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Gains And Offsets

456 Views 7 Replies 3 Participants Last post by  RONM
Could anyone be so kind to explain what color gains and offsets are

and what effect they have.Also can you adjust these using avia or dve

calibration dvd's with any accuracy?
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I did a couple of searches to find a good source but I came up short. It took me a while to understand it so I'll share what I know. The way I think of it is that the gain refers to the way the higher values of the IRE scale (say 70 to 100 IRE) are produced using the three main colors of red, green, and blue. The offset is the way the three colors are used to produce the lower IRE values (say 7.5 IRE (NTSC) to 30 IRE). I know that there is more to it (involving voltage and other complicated things) and I would welcome anybody else's input. The color temperature is what the gray scale looks like and should be neutral (D6500K - the standard that is used on the monitors during film and tv production). It should not show any color in it just grays and black. If there is a reddish tint to it it is said to be warm (which actually refers to a lower temperature - 5000K or so). If there is a bluish tint to the gray scale it is said to have a cooler color temp (which is actually a higher temperature - around 9000K to 11000K). Remember that the blue part of the flame is actually hotter but blue looks 'cool'. The way most display devices are evaluated involves checking the color temperature throughout the gray scale.

If you have a way to measure the color temperature you can adjust the gains and offset for each color to achieve a constant D6500K (the standard). The problem is that it is fairly expensive to get it done. You can try it visually, but the best way is to use a color analyzer, which is very expensive. It is unfortunate that the color temperature and gray scale tracking is not 'correct' off the shelf. The marketing departments have convinced them to get the brightest picture with the most contrast and the worst color will sell the most! Sorry, a bit of a rant there.

Hope this helps a little. I would suggest doing a little surfing around this site and others. I have been an enthusiast for a couple of years and I know I am just scratching the surface. Avia is a great resource and just about anything written by Guy Kuo is valuable.

Good luck and keeping asking the questions. I think these things are crucial to understand.
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Thanks for the response.Your explanation seems intelligent and well

thought out.However I honestly still have no clue as to what these

adjustments do and obviously do not have the equipment to properly

adjust them anyway.Guess I will just leave at the default settings.
The gains and offsets work exactly like the contrast and brightness settings for a TV, except they apply to the individual panels (Or color time segment, in the case of DLP.)

They map a function

Panel drive voltage = (Gain * signal input) + offset (for LCD or LCOS)

Duty Cycle = (Gain * signal input) + offset (for DLP)

For the panel corresponding to each color.

These can (and should) be adjusted to maximize color accuracy and contrast, but since you need special equipment to properly do this, most just leave them alone.

I personally have set these by eye, but I wouldn't recommend that for all but the most meticulous enthusiast, as it requires a huge amount of trial and error.

For gross color problems you will need to adjust these parameters. (My Sanyo Z1 was way too green out of the box, so I had to crank down the Green panel offset to fix this.)
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Thanks Golfnz34me.

I have fooled around with these settings,mostly red gain and offset as

flesh tones were unnatural.I simply took close up scenes of faces in a

normal lighted setting(Peter Parker at the mueseum beginning of

Spider Man)and got it to where it looked right to me.Found scenes of

grass from other dvd's and blue from sky shots,anything where I had

a definite frame of reference of what that color should look like in that

setting.The settings din't change much from the originals but I think it

an improvement.

Anything on AVIA that can do better job than my own eyes for

color gains and offsets?
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Personally I have found the grey step patterns most useful. It is easier for me to see when grey looks a little too "colored" then it is to try and match colors. I just use the biases to adjust the low end of the grey scale and the gains to adjust the high end. Just be sure to do the low end first!

You'll find that the biases are much more critical than the gains, and unfortunately they are also harder to adjust. One way to get very close is to adjust the bias one way until it is obviously too red/blue/green. (Use the second and third-to-darkest grey segments for this) Then adjust it the other way until it is obiously not red/blue/green enough. The midpoint of those two will get you pretty close. Then do this with the gains using the high IRE windows once you've got the biases set, and then double check using skin tones.

It still won't get you as close as a colorimeter, but at least you can get close.
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