HDTV calibration - white clipping - Page 2 - AVS Forum | Home Theater Discussions And Reviews
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post #31 of 38 Old 10-27-2016, 09:45 AM
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Hi MaaZeus,

You are an exception to this forum when you state "I am firmly in a camp that thinks that in video signal the WTW and BTB data is garbage and should not exist just like there is no data above 255 and below 0 in PC levels". So many believe that there is content above 235. If one truly understands the basic anatomy of a video signal and how it is manipulated by the time it reached your display this thread would not exist. With the exception of minor blips there is no content above 235 and if one could see these blips their appears would only be white. The biggest problem is many so not know the standards. Rec BT 709 clearly defines the luma settings. I have attached an image file showing the parameters (Note the values for Black & White). Please show me where it states that the Luma is to be set to 255. To save myself from having from having to make a future reply "Yes, the document does state that excursion can occur above the 235 level. This is in place for such things as "Luma Gamut Error" or various other forms of gamut errors. DVD encoders and television broadcasters strictly adhere to "Broadcast Legal Standards". Individuals such as Colorists will do their work in a full swing setting (0~255) but will process the final product through a legalizer which will convert the product from a "full swing" to a "studio swing" (0~255 ===> 16~235).

Dominic Chan mentioned the document, "EBU TECH 3320 ("USER REQUIREMENTS FOR VIDEO MONITORS IN TELEVISION PRODUCTION, VERSION 3.0" Section 5.1, Note 2)", and it should be noted that it applies to television production studios only. I have attached another image file showing the scope of this document. If one were to apply the standards and conditions for their personal usage one needs to determine what type category their display complies too and that their display must be able to switch between 6504K (D65) and 2600K. As to the need to track from 941~1019, this is for the aforementioned errors, as well as, for the needed corrections that must be dwelt with on the fly.

I personally do not recommend the usage of the pluge patterns from AVSHD 709. The documentation is ambiguous at best. There is no distinction between broadcast legal settings and PC settings. By what many think to believe to be the proper way to use these pluge patterns is not correct. Here is a little experiment for some. First, set the contrast with another pluge pattern (say Disnyls WOW) and the retest with the pluge pattern from AVSHD 709 and compare the results. The setting of contrast by turning down the contrast control so that one can see all of the bars is in fact retarding the video image by 10% and in doing so diminishing the contrast ratio. I know why people use the AVSHD 709 patterns and that is because it is free. I also know that what I am stating now will not change anyone so what is really needed is the proper instructions as to how the use these pluge patterns. Here is a link on how to use them properly.

Contrast


Brightness



Note where the clipping occurs on both examples.


If you want to know what the standards are for setting up Contrast & Brightness here is the official documents defining the parameters.

Rec BT-814

Rec BT-815


These documents mention the usage of a waveform. I am always surprised by those dabbling in the field of video calibration and don't have a clue of the basic knowledge of the anatomy of a video signal or what a waveform or a vector-scope does. If they did the mis-information that is passed back and forth on this forum would cease with regards to the setting of contrast and brightness. Even making an attempt to view a couple of Youtube videos on how to read a waveform may enlighten you as to what Luma is or the effects of Gamma.

Congratulations on your insight "I am firmly in a camp that thinks that in video signal the WTW and BTB data is garbage and should not exist just like there is no data above 255 and below 0 in PC levels.

Good luck and remember "The truth will set you free".
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Last edited by randal_r; 10-27-2016 at 01:07 PM.
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post #32 of 38 Old 10-28-2016, 10:38 AM
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Quote:
Originally Posted by randal_r View Post
Hi MaaZeus,
*snip*
Thanks! I was doubting myself for a second but its nice to have an assurance that I am onto something, although honestly I have too severe case of ADHD to ever be able to focus on long documentaries like the one you posted.

