Video Calibration Q&A - Page 3 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #21 of 77 Old 08-30-09, 04:46 PM
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Re: Video Calibration Q&A

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post #22 of 77 Old 11-23-09, 09:34 PM
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Re: Video Calibration Q&A

So I've been trying to find the time to play around with some of my TVs settings and my colorimeter to try and answer some of my own questions regarding exactly how some of the "classic" calibration graphs are affected by each of the different controls... suffice to say my life just keeps getting busier and I can't find the time, so I thought I'd begin picking the brains of those who already know. Not as much fun, but certainly easier, and then others can see and learn as well.

So this might be the first of many, but I'll space them out to give you guys a break...

Let's take the normalized luminance curve (% luminance vs. % stim) and how it's affected by changing the gamma of a display... I've often thought of this curve as a rope strung between two points, those being black (0% stim) and white (100% stim)... The analogy goes that as you increase gamma, it's like paying out more string, the curve hangs lower, as you take in more string, you decrease gamma, the curve hangs higher...

Taken by itself, is there any particular flaw in thinking about it in this way?

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post #23 of 77 Old 11-24-09, 08:24 AM
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Re: Video Calibration Q&A

That would be a good rough analogy. The important thing to get about gamma, however is that there are many variables at play that can be very different with different sets. Gamma needs to be thought of as a complete curve that can be complex and may not be described by a single average value. The whole rope is important, and the stiffness of the rope may vary along its lenght. Gamma may interact with other adjustments, and the exact nature of those interactions may be very different on your set than others.




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post #24 of 77 Old 11-24-09, 01:10 PM Thread Starter
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Re: Video Calibration Q&A

As lcaillo points out, it's a good rough analogy. However, depending upon where you are at in the stimulus level, you may or may not want to make different types of adjustments. In some cases, you may want to elevate the "average" gamma value to increase shadow detail. Other people may want to lower it to improve black level. The interplay of shadow detail and black level may differ entirely from what you want to do in the mid-tones and highlights. It's far better to get a reasonably granular view of gamma to correlate what you are seeing, what you want to see and the effects of specific changes to converge the two.

CalMAN -- Home Theater Calibration Software
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post #25 of 77 Old 01-03-10, 10:52 AM
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Re: Video Calibration Q&A

OK. So your responses made me think so hard it tok way longer to ask the nedxt question than I had intended... Thanks!
I appreciate the detail of not all machines operating the same way, and some being "stiff" at certain levels and the like, I hadn't thought about that... it's my hope to better understand how things should work in the theoretical world, as well as how often/not that applies in the real world...
To that end, I think you both said, yes, that's a good theoretical model even though it holds up only to some degree in the real world...
So the next question is, I believe I've read somewhere that on some displays that don't have gamma adjustments per se, that sometimes to can slightly tweak gamma by adjusting brightness and contrast, so I ask, using the rope analogy, if you increase contrast, looking at a luminance curve (not yet normalized) as you increase contrast you would expect the high end of the curve to rise....
The question is, as it rises, what happens to the length of the rope, does it stay the same, giving less contour to the curve? Or does it elongate, but not proportionately, so there's less contour to the curve? Or does it elongate ot the point where the contour stays the same?
I'm sure you can guess the next question, which is what then happens when looking at the normalized luminance curve, but I think I've answered that one already... I think the answer will be that in the ideal world, the since you're not ajdjusting it specifically, the gamma would stay the same, and the before/after curves would overlap, assuming you weren't driving one into run-out or the like...?

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post #26 of 77 Old 01-03-10, 11:10 AM Thread Starter
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Re: Video Calibration Q&A

Quote:
glaufman wrote: View Post
OK. So your responses made me think so hard it tok way longer to ask the nedxt question than I had intended... Thanks!
I appreciate the detail of not all machines operating the same way, and some being "stiff" at certain levels and the like, I hadn't thought about that... it's my hope to better understand how things should work in the theoretical world, as well as how often/not that applies in the real world...
To that end, I think you both said, yes, that's a good theoretical model even though it holds up only to some degree in the real world...
So the next question is, I believe I've read somewhere that on some displays that don't have gamma adjustments per se, that sometimes to can slightly tweak gamma by adjusting brightness and contrast, so I ask, using the rope analogy, if you increase contrast, looking at a luminance curve (not yet normalized) as you increase contrast you would expect the high end of the curve to rise....
The question is, as it rises, what happens to the length of the rope, does it stay the same, giving less contour to the curve? Or does it elongate, but not proportionately, so there's less contour to the curve? Or does it elongate ot the point where the contour stays the same?
I'm sure you can guess the next question, which is what then happens when looking at the normalized luminance curve, but I think I've answered that one already... I think the answer will be that in the ideal world, the since you're not ajdjusting it specifically, the gamma would stay the same, and the before/after curves would overlap, assuming you weren't driving one into run-out or the like...?
Since in theory changing either the brightness or the contrast control ought not to change the gamma for reasonable values of all three, there really isn't a hard-and-fast rule. A properly engineered display will, if you compress the dynamic range too much, lower the effective gamma to maintain evenness between black and white. In other words, the gamma will fall as black and white get closer together (e.g., brightness too high, contrast too low). That being said, with current technologies, a well-engineered display ought never to get that far short of "stupid human tricks".

Ideally, you would want the converse to be true, as well. As the dynamic range increases, the gamma value increases -- at least up to a point. If you have a "bat cave", then you can appreciate and use an average gamma that is quite a bit higher than one meant for a living room with a fair bit of ambient light. Ambient light requires an elevated black level to maintain a visually desirable amount of shadow detail, and given a fixed maximum brightness, this means that you end up losing dynamic range as a result.

CalMAN -- Home Theater Calibration Software
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post #27 of 77 Old 01-04-10, 07:49 AM
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Re: Video Calibration Q&A

So then, on a properly engineered display, for "reasonable" values of contrast and brightness, you might expect increasing contrast or decreasing brightness to increase the gamma slightly, but probably by very little? If so...
Would both affect the whole range equally, or would contrast affect more the gamma at the high end and brightness more at the low end?
And how many of modern displays are "properly engineered" in this way, as in, how high end do you have to go to find this behavior vs how low end do you have to go to find these controls affecting gamma more than they should, or in ways they shouldn't?

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post #28 of 77 Old 01-04-10, 08:19 AM
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Re: Video Calibration Q&A

I would consider a properly engineered display to be one that accounts for non-linear luma in the contrast and brightness controls and has no effect on gamma. In practice that relationship is hard to get right, as there are multiple variables at play, not just the video relationships, but also the display technology and its implementation.

In practice, it just depends on the display. It is very hard to generalize. The effects vary from one manufacturer to another and even within brands considerably.




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post #29 of 77 Old 01-04-10, 08:59 AM
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Re: Video Calibration Q&A

Is that one of the contributions that make the Pio Elites look so good?

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post #30 of 77 Old 01-04-10, 08:44 PM Thread Starter
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Re: Video Calibration Q&A

Quote:
glaufman wrote: View Post
And how many of modern displays are "properly engineered" in this way, as in, how high end do you have to go to find this behavior vs how low end do you have to go to find these controls affecting gamma more than they should, or in ways they shouldn't?
Good video engineering isn't necessarily cost-prohibitive on the BOM, AFAIK. What might push it higher into the food chain is the cost of the people and giving them the time to do it right.

CalMAN -- Home Theater Calibration Software
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