Magic mirrors

DLP is currently the dominant digital projection technology in rear-projection TVs. At the heart of every DLP display is a DLP chip, also known as a Digital Micromirror Device, or DMD chip. The DMD chip was invented by Texas Instruments in 1987.

A simplified view of a lamp-based DLP light engine. A high-powered lamp (out of view, left) shines light through the segments of the spinning color wheel. The colored light strikes the DLP chip, creating the full-color image that is projected through the lens. DLP TVs that use LEDs or lasers for illumination don't require a color wheel.

Though small (less than an inch on a side), a DLP chip can have over a million hinge-mounted microscopic mirrors (see photo, below). The gaps between mirrors are also tiny, allowing the mirror array to create images that are sharp and seamless.

This Texas Instruments DLP chip (left) uses 921,600 tilting microscopic mirrors to create high-definition images. Each individually controlled mirror is a single pixel in a 1280 x 720-pixel (720p) array. Pictured at right, an ant's leg against a mirror array.

Each individual mirror can tilt independently, and the direction and degree of tilt determines the shade of gray created. If a mirror tilts to the "ON" position, the maximum amount of the lamp's light is reflected toward the lens — the pixel appears white. If the mirror tilts in the "OFF" position away from the lamp, no light is reflected and the pixel appears black. The micromirrors are switched on and off thousands of times each second by a digital clock. The intensity of light output from any given mirror depends on the ratio of its "ON" cycles to its "OFF" cycles. This system can generate up to 1024 shades of gray, to provide smooth transitions from light to dark areas in the same scene.

What the heck is "wobulation"?

Wobulation is a resolution enhancement technique originally invented by Hewlett-Packard for its ink jet printers. Texas Instruments employs wobulation in the image chips used in all current 1080p rear-projection DLP HDTVs. Like interlacing, wobulation shows half the picture at a time, but displays the two halves so rapidly that our eyes combine the two into one seamless whole. A 1080p DLP TV can display images with 1920 x 1080 pixels, yet its image chip only has 960 x 1080 mirrors. Half the image is displayed, then a separate pivoting reflective panel called an "actuator" shifts the display a half pixel's width to the side. This all happens 120 times per second, in order to generate 60 full frames per second, for a smooth progressive-scan image. Texas Instruments calls this technology SmoothPicture™.

How DLP does color

As with all color displays, the entire color spectrum can be created from just three fundamental colors — red, green, and blue. Most DLP-based TVs create color through the use of a spinning "color wheel" with red, green and blue filter sections.

The color wheel is positioned between the lamp and the DLP chip (see illustration at top of page). Internal processing precisely regulates how fast the color wheel spins, ensuring that the correct color of light hits the right mirrors at the right moment. When you combine the DLP chip's grayscale with the colored light created by the color wheel, you get a palette of over 16 million colors.

Because the colors are projected sequentially rather than all at once, it's up to our brains to assemble this color information into a continuous whole. And for most people, it works beautifully, resulting in smooth images with vivid, natural colors. But the color wheel causes a few people to occasionally see "rainbows" — brief multicolored flashes of light on the screen, seen mostly when the viewer's eyes dart away from the screen. Most people don't see rainbows, and relatively few who do are bothered by them.

A recent innovation that improves DLP color performance

As we mentioned above, if you're one of those people who do see rainbows and find them distracting, you might be interested in one of Samsung's higher-end DLP TVs that replace the lamp-and-color-wheel system with three colored LEDs: one each for red, green, and blue. These super-bright LEDs flash on and off at speeds much higher than a color wheel can achieve. In our experience, these LED-illuminated TVs solve the rainbow issue for most sensitive viewers. LED-based TVs also reach full brightness more quickly on start-up, while also consuming less energy.

Samsung's LED-illuminated DLP models use super-bright red, green, and blue LEDs to provide both illumination and color, replacing the lamp and color wheel found in conventional DLP TVs. LED benefits include cooler, quieter operation, lower energy consumption, and quicker start-up.

Bottom line: rear-projection HDTVs still make sense for many viewers

Anyone looking for a large-screen HDTV to serve as the centerpiece in a home theater should certainly check out the latest DLP models. They're easy to set up, and their compact cabinets make them more room-friendly than old-fashioned tube big-screens. And when you compare them to flat-panel TVs with similar screen sizes, rear-projection models deliver impressive bang for the buck.