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Copyright © 1995, 1996, 2000
A. Thomas Heald
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Words 3900
Article 2840
Sidebars 1060

 

The Next Wave
by
Tom Heald

Lead Feature Article in Video Magazine, Sept. 1996

 

Digital and HDTV are getting all the headlines, but behind the scenes TV manufacturers are busy perfecting replacements for the old boob-tube and projection TVs. Plasma Display Panels (PDP) appear to be the heir apparent to conventional CRTs while Digital Micromirror Devices (DMD) solve most of the problems inherent with today's projection TVs, but Liquid Crystal Displays (LCD) continue to get better and are challenging both direct-view and projection systems. While each approach is as different as different can be, their theme is the same: Bigger is Better, but Price is Paramount. Their goal: To give us a family of direct-view TVs in the 40 to 60-inch range and wall-size projection systems that are superior in every way to today's TV sets.

TVs using CRTs have reached their practical limit. If the width of a 30-inch TV is doubled to 60-inches, its depth will also double, and its weight will increase at least four times. This clearly violates the law of physics that states: If the depth of a TV exceeds 30-inches, it won't fit through the door. Not only that, you will need a forklift to move it, and it will generate a magnetic field strong enough to pull nails out of the wall.

Plasma Displays

Color Plasma displays are conceptually quite simple. Three tiny fluorescent lamps as small as .2 mm (.008 of an inch) make up each pixel. They work like the fluorescent lamps in your kitchen, only each cell has 256 levels of intensity and produces a red, green, or blue light. This combination allows each pixel to produce a range of 16 million colors.

A PDP's tiny cells are arranged in a grid with one set of control wires running horizontally across the screen and another set running vertically down the screen. By applying a voltage to one horizontal and vertical pair of wires, a single plasma cell will fire where the two wires intersect. To start a picture on the top row of the screen, the top horizontal control wire is selected. Then sending a unique voltage pulse to each of the vertical wires fires the appropriate red, green, and blue cells across the top row. Moving the voltage to the second horizontal control wire, and firing each vertical wire produces a second row of colors. The painting continues one row at a time until the entire picture is done, and in this way, the PDP displays sixty pictures every second.

The process is similar to a conventional TV using a CRT. Instead of magnetic coils sweeping an electronic beam over the screen, PDPs achieve the same result by cycling the voltage across the horizontal and vertical control wires. Without the bulky deflection mechanism -- used to deflect the electron beam across the face of a CRT -- plasma displays only have to be thick enough to make them rigid.

That makes the thin and trim PDP ideal for use on walls, in information kiosks, at trade shows, and wherever space and weight are at a premium, such as airplanes. While a hefty 35-inch CRT weighs in at around 240 pounds, a much larger 42-inch PDP is a trim 80 pounds or so. However, it's no lightweight when it comes to picture quality! Brightness, color, and contrast are fast approaching parity with its heavyweight cousin.

PDP History

It hasn't always been that way. Professors Bitzer and Slottow invented the first plasma display in 1964 at the University of Illinois. The late 70s saw the appearance of the first monochrome plasma displays which were unique in two ways. They were flat, and they were orange.

Instead of phosphors, the orange-red glow of neon produces its light. Structurally they are much simpler than color PDPs that use individual phosphorescent cells to produce light. Monochrome displays use two glass panels, one with horizontal and the other with vertical control lines. Once sealed and filled with neon gas, a voltage across a pair of horizontal and vertical control lines causes the neon to glow at the point of intersection. While the resolution of these displays is good, they have poor contrast, and it takes a unique person to warm to the bright orange glow of neon.

The early 80s saw the development of the first color plasma displays which have just recently started hitting the market. Though the same resalution as conventional TVs, these early units lacked the contrast, brightness, and color depth of a CRT, but they were thin, flat, distortion free, and expensive. More recently, five major vendors -- Fujitsu, Matsushita, Mitsubishi, NEC, and Pioneer -- have shown color plasma display panels measuring 40-inches or more diagonally, and at least 16 other companies are actively developing PDPs.

Most of these prototypes have a resolution of 640 by 480 pixels for displays with a 4:3 ratio and 852 by 480 for panels with a 16:9 ratio. However, it is expected that most production plasma display panels in the 40 to 60-inch range will have a resolution of at least 1920 by 1080 to match the proposed HDTV standard in the US.

When and How Much?

The current price of PDPs is about $400 per diagonal inch. That makes a 21-inch panel a pricey $8,400 and a 42-inch model an astronomical $16,800. However, in a few years large scale production should bring the price down to around $100 per diagonal inch.

All of the major players have announced plans to build manufacturing plants for PDPs. Just last year Fujitsu announced its intention to spend $600 million on PDPs, and then NEC immediately up the ante to $800 million for their PDP budget.

