Digital Display Technology - Seminar Report

Abstract

In the present world, the display technology is usually analog. But, we present the future technology i.e., the digital technology. The analog display technologies limit the full potential of digital video. So, the digital display devices will be in the market soon. In our paper, we emphasize on Digital Light Processing (DLP). This DLP display technology manipulates the light digitally to produce all-digital  images. This technology reproduces color images with precision accuracy in a seamless fashion. The picture appears seamless because the mirror pixels used in a DLP system have very tiny spaces. This in turn gives high perceived resolution. The DLP systems are inherently reliable. DLP is a reflective technology, because the image becomes brighter and brighter as the resolution increases. As this is a reflective technology, it is more efficient than competing systems. Hence the world will definitely move over to the new technology i.e., the digital technology and we are sure that the digital technology will have a promising future.

Introduction

In this information age, we are surrounded by computers, faxes, modems, pagers, and cellular telephones. These products all rely on digital technology. More recent advances have produced digital cameras, digital camcorders, digital satellite systems (DSS), and the widely anticipated digital video disk (DVD) players, which play digital source material of unprecedented image quality. And of course, who could forget the Internet, the seemingly endless forum of digitized information. Before long, digital VCRs will make their way into the homes of millions of consumers, offering yet another digital solution.

Although many of us realize the benefits of this digital technology, the majority of video displays are still based on the old analog cathode ray tube (CRT), which recently celebrated its 100th anniversary. Why would someone want to go to all the trouble of preserving digital video and graphic data only to have it converted to analog before it is viewed? Digital Light Processing (DLP) offers a solution to this problem of unrealized image quality potential. DLP, invented by Texas Instruments, is the new digital display technology. It is the final link in the chain that makes possible a completely digital video information structure. This is especially important as TVs and PCs converge to provide central information and communication media windows.

Digital Light Processing (DLP)

When we compare DLP to today’s display technologies, it’s easy to see why DLP has a promising future. Because the technology is digital, DLP is able to reproduce life-like color images with precision accuracy. Seamless picture reproduction, high brightness, inherent reliability, the ability to show PC graphics and TV video, and other DLP advantages are discussed below. 

How DLP Works?

DLP display technology digitally manipulates (or processes) light to produce film-like, all-digital images. DLP integrates a projection lamp and an electronic video signal from a source such as a VCR or computer, and the processed light produces an all-digital picture.

The key to this complete digital process is the Digital Micro mirror Device (DMD), a thumbnail-size semiconductor light switch. The DMD consists of an array of thousands of microscopic-size mirrors, each mounted on a hinge structure so that it can be individually tilted back and forth. When a lamp and a projection lens are positioned in the right places in the system, DLP processes the input video signal and tilts the mirrors to generate a digital image.

Imagine that you are in a football stadium at night and it’s half-time. All the lights in the stadium are turned off; there is a blimp floating a few hundred feet above the field. 

As part of the half-time show, a powerful spotlight placed at the 50 yard line blasts light into the stands. All the fans in one area of the stadium are asked by the announcer to hold up the reflective seat cushions that were in their seats before the game. Each reflective cushion has a different number on the back. The announcer asks you to tilt your seat cushion so that the light from the spotlight reflects directly up to the blimp each time your number appears on the scoreboard screen. If your number is not displayed, you are to tilt the cushion away from the blimp and direct light down to the field. Each fan in the stands now controls a  pixel of light. You can imagine that if some fans reflect the light toward the blimp, an image of some sort will appear on its side. 

Now imagine a viewer looking toward the stadium and the blimp from a remote distance. When looking at the blimp, he or she will see an image on the side of the blimp that is generated by the sports fans tilting their reflective seat cushions and reflecting light onto the side of the blimp.

 DLP technology accomplishes this same task, but it does so by processing light that is focused onto the DMD . At speeds greater than 1,000 times per second, the mirrors are electronically tilted. Light from a lamp is digitally reflected from the DMD, through a projection lens, and onto a screen. Color is added through a color wheel filter system.

