LCD and DLP

The Technical Differences
between LCD and DLP

LCD (liquid crystal display) projectors usually contain three separate LCD glass panels, one each for red, green, and blue components of the image signal being fed into the projector. As light passes through the LCD panels, individual pixels ("picture elements") can be opened to allow light to pass or closed to block the light, as if each little pixel were fitted with a Venetian blind. This activity modulates the light and produces the image that is projected onto the screen.

DLP ("Digital Light Processing") is a proprietary technology developed by Texas Instruments. It works quite differently than LCD. Instead of having glass panels through which light is passed, the DLP chip is a reflective surface made up of thousands of tiny mirrors. Each mirror represents a single pixel.

In a DLP projector, light from the projector's lamp is directed onto the surface of the DLP chip. The mirrors wobble back and forth, directing light either into the lens path to turn the pixel on, or away from the lens path to turn it off.

In very expensive DLP projectors, there are three separate DLP chips, one each for red, green, and blue. However, in DLP projectors under $20,000, there is only one chip. In order to define color, there is a color wheel that consists of red, green, blue, and sometimes white (clear) filters. This wheel spins between the lamp and the DLP chip and alternates the color of the light hitting the chip from red to green to blue. The mirrors turn on and off based upon how much of each color is required for each pixel at any given moment in time. This activity modulates the light and produces the image that is projected onto the screen.

The Advantages of LCD Technology

One benefit of LCD is that it has historically delivered better color saturation. In most single-chip DLP projectors, a clear (white) panel is included in the color wheel along with red, green, and blue in order to boost brightest. This tends to reduce color saturation, making the DLP picture appear not quite as rich and vibrant. However, some of the DLP-based home theater products now have six-segment color wheels that eliminate the white component. This contributes to a richer display of color. And even some of the newer high contrast DLP units that have a white segment in the wheel are producing much better color saturation than they used to. In the last couple of years DLP technology has gotten much better at color saturation and accuracy than it used to be. Overall however, the best LCD projectors still have a slight performance edge in this area.

LCD also delivers a somewhat sharper image than DLP at any given resolution. The difference here is more relevant for detailed financial spreadsheet presentations than it is for video. This is not to say that DLP is fuzzy--it isn't. When you look at a spreadsheet projected by a DLP projector it looks clear enough. It's just that when a DLP is placed side-by-side with an LCD, the LCD typically looks a little bit sharper in comparison.

A third benefit of LCD is that it is more light-efficient. LCD projectors typically produce significantly higher ANSI lumen outputs than do DLPs with the same wattage lamp. In the past year, DLP machines have gotten brighter and smaller--and there are now DLP projectors rated at 2500 ANSI lumens, which is a comparatively recent development. Still, LCD competes extremely well when high light output is required. All of the portable light cannons in the 15 lb weight class putting out 3000 ANSI lumens or more are LCD projectors.

The Weaknesses of LCD Technology

LCD projectors have historically had two weaknesses, both of which are more relevant to video than they are to data applications. The first is visible pixelation, or what is commonly referred to as the "screendoor effect" because it looks like you are viewing the image through a screendoor. The second weakness is not-so-impressive black levels and contrast, which are vitally important elements in a good video image. LCD technology has traditionally had a hard time being taken seriously among many home theater enthusiasts (quite understandably) because of these flaws in the image.

Three developments have served to reduce the screendoor problem on LCD projectors. First was the step up to higher resolutions, first to XGA resolution (1,024x768), and now to widescreen XGA (WXGA, 1365x768), a format found on the Sanyo PLV-70 and Sony VPL-VW12HT. Standard XGA resolution uses 64% more pixels to paint the image on the screen than does an SVGA (800x600) projector. The inter-pixel gaps are reduced in XGA resolution, so pixels are more dense and less visible. Then with the widescreen 16:9 machines, the pixel count improves by another quantum leap. While an XGA projector uses about 589,000 pixels to create a 16:9 image, a WXGA projector uses over one million. At this pixel density, the screendoor effect is eliminated at normal viewing distances.

Second, the inter-pixel gaps on all LCD machines, no matter what resolution, are reduced compared to what they use to be. So even the inexpensive SVGA-resolution LCD projectors have less screendoor effect than they used to.

The third development in LCDs was the use of Micro-Lens Array (MLA) to boost the efficiency of light transmission through XGA-resolution LCD panels. Some XGA-class LCD projectors have this feature, but most do not. For those that do, MLA has the happy side effect of reducing pixel visibility a little bit as compared to an XGA LCD projector without MLA. On some projectors with this feature, the pixel grid can also be softened by placing the focus just a slight hair off perfect, a practice recommended for the display of quality video. This makes the pixels slightly indistinct without any noticeable compromise in video image sharpness.

Now when it comes to contrast, LCD still lags behind DLP by a considerable margin. But recent major improvements in LCD's ability to render higher contrast has kept LCD machines in the running among home theater enthusiasts.

The Advantages of DLP Technology

There are several unique benefits that are derived from DLP technology. One of the most obvious is small package size, a feature most relevant in the mobile presentation market. Since the DLP light engine consists of a single chip rather than three LCD panels, DLP projectors tend to be more compact. All of the current 3-pound miniprojectors on the market are DLPs. Most LCD projectors are five pounds and up.

Another DLP advantage is that it can produce smooth, high contrast video. DLP has been well-received in the home theater world primarily due to two video quality advantages that were lacking in LCDs—better contrast and the lack of pixelation.

While both technologies have produced improvements in contrast in the past year, DLP projectors still have a commanding lead over LCDs in this regard. Leading-edge LCD projectors like the Sony VPL-VW12HT is rated at 1000:1 contrast, and Sanyo's PLV-70 is rated at 900:1. Meanwhile, the latest DLP products geared toward home theater like NEC's HT1000 are rated as high as 3000:1. Just a year ago the highest contrast ratings we had from DLP were in the range of 1200:1.

