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Discussion Starter · #1 ·
I have been reading this forum on and off for a while but I am still not clear on a few basic functions on CRT projector.


I have read that the Brightness is the black level and the Contrast is the white level. This seems to be what the adjustments do, but they also intereact with eachother.

If I lower the contrast, the black level on the projector will also be lowered even though I do not touch the brightness control.

Is there any diagram/graph that shows how the controls interact?


I have a NEC projector which has a kelvin/white-balance control.

For each tube there is a Black and White level. How is this related to the brightness and contrast?


I also see posts about bias, gain and G2. I know these are service level adjustments, but somehow are lowerlevel components of the projectors brightness and contrast?


I upgraded my video card with a radeon card which has a gamma control.

I think I had read once that CRT projector does not have 1.0 gamma, but I cant seem to find the post I saved. What gamma level should be used for a CRT projector?

I am confused because a CRT computer monitor should be set to 1.0 gamma so why should a projector be different? Or do I have that wrong?


I am asking these basic questions because I have been unable to locate the answers when doing a search. I seem to get a tun of great info, but not quite what I am looking for.


If you can help to make some of this clearer or point me to another thread I would appreciate it. Thanks in advance.
 

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Hello


first thing to do would be to get the so called scoped settings for your Radeon drivers. Do a search in the HTPC forum for scoped and your drivers and I am sure you will find plenty information.


Once you put in the scoped settings into the driver all com ming adjustments should be made on your CRT.

I only did bump the gamma two more steps in the scoped settings for my Radeon 7199 driver.



Do a search on G2 or black level to gain information how to adjust G2.

Do a search on blue focus to learn how to slightly electronically defocus the blue tube to improve greyscale tracking.

On the CRT you should adjust brightness and contrast using Video Essentials or AVIA dvd.


Good luck

:)
 

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As you have figured, Black level is Brightness, Contrast light output.

G2 is the point at which your guns turn on and off

Brightness is a "fine tuning adjustment" for G2...so adjusting brightness changes the point at which the gun (or guns) turn on and off

While Contrast does seem to effect Brightness, I'm not sure why, as Contrast is basically the dynamic range adjustment. It allows the gun (or guns) to be adjusted for light output which increases or decreases the light output range (or dynamic range)

Brightness does effect Contrast/ light output...if you lower Brightness, you have lowered the point the gun turns on and off, which in turn lowers the max light output...Contrast slides with Brightness

Gamma is basically the Grayscale curve as it relates to shadow detail. The graph should be a straight, linear line from 0 to 100 ire, but it is not, it usually droops in the middle...gamma is used to adjust that droop.
 

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Discussion Starter · #4 ·
Thanks for the info.


If I were to make a graph with a point at 0 ire at the left and a point at 100 ire at the right, and a diagonal line connecting them..

Would the contrast control be moving the point at 100 ire and changing the angle of the line? And the brightness would be moving the entire line up or down (both points)? Is this right?


I know the gamma will make a hump in the line. Is there a fixed amount of "hump" that should be applied to compensate for a particular crt projectors? Or does this all depend on the other settings on the projector and no fixed gamma number can be used for a particular model?
 

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Tip,

The diagnal line concept is good and contrast would be tilting the right side of the line with the left side kind of fixed, the brightness would tilting the left side with the right side sort of fixed.

Gamma is going to be a function of how well you set up your gray scale. Generally the hump is in the middle of the diagnal line (so it has a bit of curve to it) and your goal is to minimise that hump. This is generally done with gain (for the bright side) and bias (for the dark side ) as well a blue defocus. Some CRT's have less Gamma "built in" than others. The Barco's are a prime example. Sony's and NEC's you have to work on a bit to compensate for the gamma.


Terry
 

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If you plot signal level on the x axis and light output on the y axis, you end up with a curve which rises from left to right.


"brightness" or "black level" adjusts the bottom left end of that curve. G2 also affects the bottom left in basically the same manner as "brightness" but is a service level control. You can think of G2 as the serviceman's "brightness" control which is adjusted with proper technique so that the normal user control for brightness can work correctly and keep the CRT guns at proper voltage levels. You should rarely need to adjust G2 as an end user.


"Contrast" basically sets how much the system amplifies the video signal. Thus it affects mostly the upper right end of the curve because that's is where its effect is largest. We typically try to set it so light output is sufficient, but the guns are not being overdriven.


Given only the two ends of the curve as set by "brightness" and "contrast", you haven't defined how the light output curve is shaped between the two ends. Gamma is a number which roughly describes the shape of the curve. CRT displays have a non-linear relationship between input signal and light output. That curve is roughly light = input ^ gamma where gamma is about 2.2. This curve is due to the physical properties of the electron gun and was noted early in the development of video systems.


However, capturing a scene on camera and then showing that image on a display is basically an exercise in recreating the linear light relationships in that scene on a non-linear display device (the inherent CRT gamma of 2.5). The problem of making the entire chain linear could have been solved in several ways. One would have been to equip every TV with a gamma correction circuit to make the display behave in a linear manner. That would have been an expensive proposition because every TV would need such a correction. Instead, the overall system was made "linear" by applying an inverse gamma (1/2.2) to the camera's signal so the two gammas essentially cancel out. The system then is linear.


