Digital Light Processing:Behind The Hoopla

Feb 1, 1998 12:00 PM, Peter H. Putman

Of all the current large-screen display technologies being peddled today, none has gotten more press coverage and "buzz" than Texas Instruments' Digital Light Processing , or DLP as it is commonly known. Full-page advertisements in leading business magazines have trumpeted the superior image quality of this all-digital system. Industry publications have waxed poetic about DLP being the only true digital imaging process on the market. Even TI officials have jumped on the bandwagon, making claims about the imminent doom of CRT-based imaging systems as DLP light engines take their place.

If we were to believe everything we have heard to date about digital light processing, we should be dumping 3-gun CRT projectors in dumpsters and tossing out our LCD projectors with yesterday's newspapers. But that isn't happening...yet. Maybe this would be a good time to cut through some of the hype and take a closer look at DLP.

THE MIRROR HAS MANY FACES At the heart of a digital light processing projector is TI's patented Digital MicroMirror (DMD) device, a small RAM chip with an array of mirrors mounted on its surface. Each of these tiny mirrors tilts in response to varying electrical charges on the mirror's mounting substrate. Depending on the degree of tilt, individual mirrors reflect a greater or lesser amount of light from the projection lamp.

Because the degree of reflection varies from 100% white to black - and all steps in between - a wide grayscale can be created. Using thousands of mirrors, a grayscale image starts to appear, resembling the individual dots in a screened photograph. Move those mirrors fast enough so that a new picture can be refreshed 60 times per second, and you can now reproduce full-motion black and white video.

The last step is to add color, accomplished with either color wheels or dichroic mirrors and combining prisms. Throw in a projection lens and some signal processing and you've got a digital light projector, or DLP projection monitor. Other than the fact that DLP is a reflective technology, it sounds an awful lot like LCD projection in many ways - doesn't it?

That's what many dealers, designers, installers and end users have concluded. Since INFOCOMM '95 in Dallas, we've had plenty of opportunities to compare the image quality of DLP versus LCD. Back then, the brightness and picture quality of the first DLP projector (nView's Diamond D-400) surpassed most of the desktop 640 x 480 and 800 x 600 products on the market. Today it's a different story, as portable and desktop 800 x 600 and even 1024 x 768 polysilicon LCD projectors have pushed DLP projection to the back of the line in for all but high-end applications.

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THE DOWNSIDE There are two problems that have impeded the growth of DLP technology. The first problem involves color imaging: Because of the size of the original DLP light engine, it was impossible to adopt a three-chip dichroic color system and keep the size and weight of the projector small enough to appeal to the portable and desktop projection market segment. Instead, a single DMD with a color wheel synchronized to mirror movements was employed. (This color wheel is still used in the new In Focus Lite Pro 420 and Davis PowerLite ultraportable projectors.)

The quality of color obtained from the wheel is not nearly as good as that from a dichroic filter arrangement, especially when it comes to color saturation levels. Not only that, there is a noticeable strobing effect from the color wheel when you blink your eyes, resulting in a rainbow- like effect on otherwise white or gray areas of the image.

The second problem is Texas Instruments' inability to ship 1024 x 768 DMD devices to projector and monitor manufacturers, a serious setback in the adoption of DLP for higher-resolution workstation and graphics applications. These market segments want true pixel-for-pixel reproduction in their displays and prefer not to scale resolutions down to fit smaller panels. The imaging devices used in DLP projectors available now are the same ones that were used two years ago, and offer a maximum native resolution of 848 pixels by 600 pixels. (In that same two-year period, the LCD projector market has added 800 x 600, 832 x 624, 1024 x 768 and even 1280 x 1024 amorphous and polysilicon imaging panels.)

THE UPSIDE To date, DLP has been less than successful in its battle with LCD technology in the under $10,000 segment. However, TI has done much better in the high-end light valve product category. Here, the size and weight of projectors are not as much an issue as is their light output and projection lens options. All of the light valve DLP projectors have adopted the three-chip imaging system with dichroic filters and prisms, marrying them to metal halide and xenon projection lamps.

Although DLP light valve projectors are also limited to 848 x 600 resolution, they are more than a match for LCD light valves in terms of image quality. In fact, the xenon-lamped DLP projectors tend to produce far better colorimetry than metal halide-lamped LCD projectors. The best images I've seen have been produced by Digital Projection's Power 4dv and 5dv, with Sony's VPD- S1800U, and Electrohome's Vista Pro series close behind. The only comparable product for video image quality would be Hughes-JVC's CRT-based Image Light Amplifier .

For multimedia applications, these light valve DLP projectors have superior video signal processing than lower-priced desktop models, resulting in cleaner, sharper pictures with less noise and artifacts. It's a real contrast to the full-frame image scaling used in desktop DLP designs, which frequently results in blocky, dithered video with noticeable pixel structures.

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DIGITAL VS. ANALOG As for DLP being the only true digital imaging system for large-screen displays, there is a bit of truth to that statement. While both LCD and DLP display panels use digital addressing to activate individual mirrors/pixels, both must convert an analog signal to do so. In theory, the DMD could handle digital modulation directly from a variety of program sources, if the appropriate digital "handshake" existed.

If you want to split hairs, the true digital difference with respect to a DLP projector is that its DMD does not respond in any analog fashion to the effects of light and heat, and to a lesser extent, vibration. In contrast, LCDs can be and are affected by intense light and heat, causing "drift" which affects their switching times and light-shuttering ability. What's more, the response of the individual liquid crystals in an LCD pixel is always a linear (analog) response to the voltages present at their switching transistors.

If anything, DMD devices have an edge over LCDs in their greater efficiency at processing light. Depending on the type of LCD used, 80% to 95% of the light energy from the projection lamp is absorbed and never makes it to the screen. DMDs do considerably better, losing about 40% of the projected light with monochrome images. However, add a color wheels and more light is absorbed.

LEVELING THE FIELD If you wanna play with the big boys, you gotta have the right equipment. That's exactly the case with DLP technology, which has a ways to go before it can achieve parity with other flat-screen projectors. Several LCD manufacturers showed desktop and fixed-mount 1024 x 768 polysilicon projectors at this year's COMDEX show, all with CRT-style composite and component inputs. Add to this the half a dozen other XGA models that are already shipping since INFOCOMM '97, and you can see why TI is under the gun to ship XGA DMDs.

The folks in Dallas also can't afford to wait too long to follow up with DMDs sporting resolutions of 1200 or more pixels. JVC has already shown a new 28 pound workstation and graphics projector, featuring a .9" 1365 x 1024 pixel ILA engine and 250 watt xenon lamp and offering the full boardroom-style connector complement. Barco unveiled the BarcoReality 9200 LC at INFOCOMM, and it uses amorphous 1280 x 1024 panels with dichroic color.

One last development that will help tremendously is TI's decision to have projector manufacturers design their own light engine components. This could result in a smaller 3-chip dichroic array, which could compete head-to-head with the desktop and fixed-mount XGA LCD projectors and start making a serious run at the traditional 7", 8", and 9" three gun CRT projector stronghold.

PP 12/10/97 Revised 1/23/98 Ver 1.0

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