Perhaps RGB results in ground loop when tapped, but signel going through the encoder for other signels has a different better connection the ground that doesn't result in a g
You can run different Genesis 2's with an A/V cable and Dreamcast mixed together, when one is off there is either no effect or the picture is at half brightness depending which is hooked up (faulty A/V switchbox actually mixes 1 & 2 input ports but not the last two 3 & 4). The point is different components react differently because of circuitry or other issues. I've noticed there was problem with the A/V switchbox with my MD2 V1.8, but my MD2 VA2.3 never shows it unless you turn both the DC and it on. Same damn cables, too. You can use a Y-cable (which is designed for mixing the L/R audio together for mono devices) to simulate the effect it has on Video (but combining to different Video signels as impendence mismatch happens when one is off but the other is on and the picture scrambles and becomes a horrible mess when both is on because the signels conflict). I've seen shitty TVs, mix both video signels from the front and back jacks (my brother had such a set a long time ago).

