I have very old jpg-photos with 4 and 5MP and an UHD-monitor to watch. No question, I am lost with the source quality.
Any hints what I could try? I looks like to sharpen them a lot more than normal could be helpful with upscaling.
I think it is forbidden to use denoise in this case.
what you can do? accept that they are low quality
Just share a couple… When I started to use DT I went back and tried to edit my whole collection and it went back to some old point and shoot nikon’s and sony camera’s …I was able to improve them. They are improved memories but still not great photos 
My old 10D photos are not good because I was a bad photographer back then but the quality of the files is still fine
Oh I so hope to ever reach that stage but so far it’s not looking good.
Here they are: Improved memories of 4MP photos with upscaling
That is the goal.
I would suggest keeping the edits light. Also look at the old display referred modules as they are well suited for JPGs who’s look is baked in.
The levels control or the RGB curves (sorry if I got the name wrong) are worth a try. Sorting out your black and white point and perhaps tweaking contrast can help a lot.
I don’t think there is any input upscaling, so perhaps use another app to upscale the image before processing, then you might get away with a little sharpening.
Interesting. Maybe Imagemagick with sharpen-option is an idea.
I have only 1 big problem with editing, which is sharpness, the other edits are more or less ok.
No idea, what is wrong with the images, maybe wrong focus of the camera, a few are sharp, while most are not. So unsharp photos with 4MP upscaled are frustrating. Unfortunately these have a big personal value.
Maybe someday AI can help.
Did a quick test, this seems a great idea. I need to find out the options with Imagemagick, whcih make sense.
Ha, this post got me to finally revive my trusty old 10D and create a noise profile for darktable 
Add Canon EOS 10D noise profiles by piratenpanda · Pull Request #20256 · darktable-org/darktable · GitHub
After I have learned what to do with 4MP JPEG, the challenge will be to edit 6MP RAW (PEF) for UHD monitors.
But 1 step after another. I see things can be improved and of course this is not perfect.