The lenovo 81SS Flex 14/S540/S340 series of laptops only officially supports 1920x1080p HDMI out. By using AMD custom resolution, we are able to push 3440x1440p resolution displays.
The issue with HDMI 1.4 or low powered display outs is that it limits the amount of bandwidth required to push full 4K, or refuses to negotiate full resolution support on the ultrawide over HDMI. The Vega 8 base graphics should be able to support 4k, as well as ultrawide 21:9 high resolution ratio displays. If the laptop had USB-C display out, there wouldn’t be an issue.
To fix this, we need to set a custom resolution within AMD’s Radeon settings. The issue here is that it’s bugged – setting refresh rate automatically to 60,000. Furthermore, it calculates a bandwidth that exceeds what is supported by HDMI 2.0.
To start, install CRU (Custom resolution utility). We will use this to calculate appropriate timings and porches for the custom resolution within AMD Radeon settings.
Since I had issue with the CB342CK display by Acer, I will be using it as reference. The utility automatically loads the details supported by the display, and we can use the edit feature to show all the settings it would use.
Copy + paste these values into Radeon setting’s custom resolution section.
The next step is to set the refresh rate and the G.pixel Clock. Set the refresh rate to 75 (or the max supported). The G. Pixel Clock here is where the error message “the custom resolution is not compatible with this display” showed up if configured incorrectly. Set this to a value less than 300,000.