LARGE-SCREEN DISPLAYS: flatter and flatter

May 1, 1999 12:00 PM, Pete Putman

The impact of flat-matrix imaging cannot be underestimated. One major manufacturer is discontinuing the sale of 40 inch (1 m) and larger rear-projection multisync monitors in favor of plasma display panels. Another has abandoned the projector business entirely for PDPs, and a third is moving away from direct-view CRT monitors to plasma. Additionally, 42 inch (1.1 m) versions are priced competitively with 40 inch and larger retro projectors, and they take up less space. Even the sole 50 inch (1.3 m) PDP available (Pioneer's PDP-V501x) is cheaper than most 50 inch to 72 inch (1.8 m) XGA retros.

In each projector market segment (including videowalls), virtually every leading manufacturer has embraced Texas Instruments' DLP imaging, from 6 pound (2.7 kg) ultraportables to lightvalves exceeding 100 pounds (45 kg). LCDs are having the same impact. The installation/integration projector segment is getting more crowded every day with manufacturers, and several models now offer SXGA (1,280x1,024) resolution.

What does this mean? For one thing, the extensive use of flat-matrix imaging has simplified the installation, calibration and day-to-day maintenance of projectors and monitors. In many cases, all that will be required is the replacement of a projection lamp and periodic cleaning of air filters. This simplification and market competition have combined to decrease the costs of both acquisition and ownership.

Another impact has been felt in performance. Five years ago, you would have had to specify a large, heavy projector, ceiling brackets and detailed installation to achieve an RGB image of at least 1,000 lumens. Now, you can buy a box smaller than a desktop computer that weighs 15 pounds (6.8 kg) or less to do the same thing. Interestingly enough, that behemoth from 1994 could handle video, VGA graphics and maybe SVGA, if you got lucky, but no more. Today's brown box projectors can often sync all the way up to UXGA and yield an acceptable picture.

LCDs, DMDs and plasma have simplified the process of purchasing and installing large-screen displays considerably-so much that many customers are able to do a lot of the work themselves. How much knowledge does it take to put a 2,400 lumen projector on a roll-around cart with a universal interface/switcher? How hard is it to unbox and attach brackets to a 42 inch (1.1 m) plasma monitor and hook up a few cables?

For all the convenience that flat-matrix displays provide, they do have a downside. Because LCDs, DMDs and plasma come with a pre-determined resolution, they are not as agile as CRT engines when it comes to handling a wide range of scan rates and input resolutions. That means products like video scalars and high-quality scan converters must be bundled with these imaging technologies to get the best results.

Plasma panels have notable shortcomings in both video quality and variable-resolution scaling. Until Sony bundled its DSC-1024 scan converter into a 42 inch plasma last year, they were essentially useless for anything other than wide VGA images or those measuring 852/853x480 pixels. Now, all major plasma manufacturers are introducing advanced scan conversion to provide 800x600 and 1,024x768 compatibility, a given with older CRT-equipped retro projectors.

Is this intolerable? Not really. Most multimedia monitors have insufficient dot pitch (the diameter of the CRT spot) to handle high-resolution images. Using computer monitors as a baseline, we would like to see a dot pitch of .25 mm to .28 mm when displaying 1,024x768 graphics on a 19 inch (483 mm)) monitor, meaning an equivalent dot pitch of about .4 mm on a 27 inch (686 mm) multiscan monitor and .55 mm on a 37 inch (940 mm) monitor.

The real world? The best 27 inch CRT multiscan monitors have a dot pitch of .6 mm, more suitable for viewing VGA and possibly SVGA. Also, 37 inchh monitors can do between .75 mm to .8 mm, too coarse to view true XGA or SXGA. Sony's engineers figured this out a while ago and decided that scan-converting high-resolution images down to VGA-resolution monitors did not turn out any worse than trying to display those images with coarse dot sizes.

Contrast this with a 42 inch plasma screen. A 4:3 aspect ratio image displayed on it has a diagonal image size of 35 inches (889 mm), about the same as a 37 inch multiscan monitor, but the 42 inch (1.1 m) plasma has coarser resolution 1.08 mm about twice what we want to see in a monitor of that size. Although 50 inch plasma monitors have larger pixel counts-typically 1,280x768 pixels the pixels are still too large; .75 mm would be preferable.

There are three variables at work here screen size, pixel pitch and dot pitch. Even though a plasma display can show a high-resolution signal, you may not be able to perceive all of the resolution in that signal. That will begin to change at INFOCOMM with the introduction of higher-resolution panels from Sony, Sanyo, Hitachi and others. These panels will add more confusion to the mix, however, by using non-square pixels in both 4:3 and 16:9 sizes.

Sony's new PFM-510A1W plasma panel uses 1,024x1,024 non-square pixels, each of which measures .9 mm by .51 mm (that scales to 16:9). The panel size, however, is unchanged with a 42 inch diagonal viewing area. Closer to optimum, but still a little coarse. Hitachi has shown a 1,024x768 40 inch plasma panel, but its pixel pitch is closer to 1 mm. Ditto Sanyo, who also featured a 1,024x1024 16:9 panel at least year's COMDEX show.

It is unrealistic to expect that plasma pixels will become a lot smaller than their current sizes. Given the PDP's unbeatable combination of big screens and light weight, smaller pixels will not be a burning issue for manufacturers; their answer will be to make the panels larger. If you are considering a plasma specification, it would be wise to view the different types of graphics that will be shown on the panel to see how they hold up through the compromises in pixel size and pitch.

Although pixel pitch is not a problem for flat-matrix technologies like LCDs and DMDs, the display of video is definitely a headache. Unfortunately, the best video scaling circuits are found on only the most expensive projectors. If you are considering an LCD installation projector and have a client who is fussy about video image quality, an external scalar is a must.

One trick is to use an external, high-quality line doubler to feed the converted video in as an RGB signal and let the projector scale the image to its full resolution with its internal electronics. Even better, bypass the projector's scaling engine and use an outboard third-party scalar, which will give you more control over picture quality, image size/position adjustments and custom horizontal scan and vertical refresh settings.

Higher-quality scalars also mean greater depth when it comes to color sampling, an absolute must when playing video through plasma monitors. These panels suffer from a unique problem; the response of their individual red, green and blue cells is not always linear across a full grayscale. You may get more of a red color bias in areas with low gray levels. Sometimes, the effect is so pronounced that the image appears to be slightly posterized, resembling a 256-color image and turning dark picture areas lighter.

Based on my tests on 40 inch, 42 inch and 50 inch plasma panels, an outboard scalar is the way to go. Even inexpensive scalars make a noticeable difference in video quality on 42 inch and 50 inch plasma monitors, reducing pixel scaling artifacts and providing a smoother grayscale ramp from dark to light. The plasma monitor is happier as it is being fed an RGB signal with a corrected grayscale at the PDP's native resolution.

When you demo a plasma monitor or flat-matrix projector at INFOCOMM (or anywhere else), have the dealer or rep bring along an outboard scalar so that you can do a critical A/B comparison of video signals. True, the scalar will add to the cost of the installation, but a client spending $12,000 to $22,000 for a flat-screen television or an installation projector will have high expectations.

We are just getting started when it comes to flat-matrix imaging. I have no doubt that many of these technologies will completely replace traditional CRT imaging over the next decade, provided that the problems just mentioned are thoroughly addressed. How close are we to an all-flat display era? Come to Orlando in June, and you will get some clues.