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Before You Call

LCD vs DLP
 
Liquid Crystal Display  vs
                              Digital Light Processing

The Great Technology War: LCD vs. DLP  (kind of like....Beta vs VHS...)


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, and 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. While they have manufactured some fine products with this
technology, and the release of JVC's the new LCOS-based DLA-SX21
is eagerly anticipated, the discussion of LCOS technology is too much
to include in this article.

One of the main concerns in delivering multimedia presentations
 has always been unreliable or unsuitable projection equipment.
 Fortunately LCD technology has advanced significantly from the
 first dim VGA LCD panels and good quality, high resolution,
projectors are now available. Where panels sat on top of a high
powered OHP, projectors now contain their own light source.

The Technology

LCD

Traditional Liquid Crystal Displays consist of two plates of glass
with a layer of liquid crystal material in between. When a charge
is applied to the cell, the crystals can rotate the plane of polarized
light, effectively acting as an on/off switch for the light. Color is
created by using three cells for each pixel, one for each of the
primary colors. Early models were passive, while most new
projectors use an active matrix, which has a thin-film transistor (TFT)
behind each element. Until recently, the TFT layer in most projectors
and panels was amorphous silicon. These panels have a much higher
refresh rate than passive LCDs and are suitable for displaying video
as well as still images.
There are a number of problems with this type of display, however:
bulletCreating large active matrix LCD screens is difficult, the larger
 the screen, the more transistors involved and the more likely
some of them are to fail.
bulletA very strong light source is required, as the transistors absorb
 much of the light.
Newer active LCDs are made with polysilicon rather than amorphous
 silicon. These are much smaller than traditional LCDs, are more
transmissive (i.e., brighter) and have higher switching speeds, but
are more expensive to produce. These projectors usually use three
 LCD panels, rather than one. The light is split into red, green and blue
components by dichroic mirrors, with each color passing through a
separate LCD panel driven by the corresponding component of the
 video signal. A prism is then used to recombine the light before it
leaves the projector. Using three panels results in better colors than a single panel.

Digital Light Processing

Projectors that do not use LCD technology, but Digital Light Processing,
based on Texas Instruments Digital Micro mirror Device, are now
becoming available. This is an SRAM (Static Random Access Memory)
chip, covered with microscopic aluminum mirrors, each equating to a pixel.
Charging one of the underlying memory bits causes the mirror to rotate due
 to the electrostatic charge, so acting as an on/off switch.
DLP has a number of advantages:
bulletAs reflected light is used, the image can be much brighter than with
 standard LCD projectors
bulletBecause the mirrors are so small and closely packed the image
produced is very high quality, with little pixellation.
bulletThe digital switch is very efficient, resulting in less noise and flicker.
In order to produce color images, a spinning color wheel is used to provide
sequential red, green and blue light for the projector, synchronized with the
mirror movements. The color wheel absorbs two thirds of the light at any
 one time, i.e., only the red, green or blue component is transmitted.
 Therefore in projectors where the color wheel can be removed from the
 path to give grayscale images, the brightness will increase up to threefold.
 Three chip DLP systems are also possible. These are equivalent to the
 three panel LCD systems, with dichroic mirrors and prisms used to split
and recombine the light. Since all the light is reflected, much brighter
 images are possible.

Lamps

There are two types of lamps used in projectors, tungsten-halogen
and metal-halide. The metal-halide lamps are more efficient, providing
 greater brightness with lower power consumption. It is important to note
 that the lamp wattage does not reflect its brightness, this is measured
in ANSI lumen, a value measured at the screen which takes into account
 the image size as well as brightness. For example, a 400W
tungsten-halogen lamp may produce 200 lumen, while a 250W
 metal-halide lamp may produce 450 lumen.

Tungsten-halogen lamps are filament lamps with a low pressure halogen
gas atmosphere. They run at a much higher temperature than normal
lamps, so producing a whiter light. The color temperature will shift
during its lifetime, however, causing it to look dimmer and more yellow.
The higher temperature and low pressure atmosphere both help to
prolong the filament, and so lamp, life.

Metal-halide lamps use a high voltage discharge between two
electrodes contained in a low pressure halogen and mercury vapor
atmosphere. Initially a high current is drawn, which is reduced as the
lamp warms up. These lamps are very efficient and produce a very
white light, which is maintained throughout its life, though it will get dimmer.

Although the metal-halide lamps are initially much more expensive,
 they last much longer, 2000 hours compared with 25-70, making
the metal-halide cheaper in the long term.