But there is another point I meant to bring that I forgot. The 8-bit. The maximum white it can output is 255 for each Red, Green and Blue channel. Video range of 16-235 is still in the realm of 8-bit and even if the numbers might hint otherwise the video 235 is the same as PC 255, it just has less steps between the extremes as I mentioned before for bandwidth reasons or some other legacy stuff. And 8-bit digital display be it TV or monitor still works in 0-255 range internally and if it receives a signal of 235 for some pixel and it knows it is receiving a video signal it should scale it up and output full 255 instead, the maximum of 8-bit.
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post #33 of 38 Old 10-28-2016, 02:11 PM
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Hi,

Thank you for the reply.

You are correct regarding the maximum output for the RGB channels can be 255. In the PC realm all video signals parameters are 0-255 (in reality 255 is not used as it is reserved for one of the many sync signals found in a video signal). In the broadcast legal realm the video signals parameters are 16~235. The reason why DVDs and Broadcast material utilize 16~235 are too numerous and chapters could be written on this alone. Many feel that because television broadcasting went digital that the system can now do much more and all the conditions that many label "legacy" have been replaced by the waving of the digital wand; this is far from the truth of the matter. By switching from an analogue system to a digital system many of the old issues have been eliminated but in turn new ones have been introduced. The issues of the analogue were elegant in nature such as slipping down a slope while the digital issues can be more extreme more like falling of a cliff. Despite the fact that consumer based displays are now digital in nature displays like the ones that we own, do not rescale an incoming video signal on the fly. If they did such a process the the need for video legalizers that are found in post production studios would not be needed. A display will only handle an incoming signal in the manner in which it was calibrated to.

I am attaching a number of image files that might help in clarifying this subject. The image "rangerraw" illustrates a video signal in 0~255 and 16~235 (note the differences). To assist you keep in mind that on a waveform 0% = black and 109% = reference white (0~255) and the "Broadcast Legal Luma" shows that the video range is from 0% to 100% for 16~235. If one were to view a broadcast signal on a display that was calibrated to a 0~255 the whites would appear greyish and dull and the blacks would appear lighter and washed out. See attached image "a". If the signal where a PC level (0~255) the blacks will appear crushed and the whites will clip, blooming can occur, colors will bleed and the presence of a possible audio humming.

Now turning to the calibration as many seem to think is the proper way to adjust contrast, where the Luma is set using the white pluge from the AVSHD 709 test patterns. Black is set to 16 and the contrast is adjusted so that all the bars can be seen. This method causes the Luma to be rolled down by 10% in order for all the bars to be seen. What results is a video image illustrated in image "b" (also see "waveform b"). Now if the calibration is set so that the Luma is set to 16~235, then the results will be as illustrated in image "c" (again see "waveform c"). The best way to see the differences it to set up the images so that you can toggle quickly between "b" & "c".

The documents Rec BT 814 & 815 are worthy of reading. No where will it state that contrast is to be set to 255.

If you should look into this deeper, another scale for a waveform is 0 mV (reference black) to 700 mV (reference white).

Thanks again for your reply.
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post #34 of 38 Old 11-26-2019, 01:35 PM
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Originally Posted by Scott_Arm View Post
Thanks for the reply. I'm using an Xbox One. Displaying the test patterns on my tv through a USB stick attached to the Xbox One. The Xbox is set to standard (16-235) and my tv is set to match. I can't remember what the Samsung setting is labelled (Normal?), but I know it's the right one.
Sorry to revive such an old thread but I am also using an Xbox One and having the same problem with the AVS709 WhiteClipping pattern showing all bars as you were experiencing. I'm going to play the pattern from a different player if I can find one on the same HDMI port.

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post #35 of 38 Old 11-30-2019, 04:33 AM
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Originally Posted by TimHuey View Post
Sorry to revive such an old thread but I am also using an Xbox One and having the same problem with the AVS709 WhiteClipping pattern showing all bars as you were experiencing. I'm going to play the pattern from a different player if I can find one on the same HDMI port.
Hi, what TV model do you have?

SDR Movies mastered for REC.709 2.4 gamma and 100 nit (reference/night mode targets), when you set your contrast you have to allow headroom above 100% Reference White (and not to clip), so to be able to see details up to 109% Super White.

