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post #1 of 13 Old 06-17-2017, 01:42 AM - Thread Starter
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White clipping pattern

Hello,

I am using the AVSHD calibration disc available here AVS HD 709 - Blu-ray, HD DVD, & MP4 Calibration - AVS Forum
on Sony 4k x85c
The black clipping work just fine but I am unable to raise the contrast high enough to set white level so 230-234 or higher flash.

The current setting is cinema pro preset with DCI color space

I am using htpc using intel graphic cards and I increase the contrast level on the graphic card setting from 50 to 51.
Then the white clipping works but cannot see the four distinct color on the attachment picture
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post #2 of 13 Old 06-17-2017, 02:33 AM
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Quote:
Originally Posted by tomyan112 View Post
Hello,

I am using the AVSHD calibration disc available here AVS HD 709 - Blu-ray, HD DVD, & MP4 Calibration - AVS Forum
on Sony 4k x85c
The black clipping work just fine but I am unable to raise the contrast high enough to set white level so 230-234 or higher flash.

The current setting is cinema pro preset with DCI color space

I am using htpc using intel graphic cards and I increase the contrast level on the graphic card setting from 50 to 51.
Then the white clipping works but cannot see the four distinct color on the attachment picture
When you are using video file patterns created for Video (TV Legal Levels aka 16-235) using a PC software media player, the software player it expanding the Video Levels to PC Levels (16-235 -> 0-255) by default.

If it will not expand then you will see the Black as 6.3% Gray; since Black @ Data Levels (PC) is 0....while @ Video (TV Legal) Levels is 16.

But when a software player will expand from 16-235 to 0-255 then all information 1-16 becomes Black 0 and all information 234-254 becomes 255. This is the reason any pattern with flashing bars with below black 1-15 or above reference white (236-254) will clip (not flashing).

If you will try to set your brightness level adjusting brightness (increasing) until you see the 17 bar to barely flash, because you can't see what is happening to below 16, there is very possible to increase your black level, so your black will be lifted and you will loose contrast ratio due to poorer (lifted) black performance.

You can configure your PC software player to don't expand the Video to PC levels, set your VGA to output PC levels and your TV/Projector to accept Video Levels, doing this you will able to see all flashing bars to do proper adjustments.

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post #3 of 13 Old 06-17-2017, 05:31 AM
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Quote:
Originally Posted by tomyan112 View Post
Hello,

I am using the AVSHD calibration disc available here AVS HD 709 - Blu-ray, HD DVD, & MP4 Calibration - AVS Forum
on Sony 4k x85c
The black clipping work just fine but I am unable to raise the contrast high enough to set white level so 230-234 or higher flash.

The current setting is cinema pro preset with DCI color space

I am using htpc using intel graphic cards and I increase the contrast level on the graphic card setting from 50 to 51.
Then the white clipping works but cannot see the four distinct color on the attachment picture
Despite the settings that manufactures may place within a display, DCI is not a standard meant for video. For one it has a completely different white point. This standard is only set for movie cinemas.

Rec 709 is presently tho only standard for displays today. Until displays can reach the Rec-2020 parameters, it is nothing more then a goal for the industry to achieve.
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post #4 of 13 Old 06-17-2017, 11:44 AM
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Originally Posted by tomyan112 View Post
The current setting is cinema pro preset with DCI color space
As Randal pointed out, for SDR you aim for REC.709 ColorSpace, it will be a good idea to measure with color gamut patterns (primary/secondary colors) or Saturation Sweeps all available presets of your device (or select REC.709 if it has it there) to see which one is closer to REC.709 and use that one.

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post #5 of 13 Old 06-18-2017, 02:29 AM - Thread Starter
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Originally Posted by ConnecTEDDD View Post
When you are using video file patterns created for Video (TV Legal Levels aka 16-235) using a PC software media player, the software player it expanding the Video Levels to PC Levels (16-235 -> 0-255) by default.

If it will not expand then you will see the Black as 6.3% Gray; since Black @ Data Levels (PC) is 0....while @ Video (TV Legal) Levels is 16.

But when a software player will expand from 16-235 to 0-255 then all information 1-16 becomes Black 0 and all information 234-254 becomes 255. This is the reason any pattern with flashing bars with below black 1-15 or above reference white (236-254) will clip (not flashing).

If you will try to set your brightness level adjusting brightness (increasing) until you see the 17 bar to barely flash, because you can't see what is happening to below 16, there is very possible to increase your black level, so your black will be lifted and you will loose contrast ratio due to poorer (lifted) black performance.

