Adjust Bias/Gain up for grayscale calibration? - AVS Forum
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post #1 of 25 Old 10-08-2012, 09:00 PM - Thread Starter
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Hello all,

I have been fussing with display calibration a bit now but am confused about something related to the grayscale calibration. I have a Sharp 732u that has two point grayscale adjustment capability. Since I am in a dark environment, I am using a gamma target of 2.4 and have already adjusted brightness and contrast so they behave well on ALL test patterns (found I had to turn contrast down a bit as I was crushing some colors). So here is my problem, if I start from zero (knowing that I will not adjust green since that basically adjusts contrast) I find that both my red and blue colors are low compared to the absolute target value. Therefore, I added to the bias/gain for the low and high end which brought them really nicely in line with green with all three colors having a very small divergence from nominal by about 2%. Thats all great. However, I read in a few places that you should actually never add to the grayscale adjustment knobs - only turn them down. Could someone explain the reasoning behind this? If this is the case, I really don't know what else I could do - as going for this particular gamma target and working with the acceptable contrast really only leaves me that starting point - unless I am missing something? Any help or clarity here would be greatly appreciated, I just want to make sure I am not messing anything up! Thanks.
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post #2 of 25 Old 10-08-2012, 10:07 PM
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Your misunderstanding is thinking that green is the contrast control.

All three channels are individual contrast control.

If you are close to clipping individual channels, the gains are the individual contrast controls for those channels. If red is the lowest channel, but it's not clipping yet, feel free to turn it up a few clicks.

In the end all that's important is getting to D65 and not clipping. Whether that's at contrast 98 with Gains - R +2, G -0, B -2, or contrast 100 R 0, G -2, B -4 doesn't really matter.

Also you absolutely can go back and double check the contrast setting after doing the gains to see if you can notch contrast back up a click or two after turning down the gains. Then after that you can check the gains again to make sure white didn't shift too much.

Calibration can be quite the iterative process.

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post #3 of 25 Old 10-09-2012, 07:57 AM - Thread Starter
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Originally Posted by sotti View Post

Your misunderstanding is thinking that green is the contrast control.
All three channels are individual contrast control.
If you are close to clipping individual channels, the gains are the individual contrast controls for those channels. If red is the lowest channel, but it's not clipping yet, feel free to turn it up a few clicks.
In the end all that's important is getting to D65 and not clipping. Whether that's at contrast 98 with Gains - R +2, G -0, B -2, or contrast 100 R 0, G -2, B -4 doesn't really matter.
Also you absolutely can go back and double check the contrast setting after doing the gains to see if you can notch contrast back up a click or two after turning down the gains. Then after that you can check the gains again to make sure white didn't shift too much.
Calibration can be quite the iterative process.

Sotti,

Thanks for the response - this does make sense to me. I guess what I was getting at with green is it is by far the biggest component of contrast (like 70 something percent of white) so I understand the desire to not to mess with it, especially since every time I do it just seems to move the RGB values up or down but it doesn't really seem to help bring anything in line in my experience.

As far as bumping up the gains is concerned, if I understand you right there is nothing really "bad" about bumping up red or blue to get close to the absolute target line - you just have to be aware of the fact that this too will effect your contrast to a degree (especially with those individual colors) and you should go back and double check white, black, and color clipping patterns to make sure all is still well. Does that sound about right?
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post #4 of 25 Old 10-09-2012, 08:14 AM
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Originally Posted by mjodotcom View Post

Sotti,
Thanks for the response - this does make sense to me. I guess what I was getting at with green is it is by far the biggest component of contrast (like 70 something percent of white) so I understand the desire to not to mess with it,

No Green is the biggest factor in luminance.
Your contrast setting is equal parts red, green and blue.
If blue is close to clipping and green is far away, turning up contrast till green is close will cause blue to clip and cause a color shift.

When calibrating gains/contrast one channel should be just at clipping and the other two will be below clipping. That is the ONLY configuration that maximizes contrast.

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Originally Posted by mjodotcom View Post

As far as bumping up the gains is concerned, if I understand you right there is nothing really "bad" about bumping up red or blue to get close to the absolute target line - you just have to be aware of the fact that this too will effect your contrast to a degree (especially with those individual colors) and you should go back and double check white, black, and color clipping patterns to make sure all is still well. Does that sound about right?

