October 2003

DLP Video Displays: Pros and Cons

Digital Light Processing (DLP) is a display technology developed by Texas Instruments. DLP imaging systems employ either one or three large (large for electronic chips, that is) devices called Digital Micromirror Devices, or DMDs. The micromirrors are mounted on the DMD chip and tilt in response to an electrical signal. The tilt directs light toward the screen, or into a "light trap" that eliminates unwanted light when reproducing blacks and shadows.

Micromirrors "paint" light onto the screen at high frequencies. For each video frame, the micromirrors can be tilted toward or away from the screen for hundreds or thousands of "cycles." This "cycling" of the micromirrors is too fast to be detected by the eye -- except in regard to how bright or dark it makes each pixel of the image. The more cycles the mirror undergoes -- i.e., tilted toward the screen -- the brighter the pixel will be. The more cycles the mirror is tilted away from the screen, the darker the pixel will be.

Getting color in the image when you have one DMD in the DLP display device requires synchronizing the tilting of the mirrors with the rotation of a color wheel that has filters for the three primary colors: red, green, and blue. The color wheel rotates in front of the white light source, causing the light hitting the DMD to cycle through a repeating pattern of red-green-blue-red-green-blue. The DLP display device controls how many cycles each of red, green, and blue light each pixel displays. For example, let’s say there are 1000 micromirror cycles per pixel. If the display device tells a pixel to display purple, that pixel’s micromirror would tilt toward the screen for 500 red cycles, 500 blue cycles, and 0 green cycles.

If the DLP display device has three DMDs, the white light is split into colors by a prism -- there is no rotating color wheel. All three colors can be sent to the screen at the same time; there are many more micromirror cycles for each pixel in each frame. Images from three-DMD products are thus brighter and/or larger than you can realistically get from a single-DMD product.

Today’s highest-resolution consumer-grade DMDs contain 1.3 million micromirrors, which approaches the maximum HDTV resolution of 2 million pixels. You’ll find the highest-resolution DMDs only in the newest, most expensive DLP products. Lower-resolution DMD devices lend themselves to lower-priced products. For example, one $3000 DLP projector has 0.5 million pixels in its single DMD. You’ll have to spend considerably more to get a higher-resolution DMD, and even more to get a projector with three high-resolution DMDs. Computer-grade DLP projectors are available for as little as $1200. However, these are unsuitable for high-quality home-theater displays due to a lack of functionality required for the display of DVDs, conventional TV, or HDTV.

Computer-grade DLP projectors will typically support only one resolution, such as 1024x768 -- but other than computer video, no video source has that resolution. DVD and conventional TV are of lower resolution than that, while HDTV is higher. To display a DVD, TV, or HDTV signal, you must scale the source up or down to match the projector’s 1024x768 resolution. That capability is missing from computer-grade DLP projectors.

For more information, check Home Theater & Sound’s interview with Ian McMurray of Texas Instruments. In easy-to-understand text and graphics, McMurray explains the basic operation of DLP devices in a little more detail.

DLP products come in two main categories: front projection and rear projection. Front-projection DLPs work much as a slide projector does, throwing images onto a separate screen some distance from the projection device. Rear-projection DLPs somewhat resemble conventional rear-projection cathode-ray tube (CRT) displays, but the DLP models tend to be lower to the floor, with much thinner cabinets than their CRT equivalents.

When buying a DLP display, it’s best to be aware of the technology’s advantages and disadvantages before taking something home and finding out it won’t do what you expected it to do. Many times, the salesperson won’t volunteer the technology’s limitations -- it’s up to you to be prepared with the knowledge and questions. To help you to make a purchase decision, here are DLP’s pros and cons.

DLP Pros

Both front- and rear-projection DLP systems are small and lightweight compared to their CRT counterparts. DLP images are usually bright, with relatively high contrast ratios of 600:1 or better. This means fairly good reproduction of blacks -- better than plasma screens, but not as good as a comparably priced CRT device.

DLPs deliver bright, clean whites. Images can be relatively large (front projection) and still not have a visible pixel structure, provided the projector has a bright light source. DLP displays with three DMDs can produce images of very high quality -- albeit at very high cost.

Six-segment and/or faster-rotating color wheels for one-chip DLPs reduce the appearance of "rainbow" artifacts due to high-speed eye motions -- i.e., when your eye scans quickly left or right to see something happening on the other side of the screen. The rainbow effect appears because your eye moves fast enough to become partially synchronized with the rotating color wheel -- you momentarily see only the red, green, or blue pixels being displayed on the screen. This effect is not visible unless you rapidly move your eyes from side to side. The color wheel rotates at high enough speeds that your eye-brain interface can’t detect it when viewing images from straight ahead. Some newer single-DMD displays spin their color wheels faster than earlier models. This means you have to move your eyes from side to side much faster before you’ll see any rainbow artifacts -- so fast that the rainbows may be eliminated completely for some people.