You can configure your PC software player to don't expand the Video to PC levels, set your VGA to output PC levels and your TV/Projector to accept Video Levels, doing this you will able to see all flashing bars to do proper adjustments.
You are right. It is the limited range issue.
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post #6 of 13 Old 06-18-2017, 06:46 AM
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Hi Ted,

I would like you to give your wisdom on a matter. Here it is; "Should contrast be set to 254 with no clipping (as some have suggested), or set to 235 and have this signal clip everything above"?
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post #7 of 13 Old 06-18-2017, 10:18 AM
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Hi Ted,

I would like you to give your wisdom on a matter. Here it is; "Should contrast be set to 254 with no clipping (as some have suggested), or set to 235 and have this signal clip everything above"?
Hi Randal,

I prefer to leave headroom for safety reasons, so not not clip for everything above 235, usually users are checking only Contrast pattern with classic Gray flashing bars, it's better to check WRGBCMY bars flashing and additionally Color Luminance Clipping patterns with RGBCMY Bars. There a lot of times one pattern to be ok and checking the other 2 to see clipping sooner. Having headroom improves all the levels near 100% of the signal also.

Focusing to consumer Blu-Ray Mastering; post production studios are working the movies with peak output 100-120nits with full range (0-255 RGB lets say in 8-bit for example) and when the content is ready they are scalling RGB-Full to RGB-Video....(until now the all levels are inside 16-235..so max level is 235, can't be higher).....but after that the signal is encoded and compressed to delivery using YCbCr (where some pixels of RGB Video can have different levels (higher from legal levels during conversion...even encoder settings can alter the levels) after YCbCr conversion....and when you playback the movie from your player is converting the signal from YCC 4:2:0 -> YCC 4:2:2 or YCC 4:4:4 or RGB-Video and when it enters to your display its converted again (for processing) until it re-converted at the final stage to RGB to go to the panel.

So after all these conversion, add the rounding errors, add player inaccuracies to their output, add player/display to conversions..... maybe the 235 will go to 236 or 237 or 238 etc....or more..

Now all the displays in the market have a lot of higher peak output (300-400nits calibrated easily) from the peak output movies (SDR 100-120nits) has been mastered, you are not reducing the display contrast or peak output when you sacrifice some contrast controls values reduction to avoid clipping (for safety reasons) for above reference white or color luminance...like it was happening before years when we had plasma's (where some of them had lower peak output from 100-120 nits)...when we were clipping to able to get higher calibrated peak output (so higher contrast ratio).

Look this example, I got the change to measure the output of Pioneer BDP-LX91 Blu-Ray Player (2009 model with $2.500 price player) for digital errors.

I used my Blu-Ray (1080p) calibration disk loaded to Pioneer's drive and the CalMAN's ColorChecker Classic (+Primary/Secondary Colors with 100% Saturation / 100% Stimulus Level) Chapter with DVDO AVLab TPG Color Checker function (where it displays the digital level of the selected pixel on screen) to measure it's pattern output for digital errors when you playback a blu-ray.

Here is an example about what errors can introduced during a colorspace conversion (from YCbCr 4:2:0 to RGB-Video):

Since content is 4:2:0, the conversion from 4:2:0 to 4:4:4 requires vertical upsampling and horizontal upsampling. This is usually performed in two steps, from 4:2:0 to 4:2:2 (vertical upsampling) and then 4:2:2 to 4:4:4 (horizontal upsampling) and then to RGB-Video.



As you can see to the 100% Blue, from 235 it became 239, and it was a reference player (inside to the TV also, errors can be added), there a lot of that kind of colorspace conversions or internal processing that can add errors to the signal and alter the levels, this is why it's important to calibrate from your player, to be able to measure and fix any errors introduced during colorspace conversions, to have the whole video chain calibrated.
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post #8 of 13 Old 06-19-2017, 08:14 AM
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Hi Ted,

Thanks for the great info.

The setting of contrast seems to be a mystery to some on the forums. It seems that their only reference as to how to set the contrast level is what is described in the documentation in the AVSHD-709 manual. Even some commercial establishments have adopted this method. The question that comes to mind is, "Is it the correct method"? I posed this question to you because I respect the knowledge that you have regarding video calibration and I was hoping to clarify some issues through this interchange.

Since REC-2020 standards have not been met (with the exception for a P3 laser projector), to date I am going to restrict my comments to the realm of the REC-709 HD standards.

I agree a 100% that the mastering of video content is done in a full swing format and once the content has been edited and color corrected, it is then converted to a studio swing format. One could write a number of chapters explaining the reasons why and the benefits of the procedure. Blu-Ray mastering of video content for commercial home viewing focuses at the level of 75%~90% where this allow sufficient room for highlights. Video content that exceeds the safe values are rejected and reedited. See image "Safe Level" and note the Red arrows illustrating where errors occur. The image "Video Scale" illustrates the breakdown of the various levels that are expressed within the industry. In your example the video content was not within the norm as almost never is the video content presented at the full 100% level but your results are interesting. I am not familiar with the video equipment DVDO AVLab TPG as I use the AccuPel 5000 and will soon to be using the 6000 version in a week's time. My latest piece of equipment is the Tektronix WFM5250 waveform monitor.