This sudden substantial boost in contrast is derived from Texas Instrument's newest DLP chip design, which increases the tilt of the mirrors from 10 degrees to 12 degreees, and features a black substrate under the mirrors. These changes produced a significant advance in contrast performance that simply did not exist a few months ago.

Reduced pixelation is another competitive advantage of DLP technology. In SVGA resolution, DLP projectors have a muted pixel structure when viewed from a typical viewing distance. Conversely, SVGA-resolution LCD projectors have a clearly visible pixel grid. For this reason, we don't recommend SVGA-resolution LCD projectors for home theater use except for those on the most limited of budgets.

In XGA and higher resolution, DLP technology completely eliminates pixel visibility from a normal viewing distance, and it does so more effectively than the improved current state of the art LCD machines. So in this aspect DLP continues to hold its historical competitive edge.

A Potential Problem with DLP: The Rainbow Effect

If there is one single issue that people point to as a weakness in DLP, it is that the use of a spinning color wheel to modulate the image has the potential to create a unique visible artifact on the screen that folks refer to as the "rainbow effect," which is simply colors separating out in distinct red, green, and blue.

How big of a deal is this? Well, for those few who can see the rainbow effect, it is huge problem that creates distractions which in some cases make the picture literally unwatchable. Fortunately, the vast majority of the population is not sensitive to it. If everyone could see rainbows on DLP projectors the technology never would have survived to begin with, much less been embraced by so many as the preferred technology for home theater video systems. Nevertheless, it is something that needs to be discussed and put into perspective.

The first generation DLP projectors incorporated a color wheel that rotated sixty times per second, which can be designated as 60Hz, or 3600 RPM. So with one red, green, and blue panel in the wheel, updates on each color happened 60 times per second. This baseline 60Hz rotation speed in the first generation products is also known as a "1x" rotation speed.

Upon release of the first generation machines, it became apparent that a small but vocal percentage of the population was seeing rainbow effects. So in the second generation DLP products, the rotation speed was doubled to 2x, or 120Hz, or 7200 RPM. The doubling of the refresh rate reduced the margin of error, and so reduced the visibility of rainbows. It also reduced the number of people who could detect the rainbows at all.

Today, many DLP projectors being built for the home theater market incorporate a new six-segment color wheel which has two sequences of red, green, and blue. This wheel still spins at 120Hz or 7200 RPM, but because the red, green, and blue is refreshed twice in every rotation rather than once, the industry refers to this as a 4x rotation speed. This further doubling of the refresh rate has again reduced number of people who can detect them visibly. Yet there is still a small fraction of the population (we'd guess less than 1%) who are sensitive to them.

By the way, you can sometimes cause rainbow artifacts to appear by spreading your fingers and waving your hand frantically in front of your face while watching a DLP-created image. I was recently viewing a demonstration of the PLUS Piano HE-3200, a beautiful DLP projector with a 4x speed wheel. Very few people can detect any rainbow artifacts on this machine. But there was a dealer present, waving his hand like a madman in front of his face and declaring, "AHA! The same ole rainbow problem." Indeed, if you enjoy waving your hand spastically in front of your face between dips into the popcorn bowl, we suggest you try LCD.

How seriously should you take the rainbow issue?

If you've seen earlier generation DLP machines and detected no rainbow artifacts, you won't see them on the newer machines either. The large majority of people can't see them at all on any of the current machines. However there is no way for you to know if you are one of the unlucky few that may be bothered by them without sitting down and viewing one.

Therefore, if you think you've identified a DLP projector that is just right for your needs but you are not sure whether rainbows will be a problem, there is an easy solution. Find an alternative product that is either LCD or LCOS based that would be your second choice if you find that DLP is a problem. Then find a customer-service oriented dealer who sells both models, and who will allow you to switch the DLP product for the alternative after testing it out for a week or two. There are a number of service-oriented Internet dealers who will be happy to make such arrangements, and there are plenty who will not. But if you choose a dealer who is more interested in your satisfaction than he is closing a quick deal (and they are definitely out there), you will end up with a thoroughly satisfying solution in the end.

The Current State of the Art

The largest developers and manufacturers of LCD technology are Sony and Epson. These companies have no interest in standing by and letting Texas Instrument sweep the digital projector market with its competing DLP technology. So competition has driven both the LCD makers and Texas Instruments to improve their respective products in the ongoing battle for market share.

While LCD technology has made notable improvements in contrast over earlier generation machines, DLP maintains its lead in contrast performance, while LCD projector makers have continued to emphasize latent advantages in color fidelity and image sharpness for data display. Conversely, DLP color accuracy and saturation has improved significantly this year, so color performance on the latest models is much better than it used to be.

Both LCD and DLP are evolving rapidly to the benefit of the consumer. The race for miniaturization has produced smaller yet more powerful projectors than we might have even imagined possible just a couple of years ago. Light output per pound has increased dramatically. And video quality on the best LCD and DLP projectors now surpasses that available in a commercial movie theater.

For mobile presentation it is hard to beat the current group of 3-pound DLPs on the market. However LCD products like the Epson 730c at 4.3 lbs make it clear that LCD is still a very strong contender in the mobile presentation market. And for larger conference rooms that require higher light output and greater connectivity, LCD technology holds a commanding lead.

When it comes to home theater, DLP has continued to make competitive advances in color, contrast, and image stability that have served to make it the preferred technology for home theater systems. But the fact is that both DLP and LCD continue to improve, and both are capable of delivering much higher quality video for home theater than they ever were before.

Which technology is the best? Well, it depends.