สงคราม LCD&DLP
Evan Powell, December 7, 2005
ProjectorCentral.com
(NOTE: This is the seventh update of an article originally posted four years ago. The technologies continue to evolve quickly, hence the periodic updates.)
Introduction
If you are new to the world of digital projectors, you won't have to shop around very long before discovering that the terms "LCD" and "DLP" somehow refer 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, so you can make a good decision about which will be better for you.
By the way, there is a third significant light engine technology called LCOS (liquid crystal on silicon). It is being developed by several vendors, most notably JVC, Sony, and Hitachi. Several outstanding home theater projectors have been manufactured with this technology. However the discussion of LCOS technology is beyond the scope of this article.
The Technical Differences between LCD and DLP
LCD (liquid crystal display) projectors usually contain three separate LCD glass panels, one each for the red, green, and blue components of the video signal. 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 most DLP projectors under $15,000 there is only one chip. In order to define color, there is a color wheel that consists of (at minimum) red, green, blue, and sometimes white (clear) filters. This wheel spins in the light path between the lamp and the DLP chip, and the filters determine the color of the light hitting the chip. The mirrors tilt away from or into the lens path 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. In addition to red, green, blue, and white segments, some color wheels now use dark green or yellow segments as well, and we are about to see the introduction of color wheels that contain all six primary and complementary colors--red, green, blue, cyan, magenta, and yellow. According to Texas Instruments, this enhancment, known as "BrilliantColor(TM)", will boost color performance on single chip DLP projectors to new levels. It should begin to appear on new models at CES in January.
Performance Advantages: LCD vs. DLP
Both of these technologies have been evolving rapidly over the last five years. Both of them are much better than they used to be, and the radical differences we used to see between them have narrowed substantially. There are still noteworthy differences, but the story is a classic example of how open market competition drives improvements in technology.
Historically speaking, one traditional benefit of LCD was that it delivered better color saturation than was possible from a DLP projector. That was because in most single-chip DLP projectors built for the commercial presentation market, a clear (white) panel is included in the color wheel along with red, green, and blue in order to boost light output. Though the image is brighter than it would otherwise be, this tends to reduce color saturation, making the DLP picture appear not quite as rich and vibrant. This is not an issue with data display as colors are plenty vibrant with a data signal. But it is something to consider if you plan to use the projector for high quality video display.
To compensate for the lack of color saturation and to improve color accuracy, many of the DLP-based products made for home theater now have six-segment color wheels that feature at minimum two sets of red, green, and blue filters. Meanwhile the white segment is eliminated. And some wheels have seven or eight segments to include a dark green and or yellow filter in addition to the two sets of red, green and blue. These wheels boost color accuracy and color saturation while sacrificing light output. This is a good trade-off for video applications because high quality front projection video depends on high image contrast and color saturation, not lumen output. Front projection systems are always best viewed in a dark environment where high lumen output is not required, and can actually be detrimental. Thus the home theater-oriented DLP projectors have closed the gap with LCD in the area of color saturation, whereas the DLP products built for high-brightness commercial presentation still tend to have a weakness in this area.
Another area where these two technologies have differed is in image sharpness with data applications. LCD can usually deliver a slightly sharper image than DLP at any given resolution. The difference between the two is noticeable in the display of detailed computer data, like financial spreadsheets. However, there is usually no visible difference in the sharpness of video material that is uniquely attributable to the technologies. This is not to say that DLP will project a fuzzy spreadsheet--it doesn't. When you look at a spreadsheet projected by a DLP projector it looks sharp 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 a bit sharper in comparison.
A third traditional advantage 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. Thus LCD products dominate the bright end of the performance range. However, brighter DLP models are beginning to encroach upon LCD here as well. Just over a year ago, in September, 2004, there were 60 projector models in the database rated between 3000 and 6000 ANSI lumens at retail prices under $10,000. All of them were LCD projectors with the exception of one DLP model. At this writing, there are 110 models in this category, 79 of which are LCD and 21 are DLP. So DLP has made some incursions into the high brightness niche. Still, within this group of 110 models, 25 of them are rated above 4,000 lumens, and all but one of those are LCD products. So LCD continues to maintain a significant competitive edge in light output per dollar spent.
LCD projectors have historically had three weaknesses, all of which are more relevant to video than they are to data applications. The first is visible pixelation--the ability to see the discrete pixels on the screen. The second is commonly referred to as the "screendoor effect" because it looks like you are viewing the image through a screendoor. The screendoor effect is caused by the space between the pixels, known as the inter-pixel gap. The third weakness is not-so-impressive black levels and contrast, which are vitally important elements in a good video image. LCD technology has in the past had a hard time being taken seriously among some home theater enthusiasts (understandably) because of these flaws in the image.
