Guy Kuo
Supporting Actor
- Joined
- Mar 6, 1999
- Messages
- 581
Begin Rant Mode
I keep seeing this time and time again when people learn about red push problems, cut and drive controls in the service menu, put the two together in a flash of brilliance and try to fix red push by cutting down the amount of red using the red drive controls. Even if you are not currently thinking about doing something in the service menu, tuck the following tidbit away for later.
==================================================
Grayscale and Color Decoding are Not the Same Thing
Do not confuse grayscale (aka white balance or color temperature) with color decoding problems like red push. You must not change controls for one to fix the other. I frequently read of someone trying to correct red push (excess reddish color in red color objects) by displaying color bars and turning down red drive to fix the red. If you are doing anything like that or are contemplating such a manuever, don't. You've fallen into the newcomer trap of confusing the amount of red in the grayscale with the amoutn of red in red colored objects. You've gotten grayscale and color decoding confused with each other.
The video image is basically a grayscale one upon which colorization is added. The color of the underlying grayscale is a specific ratio of red, green, and blue. That ratio ideally creates a color of gray known as D65. If you alter the cuts and drives of a display, you are altering the color of the underlying grayscale image. That grayscale must be kept completely neutral or else any tinting you create would affect all images. If you drop the amount of red you decrease the amount of red globally. Even things which are supposed to be neutral gray would lose the amount of red in them.
Chroma or color information is decoded and used to colorize the underlying grayscale image. The user controls saturation and hue adjust how color decoding is done. If you visualize it in terms of a color wheel. Saturation changes the intensity of colorization. Hue basically alters the angle of the color wheel used to assign colors. Color decoding which causes red push simply means that colors near red are intentionally overemphasized. Note that altering chroma controls does not change the underlying grayscale's color. It changes the colorization which is added.
So you see you have two very different things which shouldn't be mixed up.
Grayscale
Color Decoding
The color of gray is examined using things which have no colorization like gray windows, ramps, and field patterns. The idea is to set the grayscale to a neutral D65 using the cuts and drives. Cuts affect mostly the dark end. Drives mostly the bright end. One uses a colorless test pattern because you don't want anything to change the on screen color except the color of gray. BTW, it is quite difficult to set grayscale accurately without some sort of instrumentation.
Color decoding is how the colorization information gets interpreted and then added atop the neutral grayscale image. The user controls can be used with color bar patterns to make adjustments. Color bars are useful because they contain fixed amounts of colorization which are equal for red, green, and blue (for each portion of the pattern that contains that color). Since the gray portion of the pattern has zero colorization but has a brightness of gray exactly composed of the amount of red, green, and blue which should be in the colored portions of the pattern. One can use the gray as an unchanging point of comparison as color decoding is adjusted. In some displays there are color decoding axis and gain controls which can be used to make a color decoder behave in a more NTSC standard fashion and eliminate red push. Not all sets have such controls.
If the difference between grayscale and color decoding is still unclear, try this analogy.....
You are in charge of art displays across the country and want to ensure that the art looks the same in every one of your galleries. To make that happen you choose a standard color of lamp (D65) for all your galleries.
Think of the color of gray (the grayscale) as the light of the lamp which is illuminating a painting. If the artist chose paint with too much red for faces (decoder has red push), a newbie art corrector could try "fixing" the excess red by reducing the red the lamp light. That would fix the excess red problem in faces but you no longer neutrally illuminate the painting. Everything would be badly tinted because you have changed the underlying white balance. That is what adjusting the cuts and gains does.
The real solution is to convince the artist to always use paints that actually look the right color under the neutral standard lamp (D65). That's basically what you do by adjusting the saturation, tint, and color decoder axes & gains correctly. You have selected paints which are the same color used by artists everywhere when they paint under a D65 colored lamp. This fixes the problem reds without ruining everything else.
A few artists still insist on using the wrong color of paint. In such cases you force the audience to stay in a bright red room to make them less sensitive to seeing red when they actually see the incorrtly colored painting (turn down the saturation). This hides the flaw, but doesn't really correct it.
