Avia before or after grayscale... or not at all?

Discussion in 'Archived Threads 2001-2004' started by Patrick G, Mar 4, 2002.

  1. Patrick G

    Patrick G Second Unit

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  2. Guy Kuo

    Guy Kuo Supporting Actor

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    Before and during the grayscale adjustments, but usually not after. Setting the user controls can alter the grayscale balance so you really would like to get your grayscale set at the contrast and brightness setting(s)you will be using.
     
  3. Patrick G

    Patrick G Second Unit

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    What do you think about setting all user level menu params to 50% (aside from Sharpness), and then adjusting the contrast, brightness from the service menu? I will obviously be adjusting color from the service menu, using the cuts and drives, so could I also set color and tint to 50% in the user menu, or do I need to set color and tint using Avia?
     
  4. Guy Kuo

    Guy Kuo Supporting Actor

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    It is reasonable to set the user controls for brightness, contrast, saturation, hue, and sharpness to center and do the adjustments of those parameters using service mode "sub" controls. That assumes that they are present in your particular display. I am fairly concerned because you mention adjusting the "colors" using cuts and gains as though they can perform things analogous to changing saturation and hue. Cuts and gains are used to alter white balance and grayscale, NOT the chroma processing adjusted by saturation and hue. I may be misreading your latest posting, but if you have grayscale and chroma controls confused then I would suggest taking pause and reconsidering whether or not you should be in the service menu. Saturation and hue are not altered by using cuts and gains.
     
  5. Patrick G

    Patrick G Second Unit

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    Guy, I guess I am a little confused about how saturation and hue interact with the cuts and gains. I thought that adjusting the cuts and gains would override any saturation and hue settings. So, I do need to set saturation and hue with Avia before doing grayscale, right? If I set them afterward, would it change my color temp?

    One more question. How important is it, if at all, to do the 56-point convergence before grayscale and/or Avia?
     
  6. ThomasL

    ThomasL Supporting Actor

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    Patrick, Guy is the expert here, so I'll defer to him for the correct detailed explanation (and to correct anything I say that is wrong [​IMG] ), but from what I know, CUTs and DRVs settings inside the service menu affect your sets white balance/grayscale. Avia has a nice set of patterns for checking your sets grayscale. These do not affect overall color saturation for any of the basic colors - red, green blue. If you change these to attempt to fix a color decoder problem such as "red push", all you'll be doing is affecting your grayscale. Depending on what material you use to do the adjustments it may look like you've improved things but I can guarantee that something else will look screwed up. In theory, the grayscale parameters should not affect your red/green/blue color decoder levels (when viewed through the color decoder pattern in Avia). At least this is my experience. Contrast and brightness will affect your grayscale though i.e. grayscale adjustments interact with these contrast and brightness (white/black level). It is easier to set them first and then tweak your grayscale and then go back and double check them.
    hope this helps,
    --tom
     
  7. Guy Kuo

    Guy Kuo Supporting Actor

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    Tom pretty much has it. I'll expand a little more..

    Grayscale and Color, Don’t Get them Confused.

    Guy Kuo

    A perennial misunderstanding of those newly acquainted with calibrating their displays using bias and cut controls is that this will allow them to correct problems in chroma like red push. The very bright idea that one can cut down excess red color in image by going into service mode and turning down red gain well, simply isn’t. If that seems like a reasonable idea then you have grayscale and color confused with each other.

    Think of the video image as being represented in different ways called color spaces. One color space is that of red, green, blue. We can express a visible stimulus as a combination of intensities in red, green, and blue. Simple enough. We can also use a color space which has another set of axes called luma, saturation, and hue. In this color space we have luma indicating how bright something is, saturation tells us how colorful it is, and hue represents what color something is. This conceptual difference in how image is being represented in a system is the crux to knowing why you can’t change saturation, hue, or color decoder problems by adjusting bias and gain controls for red, green, and blue. When you adjust bias and gain controls you are working in the RGB color space. When you want to alter saturation or hue, you want to change things in the luma, saturation, hue color space.

    Or perhaps it would make more sense to think of the video image as being composed of an underlying grayscale image onto which colorization information is added. The bias and gain controls set the color of the underlying grayscale image. Saturation and hue work on the coloration information which is overlaid upon that grayscale image. So let us say that your system reproduces bright red and too red and you want to reduce red saturation. If you do that by dropping saturation then things which are COLORED get reduced in colorfulness. But notice that things which are not colored are not affected. If you take the would be calibrator tact of dropping red gain you would decrease the amount of red in everything, including things which are not colored. You would fix that particular red problem but create a minus red problem for everything else in the picture because the red gain control globally alters the color of the UNDERLYING gray image.

    So no, bias and gain controls for red, green and blue do not normally interact with saturation and hue. They are separate issues. If chroma adjustments like color decoder axes and gains are available, then you might be able to correct color decoder problems. Not all displays have that type of adjustability. You would have to check with the specific service manual for your display to possibly find out. A professional calibrator may have access to more service level and calibration documentation than normally available.
     

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