Gamma, Black Level, Contrast & HDTV Displays - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 14 Old 03-21-2004, 08:31 AM - Thread Starter
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post #2 of 14 Old 03-21-2004, 10:40 AM
 
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You may find this thread informative:

https://www.avsforum.com/avs-vb/showt...hreadid=381653
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post #3 of 14 Old 03-21-2004, 11:24 AM - Thread Starter
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Thanks for the reply Chris. That is precisely the sort of info I was looking for. I've added a link for it above. If anyone else has other good links on Gamma, etc. for HT, I'd welcome them too.

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post #4 of 14 Old 03-21-2004, 12:10 PM - Thread Starter
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Here's what prompts my question on Display Gamma btw. I'm using an HTPC for input to my Sony D/V CRT via DVI. And I notice that neutral greys (RGB=127) frequently appear brighter on the Sony than on my other computer monitors. This is regardless of the ambient light (although it becomes even worse in dark surroundings). I am assuming that this is because the TV has different gamma than the computer monitor. The question is which is right for digital video applications like DVDs for example?

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post #5 of 14 Old 03-21-2004, 12:20 PM - Thread Starter
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I'm also suspiscious that some digital sources are designed for different gamma displays than others. I had a very difficult time getting Finding Nemo, for example, to look decent on my D/V CRT at first. My suspicion is that it had minimal undercorrection, and was designed explicitely more with low-contrast computer-like displays in mind... while many other film-based DVDs seem to thrive on the higher contrast on my D/V CRT.

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post #6 of 14 Old 03-21-2004, 03:56 PM - Thread Starter
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Do Direct-View CRTs have the ideal Gamma, Contrast & Black modeling for digital content like DVDs & HDTV?

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post #7 of 14 Old 03-21-2004, 04:56 PM - Thread Starter
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More thinking out loud...

One issue is the display gamma vs. system gamma.

Another issue is undercorrection of the source. This appears to vary noticeably from source to source and provider to provider (as you can tell from looking at any DVD with murky blacks), although it's probably supposed to result in a net gamma of 1.1-1.25.

My understanding is that undercorrection was originally done to compensate for darker surrounding illumination and high contrast on TVs. I.e. to dampen the midtones and make the picture seem less overpowering in low ambient light like a living room.

If you want to achieve pleasing midtones on your display, then you have to take the following things into consideration:

1) Darkening of midtones at the source (ie camera based gamma under-correction).
2) Difference between system & display gamma. IOW: are midtones being overdriven or under-driven because the display gamma doesn't match the system gamma, and if so, how much?
3) Ambient light level / simultaneous contrast.
4) Contrast (and black level) of display.

When you consider all the different levels of visual compensation going on here, it's easy to see how the picture on HDTVs can turn to crud. All of these issues probably need to be considered though to achieve an optimal image.

Some TV's have adjustable gamma or different gamma/black enhancement picture modes too, complicating the situation even more.

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post #8 of 14 Old 03-22-2004, 12:57 AM
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To correctly set the gamma on my plasma it crushes the low blacks, so those calibration discs which tell you to adjust the brightness so you can see the almost black bars/boxes make the mid greys too bright...
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post #9 of 14 Old 03-22-2004, 01:35 AM - Thread Starter
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Thanks for the input cybersoga. This stuff frustrates the heck out me. Whenever I think I'm getting a grip on it. Some new wrinkle comes along, and it all seems to crumble again. :)

I'm beginning to think for example that the reason the midtones look so bright on my D/V CRT's is due more to contrast being set too high than to some "error" in the gamma. The higher the contrast, the brighter the "gamma" (midtones) seems to become on my CRT.

Since I can't find any way to compensate for this in the display itself, I'm beginning to think that the midtones might be one possible yardstick for adjusting the display's contrast. Unfortunately the contrast has to be set pretty low to get a (linear?) response where neutral greys actually look "grey". :) Maybe it isn't necessary or desireable to set contrast this low for everything though.

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post #10 of 14 Old 03-22-2004, 07:53 AM - Thread Starter
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EUREKA!! I think I might possibly have figured something out (hopefully) on this gamma nonsense.

After making the above observation I tried a little experiment. I used a recent DVD (not Finding Nemo), and played it via an HTPC on my Sony 34XBR800 D/V CRT with the Contrast set very low so that the midtones (based on a greyscale test pattern) looked very neutral/linear. The DVD looked rather dark and murky.... Then I brightened the gamma in the DVD using the Display Properties/Video Overlay Gamma control (set to 1.2), and voila the shadow detail became visible again, and a watchable and very 3D picture results.

Then I tried the reverse, I increased the Contrast on the display until midtones/neutral greys looked too bright and I set the Overlay Gamma back to unity (1.0). Once again... a nice result. Dynamic range is increased in the image, but shadow detail looks good, and the image was very watchable with no overlay gamma correction.

(BTW, in both cases I also adjusted Brightness until the black level of the picture matched a 0 voltage area of the screen, to insure that elevated blacks were not a contributing factor.)

