Good call. The grabber displays a vertical bar pattern when there's no signal detected, hence the RGB pattern. I got it to work with a standard 4K signal, although with a delay that I need to figure out how to fix - I can't set the frames per second to anything above 5. The bigger problem is that it doesn't look like I can pass a 4K HDR signal to the grabber (it just displays the bar pattern), and the splitter I'm using (link) cannot down-convert an HDR signal to a standard signal without completely washing the image out. Any thoughts on how to make this setup work with HDR?