I tested a cable from
www.retrogamingcables.co.uk yesterday. To my surbprise the buzzing is almost non existent.
With the volume at maximum and adventure island game at pause, there is only a slight background noise that doesn't classify as the regular hum/buzz thing of the rgb modded pcengines.
I opened the scart and din parts of the cable.... nothing special there. The cable shielding part goes to the sound ground and the pin ground on the composite ground pin of the scart connector. No other ground connections anywhere else in the scart connector (metal shell, RGB grounds etc).
The cable part doesn't look like the super high quality ones (individually shielded wires).... Its not thick and the wires inside it are not particularly thick either.
Maybe afterall it has to do with being of good quality and it being only 1,5m instead of the longer ones I have made myself and experimented on (3m) using high quality cable.