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I was wondering if it's possible to dim a VGA signal. Assume a PC with a VGA output, which is sending a VGA signal to a screen.

Would it be possible by placing a microcontroller (or raspberry pi) in between, which passes the signal through but is doing some transformation on the pixel-carrying signals RGB - leaving the hsync, vsync signal intact?

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  • \$\begingroup\$ VGA signals are high-speed analog single-ended signals on the R, G, and B pins. It would not be prudent to try to use a microcontroller or Raspberry Pi as a means of attenuating the signals, but an appropriate analog circuit (possibly with a control input from a slower DAC) will do the trick. Since I am not well-versed with analog I will leave this to be discussed by others members of the community that understand it better and can give better advice. \$\endgroup\$ – ζ-- Sep 8 '16 at 19:44
  • \$\begingroup\$ Nope, not possible. VGA uses analog signals of much higher frequency than any uC or Pi could digitize. Maybe with some kind of (USB?) VGA video grabber, if such thing exists. Even then, it's quite a lot of data to process each second (640*480*RGB*60Hz=55MB/s). \$\endgroup\$ – JimmyB Sep 8 '16 at 19:49
  • \$\begingroup\$ Get a different screen, I wouldn't want to spend a few weeks to get this to work, I don't think anyone's time is worth that. Secondly, if your driving this from a PC why not adjust the gamma property of your video card? \$\endgroup\$ – Voltage Spike Sep 8 '16 at 19:53
  • \$\begingroup\$ What do you mean by "dim"? If you divide the VGA R-G-B voltage signals by any ratio, you will reduce the contrast of the resulting image,not necessarily its brightness. Are you intending to feed the signals to an LCD display, or a CRT display? There is a major difference between these types of displays related to image "brightness". What is the goal you want to achieve by "dimming" the signals/image? \$\endgroup\$ – FiddyOhm Sep 8 '16 at 20:44
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You wouldn't save any power on anything besides a CRT or OLED (unless you're going into a TV that has some image based/dynamic contrast stuff going on). the VGA R/G/B voltages represent the intensity of each color channel, but on most LCD panels the backlight will be on regardless with the same brightness and power drain.

If you simply want to dim the screen, it's pretty easy to put a variable gain amplifier on the R/G/B signals. In fact, dedicated chips exist to do this (or at least they used to - the parts might be hard to get these days, for example http://www.nteinc.com/specs/7000to7099/pdf/nte7081.pdf).

Note that the chip I linked has -

Three Externally Gated Comparators for Brightness Control

You can use the RPi/uC to generate some DC and then use that to control brightness per channel.

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  • \$\begingroup\$ Amazingly, available at Amazon (along with Newark, but not Digi-Key) \$\endgroup\$ – tcrosley Sep 8 '16 at 21:40

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