| Video Noise November 2003
DLP Video Displays: Pros
and Cons
Continuing the series of articles
from last month’s discussion of DLP, this month we
tackle the ever-more-popular LCD. Though it was some time before LCDs were used for video
displays, the LCD has been with us now for more than 25 years -- it’s the oldest
video display technology after the 50-year-old direct-view color CRT. Still, LCD displays
have come a long way since the first devices appeared more than a decade ago. In
today’s desktop computer market, the flat-panel LCD monitor is quickly replacing the
venerable CRT. And the LCD was one of the key technologies that made laptop computers
popular, even possible.
Direct-view flat-panel LCD monitors are essentially screens
for laptop computers that are repackaged and sold as TV monitors. These are most often
13" to 20", measured diagonally. They may look cool, but their small sizes keep
them from being what you’re going to look for in a home-theater display. Users of
laptop computers will be familiar with the image quality offered by these flat-panel
displays -- they can look spectacular, but it can be difficult to get all shades of all
colors reproduced accurately.
| LCD Pros and Cons at a Glance Pros
- Perfect alignment of red, green, and blue pixels for zero
geometric distortion.
- No convergence issues, as are common in CRT products.
- Relatively long-lasting (6000 to 60,000 hours) lamps that are
usually easy to replace by the owner.
- You can’t "burn" an image into an LCD screen as
you can with CRT or plasma screens. (Onscreen network logos and video games are the most
common source of burn-in problems.)
- LCD panels are long-lived.
- Rear-projection products are lighter, smaller, and thinner
than CRT rear-projection displays of the same screen size.
- Blacks are as good as, or better than, blacks from plasma
displays.
Cons
- Replacement lamps can be moderately expensive ($100 to $400).
- LCD panels are susceptible to damage by heat, so cooling fans
are critical for the long-term life of an LCD panel, and fans can be noisy.
- If the image is sufficiently large in relation to the size of
the LCD panel, you may be able to see a texture in the image, caused by the tiny spaces
between pixels. This has been improved dramatically in the last five years. It is invisible in
some products, almost invisible in others, but still visible in the worst.
- LCD pixels may not switch on and off as quickly as plasma,
DLP, or CRT pixels; this can slightly blur image motion. There has been lots of improvement in
this area in the last five years, though the problem is still visible in some models.
- Rear-projecting LCD displays are best viewed in a darkened
room. They can tolerate a little ambient light, but daylight will wash out the images
significantly. Front-projecting displays are for use in darkened rooms only.
- Blacks will typically not be as dark as blacks from similarly
priced CRT displays, making LCD images look a little softer and less detailed, especially in
the shadows.
- LCDs have nonlinear contrast curves: If 5V makes the pixel
black and 0.01V is where the pixel is completely transparent (white), 2.5V does not, as you
might assume, produce a "middle gray." Each manufacturer has to have some method of
linearizing the response of its LCD panels. How well or poorly this is done affects the final
image quality significantly in regard to accurate color and tone-scale reproduction.
Manufacturers who use more complex and effective multi-point linearization schemes will
mention this in their advertisements and/or product specifications.
- Image quality is greatly influenced by the quality of video
scaling used by the manufacturer; better scaling almost always increases a product’s
cost.
...Doug Blackburn
db@hometheatersound.com |
|
|
Most LCD projectors have three LCDs, a white light source,
and a prism and/or dichroic mirrors to break the white light into the desired frequencies
of red, green, and blue. LCDs tend to have good image quality, without the rainbow
artifacts DLP products can exhibit when there is only a single DMD chip and a rotating
color wheel. LCD panels are sensitive to heat, and the lamp produces plenty of that.
Excess heat is removed by using heat-absorbing glass and/or infrared filtering in the
light path near the lamp, as well as one or more cooling fans.
Like rear-projecting DLP displays, rear-projecting LCDs are
smaller, thinner, and lighter than similarly sized CRT displays. As this article is
written, the lowest-priced rear-projecting LCD product I’m aware of has only a single
LCD panel producing all the colors. This will compromise image quality in ways that depend
on what technology is used to supply red, green, and blue light to the single LCD. If
you’re shopping for a low-cost LCD product, be sure you know if you’re getting
one or three LCD panels. That low-cost, rear-projecting LCD monitor might not be such a
good deal if the images are too dim, or full of artifacts.