The market for large color plasma displays is enormous. NEC has projected that annual worldwide sales of PDP will reach 10 million units by the year 2002, or about 5% of the 200 million TV sets expected to be sold that year. Other estimates are more conservative, but the consensus is that sales of PDPs should be close to $5 billion by 2002.

Problems?

The key, as always, to lower prices is high volume. Producers are developing novel manufacturing techniques to minimize costs. Sandblasting is being used to cut fine grooves or cells into glass panels, and they are looking at techniques based on inkjet technology to apply phosphors to the cells. And then there is the green problem.

The decay time for the most commonly used green phosphor (25-30 ms) is long enough to cause "smearing" in rapid moving video images at high scan rates of 72 frames per second or more. However, experiments with the phosphor's formula promise to reduced the delay time to acceptable levels (5 ms), and other weird mixes of rare-earth elements also look encouraging.

Another technical challenge is the high price of the electronics in a PDP. Like Liquid Crystal Displays (LCD), PDPs use ultra-high-speed pulse driving LSIs (large scale integrated circuits) to drive each cell. However, the voltage used to drive the cells is much higher for a PDP than a LCD. This makes the cost of these high-voltage drivers about four times greater than its low-voltage cousins. The next generation of drivers should reduce the cost by half, and by the end of the decade the industry expects to have PDP drivers that are cost-competitive with LCD drivers.

The bottom line is that PDPs have a relatively low manufacturing and materials cost. Given high volume, they should become price competitive with the larger, high-resolution CRTs used for television and computer displays. Over time, flat panel displays will clearly replace CRTs in direct view applications. The only question remaining is whether that flat panel will be a Plasma or a Liquid Crystal display? Clearly, the television industry is betting big bucks on Plasma.

Magic Mirrors

Digital Mirror Devices aren't new; they've been around for thousands of years. Back in the days of yore, Roman Legions used polished shields to send coded messages. Today Texas Instruments (TI) has found a way to put 2.2 million mirrors onto a postage stamp size chip to project digital images. Though the mirror is smaller, it works the same way as that polished shield: When tilted away, we don't see anything, but when tilted toward us we see the full brilliance of the sun, or any other light source pointed at it.

TIs Digital Micromirror Devices (DMD) work the same way only much faster. When switched off electronically, the mirror reflects virtually no light toward the screen; however, we see 90 percent of the light when it switches on. To project an image, an array of mirrors representing each pixel are all switched on or off at the same time.

A Digital Light Processor (DLP is used to project a TV signal onto a screen. It contains the electronics, light source, optics, and DMD necessary to project a digital image. The DMD measures 15 by 13 mm (0.6 by 0.5 inches) and is configured as a 768 by 576 pixel array of mirrors. This makes it compatible with both the 768 by 576 pixel European, PAL TV format, and the 640 by 480 US, NTSC and VGA formats. The projector simply focuses on the mirrors needed while the others are permanently masked or wired off.

If the TV signal is a standard analog signal, each line is digitized into 640 pixels, whereas it simply reads digital signals into a frame buffer. In either case, the frame buffer will contain all of the pixels necessary to recreate a single picture, and each pixel can reproduce 256 levels of red, green, and blue.

While receiving each frame, a constant light source shines through a color filter that rotates once per frame. When red light is shining on the DMD, the red information from all of the pixels is fed into the DMD. If the red value for any mirror is 0, it will not switch on. If the value is 25 then the mirror will switch on for 10 percent (25/256th) of the red cycle, 180 will be on for half the time, and 256 will be on for the full red cycle. During the green phase, the DMD uses its memory logic to turn each individual mirror on and off independently, so that each mirror reflects 0 to 100 percent of the green light that strikes it. Next comes blue, and by displaying all three colors in 1/60th of a second, our eyes see them blended into a single picture.

While projecting the first frame, a different frame buffer is receiving the next picture. But there is a problem. Each frame contains half of an interlaced TV picture. That is to say, every other horizontal scan line goes into the first frame, and the alternate 240 scan lines go into the second frame. Rather then wait for both frames to arrive, the processor interpolates each frame and fills in the missing scan lines, thereby doubling each frame to its full 480 lines. This not only eliminates any flicker, it increases the brightness, and actually improves the picture quality.

For larger applications a Digital light Processor with three DMDs is used. Its DMDs project all three colors for the full frame cycle instead of a third of the cycle as with a single DMD. The result is a threefold jump in brightness, and because each color is projected all the time, the DMDs can be programmed with 1024 levels of intensity for more than a billion colors.

