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May 01

Doug Blackburn – ABL/APL Behavior

From a Thread at AVS Forum

You have to know what the “viewing angle” of your meter is… many meters have a VERY large angle of view and if you use a very small window size, the meter “sees” some of the black screen AND the pattern. That is NOT good. Any meter that is supposed to be very close to the TV probably has a very large viewing angle and using a small pattern will not work. Some meters “see” almost 90 degrees (45 degrees to the left, to the right, to the top and to the bottom).

You people (you and the people you are reading “information” from) are trying to make Cell Light into something that it is not. It is NOT a control that you should use.

Pick the mode on the Samsung that you want to calibrate (usually Movie mode). Set Cell Light to maximum. Set contrast to achieve about 33 fL (113 cd/m2). Then measure the different Gamma settings in the Samsung TV and use the Gamma setting that produces the AVERAGE gamma (all 10 points averaged together into 1 number) that is closest to your Gamma target (should be 2.25 unless you have a good reason to use some different Gamma target). Then use the 10-point controls to make every step in the grayscale equal to your 2.25 gamma target.

Trying to “beat” (or avoid) brightness limiting in plasma TVs is not good because the TV responds differently when the brightness limiting is active or inactive. When you use 10% window sizes, you get some of the effect of brightness limiting included in your calibration. This is a good thing, not a bad thing. Some people seemed to be obsessed with eliminating all of the plasma TV’s brightness limiting during calibration… I believe that is THE WRONG way to calibrate plasma TVs. Yes, it would be good for plasma TVs to NOT have brightness limiting. But plasma technology (today) requires brightness limiting for the TVs to operate reliably for many years. Since the TV WILL have brightness limiting while you are using it, the brightness limiting should be part of your calibration process. Window sizes of 10% are the best compromise because they are small enough that brightness limiting does not dominate the calibration, but they are large enough that you do have SOME (small amount) of brightness limiting.

You have mentioned 120 cd/m2 several times… you do understand that you will only measure that value for 100% white with that number with 1 size of 100% white pattern? If you change the size of the pattern, you change the measurement… a smaller pattern will produce a higher measurement. A larger pattern will measure a lower number. So saying you measure 120 cd/m2 is only good for one size of pattern. If you measure 120 cd./m2 with a 4% window, you will measure maybe 60 cd/m2 with a full-screen pattern and maybe 90 cd/m2 with a 10% window size. So when you are talking about a plasma TV, your 100% white measurement does not mean very much when you do not include the size of the window pattern that you used. I believe that if you calibrate with a 4% window, you will NOT include enough of the APL range of the TV during your calibration and your ending calibration will be LESS good, rather than “better” as some people are trying to say.

Your point about use 1 size of pattern for calibrating and you are only linear for that 1 APL (average pcture level — this is very different than ABL… average picture level is an average of the luminance of every pixel. So if 90% of the pixels are black (0) and 10% of the pixels are 100% white (I will use 100 cd/m2 to keep the calculation easy), your APL will be 10 cd/m2 for the 10% screen-size patterns when you measure 100 cd/m2 for 100% white. When you view many hours of TV content and measure the APL over many hours, you will get a number close to the same APL you get with windows that are in the 10%-16% size range. That is why we use window sizes in that size range to calibrate plasma TVs… the APL you use during calibration should be the same APL you experience when you view real video or movie content. You do NOT want to eliminate ALL of the influence of APL (by using very very small window sizes, like 4%, because you will NEVER view content on a plasma TV with APL that low (except maybe if there is a movie that is always dark). APL is linked to ABL but the relationship is not always what you might expect. HIGH APL usually causes the most ABL, and very small APLs mean there will not be very much ABL. But you must remember, you do not view TV or Movies with very very low APL very often. So your calibration SHOULD include some of the effect that APL has on ABL.

Your concern about making a large correction to fix gamma for some grayscale step being bad… that is not a problem for Samsung TVs. Samsung is the only TV manufacturer I know of who uses an 18-bit data processing path inside the TV. That means there is PLENTY of “space” for making large adjustments to controls without damaging the images. Other TV manufacturers may have data processing paths as small as 10-bits. I have seen Toshiba TVs where adjustments of more than 3 (range of control +/- 10) will cause severe damage to the picture because there is just not enough “space” in the digital range for the controls to do anything before running out of bits. (some earlier Toshiba TVs had control ranges of +/-30 but you could not use changes larger than +/-10 without damaging the picture badly). You will not have that problem with Samsung TVs.

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There is no one right answer to the above question — the answer depends too much on the meter (and it’s angle of view), the room (how close reflective surfaces are), and how sensitive the meter is to external reflected light.

In general, I prefer to not have a lot of extra, unmeasured, light in the room. So I use 10% windows for all types of displays… plasma, LCD, and projectors. I just keep the borders around the patterns black.

For CRT displays, Joe Kane found that an 18% window size was the best compromise and that is the window size he uses on his DVE discs, though I prefer a 10% window size for plasma because they do so much more brightness limiting than CRT displays.

But you have to keep in mind that some meters have a wide field of view and you have to keep the pattern large enough to fill that field so the meter is not seeing the black background. The meter I use has a 1-degree field of view, so I place it close to the viewing position and set it up so that the “line of sight” of the meter closely matches my line of sight when I’m watching movies.

One thing about plasma calibration… every method you use gives you a different end result. When you view content, often “different” wins over “better” (this applies to many things including video and audio evaluations) even if different isn’t really better. So every time someone tries something “different” and says “Wow! I did this and it is wonderful.” You have to question that sort of feedback and not assume it’s an accurate reflection of reality — that person may be convinced, but you can’t know the process of how they got there.

I think one of the things about using constant APL patterns for plasma getting “popular” is that people understand that in a projection display with an automatic iris, you want to put the iris in a fixed setting for calibration and it’s OK to turn the auto iris back on after calibratiion. And that is correct for projection systems. But brightness limiting in plasma displays doesn’t work like an automatic iris… the black level does not get blacker when brightness limiting is high (the main way auto iris and brightness limiting differ). With an auto-iris, if the iris is full open for 1 scene, and 50% closed for the next scene, everything in the second scene is 50% dimmer. The shape of the gamma curve doesn’t change (much, yes, it changes a little, but it’s not a big deal). In plasma TVs, black is constant and brightness limiting affects the luminance in a non-linear way. You can’t stop that from happening (except by changing to a display that’s not plasma), so you need, IMO, to include SOME of that non-linear behavior within your calibration… I find 10% windows with black borders achieve that goal… they keep you from including a huge amount of brightness limiting without eliminating all of it.

Written by Doug Blackburn