| Video Noise September 2009
What Makes a Picture Good (Accurate) or Bad? Part
Three: Color
 This
month we’ll focus on color’s role in making the display accurate. It’s a
complex topic, but the basics are easy enough to understand. And by working through the
information, you’ll know what to look for in the next video display you buy.
Color accuracy
The colors in most displays aren’t as accurate as they
should be. On top of that, few displays have the controls needed to make the color more
accurate. Fortunately, color errors may not jump right out at you because they usually
aren’t as easy to detect as grayscale errors. But once you get a video display with
an accurate grayscale, color accuracy is the finishing touch. A color error will make
yellow look too green or too orange, the sky may be too blue or too green, and a red car
may be a more intense red than in real life. When color is accurate, everything is
displayed just as it was captured. But how would you know if the grass was the right shade
of green or if the pink roses were too intense unless you saw them as they were being
photographed? That’s where color test patterns come into play. Video signal
generators or test discs contain color targets that are essentially perfect
representations of very specific reference colors. When you measure one of these color
patterns, it’s easy for software to calculate how close or how far the measured point
is from the reference point. Nobody has to remember anything, and there’s no
guesswork. You measure the color, and the meter and software will tell you if the color is
inaccurate.
Why are manufacturers building TVs with color that’s
not as accurate as it could be? They want to fool you into buying their TV instead of
another brand! One thing that easily fools potential customers is overemphasizing greens
quite a bit and overemphasizing reds a little. Surprise! These are just tricks, and
you’ll see them in most TVs. The other widespread trick is to make the grayscale too
blue so the picture pops when viewed in the typical wall-of-TVs retail display. Another
reason is cost. It takes time to make a TV accurate -- time manufacturers can’t
afford. It’s cheaper to simply set every TV to the same settings and ship them. And
that’s exactly what happens, with very few exceptions.
How to read a CIE chart
The CIE (Commission internationale de l’éclairage or
International Commission on Illumination), established 1931, sets standards for measuring
color. Over the years they’ve defined and refined color measurement numerous times.
Because of all the refinements in working toward reasonable perceptual representations of
color, there are numerous ways to show color in a chart or graph. The 1931 CIE chart (like
the example in this article) isn’t the only way to look at color, but it’s one
of two methods commonly used in reviews of video displays.
The two different CIE charts used in video display reviews
are known as xy and uv. The letters refer to the coordinates used for the
horizontal and vertical axes. The xy system was an early chart that helped map color for
the first time. In the CIE chart below, you’ll find x coordinates on the horizontal
axis and y coordinates on the vertical axis. The colored area represents all the colors
visible to humans. Of course not all those colors can be printed on paper or shown on your
computer display, so the chart is just an approximation. Having the color in the
background of the chart makes it easy to understand, but you may see these charts in
product reviews without the background color because of the complexity of printing it. In
uv color space, u is a measurement of hue (i.e., red, green, orange, blue,
or anything in between), and v is a measurement of saturation, which may be called
intensity. The uv color space is said to be more perceptually accurate, meaning it’s
closer to what we see. And uv color is a newer take on color than the CIE’s venerable
xy coordinate system. But xy coordinates are perfectly OK to work with as long as you
understand what you’re looking at. In fact, a uv color chart looks similar to the xy
chart, but the shape of the colored area is a bit different and it appears to have been
rotated counterclockwise.
 In the sample CIE chart, a deep shade
of red would be 0.65, 0.32. The gray/black triangle represents HDTV color space.
Everything inside the triangle is reproduced by the TV, and everything outside of the
triangle never exists in HDTV displays that follow the rules. Color outside the gray/black
triangle should never exist in HDTV programming, including Blu-ray Discs. You might think
the black triangle looks pretty small compared to the full range of human vision (the
colored area of the chart), but it’s slightly larger than what we saw with the old
standard-definition color system. The dotted vertical and horizontal lines cross at the
reference white point for HDTV. This is the reference point for grayscale calibration,
discussed in Part One of this series. This chart is from a calibrated display, so the
reference white point is accurate. In video display reviews, you won’t see that white
point in the center of the triangle, but that information will be available where they
show grayscale measurement data.
