Grayscale variances - different patterns, plasma calibration - AVS Forum
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post #1 of 323 Old 11-26-2012, 01:39 AM - Thread Starter
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Hi all,

Bit of a noob question here. I recently have managed to get a fairly decent image on my Panasonic 55VT30b (UK) plasma using the AVSHD normal windowed patterns, however when i check with small or large APL's the greyscale at each brightness interval has deviated from the 'original' calibration performed with windows. Is this wrong? I was under the impression that the greyscale should remain the same regardless of pattern used?
To clarify, I use a specific technique for dialing in the Panasonic given its well documented issues with calibration and this was adopted on all tests to ensure the best chance of accuracy for the final result.

Thanks for your help.

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post #2 of 323 Old 11-26-2012, 05:57 AM
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ABL will get you every time. It is impossible to perfectly calibrate a plasma display. This thread is not directly related to the 1D LUT calibrating we do with the display's controls but it very informative none the less.

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post #3 of 323 Old 11-26-2012, 07:06 AM
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To 'see' ABL in action try the following...

Using a PC with HDMI out and a graphics program feed a full-size, good contrast image with good colours and shadow details to the display.
A 'real image' from a good scene, or something you have taken with a stills camera.

Then, with the image displayed via the Graphics program, start to draw a 100% white rectangle over the image - left to right, or top to bottom, or diagonal...

As the 100% white rectangle increases in physical size the 'real' image behind it will visually change contrast/brightness/colour...

This is the effect of ABL.

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post #4 of 323 Old 11-26-2012, 07:54 AM - Thread Starter
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Quote:
Originally Posted by Light Illusion View Post

To 'see' ABL in action try the following...
Using a PC with HDMI out and a graphics program feed a full-size, good contrast image with good colours and shadow details to the display.
A 'real image' from a good scene, or something you have taken with a stills camera.
Then, with the image displayed via the Graphics program, start to draw a 100% white rectangle over the image - left to right, or top to bottom, or diagonal...
As the 100% white rectangle increases in physical size the 'real' image behind it will visually change contrast/brightness/colour...
This is the effect of ABL.
Steve

Indeed.

The long of the short of it is then that a plasma display is inaccurate regardless of what process you use to calibrate.
So this therefore leads me on the the next question - this is probably been asked dozens of times but just want to be sure.
I have used both small apl, large apl and window patterns to calibrate - some people tell me to use the smallest ones available, others say the large APLS and even others say windows. As you can appreciate from a 'noob' perspsective is there a logical approach to calibrating a plasma display in order to gain the most accurate picture and if so what is the 'recommended' formula to achieve this? Do you use small windows to minimise the effect of ABL or do you use a pattern that resemles general viewing? There seems to be arguments, even amongst the pro's as to what one should use.

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post #5 of 323 Old 11-26-2012, 08:36 AM
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I would like to know a definitive answer as well.As I have a Panny GT30 and when I calibrate THX mode with window patterns from GCD/Avs709 I get a max gamma of 2.07..

Now when I try Lg or Sm apl patterns the gamma goes up to 2.22+...

But reading many pro's comments they swear by using window 10-14% patterns as the holy grail for reference,so I have left my tv calibrated by windows pattern as I'am just as confused as you are ndaa...

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post #6 of 323 Old 11-26-2012, 09:30 AM
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Quote:
Originally Posted by PeterLewis View Post

I would like to know a definitive answer as well.As I have a Panny GT30 and when I calibrate THX mode with window patterns from GCD/Avs709 I get a max gamma of 2.07..
Now when I try Lg or Sm apl patterns the gamma goes up to 2.22+...
But reading many pro's comments they swear by using window 10-14% patterns as the holy grail for reference,so I have left my tv calibrated by windows pattern as I'am just as confused as you are ndaa...

How does your display look? Good black level, lots of shadow detail? End of the process is watch content... 2.07 2.22 not a real big difference there.. And as you know, depending on the content the display is going to change it.. So if you have a flat gamma at 2.07 with windows and 2.22 with apl patterns that is also flat, and the results are visually good.. you are done.
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post #7 of 323 Old 11-26-2012, 10:18 AM
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It is true that accurate plasmas calibration is all but impossible, due to ABL.
The best you can do is to gain an acceptable compromise.

There are different approaches to this, but getting the patch size correct from your particular display is critical.
Note: For Your Particular Display.

One approach is:

> Use a Grey-scale Ramp image - something like the attached.

