Just two years ago the average CAD monitor had 800 x 600 pixel resolution in a 17-in. CRT — and that was fine. The choices today have exploded to include 21-in. screens aimed at CAD work, wide formats that easily display two standard pages at once, and screen resolutions that reach into the millions.
And although cathode-ray tubes will predominate in the CAD world for years because of their cost advantage, several flat-panel designs are challenging the domination by providing colorful displays in sleek, space-saving packages. The higher cost of flat panels, which are usually liquid-crystal displays, is holding back their wider use. But expect that to change with economies of scale and as manufacturers more thoroughly understand the factors that affect production yields.
More than TV
The ubiquitous CRT works like this: magnetic deflection coils sweep a beam of electrons across a phosphorous-coated screen. The phosphorous glows for an instant when hit by the electrons with an intensity that depends on their number. To produce colors, three electron guns illuminate a screen covered with alternating red, green, and blue dots (one gun is aimed at the dots of each color) arranged in a triangular pattern called triads. Another color scheme uses phosphorous stripes of alternating color. Hitting the dots in a triad with different intensities produces different colors.
In the standard television tube, the screen has a resolution of 320 picture elements or pixels on each of 525 lines and a vertical sweep or refresh rate of 60 Hz. TV also uses an interlaced sweep which means one sweep of the electron beam hits the odd numbered lines and the next sweep hits the even ones. It may sound high tech but it’s technology developed in the 1930s and kept current with a steady stream of improvements.
In contrast, modern monitors are not limited to one set resolution or a specific number of lines. They have no fixed refresh rate, and they use a noninterlaced or single-pass method which means every line get repainted on each sweep of the beam. Some of the latest improvements to the durable CRT includes smaller triads, shorter tubes to consume less desk space, narrow tube necks to shrink power consuming deflection coils, and there’s even a spillover from HDTV research. Here’s a closer look at the most recent steps in the evolution of the venerable CRT.
Shrinking the dot pitch means manufacturing methods allow producing smaller dots closer together. Pixel size and dot pitch are closely related so that as the pitch decreases, the pixel count increases. This allows squeezing more information or detail onto a screen. “Screen resolution will keep going up,” says Sam Miller, manager of new product development with ViewSonic Corp., Walnut, Calif. But it’s not necessarily to the benefit of CAD applications. “Some graphic cards produce 1,800 x 1,440, but you cannot plug them into just any monitor,” he says. They’re more aimed at prepress applications in the printing industry. “The state of the art is in the range of 0.28 to 0.26 mm on the diagonal. This is the shortest distance from one dot to another of the same color, usually a row above or a row below,” he says.
“Frequencies are also going up,” says Miller. A few monitors work in the horizontal sweep rate 110 to 115-kHz range. This raises the number of frames/sec for animation, and minimizes flicker and distortion.
“What’s preferable in CRTs is to have the whole screen refreshed at least 75 times/sec to produce a flicker-free image,” says Alan Petersburg, brand manager for visual products with IBM, Research Triangle Park, N.C.
While higher resolution seems a trend, don’t try shortcuts like buying new graphic cards that promise more pixels. “Should you change to a higher addressability for more rows and columns, and the electron gun sweeps at a fixed speed, what might be 75 Hz at 1,024 x 768 drops to 60 Hz at 1,280 x 1,024. So working at higher addressabilities could produce flicker,” says Petersburg.
In addition to frequency changes, physical dimension is getting attention. Short-neck tubes demonstrate an incremental improvement to conserve desk space. The design can trim up to 4.5 in. off the length of a traditional CRT. This modification will appear on larger monitors first, 19 in. and up, to keep their size in check.
Saving energy has become as critical as saving desk space. The so-called mini or narrow necks will be showing up shortly. The name refers to a smaller diameter yoke. Smaller electron guns allow placing smaller coils around them to control the beam sweeps. “The smaller coils trim power consumption,” says IBM’s Petersburg.