Yes that sounds about right.

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post #5 of 25 Old 10-09-2012, 09:12 AM - Thread Starter
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Originally Posted by sotti View Post

No Green is the biggest factor in luminance.
Your contrast setting is equal parts red, green and blue.
If blue is close to clipping and green is far away, turning up contrast till green is close will cause blue to clip and cause a color shift.
When calibrating gains/contrast one channel should be just at clipping and the other two will be below clipping. That is the ONLY configuration that maximizes contrast.

I see, I did notice that tweaking green bumps up/down luminance big time but had always heard it was because you are changing the major component of contrast - good to know!

For calibrating contrast what I have found is the first color I clip is actually cyan, followed by green (no real issues with above white). Therefore, I have my contrast set up so I just don't clip cyan on the color step test patterns and have a little bit of headroom yet to green on the step and flashing patterns. Red and blue seem just fine so I guess no worries at all bumping them up then? Does that sound reasonable?

Taking this to the opposite end of the spectrum, what issues if any should I watch out for when also bumping up blue and red on the low end - just clipping or crushing of blacks?
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post #6 of 25 Old 10-09-2012, 09:53 AM
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Quote:
Originally Posted by sotti View Post

In the end all that's important is getting to D65 and not clipping. Whether that's at contrast 98 with Gains - R +2, G -0, B -2, or contrast 100 R 0, G -2, B -4 doesn't really matter.

Exactly, RGB high end controls are RGB contrast controls. I prefer to avoid adjusting green (contrast) and instead use the main contrast control to avoid clipping. Therefore, I use the red and blue contrast controls combined with the main one to calibrate grayscale to D65 and avoid clipping any of the color channels in the process.


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post #7 of 25 Old 10-09-2012, 12:43 PM - Thread Starter
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Originally Posted by PlasmaPZ80U View Post

Exactly, RGB high end controls are RGB contrast controls. I prefer to avoid adjusting green (contrast) and instead use the main contrast control to avoid clipping. Therefore, I use the red and blue contrast controls combined with the main one to calibrate grayscale to D65 and avoid clipping any of the color channels in the process.

So is adjusting the RGB low end controls then almost the same as tweaking brightness? If so, does the same rule apply if I am adding to red and blue on the low end to verify that I am still not crushing or clipping blacks?
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post #8 of 25 Old 10-09-2012, 12:50 PM
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Originally Posted by mjodotcom View Post

So is adjusting the RGB LOW end controls then almost the same as tweaking brightness? If so, does the same rule apply if I am adding to red and blue on the low end to verify that I am still not crushing or clipping blacks?

yes, although in near black, chromaticity mostly goes out the window, so luminance is the dominating concern for 0-5% or so. So in this instance leaving green alone, may be a good rule of thumb.

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post #9 of 25 Old 10-09-2012, 01:06 PM - Thread Starter
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Originally Posted by sotti View Post

yes, although in near black, chromaticity mostly goes out the window, so luminance is the dominating concern for 0-5% or so. So in this instance leaving green alone, may be a good rule of thumb.

Thanks for the "low" correction =)

With luminance being the main concern on the low end then is the only issue to watch out for after giving blue and red a bump up is ensuring that my brightness setting is still appropriate (or if it needs to be turned down a notch accordingly)?
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post #10 of 25 Old 10-09-2012, 01:14 PM
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Originally Posted by mjodotcom View Post

Thanks for the "low" correction =)
With luminance being the main concern on the low end then is the only issue to watch out for after giving blue and red a bump up is ensuring that my brightness setting is still appropriate (or if it needs to be turned down a notch accordingly)?

We'll when you're adjusting low you are probably looking at like 30% or so for trying to get to D65. If brightness is set correctly, then while you do RGB balance at 30% just adjusting red and blue should keep the setting about the same for brightness.

Always a good idea to double check brightness and contrast after doing cuts and gains.

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post #11 of 25 Old 10-09-2012, 01:16 PM - Thread Starter
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Originally Posted by sotti View Post

We'll when you're adjusting low you are probably looking at like 30% or so for trying to get to D65. If brightness is set correctly, then while you do RGB balance at 30% just adjusting red and blue should keep the setting about the same for brightness.
Always a good idea to double check brightness and contrast after doing cuts and gains.

