| Video Noise July 2009
What Makes a Picture Good (Accurate) or Bad?
 Four
essential factors define video-display image quality as we know it today:
1) The blackness of black and the brightness of white
(displays can be either too bright or too dim)
2) Grayscale reproduction accuracy (is each level of gray
truly neutral gray?)
3) Gamma, grayscale’s third dimension (the luminance
of each grayscale step)
4) The color accuracy of the primary (red, green, and blue)
and complementary (yellow, cyan, and magenta) colors
Of course, there are other issues, including finding the
best setting for the sharpness control, determining what to do with the various
noise-reduction settings, and figuring out all the controls that have auto or dynamic
in their names. But once those settings are correct, the four items listed above are
really all a calibrator has to work with. Video displays have very specific standards
that, if adhered to, will create accurate images that convey exactly what the director and
cinematographer had in mind. If you settle for inaccurate settings, you likely won’t
see what the director and cinematographer intended, nor will you achieve the best possible
video display.
You may notice that color temperature isn’t listed,
even though you’ve likely read about it in many video-display reviews over the years.
Color temperature simply refers to the balance of red (warm) and blue (cool). Let’s
say you have a video display with red, blue, and green in perfect balance, with a perfect
color temperature of 6500K. You can add or remove large amounts of green from that
display, and the color temperature will not change if red and blue remain perfectly
balanced. If the calibrator sets his software for 6500K and sets the grayscale to be as
perfect as possible, the color temperature will automatically be as accurate as it can be.
Calibrators, being concerned about red, green, and blue, will always get color temperature
correct simply by setting the grayscale as accurately as possible.
Black level and white level
Let’s face it: nobody would be happy with a video
display that has obviously gray blacks or whites that are too dim or too bright. The only
problem with white levels in some current displays is LCD panels that won’t get dim
enough for comfortable viewing in a dark room. If you own one of these TVs, your only
defense is to leave a light on while you view it. Thankfully, dimness is less of a problem
for panel displays these days. There are a few with special modes that can be viewed only
in a dark room, but other modes are brighter and more suitable for daytime viewing. The
brightness control sets the black level, while contrast adjusts the white level. But these
levels sometimes interact, so adjusting one requires rechecking the other.
The black levels of most displays sold today are acceptably
dark, but there’s always room for improvement. Pioneer Kuro plasma displays were on
the leading edge of blackness (kuro means black in Japanese), but the economic
downturn forced Pioneer out of the video display business just as they were about to debut
plasma displays that were so black they appeared to emit no light when displaying a black
field in a dark room. Only those who attended Pioneer’s demonstrations at CES in
January of 2009 witnessed this impressive achievement. Other plasma displays for the 2009
to 2010 model year are slightly darker than last year’s models, but they don’t
match the standard set by the quickly disappearing 2008 to 2009 Kuro models. LCD panels
are using all sorts of tricks to make the backlight darker for dark scenes in order to
appear as dark as plasma models. Some darken the entire backlight for darker scenes, while
others with LED backlight systems will dim areas of the screen that are darker. When the
backlight is dark, less light will leak through the LCD panel so that blacks look blacker.
While it may now be impossible to find a display that has
the gray-blacks of two or three years ago, there are still differences among brands and
models. It pays to research how black the TV can get because the blacker the blacks are,
the more you’ll enjoy the picture. But don’t bother with manufacturer
contrast-ratio numbers. These figures indicate nothing about how black the panel will be
with real video content. Don’t be fooled by the "black trick" on many
current displays: if an all-black screen appears, after three to ten seconds the screen
goes completely black, emitting no residual light. That will never happen with real video
content, but many displays use this trick to claim a ridiculously high contrast ratio.
Grayscale
In case you’re unfamiliar with the term, grayscale
refers to any video content (typically a test pattern) that starts at black and ends at
white. It could be a continuous ramp with no visible steps, but calibrators typically use
an 11-step grayscale pattern that starts at black and has steps for every 10 percent
increase in luminance (brightness). Black is zero percent white, white is 100 percent
white, and each step in between is a shade of gray ranging from bright gray (80 and 90
percent) to dark gray (10 and 20 percent). The goal of grayscale calibration is to make
every shade of gray neutral, meaning there should be no color noticeable in any grayscale
step. Calibrators adjust the grayscale by balancing the amount of red, green, and blue so
that each step looks as perfectly gray as possible. To ensure accuracy, calibrators use a
meter and calibration software to measure grayscale.
Unfortunately, most video displays have only two sets of
controls to balance the grayscale. A set of RGB controls adjusts the brighter grayscale
steps, and a second set of controls adjusts the darker grayscale steps. Low-cost models
may dispense with one of the two sets, leaving you with just one set of controls to
improve the balance of red, green, and blue from 10 to 100 percent. That’s a definite
compromise. And the most economical video displays may do away with both sets of controls,
leaving a calibrator with nothing to improve the balance of red, green, and blue. Asking a
salesperson about these controls will usually be pointless. To make sure your video
display has a decent range of adjustment controls, do some research through online owner
forums or discuss your options with the video calibrator who you plan to hire after
purchasing the TV. These controls are sometimes in the user menu, or they may be in the
service menu (a hidden menu that requires a special code, as it’s often capable of
completely disabling the video display). Regardless of where the controls are located, you
shouldn’t expect to set them by eye. Accurate adjustments require a meter and
calibration software.
Most displays aren’t perfectly linear when it comes to
the response of red, green, and blue. You’ll often find that one, two, or all three
colors have dips or peaks somewhere between zero and 100 percent. The art of calibrating
video displays is finessing those non-linearities so that there’s no (or minimal)
visible error at any step in the grayscale. Human vision is most sensitive to green, so
calibrators are taught to avoid green errors in favor of errors in other colors when
there’s no other option. There are very specific x and y coordinate reference points
that are the same for every grayscale step, including 100 percent white. Any deviation
from the two coordinates means a color error. If the error is large enough, you’ll
get a visible error in one or more grayscale steps. But color is a three-dimensional
concept. Not only does each grayscale step need to be as close as possible to the ideal x
and y coordinates to ensure it’s a neutral color, but each grayscale step must also
have the proper luminance. That’s where gamma comes into play.
Next month’s column will discuss the importance of
gamma.
. . . Doug Blackburn
db@hometheatersound.com |