But, wait raising the input to a power of 1/2.2 does not exactly cancel out a gamma of 2.5 at the CRT display!!! There is still an overall gain in gamma of about 0.3 for the system. This slightly raises the perceived contrast of images and compensates for the "dark surround" viewing environment of most televisions. (i.e., a small picture with a dark surrounding will look as it has less contrast than in a bright surrounding). The overall perceived effect is then linear. In many many places you'll find that the CRT display gamma of 2.5 is simplified to 2.2 so the discussion can be simplified and made easier for people to understand. Actually, I'm simplifying as well since I've ignored that the actual "ideal" gamma curve has a linear region at the dark end.


Now comes the gamma table of the computer. Basically, the CRT monitor connected to the computer already has its inherent gamma of 2.5 (but we'll simplify and call it 2.2 to match most things you'll read). Since the display already has that gamma, the computer's "gamma" table should be pretty much linear or 1.0 to make the display look like it should on other peoples machines. However, it is possible to alter the computer's gamma table to alter the gamma of the system and pretty dramatically change how things look on screen. One example of this is the Macintosh gamma of 1.8 which tends to make pictures look very different on Mac's vs PC's when move them between the platforms. Why did Apple choose an odd gamma of 1.8? It was to match the gamma characteristics of the laser printer and allow images appear on screen as they would when printed. That gamma of 1.8 still persists on Mac's. For this reason, image editing software now can embed gamma and color space information into image files so those details can be used to adjust the image for proper display when the file is moved from system to system. At any rate, the take home message is that the computer gamma table can be used to pretty dramatically alter the gamma curve of the display system.


Since we work in three primary colors, we should add to the discussion the "bias and gain" controls which are associated with each primary. You can think of these as the "brightness" and "contrast" for each primary color. Why do we need such separate controls? The ratio of the three primaries needs to be set to keep the three guns balanced relative to each other to create the D65 color of white. Without separate white balance controls such as "bias" and "gain" for each color one couldn't produce the right color ratio. One sets the "bias" and "gain" controls so that gray images are the right color of gray from dark to light. Once that underlying gray balance is correct, colored images are built upon that neutral canvas.


Projection CRT adds a little more complication because the machines drive the electron guns very very hard compared to direct display CRT. That means the normal gamma relationship begins to fall apart especially at the bright end of the scale. The gun just can't keep up with the light output specified by the input signal. This is particulary true for the blue gun since it has to work much harder than the other three. So if you plot out the blue gun's output curve you often find its upper right (bright) end flattens out instead of rising exponentially like the other less stressed guns. If you adjust the bright end of the gun outputs to create the right color of white, that mismatch in the blue gun's gamma curve means the middle of the gray scale ends up with excess blue (the so called blue hump). It's not really a blue hump but the result of pushing the flattened upper end of the blue output to match the exponential output of the other guns.


So how do we get around the blue gun problem?


a. Keep the overall output low enough to keep the blue gun below the point at which it becomes non-exponential (in common parlance we also call this staying within the "linear" range even though it really isn't "linear")


b. Increase the blue gun's output by defocusing the spot. This can increase the amount of blue light by 20 to 30% so one can run the gun at a lower beam energy and avoid more of flattened portion of the output curve.


c. Use gamma correction circuitry to compensate for the blue gun's flattened curve - essentially reshaping the curve to match the other guns. Hence, less "blue hump."


d. Intentionally set white balance so it is most correct at a slightly lower peak IRE like 90. This means that peak whites are slightly more yellow than perfect, but there is less white balance error for the middle tones where most of the image lies.


Depending on the projector, some or all of these techniques can be used. Projectors can even incorporate gamma curve adjustments for all three guns so gamma can be adjusted to match the "ideal" curve and also match the guns so they track with each other throughout the dark to light image intensity range.



Some questions to ponder and see if you understand gamma, brightness, and contrast. You should be able to answer all of these.


1. John has a new TV and complains that the picture looks washed out even though he has turned up the brightness control. Why does the picture look bad? What would you teach him about the "brightness" and "contrast" controls?


2. A display has a lower gamma than a standard CRT display. Would images appear to have more or less contrast?


3. Recall the gamma tables in your HTPC. If the projector doesn't have good gamma controls, how might you replicate the gamma controls of a high end projector?


4. Digital display devices have an inherent gamma other than the electron gun's gamma of 2.5. The physics are different so the inherent gamma of the devices don't naturally respond like a CRT. However, we seem to achieve acceptable images. How are the manufacturers of digital displays achieving a CRT like gamma?


5. You are calibrating the gray scale of a CRT projector but notice that the 70 and 80 IRE readings consistently show there is excess blue even though you have things adjusted such that the correct RGB ratio is present at 30 and 100 IRE. What would allow you to achieve a flatter grayscale response?


a) Increase red and green gun gains to increase red and green light output.

b) Decrease overall contrast and start again

c) Defocus blue gun (towards a uniform spot blob)

d) None of the above

e) B & C

f) A, B & C



Now, get off the board and go enjoy those Turkeys!
 
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