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Discussion Starter · #1 ·
Mornin, Afternoon or good Evening, I read alot about gamma adjustment on projectors and DVD players. How does one know how to adjust the gamma and where is a good setting. My projector does have gamma adjustment(NEC LT155) as does my DVD player but I don't know where to start(or finish). I do own AVIA if there is a test on there to adjust. Thanks in advance for your help!!!


Scott

(just another forum addict)
 

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Count me in as an interested party.


-robert


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Well,


About 30 minutes ago I started writing an answer and my machine locked up.


It was quite a response this will be a readers digest version.


----------------------------------------------------------------


Before getting into a LT155 specific discussion, I would like to do a general discussion about gamma first.


Gamma determines the amount of light your projector outputs as its input ranges from a full signal (white) to a zero or near zero signal (black). Your contrast and brightness controls help you set full white and full black for your source, but they don't determine the intensity of the grays in-between.


A simplified gamma equation is as follows:


y = x^G.


Where:


y = is your projector's output signal proportion.

x = is your projector's input signal proportion.

G = is your projector's gamma constant which is usually somewhere between 2.2 and 2.5.


Sample calculation - for a mid gray input signal (50 IRE = 50% of full scale = .5) and gamma value of 2.2


y = .5^2.2 = .2176 of your projector's full output.


This means for a 2.2 gamma setting a 1000 lumen projector would put out 217 lumens for a mid scale gray input. Or if your projector had a 2.5 setting it would put out 176 lumens for a mid-scale input.


CRT owners have it easy. They don't have a gamma setting. The physical behavior of the CRT's phosphur's dictate that the display will have a gamma of about 2.5. However, most source material is shot on video cameras that have a gamma of 2.2 dictated by the NTSC standard. In the recording world this is a well known mismatch and compensation in post-processing is often used to make up the gap. Also, the CRT with its high contrast ratio can still put out a very very good image.


Now for digital displays, they have a look up table for given input signal strengths. It is whatever the programmer decided to make it. It can be 2.2 , 2.5 or even a mixture, say 2.2 for a portion of input range and 2.5 for other portions of the input range.


In general the higher the projector's gamma value the more "punch" or apparent contrast it possess. The lower the value the more it brightens up dark details.


What are the LT155's gamma values? Good question. After viewing I would guess that the normal is close to 2.5, the natural 1 is about 2.2 and the natural2 is about 2.0. These are just guesses and I don't think the gamma is consistent throughout the input range.


What should you use? Well pick your poison. It is a tradeoff. If there was only one perfect value, then you wouldn't need the adjustment. Also, if the LT155 had near infinite contrast ratio like a CRT a gamma setting would be less of a concern - everything would look pretty darn awesome. But its more like 300:1 so we have to do the best we can.


My suggestions: I wouldn't use the normal because I feel it is designed to give a lot of punch to business presentations. Natural 1 I think is well suited for most video and natural 2 is suited well for movies or film based material (The Sopranos in HD would be a great example).


Now, if didn't like my "do what you think is best answer" and your still looking for the most "correct" setting. You can use Avia and a light meter to get as close as possible:


1. Set your black level correctly with brightness control and the moving "black bars" test.

2. Set your peak white level correctly with your contrast control and the moving white bars in the "needle pulse" test.

3. Now go to the 50% steps test in the gray scale ramp. The most accurate gamma setting will give the most accurate readings for each step. The first step is full white. The second step should be half a bright and so on.


I hope this helps. I personally would leave the setting in natural 1 and forget about it until you run across a dark movie that needs natural 2.


-Mr. Wigggles



Ps. Don't get gamma, gamut, and color correction confused.


Your projector's gamut is the array of potential colors that it can produce. The LT155 is great in this regard even though it could use a slightly deeper red.


Color correction has nothing to do with gamma correction. However, the calibrator might change the individual red, green, and blue gamma tables to do color correction (tricky service menu type work). The white balance controls will do the same thing for the LT155 and they are much easier to use.



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[This message has been edited by MrWigggles (edited 05-24-2001).]
 

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Discussion Starter · #6 ·
Thanks for the support!!!!!!!!! Is there anybody out there?????????????? We know this IS where the EXPERTS are. I'm not a butt kisser, just know that this is a fact.


Scott
 

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Discussion Starter · #7 ·
MR Wiggles(heavy on the mister), Looks like we were typing at the same time. Thanks loads for taking the time to respond!!! Sorry you got shut down. I know how that goes and I'm an extremely slow typer. I just glanced at your response(gotta pizza in the oven) so I will check it out and ask any follow up questions. I knew I could count on this place to get thebest picture possible.