The illustrations that you presented express the issues of color gamut errors. Since the color-spaces of the RGB is smaller then the Y'CrCb color-space this is not an uncommon issue where illegal colors are generated. Such an errors are basically invisible to the the average viewer. These errors are not restricted to the consumer level equipment as they also occur in the professional equipment.

Calibrating the DVD player in not a bad practice but should not the display be calibrated first? Here is a logic loop or a paradox for you, if your display is not calibrated, then what source equipment are you using to calibrate and determining the proper setting for the DVD player? If you are using a DVD player as a video source for calibrating the display then how do you know if the display is correct as there maybe errors within the DVD settings? I am interested in hearing any solutions that you might have regarding this loop.

Based on the AVSHD-709 manual (which is very ambiguous), the flashing bars should be seen beyond the "White Reference" point up to 254 by some. Despite what some may say, there is no video content worth seeing (if any), above 235 (see image "Safe Level"). Then there are those who feel the proper setting is that there is to be no bars flashing above the 235 White Reference point (see image "Contrast Level"). So in order to see any flashing bars above the reference point are we not retarding of or diminishing the contrast ratio to try and view something that is not there? This is the question that I was posing to you. This subject is about setting the Y' or the proper Luma setting; since this is what the Contrast & Brightness adjustment is all about. The Chroma portion really does not come into play when setting contrast.

They say that the setting of Contrast & Brightness is one of the most important adjustment one can do for a good video image. Do you feel that many lack the basic understanding of what a video signal is and how it relates to the display? One could say that calibration is the setting up of the display to render the proper image based in the incoming video signal.

I am very interested in your point of view and thank you in advance.
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Last edited by randal_r; 06-19-2017 at 01:45 PM.
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post #9 of 13 Old 06-20-2017, 06:19 AM
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Quote:
Originally Posted by randal_r View Post
Hi Ted,

Thanks for the great info.

The setting of contrast seems to be a mystery to some on the forums. It seems that their only reference as to how to set the contrast level is what is described in the documentation in the AVSHD-709 manual. Even some commercial establishments have adopted this method. The question that comes to mind is, "Is it the correct method"? I posed this question to you because I respect the knowledge that you have regarding video calibration and I was hoping to clarify some issues through this interchange.
Hi Randal,

Here are the instructions; as reference; by THX Level II:



and by ISF Level II:



...they both agree to leave headroom and not clip to 235.

Setting Contrast slider affects Color Luminance also, so a lot of times a few clicks or Contrast reduction is reducing the clip at Color Clipping Pattern also.

I have done before 5 years a lot of tests about Contrast Settings and how affect display response (beyond flashing bars visual verification) using LightSpace CMS and Quick Profing measurement run which generates a unique 'RGB Separation' Charts (which is not like the classic RGB Balance chart we know from other software) where it compares with simple words the Luminance or 21-Point Grayscale with 20-Point Luminance or Red, Green, Blue patches; so you can see there how Contrast setting effects the display performance (cross-talk between color channels/color clipping/linearity) and it's a good tool just to test output settings of a player also to see how they affect the output signal (compare RGB-Video vs. YCbCb 4:4:4)... one example it's coming to my mind is the PS3 which while all sites recommend to use YCbCr for BD playback, because RGB-Video output will clip everything outside 16-235, you will see that YCbCr output of PS3, for Green Channel, it's reducing a lot the Luminance of all 20-Point Green Luminance patches while the RGB-Video it has perfect output. For example when you display a 50% Gray pattern, if you calculate how many nits it's outputting the Green sub-pixel, when you display a 50% Green Luminance patch, (in an ideal display) the Y of Green need to have the same Luminance as reported from 50% Gray (Green only).

Not only the player is responsible about this, it's the combination of the display with specific player, because there colorspace conversions which are performed inside to the display when you send specific signal to TV's Input. This is why you have to test the complete video signal chain and not use just a reference pattern generator.

Quote:
Originally Posted by randal_r View Post
Since REC-2020 standards have not been met (with the exception for a P3 laser projector), to date I am going to restrict my comments to the realm of the REC-709 HD standards.