However, LCD has made great strides in these areas. The inter-pixel gaps on LCD panels have been reduced and physical resolution--the number of pixels on the screen--has been increased. In the early days of the digital projector industry, resolutions were low, generally at VGA (640x480) or lower. The industry then stepped up to SVGA (800x600), and then to XGA resolution (1,024x768) and higher. Many of today's widescreen format projectors use either 1280x720 or 1366x768 resolution displays. With each step up in the number of pixels that produce the image, visible pixelation and the screendoor effect have been reduced. At the new pixel densities, visible pixelation is eliminated at normal viewing distances, or rendered so subtle as to be a non-issue for most viewers.
Second, the inter-pixel gaps on all LCD machines, no matter what resolution, have been reduced compared to what they use to be. So even today's inexpensive SVGA low-resolution LCD projectors have less screendoor effect than did earlier models.
Since DLP technology creates a pixel by reflecting light from a tiny mirror, its edge definition is softer and less well-defined than LCD (this is what accounts for the slightly softer image in detailed spreadsheet presentation, but also DLP's traditionally smoother image in video). So for the most part, at any given resolution, DLP still holds an advantage over LCD in visible pixelation. However due to the recent advances in LCD technology you need to stand closer to the screen to see the differences than you used to. And the latest home theater models made by Panasonic incorporate unique filters to eliminate all traces of pixel structure despite the use of LCD technology.
Now when it comes to contrast and black levels, LCD still lags behind DLP overall. But LCD is showing impressive new strength in contrast and black level capability. Four years ago typical LCD projectors were rated at 400:1 contrast or lower while comparable DLP models were at 600:1 or 800:1. Since then, major improvements in both technologies have boosted contrast ratings to new levels. Many commercial DLP projectors are rated at 2000:1 these days, and a few models built specifically for home theater carry ratings of up to 10,000:1.
Meanwhile, in September, 2004 there were 38 LCD projectors rated at 1000:1 contrast or higher in our database. Today there are 75. And the addition of dynamic aperture control is able to boost contrast on LCD projectors to unheard of heights. At the moment there are six home theater projectors using LCD technology that are rated at 5000:1 or higher. With contrast performance in this ballpark, the new high contrast LCD projectors for home theater are more competitive with their DLP counterparts than they ever have been in the past. By and large, however, most commercial DLP products continue to maintain an advantage in contrast against LCD.
The consumer should bear in mind that while high contrast is critically important for high quality video presentation in a darkened environment, it is pretty much irrelevant in commercial data presentation in a fully or partially lit room. Once you have lights on in the room, black levels get hammered, and the real contrast on the screen usually drops to well below 20:1 no matter what the theoretical contrast rating on the projector is. So for "lights on" data presentations, adequate lumen output is essential and contrast is meaningless. Accordingly, many commercial LCD projectors continue to to be sold despite contrast ratings of 400:1 or lower. The reason is that they deliver a substantial amount of light and razor sharp data images at extremely competitive prices. They are designed specifically for commercial presentation environments in which the contrast rating is a non-issue.
Finally, one of the key advantages of DLP over LCD is small package size, a feature most relevant in the mobile presenter market. Since the DLP light engine consists of a single chip rather than three LCD panels, DLP projectors can be more compact. There are 39 DLP projectors currently on the market weighing 4.0 pounds or less, compared to only seven LCD models in this weight category, six of which are from Epson. However, all six Epson models are near the top of the list when all of the sub-4 lb models are ranked by lumen output. So LCD's advantage in light efficiency manifests itself across the entire spectrum of projection products.
A Potential Problem with LCD: Long Term Image Degradation
We believe that at the current commercial state of the art, LCD panels and polarizers will eventually degrade over time. However, the degree to which they will degrade is dependent upon a variety of factors. Some of those factors are related to the projector's light engine design and cooling system, the presence of internal UV filters, and so on. So some LCD projectors may have more of a tendency to degrade than others based on their design. On the other hand, some factors may be related to usage. If air filters are not cleaned when they need to be, the internal operating temperature will rise. Usage in a chronically warm environment may have an impact. We would not be surprised to discover that projectors used at higher elevations could be more susceptible to LCD degradation due to higher operating temperatures in thinner atmosphere.
However, this is all speculation. There is no reliable data on the subject of LCD failure rates by brand or model under various operating conditions. We know that the design of the light engine and cooling system will have a lot to do with the potential for panels and polarizers to degrade. So identical LCD panels placed in two differently designed projectors and operated in identical environments will not degrade along the same curve. Some models may begin to see some degradation in as little as 1000 to 2000 hours of use, and others may not suffer any noticeable change for many thousands of hours--long past the realistic life of the projector.
The bottom line is that there exists the possibility that those who invest in an LCD projector may find that an LCD panel and/or polarizer may eventually 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.
