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Electronic Imaging & Signal Processing

Digital Micromirrors take Tinsel Town by Storm

From OE Reports Number 183 - March 1999

The melding of micromachine and computer chip on Earth could have Trekkies watching the next Star Trek movie in a completely new way.

28 March 1999, SPIE Newsroom. DOI: 10.1117/2.6199903.0005

Figure 1. Prismatic beam-splitters and three DMD chips provide 42-bit color for the DLP.

Companies such as Texas Instruments (TI; Dallas, TX) and the Hughes-JVC Technology Corp. (Carlsbad, CA) are busily developing new technologies that expand the role of digital video beyond home television applications and portable business projectors to the big-screen theater. These systems are based either on the Digital Micromirror Device (DMD) microchip -- in the case of Texas Instruments' Digital Light Processing (DLP) technology -- or they are based on optically addressed LCD light valves from JVC. The goal is to replace the six or more massive rolls of 35-mm film, which comprise a normal two-hour feature film, with an optical disk or computer file. In the TI approach, the digital data is fed into a DLP projection head that mounts onto a theater projector lamp housing, replacing the conventional film projection head. Three digitally controlled DMD chips, containing millions of digital micromirrors, replace the conventional silver-halide-based polyester film. Digital light is projected to the audience where each viewer interprets the sensation of digital 1s and 0s as a subtle spectrum of colors varying in hue, brightness, and saturation.

The benefits of digital cinema to producers, exhibitors, and the cinema audience are many, proponents say. The distribution and projection technologies are here. The cost benefits are real. Last fall, a proof of the DLP Cinema projector concept from TI turned many of the remaining Hollywood skeptics into true believers. According to TI executives, all that remains is for the movie industry to develop industry standards for compression and encryption, a digital distribution infrastructure, and then to buy into the future.

Digital addresses film's strengths

Digital technology is already a large part of the modern film making experience. Most movie sound tracks are digital. Film editing is accomplished with the help of computer software. Computer graphics have replaced many of the volatile special effects that punctuate today's blockbuster hits. Despite all this, however, film has remained king of the feature movie because only recently has digital electronic projection technology been capable of providing the cinematic experience that the audience has adored for over 100 years. Film's basic analog nature, although as archaic as the vinyl record, contains an almost infinite variety of possible signals, giving the cinematographer the ability to create a totally unique mood for each scene.

Across the aisle, however, digital proponents point out that Van Gogh himself would have a hard time duplicating La Chambre de l'Artiste, even on a good day. What chance, then, do a pair of technicians in a film-processing lab have of duplicating the "infinite varieties" of Jurassic Park 5,000 times for the American viewing audience? Based on the stability of DLP technology, a colorist, with the help of a DLP monitor, can adjust each scene as it is transferred to digital tape, knowing that the artistic intent of the cinematographer will be conveyed perfectly to audiences around the world without the need for operator intervention.

Digital cinema proponents, such as SPIE member, TI Fellow, and DMD inventor, Larry Hornbeck, point out the virtues of digital light. Once the digital bursts of 1s and 0s emanating from the DLP projector have been cleverly manipulated to provide enough digital depth (bits per primary color), even the experts have a hard time telling the master film footage from a digital duplication. And digital technology provides some surprising bonuses. Night after night the audience can enjoy the image quality of an opening night premier. No more dirt, scratches, burn holes, or fading. No more glitches caused by the mechanical nature of the projector such as jump, weave, and focus flutter. No more projector "tweaking" to keep the image artistically true to the cinematographer's vision.

Defining what number of bits and contrast ratios are really necessary to make a seamless transition from film to digital has been a difficult task made all the more difficult because of the hundreds of millions of dollars involved annually. Profits promote opinions and Hollywood is certainly no different.

According to Bill Werner, Senior Systems Engineer of the DLP Cinema Group of TI Digital Imaging, most experts would agree that, if digital cinema can develop a projected full-on/full-off contrast ratio between 800 to 1000:1, with 42-bit linear color depth (14 linear bits per primary color), then it will approach the level of performance that a projected film print produces on opening night.

Colors and contrast

TI has taken a total approach to digital cinema. Although competing technologies convert the analog data from the master print to digital data for distribution purposes and then reconvert the data back to analog for display purposes, the DLP approach keeps the data in a digital format the whole way, from postproduction distribution to the movie screen and the viewer's eyes.

