I am scratching my head big time.
I have Windows 10, a 3090, and a Q90T.
But no matter how I toy with the settings, when I enable 120hz 4k I get vertical lines on the top quarter of the screen, and the rest is black.
HDMI cable is rated 48 GB/s, in port 4...all hardware connected as it should be.
I have tried with HDR on, off, game mode on, off, VRR on and off.
I bought this TV specifically to run 4k 120hz, and I'd hoping to find some help with settings to get it working.
I have the exact same hardware setup and have experienced the exact same problem. The TV works fine at 60Hz but I get the black screen with horizontal lines filling the top 1/4 of the screen when changing to 120Hz via the Nvidia control panel. I have tried multiple HDMI 2.1 rated cables including a pricey AudioQuest cable. The cable is not the issue. I have tried every setting on the TV and graphics card possible and also updated the TV firmware to 1460.9 - the latest version as of this posting. Nothing works. However, I can usually get it to work at 120Hz by switching the source from HDMI 4 to one of the other HDMI ports and then back to HDMI 4.
So when you see the lines at the top of the screen, switch to either HDMI port 1, 2, or 3 - then switch back to HDMI 4. Most of the time, this will stabilize the image and the TV will work flawlessly at 120Hz. On several occasions, this method did not work and I ended up with horizontal lines across the entire screen, not just the top. Needless to say, this problem is frustrating. I contacted Samsung about it and they were worthless and acted as if they did had never heard of the issue. The problem seems to be with the initial communication and setup between the TV and the graphics card. The TV is capable of working with the 3090 at 120Hz but something falls apart during the initial setup and you have to swap inputs to get it to work. Also, this happens whether or not you are in game mode and whether or not you have input signal plus turned on for HDMI 4. It also does not matter if Gsync is enabled or disabled either(I initially thought it might be something with the Gsync setting).
I am hoping a future firmware update for the TV will fix the issue but who knows. I think it comes down to a buggy implementation of HDMI 2.1 by Samsung.
I have the exact same hardware and the exact same issue. The cable is not the problem (tried multiple HDMI 2.1 cables including a pricey one from AudioQuest) and I have tried every setting on the TV and the Nvidia software. The TV works fine at 60Hz but the minute I switch to 120Hz via the Nvidia Control Panel I get the horizontal lines at the top and black screen at the bottom. Gsync on or off is not a factor either.
I have been able to get 120Hz to work by switching the source input when the horizontal lines are being displayed. When you see the lines and black bottom half of the screen, change the source input to one of the other HDMI inputs either 1,2, or 3. Then change back to HDMI input 4. This will often stabilize the picture and it works flawlessly at 120Hz and does not revert back to the distorted screen.
On a couple of occasions I have done the input swap fix and it failed to work. The horizontal lines were spread across the entire screen with a black background. Most of the time, however, the input switching works.
On the bright side, the TV is capable of working at 120Hz with the NVidia card. It's just that something is breaking down during the initial communication between the video card and the TV when 120Hz is first activated. I am on Samsung firmware 1460.9 (the latest version as of this post) and it does not fix the problem. I think the overall problem is that Samsung's implementation of HDMI 2.1 is just buggy. Hopefully a future firmware will fix this annoying problem. I did call Samsung about it and they were worthless and did not seem to even be aware of the issue.
I just wanted to say thank you.
I tried your method, several times, with no change in results. Then, as if by some miracle, I simply changed the Nvidia setting to 120hz (preparing to try your method again) and it just worked.
And now it just works.
Hopefully this thread can help someone, as SAMSUNG doesn't seem to give two cares about this confusing problem.
I really appreciate that you took the time to help me out, Ottotavius. I had a rough day today, and you just made it a bit better.
Okay Smazza, and fair enough. Here is an update.
I did get it to work, and it has continued to work.
However, the issue I now have is that the HDMI will fail (sort of). Basically, sometimes when I shut the t.v. off, when I turn it back on it won't see the device. I have to physically unplug the HDMI from the port and plug it back in.
Then I have open the menu, and re-do the 120hz settings which reset to defaults. It happens enough that it is annoying, but not smash the TV annoying.
But I have to say it's getting old.
Will try the firmware update and report back.
So that firmware update did nothing.
Upon getting up this morning and turning on the TV, I once again had to manually unplug the HDMI cable, plug it back in, and reset all the 120hz settings (turn on input signal plus, and turn game mode back on).
I know it's a first world problem, but Samsung: Fix your *****.