Digital Light: processing
Feb 1, 1998 12:00 PM,
Peter H. Putman
Of the current large-screen display technologies, none has received morepress coverage than Texas Instruments’ Digital Light Processing (DLP).Full-page advertisements in leading business magazines have trumpeted thesuperior image quality of this all-digital system. Industry publicationshave waxed poetic about DLPs being the only true digital imaging process onthe market. Even TI officials have made claims about the imminent doom ofCRT-based imaging systems as DLP light engines take their place. This wouldbe a good time to cut through the hype and look at DLP.
At the heart of a digital light-processing projector is TI’s patentedDigital MicroMirror (DMD) device, a small RAM chip with an array of mirrorsmounted on its surface. Each of these tiny mirrors tilts in response tovarying electrical charges on the mirror’s mounting substrate. Depending onthe degree of tilt, individual mirrors reflect a varying amount of lightfrom 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 theindividual dots in a screened photograph. Move those mirrors fast enough torefresh a picture 60 times per second, and you can now reproducefull-motion black and white video.
The last step is to add color, accomplished with either color wheels ordichroic mirrors and combining prisms. Throw in a projection lens and somesignal processing, and you’ve got a digital light projector or DLPprojection monitor.
Since INFOCOMM ’95 in Dallas, we’ve had plenty of opportunities to comparethe image quality of DLP versus LCD. Back then, the brightness and picturequality of the first DLP projector surpassed most of the desktop 640x480and 800×600 products on the market. Today, portable and desktop 800×600 andeven 1,024×768 polysilicon LCD projectors have pushed DLP projection to theback of the line for all but high-end applications.
There are two problems that have impeded the growth of DLP technology, thefirst of which involves color imaging: Because of the size of the originalDLP light engine, it was impossible to adopt a three-chip dichroic colorsystem and keep the size and weight of the projector small enough to appealto the portable and desktop projection market. Instead, a single DMD with acolor wheel synchronized to mirror movements was employed. The quality ofcolor is not as good as that from a dichroic filter arrangement, especiallywhen it comes to color saturation levels. There is also a noticeablestrobing effect from the color wheel when you blink, resulting in arainbow-like effect on white or gray areas.
The second problem is Texas Instruments’ inability to ship 1,024×768 DMDdevices to projector and monitor manufacturers. These market segments wanttrue pixel-for-pixel reproduction in their displays and prefer not to scaleresolutions down. The imaging devices used in DLP projectors are currentlythe same ones used two years ago, and they offer a maximum nativeresolution of 848×600. In that same two-year period, the LCD projectormarket has added 800×600, 832×624, 1,024×768 and even 1,280×1,024 amorphousand polysilicon imaging panels.
DLP has been less than successful in its battle with LCD technology in theunder-$10,000 segment, but TI has done much better in the high-end lightvalve category where the issues are light output and projection lensoptions. All light- valve DLP projectors have adopted the three-chipimaging system with dichroic filters and prisms, marrying them to metalhalide and xenon projection lamps.
Although DLP light valve projectors are also limited to 848×600 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 metalhalide-lamp LCD projectors.
For multimedia applications, light-valve DLP projectors have superiorvideo-signal processing to lower-priced desktop models, resulting incleaner, sharper pictures. It’s a real contrast to the full-frame imagescaling 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-screendisplays, there is a bit of truth to that statement. Although both LCD andDLP display panels use digital addressing to activate individualmirrors/pixels, both must convert an analog signal to do so. In theory, theDMD could handle digital modulation directly from a variety of programsources if the appropriate digital handshake existed.
If you want to split hairs, the true digital difference with respect to aDLP projector is that its DMD does not respond in any analog fashion tolight, heat and vibration. LCDs can be affected by intense light and heat,causing drift, which affects their switching times and light-shutteringability. What’s more, the response of the individual liquid crystals in anLCD pixel is always a linear (analog) response to the voltages present attheir switching transistors.
If anything, DMD devices have an edge over LCDs in their greater efficiencyat processing light. Depending on the type of LCD used, 80% to 95% of thelight energy is absorbed. DMDs do considerably better, losing about 40% ofthe projected light with monochrome images.
DLP technology has a way to go before it can achieve parity with otherflat-screen projectors. Several LCD manufacturers showed desktop andfixed-mount 1,024×768 polysilicon projectors at this year’s COMDEX, allwith CRT-style composite and component inputs. Add to this the half-dozenother XGA models that are already shipping since INFOCOMM ’97, and you cansee 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 withDMDs sporting resolutions of 1,200 or more pixels. JVC has already shown anew 28 pound (12.6 kg) workstation and graphics projector, featuring a 9inch (229 mm), 1,365×1,024 pixel ILA engine, 250 W xenon lamp and the fullboardroom-style connector complement. Barco unveiled the BarcoReality 9200LC at INFOCOMM, and it uses amorphous 1,280×1,024 panels with dichroiccolor.
One last development is TI’s decision to have projector manufacturersdesign their own light-engine components. This could result in a smallerthree-chip dichroic array, which could compete head-to-head with thedesktop and fixed-mount XGA LCD projectors and start making a serious runat the traditional three-gun CRT projector stronghold.