Figure 2. Texas Instrument's Digital Light Projector (DLP) has been designed for linear transmission to minimize required changes to conventional theater projectors. The digital housing replaces the projector lens configuration on a conventional projector lamp, requiring only a change in the retroreflector behind the lamp to maximize optical efficiency.

At the heart of the DLP Cinema prototype system lies three SXGA 1280 X 1024 micromirror arrays having a 1.25:1 aspect ratio. An anamorphic projection lens delivers the standard Academy format (1.85:1) or Cinemascope format (2.39:1) to the screen. Each micromirror is attached to a Complementary Metal Oxide Semiconductor (CMOS) electronic brain. Circuits on the underlying microchip provide the switching and support electronics for each 16-µm square aluminum micromirror. Each mirror is a sputter-deposited aluminum alloy attached to support posts by a pair of thin, compliant torsion hinges. When activated by the CMOS circuit, an electrostatic charge causes the mirror to rotate to +/- 10 degrees (on or off). The mirror's short mechanical switching times between states (~16 µs) and short optical switching time (~2 µs) enable a large color bit depth.

"With digital cinema, the frame rate is 24 fps as compared to 60 fps for television," explained Hornbeck. "So automatically we get a lift in terms of the number of time divisions we can do to produce gray scale, while at the same time avoiding the perception of flicker." Werner adds that even with the mirrors' short cycling times and the 1/24 s window of opportunity, additional processing had to be done on the prototype system to generate the 42-bit linear color depth. Without the proprietary algorithms that produce additional gray scale depth, experts claimed they could see image contouring or banding.

An example of contouring can be seen in a digital picture of a cloudless late afternoon sky. For example, a 50-foot wide picture with a 24-bit linear color depth will create a clear line between one shade of blue and another. At 42 bits, Werner claims that most of the experts have difficulty telling the DLP Cinema prototype system from film.

One of the final technical hurdles facing DLP was to increase the contrast. According to Hornbeck, by packing the mirrors closer together and darkening the substrate between the mirror gaps, TI was able to reduce the scattered light caused by the mirror gaps and increase the full-on/full-off contrast ratio to higher than 800:1. This is very important during dark scenes or the credits, when cinematographers want a true black and not a milky black background.

How much digital is enough?

The DLP system uses three DMD arrays to produce a full color image. As designed, the DLP system would only require retrofitting the 1000 to 5000 W lamp inside a conventional movie projector lamp housing with a new reflector. The light is reflected into a series of dichroic prisms that separate the red, blue, and green wavelengths. Each DMD chip modulates the intensity of the red, blue, and green light, respectively. The light is recombined by the same dichroic prisms, and projected by a single projection lens to the movie screen, and voilà: you have digital cinema.

Figure 3. The current Digital Micromirror Device (DMD) design has reduced the space between mirrors and darkened the substrate that shows through the gaps to boost the full-on/full-off contrast ratio.

Understandably, Hornbeck and Werner feel that the all-digital DMD approach is the way to reach digital cinema. Their hardy DMD microchip has been tested at more than 100,000 hours and engineering improvements to the DLP projection system have ensured that the overall system is worthy of these robust little digital microchips. Last year, the lithographic process that creates these microelectromechanical devices was moved from a specialized facility in Dallas, Texas to the company's DMOS-IV fabrication facility for standard CMOS chip manufacture. Hornbeck and Werner feel that the monolithic, rugged, and easy-to-use-and-integrate DLP Cinema projector design will smooth the maintenance and technical concerns of the average theater owner.

Other approaches to digital cinema include an interesting technology developed by Hughes in the 1980s and now incorporated into the Hughes-JVC Technology (HJT) projector. This light amplifier technology uses a low-power light source to optically address the backside of a liquid crystal/photoconductor sandwich. The liquid crystal then modulates a high-power light source incident on the frontside of the liquid crystal. JVC uses three miniature CRTs to address three liquid-crystal-based light amplifiers. Three projection lenses, one for each primary color, combine the images at the projection screen. Brightness levels for top-of-the-line HJT projectors are comparable to prototype DLP Cinema projectors.

Whether an all-digital or mostly digital system wins the day for cinematic applications, the digital revolution seems as inevitable as the tide. If, in fact, digital can match film's performance in the theater, the financial savings of distribution and copying alone will provide the backdrop for yet another digital reality.

R. Winn Hardin
R. Winn Hardin is a science and technology writer based in Fairbury, NE.