A Potential Problem with DLP: Rainbow Artifacts
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 commonly referred to as the "rainbow effect." This is simply due to colors separating out in distinct red, green, and blue because of the sequential color updating from the wheel. (Three-chip DLP projectors have no color wheels, and thus do not manifest this artifact). Basically, as the color wheel spins the image on the screen is either red, or green, or blue at any given instant in time, and the technology relies upon your eyes not being able to detect the rapid changes from one to the other. Unfortunately some people can see it. Since LCD projectors always deliver a constant red, green, and blue image simultaneously, users of LCD projectors do not have this problem.
How big of a deal is this? Well, it is different for different people. Most people cannot detect color separation artifacts at all. However, for those who are most sensitive to rainbows, they are so distracting that they can render the picture literally unwatchable, and a couple of folks on staff here at ProjectorCentral get headaches from it. Many others fall between these two extremes--they report being able to see rainbow artifacts on occasion, but find that they are not particularly annoying and do not inhibit the enjoyment of the viewing experience.
Texas Instruments and the vendors who build DLP-based projectors 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 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 color refresh rate reduced the time between color updates, and so reduced or eliminated the visibility of color separation artifacts for most people.
Today, as noted above, many DLP projectors being built for the home theater market incorporate a six-segment color wheel which has two sets of red, green, and blue filters. This wheel still spins at 120Hz or 7200 RPM, but because red, green, and blue are 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 substantially reduced the number of people who can detect the rainbows.
For the large majority of users the six-segment, 4x speed wheels have solved the problem for home theater or video products. Meanwhile, due to the higher lumen output requirements for business presentation use, most commercial DLP units still use the four-segment, 2x speed wheels. However, rainbow artifacts tend to be less of a problem in commercial presentation for two reasons. First, viewers tend to sit back farther from the screen, and there is a smaller angle of view from edge to edge. That means less eye movement is required to see the entire image, and it is eye movement that causes you to see the colors separate. Second, data presentations are static; there are no moving images that cause the eye to pan or rapidly refocus on different areas of the image in a continuous fashion. Since the eye typically does not move as much or as rapidly when viewing data images from a distance as it does when viewing widescreen video or games from relatively close up, rainbow artifacts tend to be less of a concern with commercial projectors.
If you've seen earlier generation DLP machines and detected no rainbow artifacts, you won't see them on the newer machines either. Many 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 distracted by the spinning color wheel without sitting down and viewing a DLP projector for a while. So if you are uncertain, it would be wise to arrange an audition of a DLP projector you might be interested in prior to committing to it for your home theater.
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 Instruments 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.
DLP continues to be exceptionally strong in entry level video and home theater. TI's introduction of the 854x480 resolution DLP chip created a host of inexpensive but amazingly high performance entry level home theater projectors, now selling well below $1200. Practically speaking, LCD has no competitive answer to this, and DLP is dominating this entry level market niche.
However, the next step up in price and resolution is a different story. The latest 1280x720 resolution LCD projectors released this fall directly challenge their DLP competition in image quality in a way that they never have been able to before. Black levels, color saturation, and contrast are substantially improved from earlier generations, and in some cases, especially with HDTV subject matter, it can be extremely difficult to tell the difference between LCD and DLP even when viewing them side by side. The fact that these new 720p LCD units sell for not much more than $2,000 makes them highly competitive as compared to the more expensive DLP counterparts in the same resolution class.
When it comes to commercial applications, we noted previously that LCD has a latent competitive advantage where high lumen output is required. Most of the 6000 lumen light cannons retailing for under $20,000 are LCD products. And Epson's array of extremely bright sub-4-lb portables establish a significant presence for LCD in a mobile presentation niche where DLP otherwise holds a commanding lead.
Nevertheless, DLP has emerged as a strong technology in the large volume commercial presentation market, offering a wide variety of models in any given price and performance range. For example, at this writing there are 36 XGA-resolution projectors on the market that are at least 2000 lumens, retailing for under $2,000. Of those, 24 are DLP and only 12 are LCD.
Across all price and performance categories, we currently have 353 DLP projectors listed as in production, as compared to 325 LCD models. This is highly significant--earlier this year, for the first time in the history of the projector industry, DLP surpassed LCD in the total number of projector models on the market.
Both LCD and DLP are evolving rapidly to the benefit of the consumer, and they will continue to do so. ProjectorCentral continues to recommend both LCD and DLP projectors for a variety of applications. When it comes to home theater, DLP has continued to make steady advances in color, contrast, and image stability that have served to make it a technology preferred by many for home theater use. But the competition from LCD heated up significantly this fall, with some of the latest models demonstrating that LCD products can rival DLP image quality, in many cases for a fraction of the price. This is a startling and novel development that should stimulate some aggressive price reductions on 720p resolution DLP projectors. If it doesn't, LCD is in a good position to capture a sizable new chunk of market share in the sweet-spot of the consumer home theater market.
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