Another trick is to have a six-segment color wheel instead of one with three segments. This means two sets of red, green, and blue filters alternating around the wheel. This makes the rainbow artifacts harder to see -- they’re now dimmer, and you have to move your eyes faster to see them. And if a six-segment wheel is made to rotate twice as fast as a three-segment wheel, that’s equivalent to spinning the three-segment color wheel four times as fast. The faster you spin the wheel, the harder it is to see rainbow artifacts. We may not yet have seen all the possible tricks that can be done with color wheels.

DLPs have essentially "perfect" geometry -- straight lines are reproduced absolutely straight, and circles are perfectly round. Most DLPs have lamps that are relatively expensive ($100 to $400), but that usually last several hundred hours.

Today’s DLP products are the best ever, and carry the lowest prices ever.

DLP Cons

Next year’s DLP products will be better and possibly a little cheaper than this year’s. There aren’t the big price reductions as with plasma screens, but the resolution of DMDs improves every year.

DLP displays are fixed-pixel devices that rely on video processing to convert all nonmatching video formats (i.e., most of them) to the native resolution of the DMD used in the DLP display. Video "scaling" is the process of converting the resolution of the incoming video signal to match the resolution of the DMD. As mentioned in last month’s column, So You Want Plasma?," video scaling is difficult and expensive to do right; the quality of DLP scaling tends to decrease with the purchase price.

Rainbow artifacts are easy to see in "standard" one-chip DLP products with three-segment color wheels, if the viewer’s eyes quickly scan from left to right or from right to left.

All the light the DMD(s) direct away from the screen when creating black or dark scenes has to go somewhere. If that light isn’t effectively "trapped" and eliminated, it can reappear in the image, causing a loss of contrast. Blacks and shadows won’t be as dark and detailed as they should be -- darker areas of the image may appear grayish, with washed-out detail. Each DLP manufacturer deals with this unwanted light differently, and some methods are more effective than others. Carefully evaluating dark movie scenes, and black test patterns such as the PLUGE pattern (included on most test and setup DVDs), are the best ways to determine how effectively a model eliminates stray light, to prevent it from interfering with image quality and black performance.

Many DLP products, especially those costing $6000 or less, will show fairly obvious image defects. These defects will include:

• Sparkling pixels when the camera pans up or down, with textured surfaces in the image.
  
  
• Objects moving across the screen against stationary backgrounds may have sparkling outlines, or their motion may not seem as smooth as you would expect.
  
  
• When there is a solid-colored pastel area such as a flesh tone, an off-white wall, or a light-blue cloudless sky, you might observe digital artifacts because one light level is too bright and the next lower level is too dim. In trying to make a smooth surface appear smooth, the processor will flicker pixels bright-dark-dark-bright-bright-dark-bright -- but careful observers will note that the flesh, wall, or sky appears to "squirm" or "crawl" as the pixels change back and forth between adjacent light levels from frame to frame. This artifact can be present in some digital bitstreams if compression is too severe. You may have seen this type of digital artifact in digital cable or digital satellite programming, where the digital video has been compressed to the maximum level so the provider can squeeze more channels into their bandwidth. But this artifact should not be present in higher-quality video signals such as HDTV or DVD. If it is, the display device is generating the artifact.

How well video scaling works reflects considerably on the perceived image sharpness. Expensive products with first-class video scaling look great. Lower-priced products will not appear as sharp, due to limitations in their scaling capabilities.

As of today, no consumer DLP display can produce the maximum 1920x1080-pixel resolution of high-definition television. Hi-def images must be downconverted to the resolution of the DMD; conventional TV and DVD images must be upconverted.

Equipped to choose

DLPs have progressed light-years from the first demo I saw: the Titanic sailing diagonally across the screen, vertical strips of the image being re-drawn one after the other. The ship actually moved forward in strips rather than in the smooth, continuous motion of reality. This pathetic performance was an actual commercial product -- though I can’t imagine who would have purchased a product with such obvious flaws.

Today’s DLPs range from the artifact-ridden to the extremely eye-pleasing. You can use the information in this column and some reference DVDs and a test/setup DVD to help you find a winner among a growing number of products.

 ...Doug Blackburn
db@hometheatersound.com

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