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MillimeterDigital Light: processing
Feb 1, 1998 12:00 PM, Peter H. Putman
Of the current large-screen display technologies, none has received more press coverage than Texas Instruments' Digital Light Processing (DLP). Full-page advertisements in leading business magazines have trumpeted the superior image quality of this all-digital system. Industry publications have waxed poetic about DLPs being the only true digital imaging process on the market. Even TI officials have made claims about the imminent doom of CRT-based imaging systems as DLP light engines take their place. This would be a good time to cut through the hype and look at DLP.
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 varying 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 appears, resembling the individual dots in a screened photograph. Move those mirrors fast enough to refresh a picture 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.
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 surpassed most of the desktop 640x480 and 800x600 products on the market. Today, portable and desktop 800x600 and even 1,024x768 polysilicon LCD projectors have pushed DLP projection to the back of the line for all but high-end applications.
There are two problems that have impeded the growth of DLP technology, the first of which 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. Instead, a single DMD with a color wheel synchronized to mirror movements was employed. The quality of color is not as good as that from a dichroic filter arrangement, especially when it comes to color saturation levels. There is also a noticeable strobing effect from the color wheel when you blink, resulting in a rainbow-like effect on white or gray areas.
The second problem is Texas Instruments' inability to ship 1,024x768 DMD devices to projector and monitor manufacturers. These market segments want true pixel-for-pixel reproduction in their displays and prefer not to scale resolutions down. The imaging devices used in DLP projectors are currently the same ones used two years ago, and they offer a maximum native resolution of 848x600. In that same two-year period, the LCD projector market has added 800x600, 832x624, 1,024x768 and even 1,280x1,024 amorphous and polysilicon imaging panels.
DLP has been less than successful in its battle with LCD technology in the under-$10,000 segment, but TI has done much better in the high-end light valve category where the issues are light output and projection lens options. All 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 848x600 resolution, they are more than a match for LCD light valves in image quality. In fact, the xenon-lamp DLP projectors produce better colorimetry than metal halide-lamp LCD projectors.
For multimedia applications, light-valve DLP projectors have superior video-signal processing to lower-priced desktop models, resulting in cleaner, sharper pictures. 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.
As for DLPs being the only true digital imaging system for large-screen displays, there is a bit of truth to that statement. Although 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 light, heat and vibration. LCDs can be 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 is absorbed. DMDs do considerably better, losing about 40% of the projected light with monochrome images.
DLP technology has a way to go before it can achieve parity with other flat-screen projectors. Several LCD manufacturers showed desktop and fixed-mount 1,024x768 polysilicon projectors at this year's COMDEX, all with CRT-style composite and component inputs. Add to this the half-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 1,200 or more pixels. JVC has already shown a new 28 pound (12.6 kg) workstation and graphics projector, featuring a 9 inch (229 mm), 1,365x1,024 pixel ILA engine, 250 W xenon lamp and the full boardroom-style connector complement. Barco unveiled the BarcoReality 9200 LC at INFOCOMM, and it uses amorphous 1,280x1,024 panels with dichroic color.
One last development is TI's decision to have projector manufacturers design their own light-engine components. This could result in a smaller three-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 three-gun CRT projector stronghold.
Continue the discussion on “Crosstalk” the Millimeter Forum.
