My 3090 video card was hooked to my Samsung q900r and when I switched on the HDMI 4-port in the input signal plus option I get a blank screen but I can still hear sound through the headphones.
I unplug my HDMI port 4 from the one connect then plug it back in the port 4 again and my computer has kicked me out of Windows and I have to sign back in( and sometimes it doesn't kick me out of Windows).
If I plug my 3090 through my Yamaha RX-V6A theater receiver everything works just fine at 4K60hz. I would really like to run 8k60hz directly through the TV but it looks like it's not going to happen and yes I have the latest 2.1 HDMI cables.
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When you are at this point
"when I switched on the HDMI 4-port in the input signal plus option I get a blank screen but I can still hear sound through the headphones"
OFF and ON Game mode 1 time same time you need 3 times to screen work With 4k 120hz vrr hdr then you can play any games
All people are waiting for this issue to be resolved
You need Cable like this
I've got the Q80t and having almost the exact same issue as you!
I am using an RTX 3070 and a brand new HDMI 2.1 certified cable, whenever you turn on the input signal plus on the 4th HDMI port, the TV goes blank, and after a while, it shows no signal, but the other ports work fine.
I really wanted to get 4K 120Hz with HDR and VRR working on this TV since it supports these features.
It seems like the HDMI 2.1 functionality is broken on Samsung TVs.