So why does this work?

Well here's my guess... the DVD is undercorrected so it's final gamma is ~ 1.2. And the Picture/contrast control on my Sony is working like both a white point control and a gamma control. The higher the contrast is raised, the brighter the midtones appear relative to black and white.

If the Contrast is set very low so the midtones are very neutral in appearance, then brightening the midtones/gamma in the Overlay more or less reverses the undercorrection in the DVD, giving you a more watchable picture. The Display Properties/Overlay gamma correction setting of 1.2 (surprise, surprise) compliments the ~1.2 undercorrection in the DVD, giving an end result with an apparent gamma close to unity.

On the flipside... Boosting the Picture/Contrast on the CRT until neutral greys/midtones appear too bright also performs the same function. The brighter apparent display gamma compliments the undercorrection in the DVD, and the end result is something which appears close to unity with good shadow detail and a very dynamic range of contrast, without requiring any correction to gamma in the overlay (so it's left at 1.0/unity).

If I set the Contrast somewhere in between and use only a partial correction on the Overlay of 1.1, then once again the picture appears to approximate unity, but with a bit less dynamic range.

I'll have to perform more tests to verify that my eyes aren't just playing tricks on me, but this certainly seems to work. And it would seem to explain why some DVDs look good with high contrast settings on my TV while others do not-- different levels of undercorrection in the DVDs.

And this could also explain why adjusting many displays to a linear response frequently yields a picture which is too dark-- because undercorrection in the source video is not taken into account.

I'm not sure if the contrast on other CRTs or different display technologies will work the same way, but on the Sonys D/V CRTs this certainly seems to work.

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post #11 of 14 Old 03-22-2004, 08:47 AM - Thread Starter
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The DVDs I used for this btw were Phantom Menace & AOTC. I'll have to try some others and see if they follow a similar pattern. I'm thinkin that the Hulk might be another good candidate though.

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post #12 of 14 Old 03-23-2004, 08:38 AM - Thread Starter
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The higher the contrast is raised, the brighter the midtones appear relative to black and white.
I'm not sure if the Contrast adjustment is changing the actual gamma on the CRT per se, or if the higher percieved gamma is simply an illusion created by what Poynton and others refer to as the surround effect or simultaneous contrast due probably to different pupil dilation. Either way though, the end result seems to be the same on my CRT. Increasing the contrast on the it (in a fixed low ambient lighting arrangement mind you) makes the midtones appear significantly brighter in relation to black & white.

If the change in perceived gamma is just an "illusion" created by pupillary dilation, then its surprising to me just how dramatic the change can. By simply boosting the contrast, I can change an RGB=127 tone from looking essentially neutral grey to looking more like an off-white, relative to black and white.

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post #13 of 14 Old 03-23-2004, 09:55 AM - Thread Starter
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Originally posted by ADU here.
Quote:
Quote:
Originally posted by Guy Kuo
I get the same effect even with the CRT. Whether or not pupillary constriction is the mechanism or not, the intra-scene contrast jumps up when the laptop screen lights up a significant portion of my field of view.
My theory, FWIW: removing the surrounding illumination makes the picture seem flatter for two reasons: 1) it probably makes blacks in the picture seem less deep and 2) it makes the midtones/gamma seem brighter, making the picture looked more washed out which = Poynton's surround effect/simultaneous contrast. IMO, these effects are both the result of greater pupil dilation when there's no surrounding illumination.

The gamma undercorrection applied to most video these days is actually designed to use the unnaturally bright perceived gamma in this situation: low (or no) ambient light + high contrast on the display. So the higher perceived gamma may actually be a desireable thing. If the display is adjusted so it appears to have a linear response (ie midtones like RGB=127 look very neutral grey rather than bright grey or off-white), then the undercorrection in DVDs, etc. will work against you muddying up the shadow detail.... unless you apply some add'l gamma correction somewhere to boost the midtones back up, and reverse the ~1.2 gamma in the video source. This is my opinion anyway.
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One of the problems in this whole scheme though is that gamma undercorrection is not always consistent from one video source to another. Some will use a 2.0 gamma correction (resulting in a final gamma of 1.25), while others while use a 2.2 correction (resulting in a final gamma of 1.136), while a normal computer desktop (excluding the DVD video in the overlay) may simply have a 1.0 gamma. And I have no idea what level of undercorrection is typical in HDTV broadcasts, if any.

If you adjust your display for optimal results with a 1.25 gamma source, then sources with less undercorrection (end gamma 1.136 & 1.0) may tend to look more washed out on it than they should. For the very best results across the board, you'd probably want some way to compensate for different levels of undercorrection in different sources. And there are probably several different strategies that could be used for this.

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post #14 of 14 Old 11-15-2004, 10:37 AM - Thread Starter
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I noticed recently that the Memory/Brightness control on my old Sony DVD player does in fact appear to be a gamma control, as opposed to a black level control.

Memory/Picture seems to control the white-level clipping though.

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