Deeper Knowledge: Displays

by Reads (10,854)

Desktop computers–and televisions as well–use a display technology that has been around for decades. It’s called the cathode-ray tube, or CRT, however, most people nowadays refer to CRTs simply as monitors. Handheld computers, on the other hand, utilize a newer technology called liquid crystal display, or LCD.

CRT. A CRT consists of a glass tube that contains electron guns and a screen comprised of red, green, and blue phosphor dots. The electron guns shoot streams of electrons across the back of the screen–from left to right, top to bottom. The electrons then illuminate some of the phosphor dots, creating images on the screen. These electron guns work very quickly–faster than the human eye can detect–redrawing the screen over and over again.

This constant redrawing of the screen, called refreshing, is something you can’t see simply by looking at a computer screen. That is, unless you’ve ever had the opportunity to watch a videotape replay of a computer screen on television, possibly on a news segment about computers. This is because the speed at which the television set you are watching is refreshed (called the refresh rate, or vertical scan rate) combines with the rate at which the computer’s screen in the video was being refreshed. This often slows everything down to the point where you can actually see a line moving from the top to the bottom of the screen. That’s called flicker, and is often caused by a refresh rate that is just slow enough that you can see the screen being redrawn by the electron guns.

That’s CRT technology and, as we’ve said, it’s been around for many years.

LCD. Handheld computers, however, have entirely different requirements than desktop computers when it comes to displays. They need screens that are thin, lightweight, low-power, and touch-sensitive.

CRTs, however, fall short in several of these critical areas. For starters, there’s size. The typical CRT monitor is relatively large and heavy. The rule of thumb is that a CRT is as deep as it is diagonally wide. So, a 15" screen will be at least 15" deep. Imagine a Pocket PC three or four inches thick! Practically unusable. And anyone who’s had to pick up a monitor knows just how heavy they can be.

Also, CRTs use a lot of power, which is fine if you’re plugged into an electrical socket, but tragic if you aren’t.

So to address these requirements, PDA manufacturers turned to a more recent technology called liquid crystal display, or LCD. An LCD is a display that uses rod-shaped molecules, called liquid crystals, to create images on a screen, rather than the phosphor dots and electron beams used by CRTs.

Liquid crystals are materials that are not quite solid, yet not quite liquid. They were discovered in 1888 by an Austrian botanist, but it took three-quarters of a century to come up with a practical use for them.

In 1963, a scientist at RCA in New Jersey discovered that liquid crystals in their natural state allow light to flow straight through them. But when liquid crystals are given an electrical charge they redirect, or bend, light.

That revelation provided the first indication that liquid crystals could be used in a new form of display device. And there were two things about liquid crystals that were appealing. First, they were small, so display devices using them would, potentially, be extremely thin. Secondly, unlike CRTs, displays using liquid crystals would be non-emissive. In other words, they would not generate their own light. Instead, they could utilize external lighting, including, among other things, ambient light. Therefore, they’d use small amounts of power.

Over the next few years, several additional insights were made. First, it was apparent that the rod-shaped liquid crystals had to be fashioned into a screen. Dai-Nippon Screening Company developed a method for aligning liquid crystals between two sheets of glass by first coating the glass with a Polyimide-based film.

Next, polarizing filters were added to the equation. A polarizing filter works somewhat like a shutter, letting light in or blocking it out, depending upon the angle of the light. And remember, that’s essentially what liquid crystals do — redirect light.

Finally, at some point, it was discovered that the more the liquid crystal molecules were twisted, the better the contrast. Twist the crystal 90% (called TN, or twisted nematic) and the contrast improves. Twist the crystal 140% (called STN, or super-twisted nematic) and the contrast is even better.

In 1968, RCA succeeded in creating a prototype liquid crystal display (LCD). However, the liquid crystals used in these early devices were much too unstable for manufacturing. That is, until a professor at the University of Hull in the United Kingdom discovered biphenyl, a highly stable liquid crystal material. Since then there have been major advances in liquid crystal materials, but it was biphenyl that first made production of LCDs on a widescale basis possible.

Sharp took things a step further, refining LCD technology until, in 1973, they introduced the world’s first consumer product that featured an LCD, an electronic calculator called the EL-8025. It wasn’t too long before other companies, including Seiko, Hitachi, Hosiden, NEC, Sanyo, Matsushita (Panasonic), and Sony, began exploring the use of LCDs in computer displays.

And now, more than a century after the discovery of liquid crystals, some industry analysts are predicting that LCDs will, in the not-too-distance future, displace CRTs at the top of the display technology heap. 

TFT, DSTN, and other interesting acronyms. Now that we’ve covered the difference between CRTs and LCDs, and explored a bit of LCD history along the way, let’s take a closer look at the current LCDs used in handheld computers.

Active and Passive Displays. Color LCDs come in two flavors: passive and active displays.

Active displays, the most common type of which is thin film transistor or TFT, provide a sharper, clearer image and a broader viewing angle than passive displays.

Active displays accomplish this by, among other things, refreshing the screen more frequently. They use individual transistors that control each pixel of the screen, while passive displays use a grid of horizontal and vertical wires. The first active matrix products (including a "Dick Tracy"-style wristwatch and a 1.5" color TV) were introduced in 1980 by Seiko-Epson.