TI's larger DMD measures 37 by 22 mm (1 1/2 by 3/4 inches) and has a 2048 by 1152 array of mirrors. This makes it compatible with both the proposed North American, high-definition TV standard of 1920 by 1080 pixels, and the European standard of 2048 by 1152 pixels. By using three, high-definition DMDs, these DLPs can project 1000 lumens of bright, sharp, digital imagery with uniformly rich color. Targeted for the professional market, these high resolution projectors are perfect for the large screens used in conference centers and auditoriums.

Seamless Pictures

Both DMDs use the same size mirror, 16-micrometers square (.0006 inches). By placing each mirror only 1-micrometer (.00004 inches) apart, an extremely seamless picture is created.

CRTs and PDPs can't be seamless. They must protect color purity by separating each color in a pixel; otherwise, the electron beam or plasma from one color will muddy an adjacent color. The result is a high degree of pixelation that becomes more apparent as sets get larger.

LCDs and three color projectors are more seamless. They combine red, green, and blue light into a single pixel, but still show large seams. No other projector has yet achieved as tightly packed pixels or produces as seamless a picture as TI's DMD.

DMD History

The Digital Micromirror Device is the outgrowth of decades of research. The breakthrough came in 1987 when Dr. Larry J. Hornbeck theorized that an annoying problem causing analog mirrors to stick in one position and then suddenly spring free, could be used to advantage with digital mirrors. He was right, and in early 1988 TI demonstrated the first Digital Micromirror Device with variable intensity levels. It took another four years of hard work before TI knew it had a winner.

But is it ready for prime time? TI thinks so, and has backed it up with extensive testing. Scores of DMDs have been tested by cycling its 2-million-plus mirrors over 1-trillion times without failure. This is equivalent to more than 20 years of normal use. Shock and vibration testing were also error-free, in part, because of the inherent reliability of the mature CMOS (Complementary Metal Oxide Semiconductors) manufacturing technology.

TIs strategy is not to market the DMDs directly, rather to form partnerships and develop Digital Light Processing boards for companies already in the projection display business. The DLP board contains all of the electronics and DMDs necessary to form an image, while the partner provides the matching light source and optics.

Last year TI started producing VGA resolution DLPs with a single DMD for 350 lumen projectors. This year 1000 lumen projectors have been announced using three high-resolution DMDs on a DLP board. Next year TI will produce DLP boards for use in projection TVs.

These products all combine digital clarity with bright, accurate, color-reproduction for seamless imagery. Can digital projection technology get any better than this? Probably, but nobody's talkin' -- yet!

LCDs

Liquid Crystal Displays are light switches. They work like the signal lamps used by the Navy to communicate between ships at sea during radio blackouts. Just as a Navy signalman uses a lever to open and close shutters in front of a light, an LCD uses a voltage signal to let light pass through its crystals or to block it. Some LCDs use a backlight to provide illumination, while others use a mirror to reflect ambient light back to the user.

LCDs used in portable PCs are notorious for a limited field of view. However, new LCDs used with desktop PCs and TVs have a horizontal viewing angle of 140 degrees and an optimal view angel of more than 70 degrees, which is twice as good as their portable cousins.

They manufacture Color LCDs with a red, green, or blue filter over each cell. Two configurations are commonly used: Vertical stripes of red, green, and blue, or a delta format where each pixel alternates between two triangularly shaped patterns -- red and green centered over blue, then green and blue over red.

LCDs used in projection TVs are configured somewhat differently. Three clear LCDs are used, one for each color. A light source is split into red, green, and blue via a prism or filters and directed at three LCDs. Each LCD allows light to pass through it for the length of time proportional to the intensity of the color dot it represents. By recombining the red, green, and blue light into a single multicolored pixel, pixelation is reduced by a factor of three. This coupled with an external light source allows for very bright, full-color images on a larger screen than a conventional projection TV system based on a CRT.

LCDs' Achilles' Heel

LCDs are manufactured using Large Scale Integration (LSI) technology that engineers are still improving. Small, lower-resolution displays are relatively easy to make without flaws; however, the larger the display the more potential for flaws. Every time a display's size is doubles its area becomes four times larger, and the potential for a flaw is four times greater.

Today, 14-inch displays seem to be the practical limit for obtaining high yields. However, Sharp recently unveiled a 28-inch color LCD monitor that combines two 21-inch LCDs with a virtually invisible seam. Is this the future of LCDs?

Will large, wall-hanging, direct-view LCD be manufactured in segments to compete with Plasma displays? Maybe, but the future could hold a flat panel display combining both technologies -- like the one Sony is working on.

