Having adjusted color temp and brightness, another service-code adjustment you can make on these late-model HS/XS/XBR TVs is the image gamma. Even if you don’t want to change the gamma presets, it’s good to know where they are.
Given a specific black-point and a specific white point, the gamma setting determines how bright the mid-tones are. Without getting into the math, the graph that relates the instantaneous red, green, and blue signals that control the brightness of the spot on the screen is not a straight line, but one with a deep, sunken curve. Gamma is the degree of this curvature: the higher the gamma, the more deeply the line is curved and the darker the mid-tones are.
Consequence of higher gamma = film-like look, brilliant highlights, natural 3-D appearance of contrast-lighted subjects (say, a 3-light setup on a news anchor), less shadow detail, more realism in general, sparkly things (specular highlights) really sparkle.
Consequence of lower gamma = better shadow detail, but . . . pasty-faced people, lack of “sparkle,” feeling of “flatness” in the picture. In other words, the “TV look.” Outdoor scenes can be really flat. Valuable for lifting subject matter out of murky darkness in troublesome video, however.
Calibrated studio monitors use a gamma = 2.2, but program material varies all over the place. Traditionally TVs have been adjusted at the factory for bright mid-tones and clipped whites — great for store demo and vivid color — but viewers are now seeking more realism with their sophisticated program material. The Pro mode provides that with its maximum gamma.
The Pro picture-mode setting has the highest gamma out of the box (maybe it’s 2.2-ish), and for most good material I much prefer this to the lower values available in the other picture modes. But sometimes you may want something available for a whacked-out broadcast. I’ll deal with customizing the picture modes in a separate article, but here is where the gamma settings are located.
2170P-4 #17 or 11 GAMM is the master setting, with a range of 0-3 for its four possible values. But it’s a 2-level system, and GAMM simply points to a column in a table of presets where the *real* gamma values are set. This table is 4 rows and 4 columns. The rows are 2170P-4 #18-21 (or 12-15) GAMS, GAMR, GAMG, and GAMB. So, to be clear, GAMM = 1, for example, means only what the values of GAMS thru GAMS define it to mean. You get to choose.
However, a specific value of GAMM is saved for each of the four picture modes and, for each of those, different input/scan-mode combinations, such as RF, S-video, Compressed 480i (and others), DVI 480i (and others), memory stick, etc. Only the service-codes chart will make this clear. Use it as a roadmap. Bottom line: over 50 values of GAMM can be saved for those many combinations (although only a few will likely apply to any one user).
GAMR, -G, and -B are settings for each of the primary colors (CRT guns) for red, green, and blue. Unless you are correcting a grayscale defect, one generally sets these all to the same value. 0 is the highest gamma, and the values of 1-15 increasingly lower it. Or you think of the higher values as raising the mid-tone level. (More on grayscale corrections in another article.) If you want to tinker, write down the original values.
GAMS is still a bit of a mystery to me. I suspect it means “slope” for the gamma curve, and that setting GAMS above 0 tilts the curve up at the bright end, raising highlight values along with midtones. It is hard to deduce from the Digital-Video Essentials DVD (DVE) test patterns, as some of those trigger the brightness limiter in these TVs and fool your eye. I have tried looking at grayscale strip- and chip-patterns, and I’m not confident I understand it. For now I think it’s an added slope to the curve, and I am leaving it at 0 for most purposes. I’d love to hear from others on this.
From the factory, GAMM = 0 points to the following: GAMS thru GAMB *all* set to zero, and it is assigned to the Pro setting for *all* inputs. It’s the most conservative settings and yields what most folks will likely prefer for good video material.
[ This message was edited by: kentech on 2005-04-03 20:02 ]
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