One downside to active matrix is that a typical Pocket PC’s color display, which is quarter-VGA (QVGA) or 320×240, contains more than 200,000 transistors, making it costlier to fabricate than passive matrix. However, this trade-off seems well worth it. Bottom-line, active matrix displays are clearly superior. In fact, that’s why for some time Casio used a type of TFT LCD, called film super-twisted nematic, or FSTN, in its highly-popular color Cassiopeias.

However, recent passive displays using new DSTN and CSTN technologies, and an even newer technology called High Performance Addressing or HPA, produce sharp colors that rival active displays. And again, passive displays are much less expensive.

DSTN, or double-layer super-twisted nematic, is a passive display technology that uses two display layers. This addresses a color shifting problem associated with conventional super-twist nematic (STN) displays. Still DSTN is not considered as sharp as TFT. Philips used DSTN in their color Nino.

CSTN, or color super-twist nematic, is a display technology developed by Sharp that was used in the Hewlett Packard’s Jornada Palm-size PCs. Although the original CSTN displays developed in the early 1990’s suffered from slow response times and ghosting, recent advances in CSTN have made it a viable alternative to active displays. The latest CSTN displays offer excellent response times, a wide viewing angle, and high quality color that rivals TFT displays. And at about half the cost.

Currently, active displays own 75% of the LCD market and, until recently, looked sure to capture the other quarter. However, with these new advances and their lower cost, passive displays may make a dramatic comeback.

 

Highlights and Lowlights. Despite their prevalence, LCDs are by no means "display nirvana." For one thing, there have been serious shortages in the supply of color LCDs, despite the fact that the LCD market is currently 15 years behind the semiconductor market in terms of growth. So as demand for notebook computers, PDAs, and even Game Boys increases, we’ll continue to experience shortages. Bottom line, there is simply not enough color LCD manufacturing capacity to meet demand.

Then there are the little-talked-about screen imperfections, also known as cell flaws. LCD screens, like their CRT counterparts, are comprised of picture elements, or pixels. Each pixel in a color LCD panel is comprised of three cells, or dots — one each for red, green, and blue. Typically, LCDs are imperfect, you just may not have noticed. Most LCDs have between one and six "bad cells." These "bad cells" are either stuck on or stuck off, causing either a "bright" or a "dark" pixel effect. A good method for checking your LCD is to create a totally white screen and look to see if you see any odd pixels. Then do the same with a totally black screen. But, let’s face it, a couple of "bad cells" out of 200,000 is really not that bad.

Also, there can be ghosting and streaking problems, caused by particularly light or dark images, which affect adjacent portions of the LCD screen. And since liquid crystals bend light, there is a problem with viewing angles. When you’re not directly in front of the display the image can disappear, or even invert, with dark images becoming light and light images becoming dark. This problem is more common with passive displays than with active displays.

Also, with passive displays in particular, there is a condition called submarining. This is where the screen pointer disappears for a few moments. If you’ve ever experienced it, it’s quite unnerving. Finally, the screen can jitter, caused by interference patterns created by fine patterns such as dithered images.

Now for the good news. Liquid crystal material and polarizer technology are rapidly improving, which will result in clearer, brighter displays with greater viewing angles. And, in general, LCD panels produce crisper images and have less flickering than CRTs. The reason is simple. As mentioned earlier, a CRT uses three electron guns — one each for the red, green, and blue signals — to produce a pixel. The guns shoot electron streams that must converge flawlessly in order to create a sharp image. Even the slightest aiming problem can result in fuzziness. Also, the rate at which the CRT’s guns refresh the screen can result in flickering.

LCD panels, on the other hand, use individual cells — one each for red, green, and blue — to create a pixel. Each cell is either wired or controlled by a transistor. And each cell is either switched on or off. No electron guns to aim, no refreshing, no flickering. 

The Future of Small Screens. Manufacturers, meanwhile, continue to look for ways to improve the performance and reduce the power consumption of LCDs. They’ve succeeded by improving the panel aperture ratio, reducing the power used by the driving circuitry, and making more efficient use of backlighting. Also there’s a move toward super-high-definition LCDs with better viewing angles.

However, there are also several other upcoming technologies vying along with LCDs to replace the venerable CRT.

Here are a few.

Electroluminescent displays (ELs) have fast refresh rates, good reliability, and excellent brightness, but they require a lot of power. Not very good for handheld devices.

Field emission displays (FED), or flat CRT technology, looks promising. They have excellent contrast with rich colors, no viewing-angle problems, and are much lower power consumers than CRTs.

OrganicLEDs (OLEDs) use materials that emit light (rather than typical LCDs, which absorb light). Products, including cellular telephones and cameras (such as the Kodak camera pictured at right), that use OLEDs have just begun to hit the market.

Plasma display panels (PDP) have been around awhile but are too high-priced for the PDA market.

And vacuum fluorescent displays (VFDs) use little power but are limited to low-resolution displays. Good for billboard-type displays, but not for handhelds.

Finally, look for head mounted microdisplays and new display devices created around IBM’s new flexible transistor technology to make inroads into portable device viewing. Head mounted microdisplays use less power, have better image quality, weigh less, and have better outdoor readability. All that and they fit like a pair of glasses. And flexible transistors may allow manufacturers to produce screens that can roll up between use or, more interestingly, wear around your wrist.

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