AVIA Obscura #2 - Blue Bars, Red Bars, Green Bars
Color bars are useful for checking the proper function of color decoders in a display. With NTSC displays one can vary the saturation (amount) and hue (phase relationship) of the display. Ideally, the display is adjusted to recreate the same colors as encoded in the signal.
The traditional SMPTE color bar includes not only bars but small patches of colors which are in reverse blue order just below the color bars. By examining the amount of blue (using a filter or better yet by using the display's blue only mode) one can tell if the saturation and hue are correctly adjusted. This works because the gray portions of the pattern are encoded to be an intensity of gray which has exactly the same amount of blue in the blue portions of the pattern. Since gray contains no color content, turning the saturation control up and down does not affect its blue content. This allows the gray to act as a reference point against which to compare the amount of blue is present. Calibrating saturation can thus be done by adjusting blue intensity to match the fixed amount of blue in gray. Hue is observed by comparing the amount of blue in the magenta and cyan portions of the pattern. When hue is correctly adjusted, the blue intensities of magenta and cyan are identical.
Unfortunately, some people find it difficult to accurately tell when the blue intensities are equal. The bars and patches are of unequal size and color separation artifacts can make the color transition zones blurred or of uneven darkness. For these reasons, AVIA's "Blue Bars" add flashing patches within the color bars to aid in finely discerning when the intensities are equal. Human vision is very sensitive to flashing. AVIA takes advantage of this by having users adjust saturation and hue to minimize visible flashing in blue. This allows higher accuracy than comparison of static bars and patches. For those who still prefer more traditional static comparisons, AVIA also provides traditional split color bar patterns.
Several other features are built into the color bars. You may not have noticed that the transitions between bars and patches is closer to the center of the screen than SMPTE bars. This moves the critical comparison area of the pattern further from on screen displays which often appear at the bottom of the screen when televisions are adjusted. The white reference rectangle at the bottom of the pattern includes animated white bars for detecting white clipping. The lower right black portion of the pattern has animated black bars for checking black level. These are positioned where the PLUGE pattern is on SMPTE bars, but animation avoids the optical illusions of aligned rectangle edges that sometimes make it difficult to tell if a PLUGE rectangle is visible. Animation makes visibility obvious. Also the black bars do not rely on a blacker than black component for proper use. However, the black bars in this pattern are only to be used on displays which need a high APL during black level adjustment.
One can also use SMPTE bars in red or green only, but AVIA makes evaluation of green and red primary handling by providing "Red Bars" and "Green Bars." These are used in the same manner as the blue bars, except one views the red bars in red-only and the green bars are to be viewed in green-only. The patches which need to be compared are also moved to be positioned below the bar against which comparison needs to be made. This makes intensity comparisons easier than the wide separations that arise with a blue optimized color bar pattern. AVIA also adds its innovative flashing patches to the red and green bars to enhance viewer accuracy.
Why red and green bars? If the color decoder is perfect, adjusting to blue only accuracy would make the red and green color bar patterns also appear perfect. Unfortunately, it often isn't perfect. We'll visit that in AVIA Obscura --- Color Decoder Check.
AVIA Calibration Tips #4 -- Color Decoder Check
Note: The AVIA Obscura title of this series changes now to AVIA Calibration Tips
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.