I've been told that analog video signal from computer's graphics card changes somehow, and because of that there is an "auto" button on many monitors that connect via d-sub.

Why does the video signal change?

  • \$\begingroup\$ Please explain downvotes. I'd like to improve my question, but I need some guidance. \$\endgroup\$ – user1306322 Dec 29 '12 at 14:54
  • \$\begingroup\$ Ok, where should it be then? SuperUser.SE? \$\endgroup\$ – user1306322 Dec 29 '12 at 16:16
  • \$\begingroup\$ This is a interesting question, and has good answers which highlight relevant design issues. \$\endgroup\$ – Chris Stratton Dec 29 '12 at 19:38
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    \$\begingroup\$ @user1306322 If you make it clear you mean it only as an example to explain what you mean that is one thing, but it did read as your primary question. Your accepted answer at one point related primarily to that point also, and not to how it actually works, just how to fix it. It is not bad to have a closed question, it just means it does not fit here. The specific way to fix it was an Super User question. \$\endgroup\$ – Kortuk Dec 30 '12 at 6:33
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    \$\begingroup\$ @Kortuk: According to the CommunityWiki, Every question is on-topic on somewhere on the Internet. If there is a better place for this question, I'd be happy if it were moved there. But until we find such a place, I'm voting to re-open, because the signals in this particular question seem close enough to "a communication scheme" to be on-topic here. \$\endgroup\$ – davidcary Dec 30 '12 at 15:12

Although the screen of an analog color monitor is divided into many individual picture elements (RGB phosphor triplets), there is no fixed relationship between those and the pixels generated by the computer. Instead, an analog monitor will project a slightly blurred image of the computer's pixels onto the back of the phosphor tube; if the beam passes closer to one phosphor pixel than to another, more energy will be received by the former, making it appear brighter. When two adjacent phosphor pixels are illuminated by different amounts, the eye will perceive a bright spot whose center is closer to the more-brightly-illuminated pixel.

It would be possible for an LCD monitor to use simulate that behavior electronically, and indeed many LCD television sets do precisely that. To avoid weird interactions between computer pixels and LCD pixels, however, it is necessary to have a minimum of roughly two LCD pixels' worth of "blur". When watching videos, such blur is generally not objectionable, but when displaying things like text, it's generally undesirable. To generate a clear display, it's necessary that each computer pixel light up exactly one monitor pixel. Because VGA standards were designed at a time before anyone expected pixel-accurate displays, the VGA standard does not provide any mechanism to indicate precisely where the picture data for each discrete pixel should appear in the video stream. Thus, a monitor must try to "guess", based upon the display's image content; when it guesses correctly, it will sample each pixel's value in the middle of the pixel's display time. If a screen shows a lot of detail, then there will be only one way the monitor can line up computer pixels with LCD pixels without obvious (detectable by the monitor) problems. If, however, the screen consists of nothing but large solid areas of color, then many possible mappings between computer pixels and LCD pixels would appear to "work", but only one would be correct. If the monitor guesses wrong, then when the computer shows more detail the monitor would be able to detect that and react accordingly.


The issue with the old fashioned analog video signals like your "d-sub" connector (SVGA probably) carries is that there was no guaranteed exact registration of where the pixles are to the sync pulse. In analog this didn't matter. The image would appear a little left or right, and you'd expect to adjust the monitor for that anyway.

Most modern screens display discrete pixels at fixed locations, so the electronics needs to know exactly what part of the analog signal represents which pixel. This is no problem with a proper digital video feed since each pixel is defined explicitly. When the monitor is given a analog signal, it needs to get the timing of the A/D just right to sample the intended pixels. Things would look blurry if it sampled between pixels so as to get the average of two adjacent pixels instead of individual pixels.

The auto adjust is usually the monitor inspecting the analog stream and inferring the true start and end timing of the picture information. Since this is digitally generated on the video card, it won't change as long as the card isn't reconfigured, but can be different from card to card.

I am surprised that opening the window changes anything. Perhaps the timing in the monitor is susceptible to temperature changes, but these would happen over a bit of time, not instantly when you open the window. Perhaps the different lighting makes you see things differently. I don't have a good answer as this symptom makes little sense.

  • \$\begingroup\$ Opening window doesn't have an instant effect, but over a few minutes. Also after closing it, the room slowly becomes warmer and so does the computer, it also causes blur. I guess temperature matters (in my case at least). \$\endgroup\$ – user1306322 Dec 29 '12 at 14:58

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