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I'm trying to see if my LG 32UL500-W monitor will overclock beyond 60hz. It looks like DisplayPort 1.2 handles 4k at 75hz.

When I run xrandr --verbose, it shows me the default VESA mode for 4k 60hz:

3840x2160 (0x5a) 533.250MHz +HSync -VSync
        h: width  3840 start 3888 end 3920 total 4000 skew    0 clock 133.31KHz
        v: height 2160 start 2214 end 2219 total 2222           clock  60.00Hz

This works fine. If I calculate it using cvt 3840 2160 60 -r I get the same values.

$ cvt 3840 2160 60 -r
# 3840x2160 59.97 Hz (CVT 8.29M9-R) hsync: 133.25 kHz; pclk: 533.00 MHz
Modeline "3840x2160R"  533.00  3840 3888 3920 4000  2160 2163 2168 2222 +hsync -vsync

However, this command doesn't work if I try calculating the values for anything above 60hz.

$ cvt 3840 2160 75 -r

ERROR: Multiple of 60Hz refresh rate required for  reduced blanking.

If I run that same command without the reduced blanking flag, I get wildly different numbers that don't work when I add them to xrandr and apply them to a display.

$ cvt 3840 2160 60
# 3840x2160 59.98 Hz (CVT 8.29M9) hsync: 134.18 kHz; pclk: 712.75 MHz
Modeline "3840x2160_60.00"  712.75  3840 4160 4576 5312  2160 2163 2168 2237 -hsync +vsync

$ cvt 3840 2160 75


3840x2160 74.95 Hz (CVT 8.29M9) hsync: 169.16 kHz; pclk: 904.00 MHz


Modeline "3840x2160_75.00"  904.00  3840 4168 4592 5344  2160 2163 2168 2257 -hsync +vsync

My monitor doesn't understand the signal if I try to apply those modes at either 60hz or 75hz.

Is there really no way to calculate an appropriate set of values for 75hz? I wish cvt would at least give me the option to calculate values for 75hz using -r so I could at least try them. I know 120hz is out of the question because DisplayPort 1.2 doesn't support that at 4k and there's no way my monitor will handle that rate. None of the online calculators seem to give me appropriate values either.

dwightjl
  • 111

1 Answers1

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CVT 1.1 does not generate reduced blanking values for any rate other than 60 or a multiple of 60. You can use this modified version of CVT to generate those values anyway and push your monitor to its limits, but be careful running your monitor at rates higher than its spec. It could damage the monitor.

Follow this guide to download and compile CVT: https://github.com/kevinlekiller/linux_intel_display_overclocking

I tested this on my LG 32UL500-W and it actually works all the way up to 80hz, but not 85hz. It must be pushing DisplayPort 1.2 beyond its limits a bit.

Here are the mode values that my monitor accepted:

  • xrandr --newmode "3840x2160_65.00_rb2" 567.44 3840 3848 3880 3920 2160 2213 2221 2227 +hsync -vsync
  • xrandr --newmode "3840x2160_70.00_rb2" 612.46 3840 3848 3880 3920 2160 2218 2226 2232 +hsync -vsync
  • xrandr --newmode "3840x2160_75.00_rb2" 657.97 3840 3848 3880 3920 2160 2224 2232 2238 +hsync -vsync
  • xrandr --newmode "3840x2160_80.00_rb2" 703.40 3840 3848 3880 3920 2160 2229 2237 2243 +hsync -vsync

Add the mode that you want to use and set it for your monitor. For example, I used the following command to set 75hz:

  1. xrandr --newmode "3840x2160_75.00_rb2" 657.97 3840 3848 3880 3920 2160 2224 2232 2238 +hsync -vsync
  2. xrandr --output DisplayPort-2 --mode 3840x2160_75.00_rb2 --rate 75

I tested this at testufo.com in Chrome and it identified the higher refresh rate. The only problem is that it might be skipping a few frames, but that's a separate problem.

dwightjl
  • 111