Introduction
If you are new to the world of digital projectors, you won’t have to shop around the market very long before discovering that “LCD” and “DLP” somehow refers to two different kinds of projectors. You might not even know what LCD and DLP are before asking the obvious question “which one is better?”
The answer is simple. Sort of. LCD and DLP each have unique advantages over the other. Neither one is perfect. So it is important to understand what each one gives you. Then you can make a good decision about which will be better for you.
By the way, there is a third very significant light engine technology called LCOS (liquid crystal on silicon). It is being developed by several vendors, most notably JVC and Hitachi. Several outstanding home theatre projectors have been manufactured with this technology, and JVC’s LCOS-based DLA-SX21 is currently on our list of Highly Recommended Home Theater Projectors. However the discussion of LCOS technology is beyond the scope of this article. For more on LCOS click here.
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 the red, green, and blue channels. However, in DLP projectors under $20,000, there is only one chip. In order to define colon, there is a colour 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 colour of the light hitting the chip from red to green to blue. The mirrors tilt away from or into the lens path based upon how much of each colour 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 colour saturation than you get from a DLP projector. That’s primarily because in most single-chip DLP projectors, a clear (white) panel is included in the colour wheel along with red, green, and blue in order to boost brightest, or total lumen output. Though the image is brighter than it would otherwise be, this tends to reduce colour saturation, making the DLP picture appear not quite as rich and vibrant. However, some of the DLP-based home theatre products now have six-segment colour wheels that eliminate the white component. This contributes to a richer display of colour. And even some of the newer high contrast DLP units that have a white segment in the wheel are producing better colour saturation than they used to. Overall however, the best LCD projectors still have a noteworthy performance advantage 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 unit is placed side-by-side with an LCD of the same resolution, the LCD typically looks sharper in comparison.
A third benefit of LCD is that it is more light-efficient. LCD projectors usually 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 under 20 lbs putting out 3500 to 5000 ANSI lumens 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 some home theater enthusiasts (understandably) because of these flaws in the image.
However, in many of today’s projectors these flaws aren’t nearly what they used to be. Three developments have served to reduce the screendoor problem on LCD projectors. First was the step up to higher resolutions, first to XGA resolution (1,024×768), and then to widescreen XGA (WXGA, typically either 1280×720 or 1365×768). This widescreen format is found, for example, on the Sanyo PLV-70 and Epson TW100, (two more products currently on our Highly Recommended list). Standard XGA resolution uses 64% more pixels to paint the image on the screen than does an SVGA (800×600) 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 today’s inexpensive SVGA-resolution LCD projectors have less screendoor effect than older models did. And it is virtually invisible on the Panasonic PT-L300U, which is a medium resolution widescreen format of 960×540.
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. All of the LCD projectors just mentioned have contrast ratios of at least 800:1. They produce much more snap, better black levels, and better shadow detail than the LCD projectors of years past were able to deliver.
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 higher contrast video with deeper black levels than you normally get on an LCD projector. DLP has ardent followers in the home theater world primarily due to this key advantage.
While both technologies have seen improvements in contrast in the past two years, 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. Less than two years ago the highest contrast ratings we had from DLP were in the range of 1200:1.
This boost in contrast is derived from Texas Instrument’s newer DLP chip designs, which increase 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 before.
A third competitive advantage of DLP over LCD is reduced pixelation. These days it is most relevant in the low priced, low resolution SVGA class of products. In SVGA resolution, DLP projectors have a muted pixel structure when viewed from a typical viewing distance. Conversely, most SVGA-resolution LCD projectors tend to have a more visible pixel grid. This is entirely irrelevant if you are using the projector for PowerPoint slide presentations. However, it is more problematic for a smooth video presentation. For this reason, we don’t normally recommend SVGA-resolution LCD projectors for home theater. Conversely, the revolutionary InFocus X1 is a DLP-based SVGA resolution projector. It is selling now for under $1,000 and is an incredible deal for the home theater enthusiast on a limited budget.
In XGA and higher resolution, DLP technology pretty much eliminates pixel visibility from a normal viewing distance. However, the latest WXGA resolution LCDs do so as well. So with higher resolutions, differences in pixelation are not the big competitive battleground they used to be. DLP continues to hold a small competitive edge, but the dramatic advantage of DLP over LCD no longer exists. The screendoor effect is receding into history as a problem of days gone by.
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 produce 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. Basically, at any given instant in time, the image on the screen is either red, or green, or blue, and the technology relies upon your eyes not being able to detect the rapid changes from one to the other. Unfortunately some people can. Not only can some folks see the colors break out, but the rapid sequencing of color is thought to be the culprit in reported cases of eye strain and headaches. Since LCD projectors always deliver a constant red, green, and blue image simultaneously, viewers of LCD projectors do not report these problems.
How big of a deal is this? Well, it is different for different people. For some who can see the rainbow effect, it is so distracting that it renders the picture literally unwatchable. Others report being able to see the rainbow artifacts on occasion, but find that they are not particularly annoying and do not inhibit the enjoyment of the viewing experience. Fortunately, the majority of the population either cannot detect the rainbow artifacts, or if they can they are not overly bothered by them. The fact is 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 a great technology for home theater video systems. Nevertheless, it can be a serious problem for some viewers.
Texas Instruments and the vendors who build projectors using DLP technology have made strides in addressing this problem. 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 quite a few people were seeing rainbow artifacts. So in the second generation DLP products the color wheel 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 or eliminated the visibility of rainbows for many people.
Today, many DLP projectors being built for the home theater market incorporate a 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 the number of people who can detect them. Nevertheless it remains a problem for a number of viewers even today.
How big of a problem is the rainbow issue for you?
If you’ve seen earlier generation DLP machines and detected no rainbow artifacts, you won’t see them on the newer machines either. The 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 or another regular viewer in your household are among those that may be bothered either by visibly distracting rainbows, or possibly eyestrain and headaches, without sitting down and viewing a DLP projector for a while.
Therefore, if you think you’ve identified a DLP projector that is just right for your needs but you are not sure whether this 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 won’t work for you. 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 few days. 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 in closing a quick deal (and they are definitely out there), you will end up with a thoroughly satisfying solution in the end.
A Potential Problem with LCD: Long Term Image Degradation
Texas Instruments recently released the results of a lab test conducted last year which highlighted a failure mode in LCD technology that does not exist with DLP. Given enough time, it appears that LCD panels, primarily those in the blue channel, will degrade, causing shifts in color balance and a reduction of overall contrast. The test did not include a large enough array of test units to draw any conclusions about anticipated rates of degradation under normal operating conditions.
However it is possible that those who invest in an LCD projector may find that eventually the LCD panel and polarizer in the blue channel may need replacement. This is not much of a problem if the unit is under warranty. But if it isn’t, the replacement of an LCD panel will represent an unpleasant incremental investment in your projector that you were not anticipating. (See more details on TI’s test and our thoughts on it.)
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 significant improvements in contrast performance over earlier generation machines, DLP maintains its lead in contrast. Meanwhile LCD projector makers have continued to emphasize key advantages in color fidelity, color saturation, and image sharpness for data display.
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.
ProjectorCentral continues to recommend both LCD and DLP projectors for a variety of applications. For mobile presentation it is hard to beat the current group of 3-pound DLPs on the market. However LCD products like the Epson 735c 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 a technology preferred by many 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. Both technologies have advantages, and both have weaknesses. Neither one is perfect for everything. So the technology war continues. The only clear winner in sight is you, the consumer.
by:Evan Powell, July 7, 2003
ProjectorCentral.com
