Guy, white level in Avia calibration question.

Discussion in 'Archived Threads 2001-2004' started by Pep Guidote, Mar 12, 2002.

  1. Pep Guidote

    Pep Guidote Agent

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    Hello Guy!

    How do you calibrate white level when the TV set has some sort of Average Picture Level (APL) limiter? (Note: The APL limited is only a guess, I don't know what else to call it.)

    I bought a new 34" direct view Philips PowerVision TV. Using Avia's needle pulses and steps pattern, I set the contrast to maximum (63), because even at max contrast, the two moving white bars are still visible AND there is no blooming nor bending of the lines to speak of. Also, at this point, the "whites" on the screen are a sort of white-ish gray. This is also true of the white fields, where anything over 50 IRE would cease to become whiter. (i.e. 50 IRE looks the same as 60 IRE, as 80 IRE, and 100IRE, all of them the same white-ish gray.)

    Now, when playing a letterboxed movie (especially at 2.35:1) I feel the contrast is set to high because there is some "washing out" of bright areas and maybe a little blooming. I can't really be sure it's just what my eyes tell me. "Whites" when playing a movie (or any other screen that is less than 50% white), are a bright white. This is far less prominent on full screen, or almost full screen clips, like the videos and other extras.

    So, what's an Avia user to do? Thanks.

    -Pep
     
  2. Michael TLV

    Michael TLV THX Video Instructor/Calibrator

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    Michael Chen
    Greetings

    Please note that AVIA and VE show you where the Maximum Contrast point is ... not where you need to set it for optimal viewing.

    To really determine the correct setting you need a light meter ... and for a tube set like yours ... shoot for a light output of between 20 to 30 ft-l using the 100 ire windowbox pattern.

    What you have currently done is set your TV at a point roughly equivalent to driving your car at 5900 RPM knowing the red line is 6000 rpm.

    5900 RPM is not very healthy for your car is it?

    Optimal vehicle operating level is between 2000 to 3000 rpm ... apply this analogy to the TV too. Contrast should be 1/3 to 1/2 of the actual point where the bending or blooming begins.

    Typically, this is 30 to 50% on the TV. Never higher.

    Regards
     
  3. Guy Kuo

    Guy Kuo Supporting Actor

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    Michael explains it well. There is no reason to fret if one cannot find the "red line" level. White level is more difficult to set than black level with just a test pattern, but you can still reach a safe, final level. Unlike black level, there isn't an invariant end point a pattern can fix on the screen against which to compare the rest of the pattern. Blackest possible black doesn't change on the display, but there isn't a special signal one can send to a display to make it display the brightest safe white so we have to force you to do more work.

    The idea is to always remain below the point at which CRT wear and image fidelity worsen. As Michael described, given a proper video light meter we could have you set things to a target value, but since most of us don?t run around with one, we have you look for the point at which blooming appears because that is near the point of increased wear. Once the blooming point is determined on your display, it is taken as the maximal NOT desired setting for white level. Then, we set a viewing level somewhere below that point. Your display makes it difficult to get to that point, and that is not a problem. It isn't a goal, but something to avoid.

    It may still be possible to see blooming on your display. Many people have difficulty recognizing it. The best advice I can give is to be VERY close to your screen -- perhaps two feet -- when looking for blooming. Doing this from normal viewing distance doesn't work as well. Turn white level down to a ridiculously low level. Make white actually appear gray. You should see distinct scan lines making up the picture. If you don't see sharp, distinct scan lines, then something is amiss with the display's focus adjustments. If that is the case, it needs to be serviced. You can't see the blurring effect of blooming if the beam is already defocused at low output levels.

    I'll assume you're viewing AVIA's Needle Pulses + Steps pattern or the Black Bars + Log Steps pattern. Look very carefully at the right edge of the rectangles which make up the steps pattern. Pay particular attention to the white rectangle and the one just below it. Turn up white level until you see the width of the white rectangle grow slightly larger. That is the point of blooming. Remember that point. Never go over that point.

    Again, don't fret if you still don't see blooming. It isn't a goal to achieve but something it would be nice to know not to exceed. You can still get to the right place for your display in the next step.

    Now that you have found the maximum allowable white level or even if you haven't. It's time to set viewing level. The lower, the better for CRT life, but you also need enough output to have a decent picture. Set white level down to a ridiculously low level making white look definitely gray. Slowly increase white level until white appears white instead of gray, but stay below the blooming limit which was previously determined. With any luck, white looks white well before the point of blooming. If not, then your best course is to REDUCE AMBIANT LIGHT. It is better to adapt your eyes to make the lower level appear white than to overdrive the display to compete with excessive room light. This allows for best image fidelity within the limits of the display's capability. Some people will never control room light and turn up their displays beyond the point of blooming in order to view them in high ambient light. One is free to do so, but one must remember that this shortens display life and compromises image fidelity.
     
  4. Pep Guidote

    Pep Guidote Agent

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    Hi Michael, You mean we aren't supposed to be driving at 5900 RPM?! Man, that explains all those blown gaskets. [​IMG] But seriously, thanks for the reply. I don't have a light meter handy, but is there another way to determine 20-30 ft-l without using one? (I.e. white paper and a match or something)
    Hello Guy, I knew I could count on you. Thanks. I'm now running my set at 50 on a 0-63 scale. I swear that even at 2 feet, no visible blooming occurs because the rectangle stack never really becomes white.
    Another related question: Is the white level setting different when you use the "anamorphic squeeze" option on TV's?
    I like using the "squeeze" option on my set, the resulting image is practically free from scan line spaces, the colors are more saturated, and the display is punchier. It's almost HDTV-like.
    Does a safe white level for full screen viewing remain safe when using the "squeeze?" Or does the risk of phosphor burn require than white level be set lower?
    Thank you.
    -Pep
     
  5. Guy Kuo

    Guy Kuo Supporting Actor

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    Expect to use a LOWER contrast setting when the display is in anamorphic squeeze mode. Remember, when it does that, it is squeezing the same amount of beam energy into a smaller phosphor area. If you leave the contrast as high as you do for 4:3, you'll wear it faster in anamorphic squeeze mode. Drop contrast down in squeeze mode.
     
  6. Pep Guidote

    Pep Guidote Agent

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    Again, thank you Guy for the immediate and insightful reply. I shall be using a lower contrast for squeeze mode.
    With calibration done, it's time to continue tweaking... I just bought some illustration board and some Pritt On&Off reusable sticky tape to make some MATTES! [​IMG]
    -Pep
     
  7. Kevin P

    Kevin P Screenwriter

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  8. Jeremy Anderson

    Jeremy Anderson Screenwriter

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    Out of curiosity, could one use a photographer's light meter to properly set white level? I have several laying around from my late grandfather and would love to find some legitimate use for them.
     
  9. Tony_Pat

    Tony_Pat Agent

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    That's the exact thought I had, Jeremy. I have a rather nice photgraphic light meter that can read reflected light in a narrow aperture. I wonder what 20-30 fl equates to on a photo light meter.
     

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