I agree a 100% that the mastering of video content is done in a full swing format and once the content has been edited and color corrected, it is then converted to a studio swing format. One could write a number of chapters explaining the reasons why and the benefits of the procedure. Blu-Ray mastering of video content for commercial home viewing focuses at the level of 75%~90% where this allow sufficient room for highlights. Video content that exceeds the safe values are rejected and reedited. See image "Safe Level" and note the Red arrows illustrating where errors occur. The image "Video Scale" illustrates the breakdown of the various levels that are expressed within the industry.
Error are introduced from multiple colorspace conversion until the signal will go to the display, with REC709 SDR while the issue is know, it's not so dramatically changing the digital levels as in REC2020 and HDR (between 3.2 to 6.3 dE 2000 range) where the issue is very big with HDR10 but being ignored a lot...sadly..... Dolby Vision is coming with better approach which minimize the errors introduced:

Report ITU-R BT.2390-0 (Page 34 of 40): https://www.itu.int/dms_pub/itu-r/op...2016-PDF-E.pdf

Or Dolby's Publications:

Page 7 or 16: https://www.dolby.com/us/en/technolo...hite-paper.pdf

or with nice picture example here: https://www.dolby.com/us/en/technolo...ubsampling.pdf

Quote:
Originally Posted by randal_r View Post
In your example the video content was not within the norm as almost never is the video content presented at the full 100% level but your results are interesting. I am not familiar with the video equipment DVDO AVLab TPG as I use the AccuPel 5000 and will soon to be using the 6000 version in a week's time. My latest piece of equipment is the Tektronix WFM5250 waveform monitor.
Each player is doing difference processing, for example OPPO 203 is more accurate at these colorspace conversions but consumer don't have such tools at their home to check the output levels of their player.

About digital level testing, I used 'DVDO AVLab TPG Color Checker function (where it displays the digital level of the selected pixel on screen:



The cursor box provides key information about the current color format as well as the specific color values for the currently selected pixel.

For more details:

About Bit Depth; Color Checker can recognize 8, 10, or 12 bits per pixel. Upon receiving a video stream with properly formatted info frames and video information, the bit depth indicator will change to indicate the number of bits per pixel. The number of digits in the triplet values will also reflect this.
For 8 bits per pixel, the range will be 0-255 if displaying the value in decimal or 0-FF if displaying in hex.

About Decimal/Hex; For user convenience, the display can be adjusted to show the triplet values in either decimal or hex format. This is only a display setting and does not affect the video data.

About RGB/YUV; This indicates the current color space sent by the source. Note that this is derived from the info frame and indicates to a sink how to interpret the triplet information. An R in this space indicates RGB, and a Y indicates YCbCr. In this case, it tells the user how to interpret the triplet information.
Triplet. The actual color triplet information always consists of three values, either R,G,B or Y,Cb,Cr.

This is a useful function to check output levels of a player; Dolby Monitor has as cursor mode also; and your Accupel 6000 have it in HDR mode additionally (if you got the PRO Engineering version of the 6000).

Tektronix WFM5250 waveform monitor is amazing peace of gear, at what applications it will be useful for you? It's ultra expensive kind of hardware but not able to except HDR signal which is something interesting these years as a feature, to able to check HDR10 or DV signal. You may trick it to input 10bit 2160p using HD Fury Linker to cheat that you have connected an HDR capable display to make the player lock to REC.2020 2160p HDR output (to remove the metadata, downgrade HDCP 2.2 to 1.4) for Tektronix to able to read plain 10bit signal but you have to ask Tektronix to release a firmware that it will include REC.2020 support because REC2020 RGB <->YUV has different matrix coefficients from REC709 RGB <->YUV.

Quote:
Originally Posted by randal_r View Post
The illustrations that you presented express the issues of color gamut errors. Since the color-spaces of the RGB is smaller then the Y'CrCb color-space this is not an uncommon issue where illegal colors are generated. Such an errors are basically invisible to the the average viewer. These errors are not restricted to the consumer level equipment as they also occur in the professional equipment.
If the content was delivered in discs in RGB-Video video then settings Contrast and letting more bars flashing above 235 it was unnecessary but since there involved multiple conversions from RGB -> YCbCr -> RGB (since YCbCr is larger colorspace form RGB) many YCbCr color values are mapping RGB values out of 16-235 range, this is why you need to leave some headroom to prevent clipping issues.

Quote:
Originally Posted by randal_r View Post
Calibrating the DVD player in not a bad practice but should not the display be calibrated first? Here is a logic loop for you, if your display is not calibrated, then what source equipment are you using to calibrate and determining the proper setting for the DVD player? If you are using a DVD player as a video source for calibrating the display then how do you know if the display is correct as there maybe errors within the DVD settings? I am interested in hearing any solutions that you might have regarding this loop.
From what point of view you are looking the subject? 1) from calibrator point of view who has reference pattern generator, or a video enthusiast who has bought for his setup an external pattern generator....so we are talking for less than 10.000 people in the world), or 2) for a consumer without external pattern generator, which has see that is simple to use his notebook output or a FireStick or a ChromaCast or his any Apple device or any Android device..as a pattern generator but has no idea/know if these output solutions are bit-perfect....so we are talking for some million of people who have experienced any calibration DIY procedure. For these people who don't have something which is known to be a reference then it's a waste or time (unless they test if there matching of the output of their actual movie playback device with the patch generation solution they are using, and for these people I have quite some hundred emails about problems with colors etc. related with this mismatch.

