ABL effects measured for comparison - Page 2

Originally Posted by cobhc2010 

Then why do people try to make a plasma accurate? I'm not trying to be an ass, just wondering why people bother if you can never get one accurate?

Originally Posted by buzzard767 

You calibrate a plasma as best you can. True image fidelity is not possible but they can be made to look very VERY good.

Originally Posted by cobhc2010 

Is it not true that true image fidelity is not possible on any display though? Or is it just that plasma's generally can't get as close as LCD's or other display technologies?

Originally Posted by buzzard767 

It has nothing to do with "how" to calibrate because a plasma can never be made accurate, Any time a display of any type is making decisions on its own, no twist of a knob will yield a reference image in video. That's just the way it is.

Originally Posted by turbe 

the Pioneer 9G Monitors were some of the best Plasmas in this regard - not as 'limiting')..

The result of using 1% patterns is to get higher readings across the entire grayscale. Thus, the gamma is the same. The only difference is peak output, which measures about 10% higher with 1% patterns.

The Before is 1% patterns. The After is regular (approx. 15%) patterns.

The fact that even at 10% stim the 1% pattern reads brighter tells me that this effect has nothing to do with the voltage limitations of the panel.

I think that this is an important topic, because it strikes at the core methodology for measuring both gamma and color performance. Thus, I spent some time taking some more measurements. This time I used a more accurate instrument (Minolta LS-100) and I performed tests on both the Pioneer 5020 plasma and a Sony CRT.


First, let's begin with my initial measurements on the 5020 using the Chroma 5 colorimeter. I'll use the same % difference graph that you did.





Here's the same measurements taken with the Minolta.





The % difference curve now looks a little different. There's still the same raggedness at the bottom and the output is still higher at all levels of stimulus, but other than that there's no discernible trend that that I can make out.


Here's the gamma results from the same data.





Looking at the gamma curve, there appears to be a small effect. From 70% up as the gamma for the 1% pattern becomes progressively higher until at 90% it is 0.10 higher.


Here's the data for the Sony (uncalibrated) CRT.





There is clearly a trend here. The % difference between 1% and 15% patterns becomes progressively lower as the signal level increases. Here's the gamma chart for the same data.





Here there is a small difference, but it is the opposite of what we saw on the plasma. The gamma values are slightly LOWER and throughout the entire range.


Just to make clear to those who may not appreciate this, CRTs and plasmas are voltage limited in way that requires the use of window patterns. These displays simply cannot produce the expected output at higher levels of stim using full field patterns. For example, here's the Pioneer with full field patterns (window before, full field after.





And the gamma results for the same data.





The results are clear. The voltage limiting effect produces a LOWER gamma throughout and LESS output only a higher levels of stim.


What to make of this? I honestly don't know. The data are simply not consistent. The only consistent results I see are:


1) Measurements using 1% patterns yield slightly higher values at all levels of stimulus.


2) Whatever differences we do see are very small, and are not the result of voltage limitations of plasma and CRT displays.

I certainly don't think there's anything wrong with using smaller test patterns. I am just not convinced that it provides more accurate results. Nor am I certain that the differences I am seeing are the result of any power limiting effect. In fact, since the higher readings occur even at low levels of stim, I am fairly sure that this is not what is causing it. I have no opinion about what is.

BTW, the LS-100 has a view finder that was clearly within the boundaries of the 1% test pattern area, though it did require a little effort not to drift outside of them while taking readings.

Originally Posted by turbe 

things were different with energy compliance regulations a few years ago...

Originally Posted by turbe 

This is not a new discussion either...
.

Originally Posted by TomHuffman 

Here's some data on the APL of several films I sampled some time ago.

Originally Posted by TomHuffman 

I tested my Pioneer Kuro 5020 using a fixed 10% stimulus signal. I used a standard 11% window with different surrounds, 0%, 5%, 15%, 20%, 30%, 40%, and 50%.
I measured a very small effect. Using a 0% surround the gamma was 2.31 and using a 50% surround the gamma was 2.28. This is almost certainly below the threshold of visibility. I only tested at 10% stimulus because this is where the effect, if there was one, would presumably be the strongest.

I have a couple of other plasmas here--including a VT30--that I'll test in the next couple of days, but it seems pretty clear that no such effect plagues the Pioneer plasmas.

Originally Posted by TomHuffman 

I have some interesting data.

1. I created my own simple constant APL test patterns that are just standard 11% windows with a varying surround that keeps the APL at about 25 through the entire range.
2. I then created a standard set of window patterns with a constant black surround.
3. Then I measured both on my Pioneer plasma. The results show that there is virtually no difference between the two at 10% and at 50% and above. However between 20%-40% there are significant differences. What's more, the differences are not just in gamma, but in chromaticity as well.

So now that we have established that they measure differently, the question is which is more "correct." I think that the differences in chromaticity offer a way to determine this. Since it is at the low end of the range, I can also measure full field test patterns and use those as a reference. Here's the data.

What this clearly shows is that the constant APL patterns are considerably closer in chromaticity to the full field test patterns between 20-40%. I am now a believer.
Comments?
The constant APL test patterns can be downloaded at
http://www.chromapure.com/constant.zip

Originally Posted by TomHuffman 

Since you looked at so many test pattern variations, I'll take your word for this, but if you look at the limited universe of:
1) Standard windows
2) 22% APL windows (I looked at 25%, but that difference is negligible)
3) Full field patterns
both the chromaticity values and the luminance values in the critical range of 20-40% of full fields is much closer to the 22% APL patterns that both you and I now seem to endorse, at least for plasmas, than it is to the standard window patterns, right?

