After seeing the terrible quality of the play-asia RGB cables, I decided to do a direct SCART output onto my Saturn, removing that 10-pin bastard connector and soldering the output directly to the motherboard. Currently, I have a small ribbon cable inside the machine, that carries video output from the Saturn motherboard into an internal USB connector I removed from a spare PC motherboard - it was the only small enough connector I could find with enough pins. (soldering directly to the motherboard would be a permanent solution, which I why I did this)
This shebang is connected to the SCART output, wired up according to this diagram:
... except for pin 8 (aspect ratio control) which I omitted, and the audio/composite input/output pins switched around, as I attached a female connector while the diagram is for a male one. (just exchange pins 2, 6, 20 with 1, 3, 19). Ground pin is daisy chained.
Now, the problem is, I have this wobbly washout ontop on the picture, most visible on yellow background (ex. Virtua Fighter Kids title screen). It's the exact same problem I had with the play-asia rgb cable. I figure this happens because the scart connector takes h/v sync info from the composite video signal, and the extra video data causes interference. Now, the Saturn outputs composite sync on a separate cable, and sure enough, using that in place of composite video removes the washout entirely.
However, if I use pure sync, the picture is jumping up and down! Like, randomly once or twice every second. How should I go about fixing this? What could be the cause? The cables I use are all well shielded and high quality, so that can't be the problem...
For the time being, I've connected both c-sync and composite video to the same pin, this reduced the washout effect significantly, but it's still annoying enough.
Moreover, if I switch from 50hz to 60hz, the washout effect is removed. (I've yet to try c-sync only with 60hz output). This is only a partial fix because I have quite a few PAL games which are optimized for 50hz, so I'd like to have 50hz too.