Sure, makes sense. So nothing really major to watch out for when adjusting red and blue up on the low end sounds like.

Thanks a LOT for your help sotti, it has been very much appreciated!
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post #12 of 25 Old 10-12-2012, 04:30 AM
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I have this from Chad B. he wrote it in the forum a while back , the approach that he takes , it's kind of a work flow. Perhaps it might interest you smile.gif MJODOTCOM In this case however the method is to not increase anything for the RGB but to always cut.

Start with TV in movie mode; reset to get to baselines (contrast 95, gamma 0, enhancements off, etc). (Samsung Plasma and Samsung LCD's)

Leave contrast at 95 and gamma at 0. Adjust brightness with a low APL pluge pattern. Adjust backlight for LED and Cell Light for plasma for target luminance.

Adjust 2 pt white balance, using 100% for the high end adjustment and either 20 or 30% (your preference, could also be determined by your meter's low end sensitivity) at the low end. Only cut, do not boost 2 pt WB controls. In other words, all numbers should be at or below 25. You should have 100% pretty close to perfect. Re-check brightness with low APL pluge.

Take a full 10 or 11 step run. If gamma is close to target, then proceed. If not, then select a different gamma setting to find one that's closer, and re-check brightness. Then re-measure. Repeat as necessary. After correct gamma preset is found, re-check 2 pt white balance, then re-check brightness.

Then do the 10 pt adjustment, starting at 100% and working your way down. You should not have to make an adjustment at 100%.

In the 10 pt adjustment, use G to flatten gamma, and match R and B to G.

After that is complete, re-check brightness. If a large correction was necessary at 10%, you may need to re-tweak brightness.

Take another 10 or 11 step measurement. You will find areas that need further improvement, so you will have to repeat until you are as close as you want to be or until you reach the point of diminishing returns.
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post #13 of 25 Old 10-14-2012, 06:32 PM - Thread Starter
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Well right after I think I have it understood you just have to throw a curveball in there don't ya wink.gif

I wonder if this is just following that general rule of thumb but isn't a hard and fast rule as Joel was saying above? The one big curt guide did say to adjust up or down depending on your set. Anyone else want to chime in as to what is wrong with adjusting these values up?

P.S. for what it's worth after recalibrating with bumping these values up some but leaving green alone the set looks better than ever smile.gif
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post #14 of 25 Old 11-13-2012, 10:14 AM - Thread Starter
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Anyone have any further opinions here?
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post #15 of 25 Old 11-13-2012, 11:20 AM
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This page may help: http://www.lightillusion.com/display_calibration.html

RGB Bias and Gain is covered in the paragraph on colour temp.

Steve

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post #16 of 25 Old 11-15-2012, 03:47 PM
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Quote:
Originally Posted by Light Illusion View Post

This page may help: http://www.lightillusion.com/display_calibration.html
RGB Bias and Gain is covered in the paragraph on colour temp.
Steve

Many guides say to adjust greyscale gains/drives first and then bias/cuts but your guide says the opposite.

I just calibrated my GT30 by adjusting Drives first 80% then cuts 30%...

I got great results but now you have me thinking I should try your method to see If I can get things even better.

Samsung 60F8500
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post #17 of 25 Old 11-15-2012, 03:59 PM
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Originally Posted by PeterLewis View Post

Many guides say to adjust greyscale gains/drives first and then bias/cuts but your guide says the opposite.
I just calibrated my GT30 by adjusting Drives first 80% then cuts 30%...
I got great results but now you have me thinking I should try your method to see If I can get things even better.

Both gains and biases effect each other.

To get both right you need to measure both after adjusting either.

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post #18 of 25 Old 08-12-2013, 04:49 PM
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I recently bought a Samsung UN46ES6150 HDTV & initially was not happy with the skin tones as they looked to pink. And sometimes faces looked too green & too red at the same time, but in different areas. IOW forehead may be too red & cheeks too green. And this was after adjusting the basic TV controls using the AVS709 disc test patterns.

I turned down the Contrast to 75 in order to get rid of the pinkish tint in the test pattern.
Brightness at the default 45 looked proper to me.

I did some searching about 2 point white balance & proceeded to adjust the Offsets using a 20% gray scale & the Gains using an 80% gray scale. Here's what I ended up with trying to get the patterns looking like grays without any coloration. This was done by eyeballing it since I don't have any calibration equipment.

