AVIA Newbie -- Color Decoder Check
Guy Kuo
NTSC video signals must be separated, decoded, and matrixed to form the
final red, green, and blue signals which drive the display. Professional
grade displays accurately decode the color signals and render colors
correctly. However, consumer grade televisions often break the rules and
have non-standard color decoding. This is most often seen as exaggerated
reds (red push) and wreaks havoc when one attempts to adjust colors on a
consumer display using just color bars.
Color bars are encoded such that the amount of red, green, or blue is 75%
in each bar which contains the color. For instance the amount of blue is
75% in the gray, blue, cyan, and magenta portions of color bars.
Similarly, the amount of red is 75% in the gray, red, yellow, and magenta
portions. Because the amounts of each primary are identical in the various
patches, one can compare the intensity of each color to learn how a
decoder is functioning. AVIA also includes 50% & 100% color bars for
testing of color circuitry linearity but we'll ignore those for now and
concentrate on the more commonly used 75% variety of color bars.
75% Gray has zero color difference from gray so adjusting color saturation
up and down doesn't alter its appearance. Hence, gray serves as the
reference point against which the intensity of color saturation may be
compared. Turning saturation up and down alters the intensity of the
colored portions of color bars. View the blue portions of color bars in
blue-only as you increase saturation. You'll notice that blue increases in
intensity with increasing saturation. When saturation is correctly set,
the intensity exactly matches that of gray. On a professional display with
NTSC accurate color decoding, this same saturation setting also makes the
red and green portions of the pattern match gray. Hue is adjusted by
comparing portions of the pattern that contain two primaries such as cyan
vs magenta.
AVIA has a Color Decoder Check pattern which lets you measure and
compensate for non-standard color decoding. The pattern has a gray
background against which you compare the brightness of red, green, and
blue color patches. The patches range from +25 to -25%. If the color
decoder is perfect, then the 0% patches of each color match the gray
background (when viewed in only that color). If a display has red push,
then a higher (darker encoded) red patch matches the gray background. You
can read the percentage push by finding the patch which best matches the
gray.
You may find other imbalances with the AVIA Color Decoder Check pattern,
but red push is the most important to control. This is because red push is
more objectionable to most viewers than under push or green push. A
professional calibrator can sometimes correct the color decoder axes to
achieve NTSC standard decoding, but for most sets that is not possible.
You may want to check the accuracy of color decoding of a display prior to
purchase since this problem is often not correctable. The only recourse is
to hide (not correct) the error by decreasing saturation to make the
measured red push 10 to 20%. This desaturates the overall picture but
avoids making flesh tones too orange. Leave hue alone when making this
compensation.
There are two other things to remember when considering non-standard color
decoding. Don't confuse correcting the color decoder axes with resetting
gray scale. This problem cannot be corrected by decreasing red drive
because that would alter the underlying gray scale of the picture. The
problem is with the way color DIFFERENCES from gray are being interpreted
by the display's color decoder, not with the amount of red in the gray
scale.
The second thing to remember is that this pattern and color bars are most
accurate if one turns off the other two color guns of the display when
examining each color. Color filters leak through a bit of the other colors
and falsely make the gray background brighter than it really is. This
tends to make your observations through filters about 5% lower than if
color filtering were perfect. The difference is small, but if you want
highest accuracy, turning off or capping the other two color guns is best.
Guy Kuo
www.ovationsw.com -- AVIA Guide to Home Theater DVD