Originally Posted by TomHuffman 

However, on the other side of the coin, I now agree that plasma displays--at least during a narrow range of APL--seem to perform in a way that is not mimicked by traditional window patterns and that fixed APL patterns probably do a better job at this. Having said that, the data we have looked at strongly suggests that the APL patterns remain in the 20-25 APL range (14-27 level surrounds), which is both where the deviations in gamma and chromaticity appear and, coincidentally, where a large proportion of real-world video resides as well.

Originally Posted by TomHuffman 

This is not just about window patterns. It is full fields as well, which constant APL patterns would rule out by definition.
The whole point of a grayscale run is to measure dynamic performance under a variety of conditions using a series of different test patterns. If this were not necessary, we'd just use a single gray test pattern and leave it at that. A Pluge pattern or a SMPTE color bar measures a static phenomenon (MLL and hue/chroma levels). The "questionable practice" you refer to has been in place since the advent of video itself and was developed by professionals working with international organizations devoted to laying out video standards for everyone. I wouldn't be so quick to dismiss it.
As I have said previously, I now think that there is value to these patterns for plasmas over a narrow range of operation, but let's not oversell them.

Originally Posted by rahzel 

I only have an ST50 with a 2pt system, but I prefered the windows over the APL, but many seem to like the APL patterns. Try them and see which you like better.
As Chad said, because RGB tracking and (more so) gamma changes based on the APL on plasmas, and APL changes from source to source, scene to scene... it's debatable which patterns best simulate real world content.

Originally Posted by TomHuffman 

I just spent quite a bit of time with the ST50. I was VERY impressed with this display and am going to publish a review of it in the next few days. FWIW, it did not show at least one of the artifacts I saw on the ST30--an elevated gamma at 90%--when using 10% windows. Based on my preliminary tests special windows may not be necessary (or even advisable) for this display.

Originally Posted by TomHuffman 

I just spent quite a bit of time with the ST50. I was VERY impressed with this display and am going to publish a review of it in the next few days. FWIW, it did not show at least one of the artifacts I saw on the ST30--an elevated gamma at 90%--when using 10% windows. Based on my preliminary tests special windows may not be necessary (or even advisable) for this display.

Originally Posted by HD-Master 

Forgive my ignorance here, but if your tests may have ruled out special windows...which windows does that leave us? Perhaps your review will include this, but what windows (both size and APL vs non-APL) would you recommend after your testing?
Thank you Tom!

Originally Posted by TomHuffman 

Well, standard 10% windows with a black surround. I cannot draw any definitive general conclusions about this yet. I was not looking at how the effects of dynamic circuitry are reflected in different window sizes. What I did notice was that the elevated gamma at 90% that I saw on the ST30 did not appear on the ST50, using standard 10% windows in each case.

My own view is that some combination of smaller windows and higher-than-black surrounds is the best choice, but the precise mix is yet to be determined. Also, it seems that these effects vary between plasma manufacturers and even models.

Originally Posted by TomHuffman 

BTW, I think that for everyone's benefit we should be very clear about what we are talking about. I occasionally read about an "ABL circuit" referring to the inability of plasmas to display the full brightness of a full field signal. This is not a special circuit. This is a consequence of the design of plasmas, just as it is with CRTs. You must use windows, instead of full fields, for all calibration tasks for all plasmas.
My understanding of ABL (Automatic Brightness Limiting) in some contemporary plasmas is that it is a special feature above and beyond what is described above that is part of an energy saving effort. The consequence of this circuitry is that even normal window test patterns may not provide optimal results. The effect of this, if it exists, would be seen at higher levels of stimulus.
There is another aspect of plasma design, unrelated to ABL, but which also may require adjustment to standard window patterns. This effect, which is most clearly seen in the 20-40% stimulus range, is of undetermined origin. I originally thought that it was some type of dynamic contrast circuit that lowered the output of the panel at low levels of stimulus to artificially boost contrast. However, since this effect is not seen at 10% stimulus, this seems unlikely. You can test for this effect by comparing a full field test pattern at 20% (way below the brightness limiting threshold) to a window test pattern, also at 20%. They should measure the same--as they do on LCDs--but they don't. This results in a search for the best test pattern that yields good results at both low and high level stimulus measurements.
My own view is that some combination of smaller windows and higher-than-black surrounds is the best choice, but the precise mix is yet to be determined. Also, it seems that these effects vary between plasma manufacturers and even models.

Originally Posted by Light Illusion 

However, even then the results are 'variable' - plasmas cannot ever be fully calibrated - ABL just makes that impossible.
Oh, and I have never seen similar with CRTs. They will 'bloom' with high brightness if contrast is over cranked, but that's all I have ever seen.

Originally Posted by H Stevens 

Tom, with all of this discussion lately concerning window size, accurate measurements and gamma calibration with plasma displays. How confidant are you in the Chromapure Calibration Disc that you created and offered to your customers for plasma calibration? Do you foresee any revisions to that disc in the future? Do you feel that it provides a happy medium that reflects real world content and deals with ABL satisfactorily for the DIY crowd?

Originally Posted by TomHuffman 

...which has its own problems.

Originally Posted by TomHuffman 

Well, standard 10% windows with a surround. I cannot draw any definitive general conclusions about this yet. I was not looking at how the effects of dynamic circuitry are reflected in different window sizes. What I did notice was that the elevated gamma at 90% that I saw on the ST30 did not appear on the ST50, using standard 10% windows in each case.

Originally Posted by ttnuagmada 

What window size would you recommend for the ST30? I'm having the problem you describe with gamma at 90% using 10% windows, I figured I goofed something up, but I guess it's just these TV's. Also, what window size do you use for gamut? I used 10% 75% windows, but it seems like whichever color/tint settings give me the best numbers, ends up giving me too much saturation.

Originally Posted by TomHuffman 

5% windows seem to work. Standard windows are fine with color.