Hi there.
I have an old video chip which is generating one signal as sync+luma and another with color which are feed into an RF modulator to generate a composite output signal. My goal is to bypass the Rf modulator take those two signals and generate a VGA signal or HDMi. Is it doable ? Thank you.
If the modulator originally provided RF for a television, a likely scenario as not much else used modulated RF, the scan rates are not VGA but the frequencies for your country.
It should be possible to use RGB/Sync signals directly to a monitor if it supports the scan frequencies, some VGA monitors can, some can't. I would doubt converting to HDMI would be worthwhile.
An alternate format is S-video which is not as well known. Modern equipment often lacks it although S-video can be useful when high resolution isn't an option.
It only uses a few wires so it may be compatible with your video output. The connector is 4-pin mini-DIN (or sometimes 7 pin).
There are adapter cables which plug into S-video and combine the signals to one video output usable with a tv, vcr, etc.
No rf modulator is needed.