It's not that faster when you have to measure 10 or 20 point grayscale using an external pattern generator vs. a calibration disk, if you are using the same meter, to do manual adjustments from the display menus, nowdays it's takes more time to do manual adjustments from internal menus until to find the proper menu to change settings, than measuring patches.

If you calibrate the display only with extrernal pattern generator, and when you measure from the customer source you will see difference then it will need to re-adjust your settings because the customer will use that player to playback the movies, so this is matters, you can't say to him that your player is not perfect, go out and buy an another one (which is unknown if it's better or worse from current one).

At past Tom Huffman had compared some reference patterns generators and found differences between them, so even devices which designed to be accurate, they have differences, so all these come back to calibration from the source and manual way (or Semi-Automated / Automated with my calibration disk, released for that reason before 4 years)

Using the AccuPel as the reference, Tom found the following (all dE values are in CIE94 units):

Lumagen Mini
Ave. grayscale difference: 0.8
Ave. gamma difference: 0.04
Ave. color difference: 0.7

Quantum Data 804a
Ave. grayscale difference: 0.8
Ave. gamma difference: 0.03
Ave. color difference: 0.5

Samsung Blu-ray
Ave. grayscale difference: 0.7
Ave. gamma difference: 0.01
Ave. color difference: n/a

Toms Test Link

Calibrating using a Calibration Disk you playback from the exact Blu-Ray/Media Player you will use later to watch your movies is still most accurate way of calibrating a consumer setup, calibrating and correcting errors to the whole video chain.

Here is an another example of comparing a Reference Pattern Generator - Quantum Data 780 (6.000$) with our 'Reference Blu-Ray Players' OPPO BDP-105 & BDP-103

The difference is obviously present @ Gamma/RGB Balance graphs and visually by looking the reference pictures.

Quote:
Originally Posted by randal_r View Post
Based on the AVSHD-709 manual (which is very ambiguous), the flashing bars should be seen beyond the "White Reference" point up to 254 by some. Despite what some may say, there is no video content worth seeing (if any), above 235 (see image "Safe Level"). Then there are those who feel the proper setting is that there is to be no bars flashing above the 235 White Reference point (see image "Contrast Level"). So in order to see any flashing bars above the reference point are we not retarding of or diminishing the contrast ratio to try and view something that is not there? This is the question that I was posing to you. This subject is about setting the Y' or the proper Luma setting; since this is what the Contrast & Brightness adjustment is all about. The Chroma portion really does not come into play when setting contrast.

They say that the setting of Contrast & Brightness is one of the most important adjustment one can do for a good video image. Do you feel that many lack the basic understanding of what a video signal is and how it relates to the display? One could say that calibration is the setting up of the display to render the proper image based in the incoming video signal.

I am very interested in your point of view and thank you in advance.
After setting picture size and Sharpness (not correct sharpness can affect the outline of the flashing bars to may appear that are flashing), the initial adjustment which is important is Contrast/Brightness, so having this right, you move to classic calibration or 3D LUT.

You are not reducing contrast when you will have Contrast and Color Clipping pattern flashing bars above 235 until 253 flashing with modern displays, unless you have a Panasonic Plasma which can do 80 nits max for example, at this case you sacrifice and clip to see more calibrated nits output. No with projector which can output 100-200nits and displays which can do in SDR 300-500nits, you are not loosing contrast when you calibrate for 100-120 nits target for displays or 48nits for projectors, you just are safer from unexpected clipping that can appear to specific pixels it's difficult to measure; errors will be introduced from video signal multiple processing/conversions.
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post #10 of 13 Old 06-23-2017, 01:38 PM
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Hi Ted,

Sorry for not responding back to your post sooner but I was waiting for a reply from a third party before replying.

Like most postings within many threads, things tend to go off on tangents and the original subject gets lost. The subject is; "Should contrast be set to 254 with no clipping (as some have suggested), or set to 235 and have this signal clip everything above"?

Your personal preference of allowing headroom for safety reasons to avoid clipping is interesting. The subject of conversion from RGB > Y'CrCb > RGB issues is nothing more than Color Gamut Errors but really has nothing to do with the white clipping subject.

It was my hope to deal in the realm of the standard that exist today the REC-709. You tend to bring up the REC-2020 standard which again has nothing to do with the subject. The PDF that you linked was an interesting re-read but please note the title; "High Dynamic Range Television for Production and International Program Exchange". Is this document for the end user or for the individual in the post production and broadcasting stations? There are no displays that reaches the REC-2020 standard as I write this. I submit an image from the rting website showing the attributes of the displays as they exist. Displays are still evolving and are in a state of flux and making references to things that are still somewhat theoretical is of no value.