Lamp considerations
Front- and rear-projecting LCD products use lamps that cost
from $100 to $400 USD. These can last from 6000 to 60,000 hours, depending on the type of
lamp used. Read the owner’s manual in advance to know what sort of lamp life you can
expect. Six thousand hours translates to about four years of use at four hours per day,
every day of the year. There is no reason the lamp should need to be replaced by a
technician, but the ease of lamp replacement varies considerably with rear-projecting
products. Front projectors always have easy access to the lamp.
Technology
The LCD technology is based on the properties of polarized
light. Two thin, polarized panels sandwich a thin liquid-crystal gel that is divided into
individual pixels. An X/Y grid of wires allows each pixel in the array to be activated
individually. When an LCD pixel darkens, it polarizes at 90 degrees to the sandwiching
polarizing screens. This cross-polarizing blocks light from passing through the LCD screen
where that pixel has darkened. The pixel darkens in proportion to the voltage applied to
it: For a bright detail, a low voltage is applied to the pixel; for a dark shadow area, a
higher voltage is applied. LCDs aren’t completely opaque to light, however; some
light will always penetrate even the blackest LCD pixels.
Black performance
That LCD pixels are incapable of blocking all light
might seem a fatal flaw -- until you realize that film has the same problem. Light passes
through film to make the images we see on the screen, but the blackest blacks on movie
film are not entirely opaque. In movie theaters, blacks on the screen look fairly black
because the image is projected to a very large size, which weakens the small amount of
light that leaks through the black areas on the film. As long as you keep an LCD’s
image size within the design parameters, you’ll get adequate blacks -- slightly
better than plasma’s, and comparable to DLP’s, but not quite as good as
CRT’s. A CRT’s advantage is that the phosphors on the tube are "off"
when the pixel is supposed to be black -- no light at all reaches the screen for that
pixel.
Contrast ratio
Most video display devices have contrast ratios that fall
between 150:1 and 3000:1. A higher contrast ratio indicates suitability for viewing in
brighter conditions, but even a 600:1 ratio will give you markedly better images in a
darkened room. Remember: A screen will reflect any ambient light, to the detriment of
picture quality; the lower the contrast ratio, the darker the room needs to be for optimum
viewing.
Middle-of-the-pack contrast ratios of 350:1 to 600:1 are
typical for rear- or front-projecting LCD displays. Most people will achieve excellent
brightness in a darkened room with a front-projecting LCD; rear-projecting LCDs can handle
a small amount of ambient light, but you’ll get the best performance when the room is
fully dark. If the display is to be used in a well-lit family room, with people reading or
doing other activities while others watch TV or movies, a rear-projection LCD is probably
not your best choice.
Don’t let the lifestyle photographs of LCD
rear-projection displays, with their brightly lit modern rooms or soothing outdoor
ambience, fool you into thinking that LCDs thrive in such viewing environments. They may
look at home in such rooms, but the images won’t impress anybody until you turn the
lights off.
Resolution
Samsung's LTN406 HDTV-compatible LCD television
monitor has a 40"-wide viewing area. The retail price in the U.S. is approximately
$8000.
|
The resolution of almost any current LCD
display, regardless of size, is either 1280x768 or 1280x720 -- about the same as current
DLP displays. This makes LCDs capable of displaying 720p HDTV at full resolution
(1280x720), but the highest HDTV resolution, 1080i (1920x1080), will have to be
downsampled to match the LCD’s capabilities. During this downsampling, the interlaced
1080i input signal also must be converted to the progressive 720p format, which takes
additional processing power to do correctly.
Individual LCD panels in projection products are usually in
the range of 0.75" to 1.5" (diagonal) -- generally, the larger the final image,
the larger the panels. While the panel may be larger, it doesn’t contain more pixels.
Still, larger panels pass more light per unit area, which makes for a larger, brighter
picture.
Scaling
Being fixed-pixel devices, like plasma and DLP displays,
LCDs require manipulation of every video source signal in order for these signals to be
displayed at the panel’s "native" resolution of 1280x768 or 1280x720. Lower
resolutions from cable, satellite, DVDs, VCRs, and camcorders must be upsampled to the
native resolution; the highest-resolution HDTV images must be downsampled. As with plasma
and DLP devices, the quality of the scaling has a direct impact on image quality. But good
scaling is complex and expensive; the less expensive the product, the lower the scaling
budget.
...Doug Blackburn
db@hometheatersound.com |
