INIATURE DISPLAYS
New materials promise smaller color displays
Miniature flat-panel-display technology was a hot topic among the attendees at last May`s Society for Information Display conference (Anaheim, CA). The strengths and weaknesses of the various approaches to miniaturizing displays are becoming much clearer, particularly when analyzed in the context of specific applications.
Much attention is currently focused on liquid-crystal-on-silicon technology, in which the control matrix (backplane) is fabricated on conventional complementary metal-oxide semiconductor silicon lines and the liquid-crystal layer placed on top. Most devices operate in a reflective mode, and many producers are pursuing a color sequential ap proach that requires illumination of the display with red, green, and blue light at about 180 frames/s to produce a 60-H¥full-color image.
Key to the color sequential approach is development of liquid-crystal materials that can switch at these high frame rates. Conventional twisted-nematic (TN) materials used, for instance, in notebook computers have a response speed of about 50 ms. But for color sequential techniques, speeds of less than 5 ms are necessary to avoid image blurring with fast-moving objects.
Microdisplay developers Three-Five Systems (Tempe, AZ), Siliscape (Sunnyvale, CA), and Colorado Micro Display (Boulder, CO) have apparently come u¥with TN materials that switch at less than 5 ms, although none of them are willing to reveal how they do it. Displaytech (Longmont, CO) uses a ferroelectric-liquid-crystal material with a very fast inherent speed of about 0.1 ms.
Kopin (Taunton, MA) has developed a transmissive microdisplay and has a similar need for fast response, particularly for presentation of smear-free night vision imagery for the US Army`s Comanche helicopter. With funding hel¥from the Defense Advanced Research Projects Agency, the company appears to have met initial requirements for video performance on the twin 1280 ¥ 1024 active-matrix liquid-crystal displays used in the Comanche helmet-mounted display system.
Architecture makes a difference
Architecture differences can also impact the electronics complexity, power consumption, and, ultimately, the applications for which the devices are best suited. Three-Five Systems has, therefore, formed strategic partnerships with National Semiconductor (Santa Clara, CA) and Siliscape--each offers different architectures for different applications.
Three-Five president David Buchanan notes, "The National backplane is an analog interface approach that is really targeted for projection applications. The backplane features a DRAM architecture and analog display drivers that provide a very good range of colors or gray scale."
Most projectors receive analog signals from either video or computer sources. The system processor on a PC, for instance, outputs digital information to the display buffer, but it is converted to analog format by the graphics card (see Fig. 1). The cable that connects the PC to the projector now contains analog data for driving the display.
Digital interfaces are better suited to applications in which the signals from a microprocessor are not converted to analog, but remain digital all the way to the display. The Siliscape backplane uses a digital SRAM architecture that is optimized for these applications (see Fig. 2 on p. 86).
According to Alfred Hildebrand, chairman and CEO of Siliscape, "SRAM is a binary memory, so more-sophisticated methods are required to produce gray scales, but it does avoid the high power consumption of digital-to-analog converters and analog drive circuits in a DRAM architecture. As a result, our color SVGA display can output about 35 fL of illuminance at only 40 mW of power."
For a projector, power consumption is not usually an issue, but it is for battery-powered devices. Buchanan says his firm is targeting the Siliscape microdisplays for hand-held communication markets, such as a virtual viewfinder for cell phones. "Motorola, which was the king of analog cell phones, says that 75% of its cell-phone production in the latter half of this year will be digital products," notes Buchanan, "so the infrastructure for digital is pretty well there."
The Siliscape backplane is not targeted for wide-color-palette or video applications--areas where the National Semiconductor backplane is strong. Currently, Siliscape can offer only 3 bits/color, or 512 colors, but higher levels of color are in the works. They should be ready by the fourth quarter of 1998.
Displaytech also features an all-digital interface because its ferroelectric material is bistable, that is, it is either transmissive or nontransmissive with no gray levels in between. This requires more-complex drive circuitry to display information, yet the company has been able to achieve 32K colors on its latest VGA device.
Chris Chinnock