I thank you for the lengthly reference on the subject of hardware that you forwarded but again it too does not relate to the subject.

The copy of the slide from the THX's first day lectures that you forwarded in your last posting is damaging to their credibility. In rebuttal I offer a slide from ISF's first day lecture (see image).

The standards that exist, exist for a reason but unfortunately there is still a lot of misinformation that is still being passed around which misleads the masses. The ITU documentation that relates to this subject are BT-814 & BT-815. No where in these documents will you find any instructions or suggestions that imply to set the contrast above 235. The standard for 8 bit black is 16 (0mv) and 8 bit white is 235 (700mv). Despite what some imply, white is not 254.

As I mentioned before, I had sought information from a third party source. This source was Steve at Light Illusion. Here is a link to his views on the subject (LINK). I also submit an email response from Steve on the subject.

There is volumes of information supporting the 16~235 format as the standard. The super white format which is commonly used in gaming hardware is not really utilized in broadcasting or recorded media such as DVDs. In all the research that I have done, this misinformation to set the contrast above 235 stems from the manual from the AVSHD-709 manual. This document is ambiguous regarding "White Clipping". It would have been better if it had been proof read first before distributing to the masses.

I feel that many desire to learn how to make adjustments to a display but lack the desire to learn the more obscure ares of the video realm such as what comprises a video signal and how they interact with each other and how it relates to the display. If one does not understand how a video signal works and how the displays processes, how can one calibrate without this underlying knowledge.
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post #11 of 13 Old 06-23-2017, 04:16 PM
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Quote:
Originally Posted by randal_r View Post
Hi Ted,

Sorry for not responding back to your post sooner but I was waiting for a reply from a third party before replying.

Like most postings within many threads, things tend to go off on tangents and the original subject gets lost. The subject is; "Should contrast be set to 254 with no clipping (as some have suggested), or set to 235 and have this signal clip everything above"?

Your personal preference of allowing headroom for safety reasons to avoid clipping is interesting. The subject of conversion from RGB > Y'CrCb > RGB issues is nothing more than Color Gamut Errors but really has nothing to do with the white clipping subject.
Hi Randal,

There no reference paper which say clip to 235, all available papers saying that 235 is 100% White but they are talking when you are transmitting TV-Legal signal (without counting multiple colorspace convesions).

If you read again my earlier post all words are deadly centered to the topic because the conversion of RGB -> YCbCr -> RGB is the only reason which it's required to not clipping to 235; because the conversions produce out of TV legal range values....and the example about REC2020 I posted, I did it to see how much the signal is altered from compression and colorspace conversions.

Stacey Spears has posted a lot of picture examples with still images where the YCbCr conversion to RGB is producing levels beyond 235, up to 254:

"High Definition Benchmark" BD Edition by Stacey Spears and Don Munsil
"High Definition Benchmark" BD Edition by Stacey Spears and Don Munsil

Stacey has posted also here:

''Neither PC or VIDEO color conversion will restrict YCbCr 16-235 into RGB 16-235.

Before anyone says something about Car's being an animation, I can post a frame from any HD title and show the same thing. An example I gave in the other thread was: Y 140, Cb 161 and Cb 202 would convert to R 254, G 100, B 200 using the BT.709 color matrix. If you used PC levels, which expand, you would get R 277, G 98 and B 214. Since we don't go above 255, R would get clipped to 255.''


The fact is that ISF and THX both agree to no not clip and leave headroom.

Even Michael Chen has posted about this here:

''In the ISF and THX classes, people are taught to keep all detail to 254 ...

Not sure where people are taught to clip above 235 ... certainly not in the professional end of things.

Michael Chen @ The Laser Video Experience
ISF/THX/TLV Video Instructor
The Video Calibration Education Hub


Quote:
Originally Posted by randal_r View Post
It was my hope to deal in the realm of the standard that exist today the REC-709. You tend to bring up the REC-2020 standard which again has nothing to do with the subject. The PDF that you linked was an interesting re-read but please note the title; "High Dynamic Range Television for Production and International Program Exchange". Is this document for the end user or for the individual in the post production and broadcasting stations? There are no displays that reaches the REC-2020 standard as I write this. I submit an image from the rting website showing the attributes of the displays as they exist. Displays are still evolving and are in a state of flux and making references to things that are still somewhat theoretical is of no value.
As the problem above with REC.709 exist, the REC.2020 into conversation came to show to you with data from ITU-R and Dolby how much bigger is the issue with REC.2020

The ''High Dynamic Range Television for Production and International Program Exchange'' I linked is a report/study of ITU-R. ITU-R is releasing the specifications (ITU-R BT.2020, ITU-R BT.1886, Rec. ITU-R BT.709-5 etc.) There no publication from ITU-R for consumers all are related to pro-area.

