I would like to use a μController to adjust the brightness of VGA/EGA RGB signal. My intention is to allow for a setting from "full brightness" (0% attenuation) to "completely dark/black" (whatever % necessary, probably >80%).

I assume it means that all three channels should be adjusted in parallel to the same level, but I'm also open to them needing their own separate adjustment curves to compensate for the perceived brightness of the different colours. The attenuator/s should be controlled from a (single) μC, like a PIC or similar.

A digipot could be a possible solution, although according to this app note from Maxim (https://www.maximintegrated.com/en/app-notes/index.mvp/id/3081), there seems to be at least -3dB loss for an ordinary digipot at video frequencies. It may be incorrect, or there may be better digipots that I have not found yet.

  • \$\begingroup\$ What kind of input signal are we talking about here? \$\endgroup\$ – Eugene Sh. Feb 11 '19 at 19:47
  • \$\begingroup\$ "digitally controlled video attenuator" would make an excellent starting web search... \$\endgroup\$ – user65586 Feb 11 '19 at 19:47
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    \$\begingroup\$ Welcome to EE.SE but please note that direct shopping questions will result in fairly prompt closure of your question as per site rules.You need to edit this if you want to save it from closure. Try asking for recommendations on search terms. \$\endgroup\$ – Transistor Feb 11 '19 at 19:48
  • \$\begingroup\$ @jdv: I did such a search, but what comes up is either Audio devices such as digipots, or HF/RF parts that are quite expensive. I know I didn't mention any max price, but I was hoping there were (moderately priced) parts dedicated for this task, that I had missed due to inadequate search terms. \$\endgroup\$ – Eriond Feb 11 '19 at 21:28
  • \$\begingroup\$ @Eugene Sh: Analog RGB @ <1000px/line @ 15kHz \$\endgroup\$ – Eriond Feb 11 '19 at 21:30

Yes, there are digitally controlled attenuators. They are referred to as digital step attenuators and are available from numerous semi-conductor vendors.

For a low-budget homebrew build, you could attempt to add digital control to a basic switched step attenuator. Consider the following ARRL schematic as a starting point:

enter image description here

Reference: https://www.arrl.org/files/file/Technology/tis/info/pdf/September1982_Attenuator.pdf

You would need to replace the DPDT with a solid-state alternative. CMOS analog switches may be right at the edge of having too great an on-state resistance. In which case small signal NMOS elements would be required.

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