Scott

you don't want to see my picture!!!


[This message has been edited by scottyb (edited 05-24-2001).]
 

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MrWiggles,


Thank you very much!

I had no doubt we'll get a response from you.

Love this forum, best people on the Net.


I have a Mitsubishi X30 (Plus U3-1080 clone) and although

I'm satisfied with Natural Gamma settings it's interesting

to tweak Custom gamma. It has 31 Index in Gamma value and

257 steps in each R,G,B.

So, I don't know what to do with that.


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A.G.

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Discussion Starter · #9 ·
Hey Mr Wiggles, Thanks for the help. Also, you were right I did also get white balance controls a little confused with Gamma. Should I use AVIA to adjust the white balance controls on the red. green and blue, and if so what test pattern do you reccommend. I can probably figure it out but......


THANKS,

Scott


PS thei pizza was good
 

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Scotty,


I am glad to help.


The best way to test white balance is to watch a black and white movie. If you are using the component input (requires the break out cable on the LT155), you can make movie a black and white movie by removing the Cr and Cb (or do they label there's Pr and Pb?) This will give your projector a pure black and whit input.


If you are using an HTPC and the RGB input you can turn down color controls in the DVD player software to zero.


Now simply watch the movie, if the things look a little too blue (which they will) reduce the blue channel. If the image looks a little too yellow reduce the red and green or boost blue. If it looks too magenta, reduce red and blue or raise green. If it looks a little too cyan (bluish green) reduce green and blue and raise red. etc. Try to keep the settings as close to their standard values as possible by lowering or raising the appropriate combination. These white balance controls are essentially R, G and B contrast controls. If you simply keep raising them all of the time, you will be left with a washed out image.


Also these asjustments should be done after you get your normal brightness and contrast settings tweaked to your liking. Also, avoid using single color temp slider it is essentially a dumbed down simplefication of your white balance controls (it is for wussies). Once you get your white balance the way you want, leave it alone.


Now connect your Cr an Cb back up, and now adjust your "color" setting to what you feel is appropriate. This control determines how colorful your image is. Unfortuneately, it will be necessary to monkey with this guy from time to time to get the colors correct for the particular source. This is no different than a standard TV. Fox likes to be really colorful while other stations are often quite muted. Video based material will have more colors usually while film based will usually be more muted. etc.


If there is a color "balance" difference between your s-video feed and your component input, try your best to use the "tint" control to normalize the s-video to look like the component. By design component doesn't need a tint control and the tint control should be unique to your s-video input. (or does the LT155 have seperate user settings for all of its inputs?). Anyway you get the idea.


(btw, I would chose your gamma after white balance but before color. That is just my opinion. If your white balance switchs when change gamma (it shouldn't), you'll have to make your final gamma choice at the very begining.)


Good luck,


Mr. Wigggles




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A.G.,


I would try to use one of the preset settings before going with the custom 31 position one.


for the most part follow the advice I gave Scott. Nec does the firmware for the plus projector's so they are very similar. (Plus does the optics and assembly)


-Mr. Wigggles


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Discussion Starter · #12 ·
MR. Wiggles, Again I thank you. How did you become such a wealth of knowledge. My acquaintances think I know a lot about this stuff but people on this forum(like you) leave me in the dust and I always wonder how you learn so much about different equipment and tweaking, etc... Well again my thanks and if you ever make it to Minnesota stop in my restaurant and I'll buy you a glass of water.


Scott
 

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CRT owners have it easy. They don't have a gamma setting. The physical behavior of the CRT's phosphur's dictate that the display will have a gamma of about 2.5.


Mr. Wiggles,


CRT displays all have gamma correction circuitry! The better projectors such as the Sony G90 have very elaborate gamma correction circuitry which can be addressed in the service software. The better a displays gamma circuit is designed the more linear the Grayscale alignment will ultimately be. Inexpensive projectors typically fall short of the mark in this respect and make it difficult to achieve better than average results in many cases.


A good approach to calibrating a display with a poor gamma circuit is to adjust the bias and gain circuits at points closer to each other (30 ire and 70 ire) than at the typical 20 ire and 80 ire points. The reason for this is you will have a smaller deviation at the midrange where fleshtones and more visible information lies. If the greatest deviation is at the extreme calibration points 0 and 100 then you will notice a smaller error in the calibration.

 

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I sure wish this kind of expert information could be stored for access somewhere.


