New 4K 144 Hz Monitors use Blurry Chroma Subsampling

We saw quite a lot of these new 4K 144 Hz gaming displays being shown off at Computex. Ones that are available now are the ASUS PG27UQ and the Acer X27, which are around $2000. Both displays support 4K (3840×2160) resolution and refresh rates up to 144 Hz. Many early adopters and reviewers of these displays have noticed degradation in image quality when these monitors are running at 144 Hz, while they look perfectly sharp at 120 Hz and below.

The main reason for this is the DisplayPort 1.4 interface, which provides 26 Gbits/s of bandwidth, which is just enough for 4K at 120 Hz. Display makers had to find a way to get these displays running at 4K 144 Hz, so what they came up with was chroma subsampling (YCbCr), which transmits the grayscale portion of the image at full resolution (3840×2160) and the color information at half the horizontal resolution (1920×2160).

This approach (4:2:2) works quite well in the movie industry, when it is more or less standard to ship the post processed content to movie theaters and TVs using subsampling of 4:2:0 or 4:1:1. While that works for that type of content, computer-generated content like text and operation system interface take a serious quality hit when chroma subsampling is used. So in PC games the user could see a loss in sharpness of the HUD.

Below you can see images that describe how chroma subsampling works and the effect on image quality. You can see how high-contrast edges look worse while low-contrast surfaces look basically identical.

Gamers have wanted 4K 144 Hz displays for a while, NVIDIA has been pushing hard for it too. As a consumer it seems like we’ve been waiting for these displays too, so honestly I’m not surprised that display vendors went with a compromise. The thing that bothers me is that these vendors have not said anything about this (obviously) and make no mention of it in their specification documents.

If these vendors want to avoid using chroma subsampling they could use HDMI 2.1 or DisplayPort’s DSC data compression scheme. HDMI 2.1 was only specified in December, so it is pretty far away from being used in actual monitors. With DisplayPort 1.4 DSC (Display Stream Compression) was introduced, which is actually optimized for this task, although it is not completely losses compression, the quality is much better than chroma subsampling. But there are not a lot of display controller support for DSC at this time.

For all of those out there purchasing these display I would say to run them at 120 Hz if you want a fully unmodified RGB image over the display connection.

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