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Light and Color: Projecting the Future

WK Bohannon

March 15, 2021 | This year's Consumer Electronics Show (CES) at Las Vegas was a great place take a look at many of the latest in front- and rear-screen video, DTV, and HD video projectors. Since CES exhibits products targeted for the living room, there were a lot of low-cost/high-performance products on display. Though some were built around the more familiar CRT, many were based on Plasma and LCD components and still others were built around TI's Digital Micro-mirror Device (DMD) and its associated Digital Light Processing (DLP) systems. These lower-cost DLP projectors and rear-screen TV systems are based around one-chip DLP designs that use a spinning color-filter wheel placed between the lamp and one DMD chip in an attempt to make colorful images.

Good television, whether showing standard-definition, DVD, DTV, or HDTV images, requires properly saturated colors--something that the existing "business application" one-chip DLP projectors have a hard time doing. However, several companies have developed production or prototype 480-line video or 720P HD "video optimized" one-chip DLP systems that produce well-saturated colorful images.

TI has always claimed that its one-chip DLP systems can go head to head with the more colorful three-panel LCD systems, but I've never seen it--until CES. The one-chip systems exhibited at CES produced what appeared to be well-saturated colors with video signals. These products were made by some of the biggest names in consumer electronics and TV: Panasonic, Mitsubishi, Hitachi, Sharp, Runco, Yamaha, Zenith, and Marantz as well as others not so well known in video such as In Focus, Plus, Seleco, Dream Vision, and others. So how can they do with video at CES what the one-chip business projectors shown at COMDEX only two months earlier could not?

They did it by trading brightness for more color saturation. The standard one-chip business projector uses a variety of color-wheel tricks to deliver brightness--1000-lumen images with a little 120-watt lamp. But those tricks come at the expense of color saturation. For one, the color wheels used in business projectors contain "white" segments of about 30 to 50 degrees in arc length. Obviously, white segments do not make color. They do, however, make a lot of light. All of the companies showing one-chip DLP video projectors at CES did not use white segments in their color wheels--they used only red, green, and blue segments. These "pure" RGB color wheels, done right with the proper optical system, can make good colors. Exactly how good I can't say, precisely, but as soon as I get a system to evaluate and measure, I'll be sure to let everyone know.

The business projector manufacturers don't use the same pure RGB color wheels because they are in a lumens race with the LCD projector manufacturers. It is pretty hard to market a business projector (even a tiny, micro-portable one) that produces way under 1000 lumens, so they've cranked up the white segments in the color wheels and allowed their desire for lumens to get in the way of competitive colors. As a result, about one-half of the lumens per watt of lamp power is lost when pure color wheels are used instead of wheels with white segments. However, TI claims that it should only cost about 20 to 30% in ANSI lumens with the same lamp power to make good color--if the optical systems are properly optimized with the latest DLP technology.

Some of the additional available light power planned by TI includes more digital processing and new DMDs with more tilt. TI's partners are supposedly planning to introduce products at INFOCOMM in June 2001 that will be based around DMDs with 12 degrees of tilt angle versus the current 10-degree tilt. Think about it: If the little mirrors on the DMD can tilt further between on and off, then the projector and TV set manufacturers can use faster lenses (both in illumination and projection optics) with lower "f' numbers. And just like a 35mm camera's lens, whenever you can use a lower f-number, you get much more light. This light can be used to blind the competition or to make good colors depending upon the strategy.

Besides more tilt in the mirrors, TI has other tricks, one of which is called "Spoke Light Recapture." This method is generally used in business projectors to increase light output another 10 to 20%. The Spoke Light Recapture method leaves the DMD on when the color wheel rotates between colors and that lets the "spoke light" hit the screen. According to TI, if the spoke light between all three colors is used in a video projector, then the white light (ANSI Lumens) output is boosted. If only the spoke light between two primary colors is used, then the complementary color is boosted, which allows for better color control.

Tricks like Spoke Light Recapture are controlled by the "Data Path" Formatter (DPF), and one of the biggest ASICs that TI sells as a DLP package comes with its DMD. But spoke light processing is not contained in the fast DPF chip used in the one-chip video systems that can accommodate the new 16x9 aspect chip. The one-chip video systems run their color wheels at 4 to 6 times the standard 60Hz frame rate. This reduces the color break-up, motion artifacts, and smearing seen in the 2-times frame rate color wheels used in business projectors with slower DPF chips, which do spoke light processing. As for the future, along with DMD chip improvements, TI is planning a series of new formatter chips that handle faster data rates and faster color wheel rates, and do the spoke light tricks that will increase brightness in "pure" color systems while also providing better color control.

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