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Decoding the Service Codes 06 - Brightness Limiters |
kentech
Rank: Sony Enthusiant
Joined: Mar 11, 2005
Posts: 45
From: Portland, OR
 |  Posted: 2005-04-03 21:04
Unlike many small and older large analog TVs, the image on these late-model Sony CRT-type TVs is not completely under control of the video signal. There is significant brightness limiting, some optional. (And the “Dynamic Picture Control” — brightness and contrast control — promoted by Sony is another story entirely! Another article.)
Imagine the following: You are watching a car commercial full-screen on SD in Pro mode. The car is nicely situated in a medium-bright outdoor scene beside a river. Brilliant highlights sparkle from the water, and there is a 100%-white sun-glint from the windshield. Gorgeous! Then, cut to a solid-white screen with the text “2.9% financing” in the middle. You will note that, no matter the picture mode, the white screen will be nowhere nearly as bright as that sun-glint, even though it may also be 100%-white video. And it’s not your eyes adjusting. It is especially apparent during kiddie cartoons, with their large areas of bright colors and white.
This is brightness-limiting. I was first appalled by it, having come from a 27” older TV with no limiting of any kind. But I have come to believe is a bit kinder to the eyes, especially if you like to watch bright video in evening living-room lighting. Those 100%-white screens would be blinding.
As an engineer myself, I imagine it may serve several purposes:
(1) It limits the CRT’s beam-current. My 36XS955 can be made incredibly bright, and that is a large beam-current for the poor electron-emitting cathodes and the 31.5 kV high-voltage supply to support. So it likely protects the tube and maybe other electronics.
(2) All that energy flung all at once at the aperture-grille causes it to warp or sag, and you start to get pink or green patches in the white areas. You can probably see this occasionally; I certainly do. This is probably the outer bounds of this CRT technology, and limiting this heating keeps folks from calling Sony to complain. Another CRT-protection feature, too.
(3) For aesthetics. The response of the limiter is very prompt, and I got used to it very quickly, even though I’m very picky. I don’t like being blinded, hate green/pink patches, and love the brilliant whites when they are small in area. I can live with it. Maybe Sony is just being kind!
I have not found a setting for this instantaneous brightness-limiting, and I’m tempted not to look for it. However, there are a couple of limiters that you do have access to:
2170P-4 #21 and 22, or 27 and 28, ABLM and ABLT. These act over a few seconds. ABLM (values from 0-3) is part of the BLK setting-complex (more another time) and mostly used with Vivid mode, probably to prevent excessive burn in dealer showrooms (my guess). ABLT has values from 0-15, and is set to 0 for “Others” and 7 for “Small Pic.” I’m not sure what qualifies for “small pic.” I want both at 0 for Pro mode.
Then there is 2170P-2 #4 YLMT, which seems to be a “white limit” clipping effect. YLMY = 3 is the highest level, which seems to have no effect. Lowering it to 1 or 0 causes immediate limiting of white. I haven’t tested to see if it’s a hard clip or not, in which case it would be ugly. Fortunately default on my set is 3, which means it’s not in use anyway.
I don’t know where else these limiters may be lurking in the service-code jungle. I see the 3DNR group has a CLPW and CLPP (#2, 3). Also 2170P-2 #7-9 are CLPP, CLPG, and CLPS, with columns for “Other” and “PT.” But maybe “CLP_” has nothing whatever to do with clipping.
Any suggestions are welcome. Maybe there is a setting for the seemingly-permanent brightness limiter. Maybe I would leave it alone if there were.
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KenTech
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