(a) In much the same way the image was formed on the side of the blimp, DLP forms images for video. Shining light on the DMD and tilting the mirrors creates a digital image. Color is added by placing a red, green, and blue color wheel filter system in the optical path. As the wheel spins, the mirrors are tilted on for the exact amount of time required for each color. At any given instant, only one of the primary light colors is hitting the DMD, but when the filter system spins fast enough, the colors blend to create a full-color digital image.(b) A viewer sees an image on the screen from a DLP projector.

Seamless Picture

Besides being able to generate images digitally, DLP reproduces these images in a virtually seamless fashion. The picture appears seamless because the mirror pixels used in a DLP system have very tiny spaces between them—1 micrometer gaps, to be exact. This gives DLP a fill factor, or active area, of approximately 90%. In other words, DLP comes closer than andother technology to producing an exact mirror image of an input video or graphic signal (Figure 3).

Actual photographs of (a) liquid crystal display (LCD) pixels and (b) DLP pixels. Projected images are made up of thousands of small pixels; DLP pixels (b) are more uniform and more closely spaced than the polycrystalline silicon (poly-Si) pixels seen in view (a). Based on a superior pixel structure, DLP offers a higher fill factor and better image quality.

Using a super video graphics adapter (SVGA) DLP projector (800 ´ 600 pixels) and a leading SVGA liquid crystal display (LCD) projector, the photograph of a polar bear (Figure 4) was used to illustrate DLP’s superior image quality.

Photograph used to demonstrate DLP’s seamless picture. This digitized photograph of a polar bear was projected with both a DLP and an LCD projector to illustrate the seamless, film-like picture quality of DLP (Figure 5).

The pixelated screen-door effect common to LCD projectors. The screen-door effect is gone when the image is projected with a DLP projector . Rather than looking at an image as if it were behind a screen door, the viewer sees a seamless, digitally generated and projected image. Although the resolution, or number of pixels, is the same in each photograph, DLP has a higher perceived resolution because of its seamless advantage.

Actual close-up photographs of both (a), an LCD-projected image, and (b), a DLP-projected image. An SVGA poly-Si projector (a) was compared to an SVGA DLP projector (b). The photographic image of the polar bear in Figure 4 was displayed on both projectors. The LCD and DLP photos shown here were taken under the same conditions, with each projector being optimized for focus, brightness, and color. Note the high level of pixelation in the LCD photo compared to that of the seamless DLP photo. DLP offers superior, seamless picture quality because of the high fill factor and close spacing of DMD mirror pixels.

High Brightness

Because DLP is a reflective technology, it is far more efficient than the competing systems. It means that more light gets from the lamp to the screen— an extremely important factor in high-brightness applications. In fact, thermal models predict that for a three-DMD super-extended graphics adapter (SXGA) projection system (1,280 ´ 1,024 pixels), 10,000 on-screen lumens can be achieved.

Reliability

DLP systems have passed a series of environmental and regulatory tests to simulate thermal shock, temperature cycling, mechanical shock, vibration, moisture resistance, and acceleration conditions. Most reliability concerns are focused on the hinges that tilt the mirrors from one position to another. To test reliability, DMDs were subjected to accelerated life cycle tests simulating approximately 20 years of use. Inspection of the devices after the testing showed no broken hinges on any of the devices. Texas Instruments has completed thousands of hours of life cycle and environmental testing to conclude that the DMD and DLP systems are inherently reliable.

Other Display Technologies

In addition to DLP, there are three other primary display technologies: amorphous silicon (am-Si) LCD, polycrystalline silicon (poly-Si) LCD, and cathode ray tube (CRT). 

LCDs

An LCD acts as a light shutter. It can be best understood as a mechanism capable of modulating or controlling the amount of polarized light that can be transmitted through the panel. Improvements in LCDs have led to increases in transmissivity (light throughput), but LCD technology is still limited to an analog architecture. Am-Si and poly-Si are thin film transistor (TFT) LCDs that require a transistor to control each pixel on the LCD panel. An electric signal applied through the transistor to the LCD pixel changes the pixel’s polarization. By varying the polarization, the amount of light passing though each pixel can be controlled to produce an image.

