That being said, I found the limitation of an HDMI 3D TV that overclocks to a true 120hz at 720p, 900p, and 1760x990 (all frame-skipping tested), but barely overclocks above 100hz at standard 1920x1080 timings. I previously thought this was the TV's dot clock limitation, or perhaps a cable limitation (not likely with High Speed HDMI and pixel clocks under 330-340MHz). Turns out it is a panel limitation (I think) that refuses to display above 110hz beyond a certain horizontal pixel width. The particular display I tested was fine at 120hz with 1728 horizontal active pixels, but was sketchy at 1760 and complete bust at 1780.
Here is a list of 16:9 resolutions that work at 120hz:
720p (added as default secondary resolution/refresh via CRU)
1600x900 (added as default secondary resolution/refresh via CRU)
Here is a list of dirty hack resolutions that work at 120hz:
1750x1000 7:4 / 14:8 (for comparison to native)
1740x1080 29:18 / 14.5:9 (for comparison to native)
1728x1080 8:5 / 16:10 (because calling them 8:5 monitors would've blown consumers minds...)
1600x1400 8:7 / 16:14 (for comparison to native)
1600x1200 4:3 / 16:12 (to further confuse anyone who's mind was just blown after seeing the 8:5 aspect ratio)
Here is a very dirty hack to get games to render at 1920x1080 at 120hz, while sending a reduced pixel resolution to your display:
1. use NVIDIA Control Panel's Custom Resolution (CRU won't allow active pixels to be lower than desktop pixels like NVIDIA does)
2. set "desktop resolution" fields at the top of the custom resolution screen to 1920 x 1080 at 120hz
3. select Manual Timings
4. Enter a reduced active horizontal pixel value (like 1920 to 1728 or 1755)
5. Enter a reduced total horizontal pixel value (like 1838 or 1865)
6. Test the resolution.
Similar to downsampling, this forces a 16:9 desktop resolution. Because of the reduced timings, it will either be slightly stretched on the display into a full screen image that maintains 16:9 aspect ratio (at the cost of introducing slight FXAA-like blur), or appear with a very slight letterbox, depending on the display. If the letterbox occurs, consider trying the 1728x1080 resolution instead.
Your mileage will vary.
quote from other thread discussing the issue:
trey31 wrote:Also of note, I found the clock limit of the M651d (313.99MHz) and the non-clock related restriction of 1080p at 120hz.
At 1920x1080 Active pixels with 2025x1090 Total pixels at 120hz, the dot clock is 264.87
At 1920x1080 Active pixels with 2025x1090 Total pixels at 108hz, the dot clock is 238.383
120hz fails. So does everything down to 109hz (109hz works fine as a custom resolution with even further reduced timings, +104 horizontal and +9 vertical total pixels above the active pixels; it even retains full sound. It just refuses to work as a native res with a modified EDID; so I still use 108hz.)
However, this is not the Pixel Clock limit of the panel.
At 1920x1080 Active pixels with 2025x1260 Total pixels (a +170 pixel increase of the vertical blank over the above timings) at 108hz, the dot clock is 275.562, which works flawlessly. The anomaly here is the fact that a very similar timing to the 120hz above has an 11MHz higher clock at 108hz and works fine, but nothing works between 120-110hz.
So to test a hypothesis, I created a custom 16:10 resolution of 1728x1080.
At 1728x1080 Active pixels with 1848x1400 Total pixels at 120hz, the dot clock is 310.464, which looks great and scales very well as the panel only stretches the image horizontally 192 pixels and not at all vertically.
Ok so this tells me the display is having issues with the number of horizontal pixels above a certain refresh rate, which doesn't appear to have much of anything to do with the pixel clock nor the amount of vertical pixels.
So to test the theory again, I created a custom 29:18 (14.5:9 if you want to compare it to 16:9) resolution of 1740x1080.
At 1740x1080 Active pixels with 1844x1089 Total pixels at 120hz, the dot clock is 240.9739, and it works great at 120hz. But...
At 1740x1080 Active pixels with 1844x1100 Total pixels at 120hz, the dot clock is 243.4080, and it is not working properly.
So my guess is that anything greater than 1728 Active horizontal pixels causes the display to become less and less stable between 108hz and 120hz.
4K Resolution downsample UPDATE:
Also, to update what I previously posted in the thread about 4K resolution and pixel clock, the M651d successfully passed at 3840x2160 Active pixels with 3944x2169 Total pixels at 37hz (36.7hz), and the dot clock is 313.9515
Going above 313.99MHz clock at any resolution causes screen flickering, every step of .1 above that introduces significantly more. 314.0000MHz appeared to be stable, but I encountered some very minor flicker and changed all the custom resolutions to at least 313.99MHz and have seen no issue sense then. Also, anything near 315MHz or above flickers and shakes severely.