Sidebar 1: Inside a Plasma Display Panel

PDP Figure All plasma displays work on the same principle; however, designs differ considerably. Matsushita's design uses an enclosed cell for each color in a pixel. The anode and cathode in each cell are connected to a horizontal and vertical control line. Placing a voltage difference across the cell causes current to flow through a mixture of neon and xenon gases trapped in the cell. This causes the gas to discharge a burst of ultraviolet light. The ultraviolet light excites the phosphor that gives off visible light just like fluorescent lamps. Only, here, different phosphors are used to produce red, green, and blue light.

The length of time a cell fluoresces determines the intensity of the light; however, the pulse width of the voltage is critical to its operation. If the pulse is too long in duration, the cell may arc, and if is too short it will turn the gas-plasma discharge off.

To achieve 256 levels of intensity, a series of pulses drives each cell. The initial pulse causes the gas to discharge, a series of slightly shorter pulses sustain the discharge, and a short pulse turns the cell off. In this way the intensity is precisely controlled giving each pixel the capability of producing over 16-million different colors.

Another popular design is the Barrier Rib PDP developed by Fujitsu. Other manufactures including NEC, Mitsubishi, and Pioneer are adopting its structure. Instead of individual cells, it uses columns of red, green, and blue phosphors separated by glass ridges.

The barriers between each color are critical to color purity. If any gas-plasma leaks from one to another, the colors will become muddied. Less critical is the separation of the same color between pixels.

When a voltage difference is placed across the electrodes, the current flow excites the gas in its immediate area causing it to discharge a burst of ultraviolet light that excites the phosphor close to it. Because each color stripe in a pixel is about three times longer then it is wide, there is a natural separation between the same color in adjacent pixels.

Which design is superior? Both promise to be so good that it'll be like choosing between a Farrari and a Lamborghini. In the final analyses, the lowest manufacturing cost will probably carry the day. And if it's a draw, experts will argue for years over the merits of each, while we benefit from competitive pricing.

Sidebar 2: Inside a Digital Micromirror Device

DLP Projector A mechanical torsion hinge holds each mirror flat. To turn the mirror on, a tiny electromagnet tips the mirror +10 degrees. To turn it off, an electromagnet on the opposite side tips it -10 degrees. This gives each mirror three states: on, off, and flat. However, there are a few sticky problems.

It takes a sizable voltage to turn the mirror on, and when it is reversed, there is a tendency for the mirror to try to assume its flat state. To overcome this, a bias voltage is applied to the yoke and mirror assembly. This balances out the Torsion Hinge and makes the device binary with only an on and off state. The bias also reduces the voltage needed to turn it on or off and locks the mirror in that state until the bias is removed.

To display an image, the Digital Light Processor must:

  1. Reset all of the mirrors from its previous on or off state by turning the bias voltage off so that each mirror begins to rotate to its flat state.
  2. Turn the bias voltage back on to allow each mirror to rotate to the address state currently in its SRAM memory array, either on or off (+ or - 10 degrees). The bias voltage also latches each mirror in its on or off state so it cannot respond to a new address state.
  3. Read the next bit plain into the memory array that controls the mirrors using horizontal and vertical control lines to address each pixel.
  4. Go back to step 1 and start the next cycle.
To achieve varying levels of intensity, one cycle is completed for every bit in each color level. For 256 levels of intensity, 8 bits are required. That is to say, the sum of the values for each bit (128, 64, 32, 16, 8, 4, 2, 1) can represent any value from 0 to 255. They call this 24 bit color, as 8 bits times 3 colors equal 24 bits that can produce more than 16-million colors. In the same way 30 bit color has 10 bits per color for 1024 levels of intensity and over 1-billion color combinations.

To display a single color, the mirror will cycle once for each intensity bit. When working with 8 bits, the first cycle will last 1/256th of the time allotted to a single color. The second cycle will last twice as long (2/256th), and each succeeding cycle will double in length until the last bit is on for half (128/256th ) of the color's display time.

Sounds complicated; let's look at an example. Suppose the intensity level of a red dot is 174, or "10101110" in binary. For the first cycle, the low order "0" bit will turn off the mirror. For the next three cycles, the "111" bits will turn it on. Then, for the last four cycles, the mirror will switch off, on, off, and on. Taken together, the 8 bits will have latched the mirrored on 174/256th of the time and off 82/256th of the time.

Sidebar 3: Inside a Liquid Crystal Display

LCD Figure The secret behind liquid crystals is that they tend to line up in parallel rows along a grooved surface. When sandwiched between two grooved surfaces running 90 degrees to each other, the crystals will assume a helical twist as they stack up in layers between the groves. When a voltage is applied across the grooved plates, the crystals snap to attention and align vertically.

By adding polarizing filters perpendicular to each other, light following the helical structure of the crystals is twisted 90 degrees and passes through the lower polarized filter. However, when a voltage is applied, the light passes straight through the crystals, and the lower polarizing filter blocks it.
 


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