I hope this makes it a little more clear why cuts and drives are not used to correct red push.
End Rant Mode
I keep seeing this time and time again when people learn about red push problems, cut and drive controls in the service menu, put the two together in a flash of brilliance and try to fix red push by cutting down the amount of red using the red drive controls. Even if you are not currently thinking about doing something in the service menu, tuck the following tidbit away for later.
==================================================
Grayscale and Color Decoding are Not the Same Thing
Do not confuse grayscale (aka white balance or color temperature) with color decoding problems like red push. You must not change controls for one to fix the other. I frequently read of someone trying to correct red push (excess reddish color in red color objects) by displaying color bars and turning down red drive to fix the red. If you are doing anything like that or are contemplating such a manuever, don't. You've fallen into the newcomer trap of confusing the amount of red in the grayscale with the amoutn of red in red colored objects. You've gotten grayscale and color decoding confused with each other.
The video image is basically a grayscale one upon which colorization is added. The color of the underlying grayscale is a specific ratio of red, green, and blue. That ratio ideally creates a color of gray known as D65. If you alter the cuts and drives of a display, you are altering the color of the underlying grayscale image. That grayscale must be kept completely neutral or else any tinting you create would affect all images. If you drop the amount of red you decrease the amount of red globally. Even things which are supposed to be neutral gray would lose the amount of red in them.
Chroma or color information is decoded and used to colorize the underlying grayscale image. The user controls saturation and hue adjust how color decoding is done. If you visualize it in terms of a color wheel. Saturation changes the intensity of colorization. Hue basically alters the angle of the color wheel used to assign colors. Color decoding which causes red push simply means that colors near red are intentionally overemphasized. Note that altering chroma controls does not change the underlying grayscale's color. It changes the colorization which is added.
So you see you have two very different things which shouldn't be mixed up.
Grayscale
Color Decoding
The color of gray is examined using things which have no colorization like gray windows, ramps, and field patterns. The idea is to set the grayscale to a neutral D65 using the cuts and drives. Cuts affect mostly the dark end. Drives mostly the bright end. One uses a colorless test pattern because you don't want anything to change the on screen color except the color of gray. BTW, it is quite difficult to set grayscale accurately without some sort of instrumentation.
Color decoding is how the colorization information gets interpreted and then added atop the neutral grayscale image. The user controls can be used with color bar patterns to make adjustments. Color bars are useful because they contain fixed amounts of colorization which are equal for red, green, and blue (for each portion of the pattern that contains that color). Since the gray portion of the pattern has zero colorization but has a brightness of gray exactly composed of the amount of red, green, and blue which should be in the colored portions of the pattern. One can use the gray as an unchanging point of comparison as color decoding is adjusted. In some displays there are color decoding axis and gain controls which can be used to make a color decoder behave in a more NTSC standard fashion and eliminate red push. Not all sets have such controls.
If the difference between grayscale and color decoding is still unclear, try this analogy.....
You are in charge of art displays across the country and want to ensure that the art looks the same in every one of your galleries. To make that happen you choose a standard color of lamp (D65) for all your galleries.
Think of the color of gray (the grayscale) as the light of the lamp which is illuminating a painting. If the artist chose paint with too much red for faces (decoder has red push), a newbie art corrector could try "fixing" the excess red by reducing the red the lamp light. That would fix the excess red problem in faces but you no longer neutrally illuminate the painting. Everything would be badly tinted because you have changed the underlying white balance. That is what adjusting the cuts and gains does.
The real solution is to convince the artist to always use paints that actually look the right color under the neutral standard lamp (D65). That's basically what you do by adjusting the saturation, tint, and color decoder axes & gains correctly. You have selected paints which are the same color used by artists everywhere when they paint under a D65 colored lamp. This fixes the problem reds without ruining everything else.
A few artists still insist on using the wrong color of paint. In such cases you force the audience to stay in a bright red room to make them less sensitive to seeing red when they actually see the incorrtly colored painting (turn down the saturation). This hides the flaw, but doesn't really correct it.
I hope this makes it a little more clear why cuts and drives are not used to correct red push.
End Rant Mode