Prices are also getting a trimming. “They have been plunging in the 17 and 19-in. market,” says ViewSonics’s Sam Miller. Monitors that were $800 to $1,000 a few years ago are now in the $1,200 to $500 range. “The 19-in. unit is a recent addition. It gives engineers the versatility of the 21 in. in a smaller footprint,” he says. But if you want to see what could be sitting on your desk in five years, assume price is no object and pick the technology you’d like to have. Your selection may well be a flat-panel display.
The future is flat
Several promising flat-panel technologies have surfaced but the best understood has been the TFT LCD or thin-film transistor liquid-crystal display. These are completely different from CRTs. “There is no electron beam,” says IBM’s Petersburg. “Pixels are either on or off separately. There’s no horizontal line rate to deal with so there’s no flicker. And the picture is crisper and brighter.” An LCD pixel is a small square about 0.27 mm on a side with red, green, and blue vertical rectangles.
Other differences become apparent by considering the curvature on the CRT screen. It occasionally affects CAD work that demands straight lines and perfectly round circles. The TFT avoids the problem by being a perfectly flat-screen surface.
The big advantage of TFTs is their space and power saving, the crisp picture, and lower emissions. “Consider a 20-in. tube and compare it to a TFT that you could hang on a wall, and the value of that space saving can be huge,” says Petersburg. And the 70% reduction in power is significant to most companies with hundreds of monitors installed.
Their only drawback is their price. For example, high-resolution 17-in. monitors run about $500. The 15-in. viewable TFTs which compare to the 17-in. CRTs sell for about $2,200. What might bring down the price of TFTs is a newer generation of glass that allows printing more panels at one time. “These devices are made more like the semiconductor wafers used in processors,” says ViewSonic’s Miller. “It’s like fluid-filled sandwich.” In a nutshell, semiconductors and tracks are printed on glass. Filter material and polarizers are added to generate color, drive electronics are laid around the side, and mounting hardware completes the job.
The resolution limit seems to be about 1,029 x 768 on a 15-in. unit. Screen size can be made larger but pixel size has not dropped. Nevertheless, Miller says ViewSonic will offer an 18-in. flat panel later this year and NEC has demonstrated a 20-in. TFT unit.
But TFTs are not the last word in flat panels. Watch for other technologies to creep in here as well, such as electroluminescence, a phenomena in which a solid emits light under the direct application of an electric field. Technology called field-emission displays also shows promise. They’re instantly on in any temperature, show full motion video without lag, and allow viewing within a 160° angle. So far they are produced only in small sizes. An 8.5-in. unit with 640 x 480 resolution is due this summer from Pix Tech, Santa Clara, Calif., and rumor has it that a 15-in. model will be on display later.
Tips on buying a monitor
In reference to size, most CRTs are nearly cubic so a 17-in. unit is about 17-in. deep. Flat-panel monitors, however, are only about a 6-in. deep at the base. Only a few older designs may not provide the accuracy of straight lines and round circles required by CAD work.
There is some debate as to whether a CAD monitor works best with a shadow mask or aperture grill. These devices fit inside the tube and mask the electron beams to their respective color phosphor. One argument against the aperture grill is that the monitors need two small visible wires across the screen to hold the grill in place. To answer the question as to whether the point matters or not, ask for a demonstration of CRTs side by side and decide for yourself.
As for resolution, one spokesperson remarked that engineers should not consider anything larger than a 0.31-mm phosphor pitch. Larger pitches don’t display fine detail well. And color is usually a function of the video card in the computer. Others also suggests that if you’re upgrading the monitor make sure your graphic card produces the operating sweep rate and resolution that you think you’ll use in the monitor.
Price is always a major consideration. Respectable CAD-worthy 17-in. CRTs can be had for $500 and less. CAD ready 19-in. CRTs run about $1,500. The 16:9, 24-in. wide-screen monitors list for about $4,000, while the 28-in. versions run about $10,000. And flat panels of the 15-in. variety cost about $2,200.