A few months ago, I replaced the dead fluorescent backlight of an LCD computer monitor with an 8-bit-per-channel RGB LED strip, and built a controller for it from an AVR microcontroller that talks to the same Raspberry Pi that drives the screen. This allows me to dim the display from a shell script or a manual potentiometer or whatever. So far so good.
Now that I've used it for a while in very low ambient light, I want to dim it even more, but the minimum setting for the LED strip is still too bright. Even dithering barely-on yellow (RRGGBB = 020100) vs. off for adjacent "pixels" isn't enough, and using red-only (RRGGBB = 010000) is almost useless as it only shows that channel of a color image.
The monitor itself only accepts VGA, so I have an HDMI > VGA converter between it and the Pi. And since I have that analog signal to work with, I eventually came up with this concept:
All three colors operate together, so there are effectively 2 proportional controls and 1 binary control. All managed by the AVR in addition to the backlight itself.
The sync's and other signals are wired straight through, unchanged.
I looked on DigiKey for digipots and voltage-controlled-amplifiers that would work for a ~62MHz analog signal (1920 x 1080 x 60Hz, alternating black and white), but didn't find anything even close. (I didn't look for opamps yet; I'm pretty sure that those exist.) I also thought about doing those functions in the Pi, before the HDMI > VGA converter, but decided that I'd rather like a hardware solution if possible.
Are there digipots or VCA's that can do that?
Or maybe a different circuit design or processing technique that I'm so far unaware of?
(a PWM chopper might be a challenge too: >10x signal freq with LPF gives >0.62GHz PWM)
Maybe a chip that I haven't found yet, that "just does this"?
Note: The schematic above is just a concept that would technically work with ideal parts. It has some glaring omissions (like impedance matching), and is not a final design by any means.