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Grayscale vs Color Decoder Problems

"Recently, I've seen several discussions in which novices have attempted to correct red pushing using red bias and gain controls. I'm posting this in an attempt to clarify the difference between correcting color decoder problems vs altering the entire gray scale.

Let’s review how color bars are useful in setting and checking color decoding. Looking at AVIA's Blue Bars, SMPTE Bars, or other 75 color bars one sees a series of colored bars and a 75% gray bar. The amount of each primary color in each bar is 75% of maximum. The red bar contains 75% red but none of the other two primaries. Blue contains 75% blue but no green or red. Green contains just 75% green. The cyan, magenta, and yellow bars each contain 75% of two primaries. Cyan is 75% blue + 75% green. Magenta is red + blue. Yellow is green + red. The gray bar contains all three primaries each at 75%. The key message is that each bar is encoded with the same saturation level of each primary color which is present in that bar.

By examining color bars using only one primary color on screen, one can determine if color decoding is working properly. Ideally, when viewed in blue only all the blue containing bars are of identical intensity to each other. In red only, the red containing bars are also of identical intensity. In green only, the green containing bars are identical in intensity. Of special importance is the gray bar. It contains no color difference information for the color decoder to process since it is by definition colorless. However, its gray is composed of 75% of red, green, and blue. It intensity in each primary is constant as one adjusts the color decoder's saturation and hue controls. Hence, it serves as a reference against which the other bars may be checked. Viewing in single primary is best done by cutting off or covering the other guns. Another way of doing this is to use red, green, and blue color filters. However a small amount of the other colors leak through a color filter so it is slightly less ideal. The most accurate means of checking the bars is to directly sample the waveform of the final video output stages and see the amplitude of the bars on an oscilloscope. For most people, color filters are the simplest approach.

Now you may begin to understand why adjusting in blue-only works. You see only the blue light output of the bars. When you adjust saturation, you alter the amount of color. Hue alters the distribution of the color. So to set saturation, one pays attention to the pure blue bar and the 75% gray bar. Since you know that both have the same amount of blue you adjust saturation to make them both equal in intensity. This works because the saturation control does not affect the gray bar but does affect the blue bar. Hue alters the distribution blue relative to the other colors so you look at the bars which contain blue + one other primary. That would be the cyan and magenta bars. When hue is correctly set, those two bars have the same amount of the color blue. The two controls interact so it takes both hue and saturation adjustments to get all blue, cyan, magenta, and gray bars to have identical amounts of blue.

SMPTE bars made this easier by putting patches of the comparison color under each bar. AVIA Blue Bars do that and also add flashing patches of the comparison color to take advantage of visual sensitivity to flashing and improve viewer accuracy.

That would be the end of the story if color decoders all worked perfectly along NTSC standards. If a decoder is NTSC standard, setting the hue and saturation perfectly for blue only would also set it perfectly for making green and red containing bars have relationships analogous to the blue bars. That is the amount of green in each green bearing bar would appear equal when viewed in green only. The red bars would also be equal in red content.

Just like for blue, the pure red bar has the same intensity in red as the gray bar when saturation is correctly set. The red + one other primary bars (yellow & magenta) would also match each other in red intensity when hue is correctly set. Similarly, the pure green bar has the same intensity in green as the gray bar when saturation is correctly set. The green + one other primary bars (yellow & cyan) would also match each other in green intensity when hue is correctly set.

You can compare these intensities using standard or SMPTE bars, but the colors to be compared are not paired with the correct comparison color. That's why AVIA also has "Red Bars" and "Green Bars." These have their comparison patches arranged for making comparisons for the red or green bearing colors. The Color Decoder Check pattern in AVIA is a quick way of checking 75% of all three primaries against 75% gray.

In decoders with "Red Push" setting hue and saturation to make blue bars appear correctly, sets the decoder to exaggerate red containing colors. You see this in color bars by looking at the red, blue and gray bars. First look in blue only and verify that the blue containing bars are all of equal intensity. This double checks that you've set the decoder for proper decoding of blue. Next, switch to red-only viewing and compare the red bar vs gray bar intensity. They should be identical to each other. If the red bar is brighter (in red-only) than the gray bar, then the decoder is exaggerating reds and has red push. You may wish to use the Red Bars pattern in AVIA to have gray and red physically together on screen. The Color Decoder Check pattern has patches of varying intensity red which lets you quantify the error.

Green push is less talked about because it isn't as visually glaring as red push. You check this with green-only viewing and once more compare the primary color against gray. The green patch should match the gray bar in green intensity.

Notices that nowhere in this discussion did the grayscale come into play. That's because it is a separate issue. All three primaries are checked using bars at 75% intensity. That means all the test points are at the SAME content of each primary. This eliminates the effect of grayscale imbalance. Imagine that your grayscale was set so badly that red was 50% higher than it should be. At first glance, you might think this would throw off your checks for red push. It doesn't because the red bar and the gray bar would BOTH be altered by the same grayscale imbalance . Both are at 75% red signal intensity and rise in red output TOGETHER as grayscale red content is boosted. This means you can check for color decoder accuracy even when gray scale is incorrectly balanced.

Ideally, your display would be adjusted to make gray scale correct AND color decoding correct. Reducing the amount of red gun output using the grayscale adjustment controls like bias and gain alters everything which contains any red gun output. That includes the color of gray. If your decoder has red push and boosts reds by 25% and you reduce red gun output by 25 percent, you'll get the color red to look right. Unfortunately, that means everything will be 25% lower in red, including the color of white and grays. It is better to leave the grayscale correctly at D65 and fix the color decoding axis and gains.