Your posting of gamut coverage with current display don't change the fact that REC2020 signal is altered because it will be altered even with colors inside REC709 (or any luminance or saturation level), it's not affecting only the edge of REC.2020 colors.

Quote:
Originally Posted by randal_r View Post
I thank you for the lengthy reference on the subject of hardware that you forwarded but again it too does not relate to the subject.
I posted because you wasn't familiar about DVDO and how it can be used to measure the digital levels, this is why I explained

Quote:
Originally Posted by randal_r View Post
The copy of the slide from the THX's first day lectures that you forwarded in your last posting is damaging to their credibility. In rebuttal I offer a slide from ISF's first day lecture (see image).
The page of ISF you posted is one of the initial pages, if you scroll down to further pages you will see that picture from ISF I posted. I have the page you posted also, I didn't posted because it's doesn't say something useful, all we know that reference white is 235.

Quote:
Originally Posted by randal_r View Post
The standards that exist, exist for a reason but unfortunately there is still a lot of misinformation that is still being passed around which misleads the masses. The ITU documentation that relates to this subject are BT-814 & BT-815. No where in these documents will you find any instructions or suggestions that imply to set the contrast above 235. The standard for 8 bit black is 16 (0mv) and 8 bit white is 235 (700mv). Despite what some imply, white is not 254.
There no reference in the world which say to clip to 235 and to talk about what to do when you have a source a BD-Movie content. These BT-814/815 are saying something it's know for many years (since are very old documents before 25 years) about 235 is 100% White. There no reason to use 25 year old documents are reference since we are talking for REC.709, better to talk for latest revision or REc.709 which is from 2015: https://www.itu.int/dms_pubrec/itu-r...6-I!!PDF-E.pdf

There no reason in a digital world to talk about analog signal voltage (mV) measurements and IRE, everything is digital now.
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post #12 of 13 Old 06-29-2017, 09:25 AM
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Hi Ted,

I have been out of town for a couple of days and was not able to reply to your posting. So let’s begin.

Lets address your statements;
There no reference paper which say clip to 235, all available papers saying that 235 is 100% White but they are talking when you are transmitting TV-Legal signal (without counting multiple colorspace convesions).”


I really do not understand why you would make such a statement. Ever since the early 1950’s the conversion process for television transmitting has been in existence. This was done as a compression of the video signal for the purpose of lowering the broadband width of the television signal. This process eliminated the Green channel and then transmitted the Luma and the Red and Blue channels thereby reducing the signal size. In North America it is commonly known as Y’PrPb. Before that the Luma portion was only used. Even at this point the practice of Contrast and Brightness existed. It was found that if the contrast was set higher than Blooming and audio noise issues would ocurr, to state a couple. The industry settled on a limit range of 235 which resolved this issue. Some would say that this practice is redundant with the technological advances in modern displays but lets take a deeper look. In the USA alone more than 25% of the household still have a functioning CRT display.

You make the statements;
There no reference paper which say clip to 235”.

These BT-814/815 are saying something it's know for many years (since are very old documents before 25 years) about 235 is 100% White. There no reason to use 25 year old documents are reference since we are talking for REC.709, better to talk for latest revision or REc.709 which is from 2015: https://www.itu.int/dms_pubrec/itu-r...6-I!!PDF-E.pdf

There is no “Dummy’s Guide” or “Idiot's Guide” when it comes to reference papers. These papers do not hold your hand, they state the reference parameters and standards and very little else. The wording is carefully chosen to allow for slight deviances but the intent is clear.

I feel we can both agree that the ITU organization has a reputation sufficient to categorize it as an excellent source to reflect the standards. Many of the documentations that they publish are for the industry but there are the occasional pieces the do deal with the home end user. Since you have quoted the ITU as a reference in you pass postings, we will utilize them as the standard for this discussion.

You say that the BT814/815 is an old document; this I agree. They many be of an age but they are “Tired, Tested and True”, and they are still applicable today. You were kind enough to forward a document from ITU on the REC BT-709-6 and wish to utilize it since it is the “latest version”. Lets review this document with regards to the subject of “White Clipping and the setting of Contrast & Brightness.

On page #5 of the document (see image 1), I would like you to note the values for the Black level “RGBY” and the Nomial Peak (White Level), “RGBY”. Note that they are 16 and 235 respectively. An individual from SpectraCal once tried to imply that this was wrong because the “Video Data” states 254 and the CrCb goes up to 240. Such statement are easy and commonly made when an individual does not have the basic concept of the structure of a video signal.

Let us turn to page #3 (see image 2), and let us take note of the footnote. It mentions two other documents BT-1886 and BT-2035. The footnote is self-evident and I really don’t think we need to debate the meaning of it. In BT-1886 (see image 3), we find on page #2, that this document states that Black Level = 64 (16 – 8 bit), and the White Level – 940 (235 – 8 bit).