> Then measure the overall brightness of the screen (with the probe covering all the screen - moving back and forward to get the peak reading).
> Just looking for Luma not colour readings.
> Then make a calibration patch window with 50% white and sized it to get a similar Luma reading with the probe in the same place.
> Use that patch size for the profiling (moving the probe to the correct distance for the patch size).

Also, the best results are often with a Quick Profile, not a full 3D cube profile.

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post #8 of 323 Old 11-26-2012, 11:11 AM
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Hi Steve:-) Wait, let me get this clear in my head.
Quote:
Then make a calibration patch window with 50% white and sized it to get a similar Luma reading with the probe in the same place.

You mean a white patch at 50% stimulus right? Then experiment with the patch size until the luma reading of said white patch matches that one from the measurement with the greyramp pattern?

So what is the goal here? I know that when the ABL kicks in it messes with the calibration because the screen will go dimmer. So is the goal here to have a patch size that is as bright as possible without triggering the ABL?

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post #9 of 323 Old 11-26-2012, 11:20 AM
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Greetings

Peter,

The THX mode gamma is based on test patterns in the 6.5% size. The larger your test patterns, the lower the gamma number becomes.

I did a run with boxes in the 20% size range ... and got 2.07 range gamma. Then for fun, I went to a 10% window size and the gamma went to 2.16 range.

The test patterns you use matter. No sense killing yourself over this gamma number. It actually is where it is supposed to be.

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post #10 of 323 Old 11-26-2012, 11:32 AM
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ABL is always active - it just gets stronger and stronger as the overall screen brightness increases
There is no single level when it 'kicks in'....
You can see this with the ABL test I outlined above, or by checking a RGB Separation graph after profiling.

See:

So, the idea is to get a patch size that 'best emulates' your display for average contrast content...
Then the calibration will be as good as it can based on your display's ABL.

This is why I suggest a 50% white patch (50% stimulus) and then size to get the same overall 'brightness' reading, matching it to the ramp.
This attempts to 'best match' the patch size to your specific display.

Steve

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post #11 of 323 Old 11-26-2012, 11:49 AM
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I see, so content with higher or lower average contrast will not be correct. Thanks for explaining this.
So visually this translates to: too bright dark scenes and too dim bright scenes (because gamma is not completely linear and cannot be made completely linear on plasma's)?

Which makes me wonder, does the ABL behave linear? IF it does, can't this be countered *somehow*

Sorry if my above interpretation is wrong. But that is what I make of the issue:-).

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post #12 of 323 Old 11-26-2012, 11:59 AM
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With plasmas is it mainly bright scenes that are contrast reduced, which also alters gamut (saturation).

To get a plasma to 'look' good (ie, look accurate) you often have to set gamut wider than the desired target when calibrating.
This is a trick we have used a number of times, when the actual calibration data states it is correct, but the visual results look poor.

Unfortunately, ABL is anything but linear...

And, it reacts differently to different colour ratios, due to the fact different colours have different underlying luma values for what 'appear' to be similar brightness colours...

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post #13 of 323 Old 11-26-2012, 12:15 PM
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Yes, Y is one of the properties that makes the color look right so I can imagine that the ABL would ruin the accuracy. Since there are so many color variations possible in an image, I should have seen it coming the ABL is not linear.
I read so many color science links I actually get your last sentence (bearing in mind you are many times more knowledgeable than I am on the matter:-)).

Just as I took comfort I could live with all the plasma compromises vs LCD, this comes up:mad:

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post #14 of 323 Old 11-26-2012, 01:55 PM
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On plasmas, how far back did they start using ABL circuits????

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post #15 of 323 Old 11-26-2012, 02:27 PM
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ABL has always been part of plasma technology. The amount of power needed to make a 10%-of-screen-area window pattern at 100% white that measures 35 fL have the same 35 fL measurement for a 100%-of-screen-area is substantial and causes the TVs to produce even more heat than they already produce. In "modern" plasma TVs, you'll find that if you set a window pattern that is 10%-of-screen-surface-area to read 35 fL, when you do nothing but change to a full-screen pattern, you'll probably measure something like 17-18 fL. They get away with this in plasma TVs because 17-18 fL coming off the entire screen surface still seems pretty bright to human vision. But you can certainly see the difference easily enough with the right sort of test pattern (like a split window pattern). In actual video content, the difference is much more difficult to detect. Most often you hear complaints from hockey fans that full-screen shots of the ice seem kind of dull... because they are. But you rarely see that sort of content in a movie or TV show.
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post #16 of 323 Old 11-26-2012, 03:20 PM
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Great post, Doug. I always enjoy seeing your perspective.