Light can be represented in vertical and horizontal components. If light is directed at a vertically oriented polarizer, the polarizer acts as a filter and allows only the vertical light to pass. On the other side of the system,another polarizer is positioned so that light will pass only in the horizontal direction. With no liquid crystal in the path, the first polarizer would block the horizontal light and pass the vertical. When the vertical light hits the second polarizer, it would also be blocked (because the second polarizer passes only horizontal light). The result would be complete blockage of light, producing a dark pixel. When a liquid crystal is “sandwiched” between the two polarizers, it acts as a modulator or “twister” of polarized light. By applying a voltage to the crystal, the light polarization can be altered, allowing various levels of light to pass through the system. Projection systems based on LCD technology use either a single LCD panel or three panels, one for each primary color—red, green, and blue. In the single-panel configuration shown here, small, closely spaced red, green, and blue sub-pixels make up one pixel

         

Amorphous Silicon LCD

               Am-Si LCDs are built by depositing transistors on a large glass substrate. A transistor is located in the corner of each pixel while a thin conductive grid connects to each pixel on the panel. Pixels are made up of three individually controlled sub-pixel strips (red, green, and blue) to create a pixel capable of producing many color combinations. Am-Si panels are used to create single-panel projectors, but these projectors suffer from poor image quality due to the side-by-side sub-pixel color scheme.

Polycrystalline Silicon LCD

            Polycrystalline silicon LCD, more commonly referred to as poly-Si, is   a very popular LCD technology for projection display. These LCDs are fabricated at high temperatures on quartz substrates. Poly-Si LCD panels are much smaller than am-Si panels. They have smaller transistors and greater fill factors as well, but to date, poly-Si panels are monochromatic (meaning they don’t have the color stripping found in am-Si panels). Color is created in poly-Si projectors by using three separate LCD panels, beam-splitting mirrors, and a prism system. White light is split into red, green, and blue components. Each component of light is directed to its own LCD panel, where the light modulation occurs. The modulated light is then recombined by a prism so that the pixels from each panel are overlaid on each other to produce a color image. The challenge for these three-panel poly-Si projectors is the precision alignment that is necessary to make the separate red, green, and blue image planes converge to produce a uniform, aligned picture.

Cathode Ray Tube Technology

Cathode ray tube technology is used in nearly all of today’s computer monitors and televisions. Electron beams are scanned back and forth and directed at a phosphor-emitting surface. When electrons hit this surface, light is emitted. By scanning the beams at rates faster than the eye can detect, a full image can be created. 

The problem with CRTs is that they are not digital but analog displays. CRT technology probably will be replaced in the future by emerging LCD and DLP technology. In addition to being based on an old, analog technology, CRTs also lack the brightness necessary for many larger screen applications. Further brightness limitations arise when CRT projection systems attempt to drive higher resolution video signals. The CRT’s brightness decreases as resolution increases, limiting the CRT’s potential as an optimal solution for HDTV. Because a CRT  display system usually relies on three electron guns, one for each primary color (red, green, blue), it also requires constant alignment and tweaking for optimum picture quality. 

This leads to increased setup time and maintenance costs during a product’s lifetime (Figure 7).

The CRT monitor. Input video or graphic signals are sent to three electron guns at the back of the monitor’s cathode ray tube. Each electron gun shoots a stream of electrons, one for each of the primary colors.The intensity of each stream is controlled by the input signal. The beams pass through a shadow mask to keep them precisely aligned. When the electrons strike the phosphors coating the inside of the screen, the phosphors emit light. A magnetic deflection yoke bends the paths of the electron streams so that they sweep from left to right and top to bottom in a process called raster scanning. The screen is usually redrawn, or refreshed, 60 or more times a second.

CONCLUSION

More and more of today’s information is created, edited, and transmitted in the digital domain. The majority of existing display technologies is analog and limits the full potential of digital video. At present, DLP is the only digital projection solution available to produce an entirely digital video infrastructure. As PCs and TVs converge and the digital HDTV infrastructure begins to take form, DLP offers consumers the maximum benefit of digital source material by producing life-like, digitally projected images of unprecedented picture quality. The colors are truer and brighter, the pictures seamless. Because it is a reflective technology, DLP images will become brighter and brighter as resolutions increase. The DLP technology proved to be successful even in numerous environmental and regulatory tests. Using DLP technology, leading manufacturers are bringing world class projectors to the market to serve a wide variety of digital display applicati