Front Projectors

Digital Light Processing is finally getting the recognition it deserves. It's not as hot a technology as plasma, but people are beginning to realize that it's an appetizing alternative—especially since it offers many of the strengths and few of the weaknesses of other digital display technologies. Texas Instruments is the creator and sole manufacturer of DLP chips, and their latest offering is the HD2+ (or Mustang) chip. But it all started long before the arrival of HD2+.

The DLP chip—which is actually called a Digital Micromirror Device (DMD) chip—was created in 1987 and hit the mass market in 1996. It held the promise of higher resolutions and less space between pixels, which creates the all-too-familiar screen-door effect you see with LCD. Resolution has been steadily increasing; soon you'll be able to buy a 1,920-by-1,080 DLP, but not yet. How does DLP technology and especially the HD2+ chip work? I'm glad you asked.

Light Me Up
It all starts with a bulb. Not a mere incandescent, this bulb (affectionately referred to as a lamp) is crucial in determining the DLP display's picture quality. Current lamp life is about 2,000 to 3,000 hours, although that's increasing; meanwhile, lamp prices are decreasing.

The lamp's more-or-less white light travels down a path of lenses. These lenses focus the light onto the DMD chip, which looks a lot like a computer chip with a mirror in the middle (see photo 1). In the case of a 1,280-by-720 DMD like the HD2+ chip, that mirror is actually made up of 921,600 separate mirrors that are too small for the naked eye to see (photos 2 and 3).

As with other digital displays, DLP is a fixed-pixel technology, so all video sources must be processed in order for the signal to address the individual mirrors and create a picture. When the lamp's light strikes the chip, the video processing tells certain mirrors to pivot—either toward another set of lenses and out to the screen or away from the screen, into a light absorber (see photo 4).

A DLP display creates differences in the image's gray scale (different levels of light and dark) by pivoting the mirrors at a high rate of speed. For a brighter pixel, the mirror spends more time pivoted toward the screen; for a darker pixel, it spends more time pivoted away from the screen. The latest generation of home theater chips (like the HD2+ and the lower-resolution Matterhorn) have 12 degrees of tilt. The previous generation had only 10 degrees. This may not seem like a big difference, but it allows for a far better black level and therefore a greater contrast ratio. In addition, the HD2+ chip's pivot point is filled in (you can see the pivot point in the middle of each mirror in photos 2 and 3), which allows for more light to be reflected and therefore an even better contrast ratio.

Something's Missing
Color. Depending on whom you ask, the way most home theater DLP displays create color images is either the technology's Achilles heel or just a temporary limitation. Between the lamp and the DMD chip resides a color wheel, a thin disc with transparent colored sections for each of the primary colors: red, green, and blue (see photo 5). As the wheel spins, each section filters the white light from the lamp, allowing only one color to reach the chip at a time. The wheel spins at a known rate, so the internal processor knows what color is on the chip at any given moment. The DMD chip forms the correct image for the red, green, and blue portions of the picture as each filter rotates into the light path, and these images are projected onto the screen sequentially; this happens fast enough that human persistence of vision allows the brain to merge these sequential images into a full-color picture.

Three-chip DLP displays handle color differently. A prism splits the lamp's light into the required wavelengths for red, green, and blue; these colors are then reflected off of dedicated DMD chips for each color. The advantage of this approach is that you can make each color (and therefore all of the colors that need to be reproduced) more accurate without sacrificing light output. This design also doesn't create rainbows.

That's right, Dorothy, I said rainbows. The color-wheel design creates an artifact known as the rainbow effect. Some people see it; some don't. Some people are bothered by it; some aren't. I've seen tutorials that teach people who haven't seen the rainbow how to see it. My question is, why? If you can't see it, consider yourself lucky and stop worrying about it.

What causes the rainbow effect? Your brain notices that there's really only one color on the screen at any moment and it's just being fooled into seeing a full-color image. It's a fight between the persistence of vision that makes all TV and film appear to depict smooth motion and millions of years of evolution forcing the brain to pay attention to movement and peripheral vision (blame saber-tooth tigers for that one). All I'll say to those who pride themselves on the tricks they know to help see rainbows in a DLP projector is this: Are you really going to watch a movie while doing any of those tricks? I didn't think so. For those of you who are bothered by rainbows, the incessant march of technology is your friend.

The original HD2 chip introduced a faster color wheel with six segments, which made the rainbows far less severe. The HD2+ chip is mated with a seven-segment wheel that should make most of the staunchest critics stop their whining. On the horizon is a color wheel that looks like a child's pinwheel, which goes by the moniker Sequential Color Recapture (SCR). This wheel, in which the three primary colors swirl out from the center, may eliminate rainbows all together, if we ever see it in a real product (the drawings look quite interesting).

Paint It Black, Please
Besides rainbows, DLP has the same problem that all current digital displays face: the inability to create a true black. This is because the lamp, even at low power settings, puts out a lot of light. That light has to go somewhere. Engineers have spent a lot of time developing a light path that absorbs or contains as much of the unwanted light as possible, but some still sneaks through onto the screen, making the whole image brighter (and not in a good way). The mirrors can only reflect light; while the area behind the mirrors (on the chip's surface) is painted black, some light is still reflected out of the lens, even when the mirror is tilted away from the screen.

The color and black-level issues I've discussed aren't insurmountable problems. In fact, if the measurements for the latest batch of HD2+ projectors is any indication, they may not be problems for long.