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post #17 of 323 Old 11-26-2012, 11:46 PM - Thread Starter
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Quote:
Originally Posted by Light Illusion View Post

With plasmas is it mainly bright scenes that are contrast reduced, which also alters gamut (saturation).
To get a plasma to 'look' good (ie, look accurate) you often have to set gamut wider than the desired target when calibrating.
This is a trick we have used a number of times, when the actual calibration data states it is correct, but the visual results look poor.
Unfortunately, ABL is anything but linear...
And, it reacts differently to different colour ratios, due to the fact different colours have different underlying luma values for what 'appear' to be similar brightness colours...

You see the problem I have with my Panasonic VT series is that I am unable to achieve a flat gamma without consequences ie a bad case of contouring and posterization. This seems apparent on all pattern types and especially where larger adjustments are made to a single gamma channel in order to flatten the line out. Perhaps this is the result of an aggressive ABL function - anyone have a work around?

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post #18 of 323 Old 11-27-2012, 01:07 AM
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Unfortunately, there are limits as to Plasma capabilities.
Their screens are actually about 6 bit in depth for example...
This will cause banding.

The best way we have found to calibrate a Plasma is the same we define for all calibration...

See: http://www.lightillusion.com/display_calibration.html

Basically, use the minimum of internal display controls, as they are very often of poor quality (most TV displays are developed down to a price), and then use an external LUT box for the actual calibration.

However, with plasmas you will as like it or not still suffer issues.

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post #19 of 323 Old 11-27-2012, 01:59 AM
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Is there a difference between plasma's as to how much they suffer from these effects? My plasma, a 65VT50, is easaly calibrated using different patterns. I have used windows without APL, windows with APL, different kind of APL (GCD vs AVCHD), and different size windows. With all patterns I am measuring good gamma, greyscale and gamut. But I have found that both the AVCHD APL windowed patterns and the CGD smallest windowed APL patterns give the best results.
Ofcourse my plasma suffers from ABL like any other, but as long as there are no extreme images, it all looks very good. Except for example, when I watched Fargo the other day, and the scenes that are all snow are visibly dull. But Avatar, both the scene's in the dark when Sully meets Neytiri, and the scene's with the aerial battle in the end, all looks very good.
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post #20 of 323 Old 11-27-2012, 03:15 AM - Thread Starter
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Thanks Steve

Its apparent then that actually trying to set a flat gamma on a plasma screen is essentially a waste of time since without knowing the ins and outs of the
ABL and the way it works with a particular display means you are shooting in the dark and can potentially end up making matters worse, as per my example.

Best approach then is to calibrate greyscale and gamut only, leaving the gamma alone.I have found that certainly in the case of the UK Panasonic VT30 range that colour decoding is skewed, with an expanded gamut a result ensuring both 25 and 50 hit their targets as close as possible.

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post #21 of 323 Old 11-27-2012, 03:55 AM
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Steve,

Would not limiting peak white help reduce the amount of ABL and the subsequent effects on gamma?

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post #22 of 323 Old 11-27-2012, 07:33 AM
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Limiting the peak white will reduce the amount of ABL, and this trick is used in some of the 'pre-sets' used in some plasmas that state they have 'monitor' or 'cinema' modes.

The issue is that the brightness reduction needs to be rather large, as can be seen in the graph I posted above.
And then you still get an amount of variation in brightness, saturation, etc...

In all honesty we (the professionals within Light Illusion) just find Plasmas unacceptable as displays for accurate calibration - they really can' be calibrated to levels we regard as being acceptable...

But, if you like the pictures, that is what really matters!

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post #23 of 323 Old 11-27-2012, 07:37 AM
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Quote:
Originally Posted by Light Illusion View Post

In all honesty we (the professionals within Light Illusion) just find Plasmas unacceptable as displays for accurate calibration - they really can' be calibrated to levels we regard as being acceptable...
But, if you like the pictures, that is what really matters!
wink.gif
Steve

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post #24 of 323 Old 11-27-2012, 09:43 AM
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LOL - Yup!

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post #25 of 323 Old 11-27-2012, 11:02 AM
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I am wondering about something though. Usually (let's not fight over this) plasma is touted as (currently) the best technology in terms of motion handling, response time, screen uniformity and color accuracy (this last one being debated here). This is the image I have gotten from browsing these and other forums for many, many hours. The flat-panel shootout at Value Electronic had the Panasonic TC-P65VT50 as the winner. As long as I follow display technology, the Pioneer Kuro was the screen the beaten. So how bad is this ABL thing in terms of CIE94 dE error. Is this quantifiable?

