Question for ISF tech - grayscale

Discussion in 'Archived Threads 2001-2004' started by Jim A. Banville, Jun 13, 2001.

  1. Jim A. Banville

    Jim A. Banville Supporting Actor

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    Okay, I understand how drives affect the bright end and cuts affect the drak end. My question is that when you are adjusting the high end, are you only adjusting for the balance of red, gren and blue that yields a neutral gray (D6500?), or are you also going for a certain level of saturation (or intensity)? In other words, if you had a theoritically linear RPTV, and the drive were each CRT were set to a number like "31" after calibration, if you increased each to a number like "41", would anything change?
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    Jim
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  2. ThomasL

    ThomasL Supporting Actor

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    Jim, I'm not a grayscale expert but the intensity level is measured by the IRE level I would guess. Across the grayscale spectrum, you have the same neutral shade of gray (D6500) at 10 different intensities - 100 IRE to 10 IRE. For example, on the Avia vertical grayscale ramp pattern, Avia outputs 100 IRE to 10 IRE. The intensities are correct but then one would use a color analyzer to measure what the color temperature was (i.e. D6500 being the goal). There are multiple ways to get to D6500 I would guess as well, using the CUT and DRV settings. So I would assume the goal would be to get the D6500 that contains the best balance/saturation of all three colors and still maintains D6500. Having never used a color analyzer I'm not really sure. Perhaps some of the experts can shed more light.
    hope this helps,
    --tom
     
  3. Jay Mitchosky

    Jay Mitchosky Producer

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  4. Michael TLV

    Michael TLV THX Video Instructor/Calibrator

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    6500K is a line on a graph. It is not a single point. So you can have 6500K being too green or too blue/red. D6500 is the desire point on this line where neutral is. (It's actually 6480K on the black body curve not 6500K.
    Explains why I prefer to use the CIE Chromaticity chart on the analyzer to do my grayscales. At any time, I know both the temp and the "tint" of the test pattern.
    And for Jim ...
    You also shoot for a desired light output as three setting of 5 on your linear scale would also give you 6500 but no light output. RPTV's ... typically 10 to 17 Ft-l.
    Regards
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    Michael @ The Laser Video Experience
     
  5. Gregg Loewen

    Gregg Loewen Video Standards Instructor, THX Ltd.
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  6. Jim A. Banville

    Jim A. Banville Supporting Actor

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    Thanks for the replies! Maybe I'm too dumb to get this, so let me ask it like this- say for example you just got through calibrating a RPTV. Now, say that the readings in the service menu are "50" for the drives of each CRT (50 is just a number, it could be 40, or each CRT could have a different number, I'm just setting up an easy to understand baseline). Now, if the TV was "perfect" and all three CRT's reacted identically, if you raised the drives to "60", what would change? Have you just simply raised the white level of the overall picture? Have you increased the intensity of the red, green and blue CRT's? Would you still have a color temp of D6500, but the colors would be "cartoonish"? I'm asking because I want to know, can you have a perfectly neutral grayscale, but due to the drives equally being set too high, color saturation is too high?
    thanks
    Jim
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    Jim
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  7. ThomasL

    ThomasL Supporting Actor

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    Jim, that is a very good question and the more I think about it the more I get confused. Can anyone explain how physical light output is related to IRE? I assume IRE is simply a relative scale so if in your example, if we set all the settings to 5 as Michael said, then we'll end up with a very low light (foot lamberts?) output at 100 IRE even though it may be D6500? Does that make sense? So, is it true then that when calibrating one wants to find the right mix of blue, red and green that gives you D6500 and also gives you a certain level of light output so that whites will be white but not searing white nor way too dim. Is this right or I am completely missing something?
     
  8. Michael TLV

    Michael TLV THX Video Instructor/Calibrator

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    Don't forget guys ... setting up a proper grayscale also includes setting the brightness and the contrast to their recommended intensities.
    This explains why when all is said and done, with the grayscale, we have to double check the brightness level and the contrast again. If they have changed from the desired points, we have to recorrect and redo the grayscale again. It's one big iterative process.
    As an aside ... when I do grayscale for display units in the stores, the grayscale is set up based on a high contrast setting. It's a compromise that has to be made for show room lighting. So effectively, grayscale is correct based on 100% contrast. The moment the contrast comes down, the grayscale goes out the window ...
    Having the three CRT's driving at max output does not increase colour saturation. It is merely like cranking the contrast and brightness up. You will note that the colour saturation does not actually increase when you do that.
    colour saturation and tint are generally unrelated to how high you crank the cuts/drvs.
    Regards
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    Michael @ The Laser Video Experience
     
  9. ThomasL

    ThomasL Supporting Actor

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    Thanks Michael...I think this is very interesting stuff. I never knew tv internals could be this interesting [​IMG] So, if my layman interpretation is close to the mark, you're saying that brightness and contrast effect the intensity of the grayscale and vice versa...i.e. setting the high end of the grayscale affects how intense white looks and the low end affects black. Thus brightness and contrast settings are effected but of course, when you change those you also throw off the grayscale calibration from the D6500 goal. So, I can see how this is an iterative process basically as you get closer and closer until you converge. Do you normally use the Avia/VE test eyeball patterns for setting contrast and brightness or do you have a finer method involving instrumentation?
    cheers,
    --tom
     
  10. Michael TLV

    Michael TLV THX Video Instructor/Calibrator

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    You pretty much use the instructions as described on either of the discs.
    It's television afterall. Don't take the calibration too seriously ... close is good enough. Although you have to know where close is ... [​IMG]
    Regards
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    Michael @ The Laser Video Experience
     

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