What to look for

Image Size

Image size depends not only on the projector, but also on its positioning in the room, the further away, the bigger the image. It is very important when making a comparison to ensure all the models are the same distance from the screen, and that it is a similar distance to a real use situation. Although most of the model tested had zoom lenses, in all cases these had to be at maximum magnification to produce a reasonable image size at the test distance of 2.5m from the screen.

Resolution

All the projectors reviewed below had a maximum physical resolution of 800x600. They differ, however, in how they cope with other resolutions. At lower resolutions (640x480), some are able to resize the image to fill the whole screen. To achieve a high quality image, however, resizing may not be desirable, so look for an optional, rather than auto-resize feature. At higher resolutions, some method of compression is employed. This means that data is lost, and the image quality will therefore decrease. Compression methods vary however, and it is worth checking the quality with typical screens if you plan to use this feature.

Color and Contrast

To a certain extent, determining which projector provides better color is a subjective exercise, and will depend on the exact contrast and brightness settings. However, it is worth displaying the same images through different projectors to compare. Test images should include some with a wide variation in shades, and some with large areas of solid colour.

Claimed maximum contrast ratios vary from about 100:1 to 300:1, and generally the higher the ratio, the better the image quality. Note that the type of screen used will also have some affect on the brightness of an image. Standard matte screens give the dimmest image, but it can be seen from anywhere in the room. Reflective screens are also available, which, while giving a brighter image, reduce the viewing angle.

Mounting

Where to site a projector is a very important issue. It must be at a reasonable distance to give a good quality image with a reasonable size, but must not interfere with the view of any of the audience. Most of the reviewed projectors supported three projection directions:
bulletFront - the standard viewing position. The main disadvantages are that the projector may obstruct the audience's view, or that the presenter may block the image path. All the projectors reviewed below were small enough not to cause a real obstruction, certainly smaller than an OHP and LCD panel.
bulletRear - this requires a special projection screen, and a similar amount of room behind the screen as would be required for front projection. It is usually only found in very large lecture theatres.
bulletCeiling - perhaps the best solution as the projector is not in line of sight, and is less likely to be accidentally damaged or stolen. As the projector will usually be mounted upside down, it must be possible to invert the image. In addition to the cost of the mounting, a cable booster may also be required if the projector is more than a 6 feet from the computer.

Keystoning

Keystoning occurs when the projected image is not square, e.g., the top of the image is large than the bottom. This is caused when the projected image is not perpendicular to the screen, and so can be corrected to some extent by tilting the screen. All the projectors had some degree of keystone correction built in, allowing the projector to be placed off center. As the degree of keystoning is dependent on the position of the projector, it will be different if the projector is front or ceiling mounted. While a few projectors allow the degree of keystone correction to be altered, most have a fixed correction.

Accessories

To prevent frustration later, it is important to check the projector comes with a full set of cables, including those for connecting to PC and Macs and where appropriate video and audio cables.

Scan Rate and Autosync

Autosync is a common feature which allows the projector to automatically detect the input signal from the computer and adjust its scan or refresh rates etc, accordingly. In most cases some fine-tuning will also be required to get the best image quality.

Many graphics cards allow users to change the (vertical) refresh rate to eliminate any flicker. This may cause problems if the LCD projector is not capable of working at this rate, and should be checked if the projector cannot 'lock on to' a signal.

Flat Panel or Projector?

Flat Panel LCDs do offer a number of advantages over projectors. They are cheaper, more portable, often higher resolution and tend to be more reliable. However, they require a high power OHP, produce a much dimmer image than can be achieved with a projector and usually have slower response times, resulting in poorer video quality.

 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.

 Both technologies have advantages, and both have weaknesses.

Neither one is perfect for everything. So the technology war continues, and the only clear winner in sight is you, the consumer.

Glossary

 
Active Matrix
Term used to describe LCDs which have micro-transistors that "open" and "close" each pixel.
AMLCD
Active Matrix Liquid Crystal Display
a-Si
Amorphous Silicon
Dichroic
A mirror or lens that reflects or refracts selective wavelengths of light.
DLP
Digital Light Processor, projector technology based on DMD.
DMD
Digital Micromirror Device, developed by Texas Instruments
Keystoning
Keystoning is caused when the projected image is not perpendicular to the screen, making the image appear wider at the top than bottom.
LCD
Liquid Crystal Display
Lumen
The unit of illumination on a screen or other surface.
p-Si
Polycrystalline silicon or Polysilicon
TFT
Thin Film Transistor


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