Vega 64 issues with 4K @ 120Hz?

[GoogalyMoogaly]

Hi,

Got a couple new 4K monitors recently in Black Friday sales and have just connected up my Vega 64 (again). It won't let me select higher than 4K @ 60Hz.
I was using the same cables with a GTX 980 and one of the cables allowed me to do 4K @ 120Hz (the other didn't let me go above 60Hz).
So I'm wondering are AMD cards (or the Vega ones specifically) are a little more picky with cables?
Any idea what I can do? There's no setting AMD have turned on in the control panel that I can disable to allow the higher refresh rates is there?
I believe the cables are all reasonable quality (or at least I did based on using them with Nvidia cards). Some of them are a little niche. Due to the oddity of monitors tending to have 1 DP port and a bunch of HDMI ports while GPUs tend to have 1 HDMI and a bunch of DP ports I've bought some not cheap cables that do DP -> HDMI and were stated to do up to 8K @ 60Hz (I can't link to them as they weren't bought on OcUK).

Anything I can do?

Cheers.

 

[eeii]

Not sure this will help but my Samsung monitor was not happy with cheaper cables and flatly refused to work with what I thought was ok cables Now it could be my old vega 64 was sensitive, either way the fix was to by some quality cables. From memory they were club 3d. I would also make sure the monitors are being fully recognised by the AMD drivers and install monitor drivers if needed.

 

[Mr Evil]

In order to do 4k@120Hz with full colour, both the monitor and graphics card must support either or a connector that has sufficient bandwith (HDMI 2.1 or DP 2.1), or DSC (to reduce the required bandwidth). Your Vega 64 supports neither, unforunately.

 

[GoogalyMoogaly]

Not sure this will help but my Samsung monitor was not happy with cheaper cables and flatly refused to work with what I thought was ok cables Now it could be my old vega 64 was sensitive, either way the fix was to by some quality cables. From memory they were club 3d. I would also make sure the monitors are being fully recognised by the AMD drivers and install monitor drivers if needed.

The HDMI cables might not be the best it's true, although I've had decent results from them in the past (on Nvidia cards). The DP -> HDMI cables weren't cheap (~£25 each) but I couldn't find many suppliers of those.

In order to do 4k@120Hz with full colour, both the monitor and graphics card must support either or a connector that has sufficient bandwith (HDMI 2.1 or DP 2.1), or DSC (to reduce the required bandwidth). Your Vega 64 supports neither, unforunately.

I thought DP 1.4 could support 4K@120Hz?
Oddly my GTX 980 could support 120Hz over HDMI despite it being an older version that doesn't appear to support 4K @ 120Hz!

 

[andybird123]

DP1.4 does support 4K120 but not at 4:4:4, I think it has to drop to 4:2:0

Nvidia have options for settings which could explain why that is working, you'd have to find out if the vega supports it, a quick Google shows a lot of other people not being able to get 120hz from a Vega, not sure if there are any solutions in there

Certain GPU monitor combos can be very finicky about cables too

 

[GoogalyMoogaly]

Ah, I see, I think it's all starting to make a bit of sense now.
DSC is only over DP right?

 

[Marine-RX179]

I thought DP 1.4 could support 4K@120Hz?
Oddly my GTX 980 could support 120Hz over HDMI despite it being an older version that doesn't appear to support 4K @ 120Hz!

GTX980 is HDMI 2.0 as well, so it shouldn't have enough bandwidth for 4K HDMI 120Hz.

Only thing I could think of is may be you were either not actually running at 4K or 120Hz.

 

[GoogalyMoogaly]

GTX980 is HDMI 2.0 as well, so it shouldn't have enough bandwidth for 4K HDMI 120Hz.

Only thing I could think of is may be you were either not actually running at 4K or 120Hz.

Oh yeah, it's entirely possible Windows was lying, I don't really know how to test it.
It certainly looked like it was 4K, but not 100% that it was 120Hz. As you can probably guess, not too many games would run at close to 120fps at 4K on a 980...

Edit: I will say that the other 4K screen was only running at 60Hz (which I originally blamed the 980 for, but now I'm wondering if it might have been the cable (DP -> HDMI)) and the memory clock on the 980 wasn't downclocking (and the core clock wasn't fully dropping either). If I set the HDMI monitor down to 60Hz both the clocks on the 980 dropped when idle.

 

[Eddie99]

Oh yeah, it's entirely possible Windows was lying, I don't really know how to test it.
It certainly looked like it was 4K, but not 100% that it was 120Hz. As you can probably guess, not too many games would run at close to 120fps at 4K on a 980...

HDMI 2.0 can do 4K 120Hz with chroma subsampling which it was probably using.

Chroma Subsampling: 4:4:4 vs 4:2:2 vs 4:2:0

What is Chroma Subsampling and where is this visible? Chroma subsampling is a type of compression that reduces the color information in a signal in favor of luminance data.

www.rtings.com

 

[GoogalyMoogaly]

HDMI 2.0 can do 4K 120Hz with chroma subsampling which it was probably using.

Chroma Subsampling: 4:4:4 vs 4:2:2 vs 4:2:0

What is Chroma Subsampling and where is this visible? Chroma subsampling is a type of compression that reduces the color information in a signal in favor of luminance data.

www.rtings.com

Oh yeah, pretty sure it was 4:2:2

Edit: Although even setting the Vega 64 to 4:2:2 doesn't allow it to do 120Hz

 

[GoogalyMoogaly]

OK, I've now used a DP -> HDMI cable that I know allowed at least 120Hz @ 4K on my 1070 and it won't do over 60Hz on the Vega 64, even if I drop it to 4:2:2 (which the 1070 drops to).
Not sure why, bgut it looks like I'm going to be downgrading to the 1070 so I can get the higher Hz.

Disappointing, not sure why the AMD card can't/won't do 120Hz (at 4K, it will at 1440p).