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The Great Technology War: LCD vs. DLP
Evan Powell, July 7, 2003
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
theater 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
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 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.
The Advantages of LCD Technology
One benefit of LCD is that it has historically delivered better color
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 color 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 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 better color 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 (1,024x768), and then to widescreen XGA (WXGA,
typically either 1280x720 or 1365x768). 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 (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 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 960x540.
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. |