Steve, are you only talking about the aspect of color accuracy then? What would you consider acceptable? This is not criticism I'm just wondering what I'm missing out on:)

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post #26 of 323 Old 11-27-2012, 01:08 PM - Thread Starter
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Hi Steve, other pros and enthusiasts,

Perhaps you're better positioned to explain something to me here.

Specifically in respect of Panasonic plasmas, they have a gamma option.
You can choose 2.2, 2.4, 2.6, etc etc.
Now what does this mean in real terms?
Does it mean that Panasonic have engineered their displays and implemented their gamma settings based on the sets ABL functionality?
If this is the case then that would then lead me to believe there is a specific pattern size to be used to gain a virtually linear 2.2 gamma or certain user defined gamma level, which would be the one to use for calibration.

I note that some people here and on other forums have suggested 10%-18% windows, apl's (some saying apls produce a non accurate result - how so?) in order to accurately calibrate a plasma display. Well how do they know what the right size is given how ABL impacts a calibration from the start? It's a moving target and you can really only know what pattern to use if you know how the ABL functions? Does this mean some amongst us know this info or is it a case of making a gamma graph look impressive when we post our proud calibration results in a forum such as this one?

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post #27 of 323 Old 11-27-2012, 01:31 PM
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To attempt to answer both your questions...

In our (or my, but there are a team of people in Light Illusion) experience all that plasmas provide over LCDs is the possibility of better black, but with poorer shadow detail, and better viewing angles, although IPS and LED based LCDs seem to have that covered now.

And all plasmas I have seen have much worse motion artefacts (blurring) than LCDs.

And ABL kills colour accuracy dead, and plays havoc with bright scenes becoming very dull, unless you have everything set to be dull...

So, this means the pre-set gamma setting really mean nothing - in all the tests we have done with a relatively calibrated Plasma (internal calibration, not external LUTs) the 2.6 setting gives you the best approximation to 2.2.

And for any given plasmas the size of the patch window to best manage the ABL will vary, including with the peak white setting.
I did offer possible way to 'guestimate' this. It has worked for us.

Any help?

Steve

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post #28 of 323 Old 11-27-2012, 01:37 PM
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Quote:
Originally Posted by Light Illusion View Post

To attempt to answer both your questions...
In our (or my, but there are a team of people in Light Illusion) experience all that plasmas provide over LCDs is the possibility of better black, but with poorer shadow detail, and better viewing angles, although IPS and LED based LCDs seem to have that covered now.
And all plasmas I have seen have much worse motion artefacts (blurring) than LCDs.
And ABL kills colour accuracy dead, and plays havoc with bright scenes becoming very dull, unless you have everything set to be dull...
So, this means the pre-set gamma setting really mean nothing - in all the tests we have done with a relatively calibrated Plasma (internal calibration, not external LUTs) the 2.6 setting gives you the best approximation to 2.2.
And for any given plasmas the size of the patch window to best manage the ABL will vary, including with the peak white setting.
I did offer possible way to 'guestimate' this. It has worked for us.
Any help?
Steve
Edit: Oh, I see what you're saying now about LEDs...
It's not exclusive to plasmas to have labelled gamma presets come nowhere close to actual measured gamma. JVC front PJs come to mind first, and there's been plenty of others.

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post #29 of 323 Old 11-27-2012, 01:42 PM
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All your comments are helpful and appreciated Steve. But surely, you too must have noticed most reviews favour plasma for motion handling, no blooming, good panel uniformity and low response times?
I have alway read motion blur is actually a lot worse on LCD's due to their slower response times. That is actually the main reason LCD manufactures pull tricks like 100Hz, 120Hz, I don't even know what the number has grown to now:).
Has this changes so radically over the past years? Or pehaps I've been grossly misinformed somewhere. But like it said, the best TV for this year IS a plasma.

My gear: Panasonic TH-42PF11EK pro plasma display. -- Iscan Duo video processor -- i1 display 3 colorimeter -- i1 pro 2 spectrometer
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post #30 of 323 Old 11-27-2012, 01:43 PM
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It's actually IPS that has greatly improved viewing angles, and in conjunction with LEDs have helped yet further.

The only 'TRUE' Grade One 'digital' monitor that exists today is a LED back lit LCD, from Dolby.

And do NOT get me started on JVC displays, or worse, JVC D-ILA projectors.
Just not fit for purpose...
We have had more clients return them to the supplier than any other display!

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