| Video Noise April 2008
 The Next Frontier for Home-Theater Video Displays
Improvements in home-theater video displays of all types
continue at a breakneck pace. Most people know about contrast ratio: the difference in
light level between black and white. Color temperature and gray-scale tracking are related
specifications that have been well covered in the magazines. A ten-step gray scale has 0%
white (aka black) at one end and 100% white at the other end, with intermediate shades of
dark, medium, and light gray.
When you plot the luminance (brightness) value for each
step of the gray scale on a graph, you get a Gamma curve. Gamma is a legacy issue
we’re stuck with: Because early television displays could not be manufactured with
linear response, cameras and displays were made to be intentionally nonlinear. TV displays
were darker in the mid-tones than a strictly linear device would be; to make images that
would look good on such displays, cameras had to record images that were brighter in the
mid-tones than a strictly linear image would be. In other words, when you display
brighter-than-neutral mid-tones on a TV whose mid-tones are darker than neutral, the
images look normal. If we did not have to intentionally make video displays darker in the
mid-tones, their Gamma "curve" would be a straight line with a 45-degree slope,
and their Gamma value would be 1.
To display video images accurately, a display should have a
Gamma of 2.2-2.5. Some displays do meet this requirement, and the ones that don’t can
often be adjusted to meet the specification. Good measurement results for gray scale and
Gamma are necessities for accurately reproduced images. For this, instruments are needed
that can measure small differences in color balance, and -- very important -- the
instruments must be accurate for different types of video displays. Measurements are
needed to make slight variations in the grays apparent so that adjustments can be made to
ensure that the shades of gray are not tinted with any color. For accurate images, grays
must be perfectly neutral, which means that each color (red, green, blue) is present in
equal proportion. "Eyeballing" color variations is difficult because the eye can
be fooled; you need see only a few optical illusions to know that this is a serious
concern. (One of the best color-perception illusions can be viewed at Color Perception Illusions.)
Few makers of displays have addressed with satisfactory
results one fundamental performance parameter: color space. A color space is any
three-dimensional space that defines color, and there are many ways to represent them.
Red, Green, and Blue values can be plotted on a three-axis graph to produce a cubical
color space. The point where Red=0, Green=0, and Blue=0 would be black, and the point
where all three colors were 255 would be 100% white. The eight corners of the cube would
be black, white, red, green, blue, cyan, magenta, and yellow. Every combination of RGB
values between 0 and 255 would represent a slightly different color.
 You can make different types of color
spaces by using different measurement parameters. You’ve probably seen the CIE 1931
color chart from the Commission Internationale de l’Eclairage (International
Commission on Illumination), first released in 1931 and often reproduced in reviews of
video displays in magazines. A sample is shown here. It has shades of green at the top,
blue on the left, red on the right, and a white area in the center. The chart is presented
as a two-dimensional slice of a three-dimensional color space -- like a slice from a whole
ham. Brightness or luminance -- darker and lighter shades of the same colors -- complete
the 3-D CIE color space, but this third dimension is omitted when the chart is presented
in two dimensions.
HDTV programming has a specific color space defined by the
locations of red, green, and blue on the CIE chart. This space is everything inside the
triangle created by the red, green and blue measurements. In the CIE 1931 chart, the HDTV
color space is shown as a dark triangle; the slightly larger white triangle is the color
space of the display being measured -- in this case, a plasma TV. All of the colors inside
the white triangle are reproducible by the video display. But here’s the thing: None
of the colors outside the dark triangle should be inside the video
display’s color space. If the display’s color space is too large, all colors
will be reproduced inaccurately to some degree. If the display’s color space is
smaller than the HDTV standard, all colors will be displayed inaccurately to some degree,
and some colors will not be displayed at all. To take video displays to the next level,
manufacturers must address the color-space issue. Some brands claim that some of their
models include a Color Management System (CMS), but most of these turn out to be useless
in making the display’s color space more accurate. For one reason or another, you
just can’t move colors with the provided adjustments to the points they need to be at
for accurate color reproduction. Or if you can move the red, green, and blue
primary colors to the correct reference points, other important parameters are made
nonlinear, which results in poor-quality images for different reasons.
What’s needed is a system for adjusting a
three-dimensional color space. The CIE color chart is really a single two-dimensional
cross section of a three-dimensional color space. What are missing are the darker and
lighter shades of the colors present on the chart. You can find a color on the CIE chart
using the horizontal x and vertical y coordinates. A third, Y,
coordinate representing brightness is needed to place the x and y
coordinates accurately within the 3D color space. This is critical -- if x and y
are perfect for blue, that shade of blue can still be inaccurate if it is too dark or too
light. The third axis of the graph would be perpendicular to the page or screen the CIE
chart is printed on, extending above and below it.
Such a built-in adjustment system would have a three-axis
control for each primary color (red, green, blue) and each secondary color (cyan, magenta,
yellow), and for each of these a slider for each of the three coordinates: x, y,
and Y. That’s 18 individual adjustments. Few video displays give us anything
close to that range of adjustment. Joe Kane, creator of the Digital Video Essentials
test discs, has worked with Samsung on a 720p front-projector (since discontinued) and a
1080p DLP front-projector to provide internal video calibrations for the primary and
secondary colors on all three axes. These projectors could/can produce images with nearly
perfect color spaces. Unfortunately, neither projector made much of an impression in the
marketplace and they were not widely reviewed.
Actually using a CMS will cost money above the
display’s purchase price: the buyer either must spring for his or her own measuring
device and software, or pay a professional calibrator to do the job. Fees charged for
calibrations vary but are most often between $200 and $500, depending on the services
desired, the display itself, and the individual calibrator. While many home-theater
enthusiasts have heard about professional calibration, most have never seriously
considered laying out cash for it, even though it can make worthwhile improvements in the
image quality of a video display, whether or not it includes an accurate CMS.
To understand why manufacturers do not calibrate video
displays to accurate values of color, Gamma, color temperature, and gray scale, you need
only see how long it takes a professional to do the job. It can take more time to
accurately adjust a video display than it takes to build and test the entire product.
It’s not a simple job; changing some settings unavoidably changes others, and you
often have to go through several cycles of measuring and adjusting, then remeasuring and
readjusting, before the display will produce an accurate picture. The end result is worth
it, but the economics of the marketplace don’t support accurate factory calibrations.
And consumers want low-priced video displays, and don’t seem put off by highly
inaccurate images.
Home-theater enthusiasts know better. It’s time to
step up to rigorously accurate displays that include fully functional CMS systems, and to
accept professional calibration as a necessary component of a high-quality home-theater
system.
...Doug Blackburn
db@hometheatersound.com |
