Advice on fixing gray scale problem?

Discussion in 'Archived Threads 2001-2004' started by James Zos, Jul 10, 2002.

  1. James Zos

    James Zos Supporting Actor

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    Thanks to the members of this forum I have greatly improved my Panasonic's picture. But I'm still struggling to fix the gray scale, because of a greenish tint to some scenes. Using the AVIA color decoder check I have determined that this is not a green push or decoder problem, but a gray scale problem.

    I have knocked G Cut-Off down a few notches in my service menu, but I'm not sure if this is the way to go.
    Rather than dropping the G cut, should I instead increase R and B cut?
    My understanding is that cut affects darker portions of a scene, while drive is for lighter portions. But I have no G drive option in my service menu. Anyway to get at the green in the lighter portions of the sceen?
    Any general advice about gray scale that you could offer would be most appreciated!

    Here are the service menu options I have to work with:

    R Cut-Off
    G Cut-Off
    B Cut-Off
    User Brightness
    Sub Brightness
    R Drive
    B Drive
     
  2. Michael TLV

    Michael TLV THX Video Instructor/Calibrator

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    Greetings

    Green is held constant in the bright area ... use the 2 drivers to change it.

    Will have to go to the cuts to fix up effects of the drivers on the dark end.

    Then back to drivers to fix up again ... then back to cuts ...

    etc. etc. etc.

    It is an iterative process. Get closer to idea every time.

    Regards
     
  3. James Zos

    James Zos Supporting Actor

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    Michael, thanks for your response!
    I'm still a little confused though. How do I use B drive and R drive to fix the G drive? Is there anything on AVIA that can help me with this?
     
  4. Michael TLV

    Michael TLV THX Video Instructor/Calibrator

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    Greetings
    It's the screwing light bulb principle. Can screw it in by hand or screw the house into the bulb.
    I hope I don't have to spell it out any clearer for you... cuz if I do, then you shouldn't be messing with grayscale. [​IMG]
    Think "fixed sum solutions" ... X + Y + Z = 12
    3+4+5=12 ...
    now if 3 + 4 + 5 = 15 ... and 5 is fixed ... and answer = 15 now ...
    What if the answer = 9 ...
    Regards
     
  5. ThomasL

    ThomasL Supporting Actor

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    James, I had a similar problem with a Toshiba 27A40 set and had to work with the same controls. I suspect, most likely, that you're only being bothered by the green in darker scenes. It's much harder to detect minor shades of color in the higher end of the grayscale, using one's eyes. In addition, even though the CUTs are for the low end, they all actually interact, and you'll find if you lower GCUT, you'll also be taking some green out of the high end as well. My advice would be to leave the DRVs alone for now. Use Avia's grayscale test patterns, specifically the window patterns and the stepped vertical patterns and focus in on the 20 IRE window (or slice if the stepped pattern). Does your eye see a greenish tint to it? If so, try raising the BCUT value a few notches. What change did it make? Then try the RCUT value and then lower the GCUT one or two notches. I am not familiar with the Panasonic scale on these parameters so one notch may make a huge difference or very little difference (like with the Toshiba). Basically, you want to increase BCUT and possibly RCUT to counteract the green and possibly lower GCUT a little. You will need to do this more than once, I'm afraid. Eyeballing the grayscale is very inexact. So, the first time your eyes will see the green tint and then when you begin to raise the BCUT and RCUT, your eyes will just be happy that the green tint is gone and they won't tend to notice if you've introduced a blue or reddish tint. So, make your changes, then go away from the set for awhile and come back and look at the pattern again. Basically catch your eyes offguard [​IMG] Do this a few times until you're satisfied that the color is a neutral gray. I probably did a dozen iterations over the period of a month [​IMG] It won't be NTSC 6500K, it may not even be close but if you're satisfied it looks neutral than it should look much better than how it looks now to your eyes.
    good luck,
    --tom
    p.s. make sure to write down all original values before changing any of the parameters. Just in case, your finger slips [​IMG]
     
  6. James Zos

    James Zos Supporting Actor

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    Thanks for the response, Thomas! I will try your suggestion.

    Michael - I'm afraid your response is too cryptic for me. I have no idea what you are talking about.
     

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