R-Offset = 22, G-Offset = 20, B-Offset = 25
R-Gain = 15, G-Gain = 25, B-Gain = 25

From reading this thread it appears I should not have decreased the G-Offset but instead should have increased the R & B, is that correct?

There's mention of clipping which can affect any RGB color. How would I know if an individual color is clipping? Is it safe to presume I would use the AVS709 test pattern which has all the color scales? I know when I started adjusting that a few of the brighter shades of Red all looked the same. I don't recall what they looked like when I got done but will go back & check. That being said if the brighter Reds still all look the same does that mean they are still clipping?

At the moment I am satisfied with the skin tones. But sometimes Red looks over saturated. For example a bright red sweater may have minimal details & almost look solid red. I would like to have some calibration equipment but that's just not in the budget at this time.

Any helpful hints or advice would be appreciated.
Thanks.
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post #19 of 25 Old 08-12-2013, 11:30 PM
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Sounds like you may be using standard mode and/or native color space.try movie mode and auto color space.
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post #20 of 25 Old 08-13-2013, 02:25 AM
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I'm using Standard mode, Auto color space, & Warm 1. I tried Movie mode but it was too warm for my taste & seemed to make things more reddish looking.

In this thread it appears others have experienced similar skin tone problems.
http://www.avsforum.com/t/1471980/samsung-skin-tones#post_23516739

Anyway adjusting the Offsets & Gains seemed to help a lot but maybe I should be adding two colors instead of subtracting a different one.
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post #21 of 25 Old 08-13-2013, 04:57 AM
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Get a black and white tv and in a black room match sharp tv to it,or get another tv you know looks good and match them(color control on tv at zero).

Bit harder than it sounds.Just do a bit each night.First night you get it close,second a bit closer,etc etc,then your in the ballpark.

Standard mode on the samsung here makes colors more saturated compared to movie mode.Get the grayscale good first before the next diagnostic step.
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post #22 of 25 Old 08-13-2013, 11:03 AM
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Quote:
Originally Posted by Vic12345 View Post

Standard mode on the samsung here makes colors more saturated compared to movie mode.Get the grayscale good first before the next diagnostic step.

Standard is horrid for really anything apart from gaming. Even then, I've had to tone down the colour space and gamma tweaks. As it was still too neon for gaming. Been mainly using Mascior's disc.

There's so much red and blue push in standard that the bars aren't even pulsing. I did manage to get green to flash but no matter what I do red and blue is solid. I never really bother with standard as it's so unnatural looking.
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post #23 of 25 Old 08-13-2013, 02:19 PM
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There are issues way beyond clipping that you have to understand while calibrating. Some TVs produce bad contouring if you adjust any gain or offset/bias control too much... they don't have enough bits in the video processing path and the end result is you see contour lines in solid areas or fades (like a streetlight on a foggy night, or the red fade-to-black behind the MARVEL logo at the beginning of many Marvel superhero movies). Color test patterns (red green and blue) with smooth fades from black to 100% of the color) can tell you whether you have introduced any contouring but you have to know how those fades looked before you started... some TVs can't reproduce all of the primary ramps without contouring even before you touch any controls.

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post #24 of 25 Old 08-13-2013, 06:53 PM
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Quote:
Originally Posted by Vic12345 View Post

Get a black and white tv and in a black room match sharp tv to it,or get another tv you know looks good and match them(color control on tv at zero).

Bit harder than it sounds.Just do a bit each night.First night you get it close,second a bit closer,etc etc,then your in the ballpark.

Standard mode on the samsung here makes colors more saturated compared to movie mode.Get the grayscale good first before the next diagnostic step.

I have the color turned down quite a bit. I used the AVS709 flashing color bars while setting the Samsung RGB control to Blue Mode.
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post #25 of 25 Old 08-13-2013, 06:57 PM
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Quote:
Originally Posted by xvfx View Post

Standard is horrid for really anything apart from gaming. Even then, I've had to tone down the colour space and gamma tweaks. As it was still too neon for gaming. Been mainly using Mascior's disc.

There's so much red and blue push in standard that the bars aren't even pulsing. I did manage to get green to flash but no matter what I do red and blue is solid. I never really bother with standard as it's so unnatural looking.

The colors flash for me using the AVS709 disc.
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