You can see why you need to leave 'headroom' there, with some picture examples of 'out of video legal range values' using 'Mission Impossible - Fallout Blu-Ray' movie as example there or some other frame animations of some movies using waveform monitor analysis.

Grade-1 SDR Reference Monitor/TV Calibration Targets

The viewing of accurate images depends on an accurate calibration of the display, control over the viewing environment (lightning and room decor), as well as the appropriate placement of the observer relative to the screen. (distance per content resolution and viewing angle).

The Reference Viewing Environment can be considered where color critical decisions are made, while the Home Viewing Environment is where finalized deliverables are viewed, with the intent to best match the original artistic intent, as defined by the director and colorist within the Reference Viewing Environment.



The goad of home TV calibration is the Home Viewing Environment to match the Reference Viewing Environment image.

Within the professional industries, reference monitors (Grade-1) are the standard for color critical work.



EBU TECH 3320 (Version 4.1 - September 2019) - User Requirements for Video Monitors in Television Production, defines the technical characteristics for video broadcast monitors used in a professional TV production environment for evaluation and control of the images being produced.

These specifications describing the definition of Grade-1 SDR Reference Monitor with Standard Dynamic Range capabilities.

Grade-1 Monitors are devices for high-grade technical quality evaluation of images at key points in a color grading production workflow.

Grade-1 Monitors are used for critical evaluation during post-production.

As a minimum requirement, these monitors shall have the quality properties of the image system they are used to evaluate.

It's expected that all applied technologies are state-of-the-art at this level as the Grade-1 monitor is a 'measuring instrument' for visual evaluation of image quality.

Grade-1 SDR Reference Monitor should be been calibrated and capable to produce a reference luminance level of 100 cd/m2 (nits) for 100% White (235 level @ 8-bit) patch on the screen.

Automatic Brightness Limiter (ABL) functions shall not be used for Grade-1 SDR Reference Monitors, this means that the monitor need to be capable to display 100 cd/m2 with a full field 100% Reference White pattern also.

100% luminance on the screen corresponds to a 10-bit luma signal of digital level 940, and the black level corresponds to a 10-bit luma signal of digital level 64.

100% luminance on the screen is defined as the luminance of a luma signal of digital level 940, but levels 941 through 1019 should also be correctly displayed.

The highest value of 10-bit luma signal is digital level 1019. The luma level 1019 is called 'Super-White' or '109% White'.


For the luminance gamma characteristic (Electro-Optical Transfer Function) of the screen, its recommended that a nominal value of 2.4 gamma to be used.


Ted's LightSpace CMS Calibration Disk Free Version for Free Calibration Software: LightSpace DPS / CalMAN ColorChecker / HCFR
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post #36 of 38 Old 11-30-2019, 08:13 AM
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Sony BRAVIA A1E OLED.

Every calibrator says the correct final setting on the Sony A1E for contrast is 100%. And that's fine but I'm seeing data above white and I can't get it to stop showing data that is supposed to be above white.

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post #37 of 38 Old 11-30-2019, 08:32 AM
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Originally Posted by TimHuey View Post
Sony BRAVIA A1E OLED.

Every calibrator says the correct final setting on the Sony A1E for contrast is 100%. And that's fine but I'm seeing data above white and I can't get it to stop showing data that is supposed to be above white.
Have you read all the info above?

The point of setting correctly your contrast is to be able to see up to the last flashing bar (253), without any bar to have color-shades, not to clip to 235.

It doesn't matter what any calibration is saying, its more important what is happening to you specific setup which is the results of source-TV combo.
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post #38 of 38 Old 11-30-2019, 09:40 AM
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Quote:
Originally Posted by TimHuey View Post
And that's fine but I'm seeing data above white and I can't get it to stop showing data that is supposed to be above white.
You got it backwards. The objective of setting the optimal Contrast level is to avoid clipping, not to ensure clipping.
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