What “CSync” is depends on whether you’re talking about the signals themselves or the type of sync used in an RGB cable.
The “short” explanation is that analogue video signals rely on two sets of pulses to determine when to start a new line or jump back to the top of the screen–horizontal and vertical sync respectively. PCs carry these signals on separate wires (the HV in RGBHV, known as separate sync, the scheme used by VGA); however, pretty much all other analogue video formats–RGBS (most often via SCART), RGsB (sync-on-green), YPbPr (most often called component), YC (S-Video), CVBS (composite video), and RF–send the horizontal and vertical sync signals together on the same wire, which is a scheme called composite sync, or CSync.
In the context of the cables, CSync means something a little different. Where RGBS (most often SCART) is concerned, that capital S refers to the composite sync signal being on its own wire, and can be any of composite video (the confusingly-named sync-on-composite), luma from S-Video (sync-on-luma), or it can be just the composite sync signals without any video data (referred to as CSync, or clean composite sync). Most consoles will use composite video as sync, which I believe is because doing so would allow a display that doesn’t support RGB to fall back on composite video.
So, regardless of which console you’ve got hooked up, you’re providing a composite sync signal, but, depending on the console and/or how the SCART cable is wired, you may not be providing clean composite sync; either way, both are compliant with the SCART spec.
Dreamcast and Saturn work fine when plug the scart switch directly into the tv, but picture on mega drive just jumps around and eventuelly the tv won’t even find the signal. I guess this is because of the lack of CSYNC.
Assuming the console and cable are good, the problem might be because the Mega Drive’s video output is just enough out-of-spec (high or low framerates, line-doubling gives it too many lines, etc.) for the display to tolerate it.
So my question is, i have thought about using an RGB2COMP to connect the scart switch to the Retrotink 2x pro. Will passing through the RGB2COMP reduce the degredation in image quality since there is no composite involved anymore, or do you believe the lack of CSYNC will reduce the quality still of the picture still rather than just not use the scart switch?
Using an RGB2COMP would absolutely reduce the degradation over using composite; you might lose a small bit of fidelity in the colorspace translation from RGB to YPbPr, but you’ll still end up with a sharp picture with good colors, and none of the rainbowy/shimmery BS that composite video would get you.
If you want to skip doing a colorspace conversion before digitizing with an RT2X Pro, you can get a RetroTINK 2X-SCART, which is basically an RT2X that directly takes RGB instead of YPbPr/YC/CVBS.