The Xbox Series X applies chroma subsampling when the console is set to 4k and 60Hz. This is striking, because that is not the case when 4k and 120Hz are set with an HDMI 2.1 TV. At 60Hz the console appears to go to the bandwidth of HDMI 2.0.
Television reviewer John Archer writes on Forbes that this is the case on his Xbox Series X, in combination with the 48 “LG CX-OLED TV. This television supports HDMI 2.1 with a maximum bandwidth of 40Gbit / s, which makes it possible to use the 120Hz. option in-game with the 4k resolution. At that 4k120 setting, the CX appears to use 4: 4: 4 and thus no chroma subsampling, which means that no ‘color compression’ is applied. the Xbox settings are reduced to 60Hz, 10bit color depth is maintained, but color compression is in the form of 4: 2: 0 chroma subsampling.
The application of such a color compression is striking, because for 4k60 significantly less bandwidth is required than with 4k120. At 4k, 120Hz, 4: 4: 4 and 10bit color depth for hdr, the bandwidth is 40Gbit / s, just enough for the maximum bandwidth that the CX offers. At 4k, 60Hz, 4: 4: 4 and 10bit color depth, the bandwidth is 20Gbit / s. So this is well within the maximum bandwidth that CX supports, but color compression is still applied, in the form of 4: 2: 0 chroma subsampling.
It doesn’t seem to be a bug. An earlier observation by another television reviewer, Vincent Teoh, shows that the LG television uses the maximum bandwidth of HDMI 2.0 at 4k and 60Hz. This can be seen in a video of him, in which a retrievable window of the LG television indicates that there is the signal technology of HDMI 2.0, namely transition minimized differential signaling. At 4k and 120Hz, the window indicates fixed rate link, the technology of HDMI 2.1.
The HDMI 2.0 standard has a maximum bandwidth of 18Gbit / s. At 4k, 60Hz, 4: 2: 2 or 4: 2: 0 and 8, 10 or 12bit color depth, the bandwidth is 17.82Gbit / s. On the CX this should not be necessary, as the television passes its bandwidth to the console via the EDID data. The Xbox Series X console may still assume that someone who selects 60Hz will only do so because they want to stay within the HDMI 2.0 bandwidth.
It is not clear whether this issue is limited to the CX and other HDMI 2.1 televisions from LG, or whether it also applies to HDMI 2.1 televisions from other manufacturers, such as Samsung’s. However, it appears to be coming from the console so it will appear this way on all HDMI 2.1 TVs.
Archer made his observations on the game Call of Duty: Cold War. For owners of HDMI 2.1 televisions, there is a reason not to play this game on 4k120 but on 4k60: ray tracing only works at 4k and 60Hz. It remains to be seen whether this issue occurs in the same way with other games that, like Call of Duty: Cold War, support both a 120Hz and a 60Hz mode on the Xbox Series X. That will depend on the settings of those games. This is because it is possible to avoid the limitation of the bandwidth in Call of Duty by leaving the Xbox Series X set to 4k and 120Hz and then playing the game in the standard mode. That standard mode is 4k with 60Hz and ray tracing.
This color compression does not mean that the image suddenly looks significantly uglier. It is a subtle difference, although it can be seen by paying close attention. For example, the color compression can make colors appear slightly fainter and image artifacts may be visible. Chroma subsampling is often used to reduce bandwidth. The idea here is that color resolution is sacrificed, but that is not noticeable in most natural images, because the resolution of the brightness signal is preserved.