Mr Wigggles - I hope you at least keep it, so reposting months from now when the same questions arise again can be answered with ease-


thanks for sharing your expertise


ken
 

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To do gamma correction in analog crt projectors requires

a multi-diode multi-breakpoint setup. The problems with

these circuits are that they are temperature sensitive

and they have a limited frequency response. To get one

of these circuits to perform at even 1080i levels is

tough, 720p is even worse.


To do gamma correction digitaly is a snap. There are a

few rear projection sets that do everything digitally

and therefore have the capability to do gamma correction.

If the sony G90 actually does gamma correction it would

be the only front projector i know of.
 

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Cliff,


Not all CRT's have gamma correction circuitry. Most have white balance but not gamma correction.


Please share with us a FP that has it that doesn't cost $30K


As far as direct view is concerned, I can't think of one direct view that has it.


Princeton Graphics, no.

Sony Wega, no.

Any computer monitor, No.


Once again please feel free to share specific direct view examples as well. Are these controls barried somewhere in service menus?


"All" is a pretty big word. In my summary post I am sorry I didn't mention the exceptions.


-Mr. Wigggles


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MR. Wiggles -

In your above instructions for setting white level using a black and white image you describe how to make adjustments to the different RGB channels. Should one use the Brightness RGB sliders or the Contrast RGB sliders or both?


I know that black and white levels of each color are going to interact with each other, so which control should be given priority? Should it be the Contrast adjustments because we are adjusting "white balance" not "black balance"?


(I am trying to adjust a UP-1100 if it matters. Just got the projector on Friday and even though I know that I am at the low end of what it is capable of I am still super impressed.)

Regards,

-Jeff
 

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High end CRT projectors do have gamma correction, and in some of these it is adjustable, but these circuits are intended to correct for non-linear responses of the crts and driving circuitry, so that the projector can be made to have a good grayscale and the proper gamma response curve. It's important to not confuse internal gamma curves with the display device transfer characteristics.


The Electrohome Maquee 9500 LC, for example, has a diode based circuit such as Mr. Gilmore describes on the blue crt only. In a G90 there is an electronic gamma correction circuit which affects all three colors. I don't know what sort of internal "gamma" circuitry there is in less expensive crt devices, if any. Probably none...


But again, this is internal "gamma", not the external characteristic of the projector as a display device.


For a detailed explanation gamma in display devices, see this:
http://www.inforamp.net/~poynton/GammaFAQ.html


Wm
 

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Jeff J,


Leave the UP-1100 in normal. The factory hopelessly programed the Natural 1 and Natural 2 curves incorrectly.


These settings help brighten up the image, but they generate horrible grey ramps. One part will be purplish. One part will be orangeish, etc. It is quite weird.


I would leave things pretty much stock on the colors. Use the R,G, and B contrast controls mainly. You might want to punch red up a few notches and blue down a couple but it all depends on the bulb.


George,


The Radeon in an HTPC has gamma controls when used with it's own ATI player. Most graphics cards on computer's have gamma control but the controls don't do anything for DVD playback because it is an "overlay graphic" There are a few other processors that I have heard of (I think Vigatec is one) that have gamma controls. Also some of Panasonic's high end DVD players have them. But I would say that in general CRT devices don't need it. They have the contrast ratio to make up for any gamma "problem" they might have.


As far as LCD's/DLP's are concerned, NEC has been real good about putting gamma controls in their latest products (along with their Plus counterparts). A lot of the others have gamma adjustments in their service menus like the 10HT and XP18n/XP21n.


-Mr. Wigggles


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The latest version of DScaler has a gamma factor option. If you are not familar with DScaler, it is an open source software scaler that can be used with many video capture cards to scale external source in a computer (yes, like your VCR, DVD player or LD player).


The gamma setting in DScaler is normally linear (1.0), which means:
  • At 0 IRE (black), 0% luminance is output
  • At 50 IRE (medium gray), 50% luminance is output
  • At 100 IRE (white), 100% luminance is output.

This is the default setting.


DScaler allows you to set the value higher (like the 2.2 NTSC/SMPTE standard) to produce more light output from mid-tones. For example, at 2.2 gamma:
  • At 0 IRE (black), 0% luminance is output
  • At 50 IRE (medium gray), 73% luminance is output
  • At 100 IRE (white), 100% luminance is output.


To pull this off, DScaler has to determine the intensity and perform the gamma calculations on every single pixel in the image. Needless to say, this is quite a CPU intensive activity.


If you have a well-endowed computer and a video capture card, you can perform gamma correction on any video source you care to plug in. See the HTPC forum here for more information on DScaler.
 
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