As for BT_2035 (see image 4), where this is applicable to home viewing, turn to page #4. In subsection 3.2, the reference settings for the Black and the White levels are clearly stated.

It does not take much to conclude that the reference Black is to be 16 and the White is 235. As for BT-814/815, despite the age of the documents they still seem to have their relevance and are still called on for the setting up of a display. If you search the ITU database they do label documents as being obsolete but these one are still active.

Why do you keep making references to BT-2020? This is a standard that has yet to be achieved. To state conditions that might exist in the future is still under the theoretical category. The concepts and possible implementation of the procedures that Dolby may do; it to is still theoretical. This is why I suggest we restrict the discussion to the BT-709 as it has been obtained and is the present the standard that is widely implemented.

I reviewed the links that you forwarded regarding Stacey Spears. In the first link, he suggests an error, I have issues with this. On the Histograms I can not see any points of references or scaling system. How do we know the degree of the alleged issue?

In the second link, the illustrations of the warnings in “Pink” and “Green” again are ambiguous. The images illustrate an condition that the signal is outside of the parameters. The same effect can be reproduced via a waveform monitor and setting the parameters for white to 233 or 234 and for black 17 or 18. Even though the video signal is fine an alarm will still be generated. By no means am I saying that this is what Stacey did but without addition support information everything is subject to questioning. Where is the screen showing the parameter settings?

Since your stance is the raise the white level for safety reasons as stated in your past posting, are you also going to lower the black level for the same reason due to what Stacey is suggesting?

Regarding the slide from ISF (see image 5). If memory serves me, this slide was used to convey the parameters and attributes of a PLUGE test pattern; not what you are implying.

With regards to Michael Chen. I am very well acquainted with his view points regarding the setting of contrast and brightness. A number of years ago, I flew to Calgary, Alberta Canada and took a three day personal (one on one), training course that he was offering. The fact that I completed the course dose not mean I adopted his view points.

Through a little research, I had narrowed is down to the culprit to be the AVSHD-709. No other document or PLUGE pattern at the time ever suggested to raise the white level beyond 235. I applaud the two amateurs that put together the package but they failed to adhere to the known standards and implied misinformation that has entered into the realm of tradition. Even when someone learns the truth is almost impossible to put tradition behind you.

Let look at their 3 Rules of setting contrast;

Clipping: What this thread is all about. This document is the first to suggest setting the white level higher to “make sure the entire signal reaches the display”. If they understood the structure and the workings of a video signal they would have never even suggest this.

Discoloration: Again, this is the wrong course of action. Ask yourself, “Why is there discoloration”? If you do have discoloration, note the color that is bleeding into the grayscale ramp. Now run a 10 or 20 grayscale test and note to see if that color in question is unbalanced to the other two color channels They say a picture is worth a thousand word, so here is one (see image 6).

Eye Fatigue: This is why there are two settings such as “Night Mode” & “Day Mode”; use them. The Contrast control should be not used as a volume control except under certain conditions. Michael use an example of someone who maybe drunk of one drugs. It is not a calibrator’s job to evaluate a clients present or future state of being for eye fatigue. A calibrator’s job is to get the best image possible and in most cases setting to the standards is more than sufficient.

Lastly, your statement;
There no reason in a digital world to talk about analog signal voltage (mV) measurements and IRE, everything is digital now.”

With the exception of an image that was forwarded to you to illustrate “Hot Spots”, I have kept my references to the digital realm. If you are referring to the usage of “mV”, then you need to realize that this is a scale is used by the industry and that is still in wide use. The fact is that a video signal is 1V p-p. This can be said for analog ,as well as, digital. This is the nature of the beast. Try reviewing the Tektronix PDF’s and you will note numerous usages of the voltage scale.

I do enjoy these debates and I thank you for them.
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Last edited by randal_r; 06-30-2017 at 05:40 AM.
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post #13 of 13 Old 07-05-2017, 07:07 AM
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Hi Ted,

I noticed that you did not reply. I was not sure if the conversation had come to it's conclusion.

Here is a LINK to an establishment that calibrates and evaluates displays. The link is based on "How to calibrate", and I took note of the parameters that they state that constitutes proper contrast. They state "Just lower the contrast setting on your TV to the minimum, and then increase until only lines 230-234 appear".

In the past I have forwarded to you information from a German company "Burosch". One can easily find their youtube video for setting contrast. Again, no bars above 234. Since I do not wish to get into lengthly listings of institutions that all state the same parameters for contrast lastly, "Light Illusions" (which you are familiar with), also states the same, "All transmitted video, and DVD/BluRay is 16-235, and nothing else".

So if all of these institutions all say the same on the subject of the proper way to set contrast then why do you think some would say something different?

I am interested in your opinion.
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