The colored squares at the points of the triangle represent
the primary color (red, green, and blue) HDTV reference points. The colored squares in the
middle of each side of the triangle represent where the complementary colors (sometimes
called secondary colors) should measure. The complementary colors are yellow (red +
green), cyan (blue + green), and magenta (red + blue). The white dots represent actual
measurements from a fairly typical video display. The green measurement is quite a bit
farther into the green than it should be, which means the TV’s greens will be
oversaturated and every color containing green will be inaccurate compared to the original
image. The measured red point is a bit oversaturated, but not nearly as much as the green
measurement. This slight oversaturation will make any color containing red inaccurate.
Note that the measured blue point, which is almost right on top of the blue reference
square, represents a fairly accurate blue primary. You rarely see displays with large blue
errors. The yellow point is outside of the triangle, indicating that it’s
oversaturated, and it’s displaced slightly toward green. The cyan point is also
oversaturated, but it’s displaced a little toward blue. This means shades of blue in
the sky will be a little more blue than the original images. The measured magenta point
indicates that saturation is good (the measurement is almost centered on the black
reference line, but magenta is a little too red). So this chart ultimately tells you that
blues and near-blues will be relatively accurate, but all other colors will be inaccurate
to some degree.
A perfect display would place each of the measurement dots
right on top of the reference boxes for each color. There are a few video displays and one
video processor I know of that will let you adjust each color to be perfect or very close
to perfect. But the majority of video displays will leave you with unfixable color errors.
These charts ignore color’s third dimension, luminance
(or brightness). If you remove luminance from an image, every color gets darker and the
white areas become increasingly dark shades of gray. You need a 3-D chart to show
luminance, so it’s tough to demonstrate on a 2-D computer display. But setting the
correct luminance value for each color is important, even though it doesn’t appear on
the 2-D CIE chart.
Your only hope for achieving accurate color is if the TV
has CMS (color management system) adjustments. Unfortunately, manufacturers don’t say
things like "this TV has no CMS" in their manuals, so you often don’t know
what you’re getting into. And even if the TV has CMS controls, they’re often
referred to as something else, sometimes a name the manufacturer made up. Toshiba calls it
ColorMaster even though the 2009 version lacks the inner workings necessary to
"master" the color errors they left in the TV. Samsung puts their CMS controls
in the Color Space menu choice, but you can’t access them until you’re in the
custom mode. Ultimately, it doesn’t matter if the video display has CMS controls or
not unless you’re going to have the TV calibrated.
There are two ways to get a video display calibrated. The
DIY method requires purchasing a meter and calibration software and then teaching yourself
how to calibrate. But you’ll need a good 100 hours of research and practice before
you get reasonable results. Trustworthy meters for accurate grayscale and color
measurements will range from $300 to more than $25,000. Your other option is to hire a
professional calibrator to do the work for you. Most knowledgeable independent calibrators
will charge around $300 to $400. Some retail chains offer calibration, but reviews posted
on many video forums indicate that retail calibration could leave your TV in worse
condition than if you’d simply followed the instructions on a $20 setup disc.
Remember that grayscale, gamma, and color accuracy all
contribute to video image quality, but the contributions aren’t always equal. A big
problem in any of these parameters could overwhelm any smaller problems in others, but if
none of the measurements has a major problem, having the grayscale close to perfect is
half the battle for high-quality images. Gamma and color accuracy share the remaining 50
percent about equally -- though you could debate the relationships between grayscale,
gamma, and color endlessly.
As of the summer of 2009, video displays and video
processors with usable grayscale, gamma, and color accuracy controls include:
- Some Samsung models (all AXXX models, B650-B960 models,
plasma and LCD)
- Some Toshiba models (XV series LCDs and possibly other
models, but color adjustments can’t completely fix green without causing other severe
problems)
- Some LG models
- Lumagen Radiance XD or XE video processors (they have a high
degree of control of all parameters and can make almost any display very accurate, but
they’re now selling for $5000 to $6000 MSRP)
There are also some THX Certified video displays that tend
to be considerably better than their non-THX siblings, though the THX Certified models
don’t always have adjustments to allow correction of the THX-mode residual errors.
. . . Doug Blackburn
db@hometheatersound.com |
