Fig. 6. The resolution of the monochrome Eagle display, from Display
Technologies, fits in at the practical cutoff point for color monitors.

Fig. 7. The M21PMAX has a resolution of 1,024 x 1,664 and is designed for
medical and high-end engineering applications.

Fig. 8. The 19-in. GMA 202 monitor from Tektronix has over 4 million
pixels (compared to 2 million for 1,280 x 1,600 screens) and is the
leading edge for medical imaging applications.

Fig. 9. The three main advantages of Trinitron technology over
shadow-mask technology are the use of one-gun, three-beam, electron gun,
an aperture grille, and a cylindrical screen. ??

Fig. 10. Apart from improving the beam's current/dot-size ratio, the use
of a grille in Sony's Trinitron tubes also eliminates the doming effect
found in shadow-mask tubes.

The following companies' products are mentioned in this article. For
more information, call the contact or circle the reader service number.

AEG Corp. Opto- and Vacuum-Electronics Div. Basking Ridge, NJ Andrew
Fay 908-204-8945

Clinton Electronics Rockford, IL Diane Hurd 815-633-1444

Codar Technology, Inc. Longmont, CO Steve McCann 303-776-0472

CTX International, Inc. Walnut, CA Alex Lee 714-595-6146

Digital Equipment Corp. Maynard, MA Bonnie Whitney 508-635-8290

Display Technologies, Inc. Elgin, IL Lee Kuoni 800-544-8823

Dynamic Displays Eau Claire, WI Len Stewart 715-835-9440

Electrohome, Ltd. Kitchener, Ontario Bruce Brown 519-744-7111

Image Systems Corp. Hopkins, MN Diana Scheff 612-935-1171

Sony Corp. Graphic Display Div. San Diego, CA Eric Korman
619-673-2758

Systems Research Laboratories Dayton, OH Richard Vuketich
513-426-6000

Tektronix, Inc. Beaverton, OR Sat Narayanan 503-627-7111

Thomas Electronics Wayne, NJ Doug Ketchum 201-696-5200

ViewSonic Santa Fe Springs, CA Mike Mahon 800-888-8583

Zenith Electronics CRT and Components Operations Glenview, IL
Richard Kurtz 708-450-8306