Apple tv 4th gen and YCbCr

The H.264/H.265 digital video is always encoded with 4:2:0 chroma subsampling anyway, so if you set it differently, that setting only applies to non-video screens (menus, games, etc) at best. For delivery formats, it doesn’t have the importance that it has in editing. Using 4:4:4 on a 4:2:0 signal doesn’t make it better, just 4× the bandwidth of the same information.

The higher bandwidth needs of 4K UHD needed some kind of extra compression over HDMI, which it found in allowing 4:2:0 passthrough in HDMI 2.0. The settings with the choice for 4:4:4, 4:2:2, or 4:2:0 are only available in a full HDMI 2.0 chain (all devices). The signals that do not conform to HDMI 2.0, would still use 4:4:4 or 4:2:2 that were allowed in previous HDMI versions. E.g. your 1080p video picture would go as 4:2:2 over HDMI, if there is one or more HDMI 1.4 ports (like your 4th generation).

RGB doesn’t allow chroma subsampling, as there is no chroma channel, so that is always 4:4:4. RGB High and RGB Low refers to levels range (as in 0–255 or 16–235 for 8 bit), which should match the TV input setting (even though it might be labeled differently there), to prevent crushed black levels, or reduced contrast the other way. YCbCr doesn’t allow this kind of level scaling.

Hi.

Chroma subsampling is used to reduce the video bitrate used, by reducing the resolution for the chromatic signal (colors), the luminance ("black and white") signal being always encoded at full resolution.

This is used because the human eye is more sensitive to luminance than chrominance.

- 4:4:4 means full resolution (3840x2160 for a 4K signal)

- 4:2:2 means half horizontal resolution and full vertical resolution ((1920x2160 for a 4K signal)

- 4:2:0 means half horizontal resolution and half vertical resolution (1920x1080 for a 4K signal)

Chroma subsampling is not needed in the HDMI specification for 1080p60 content, so your Apple TV 4K uses no chroma subsampling, which would be 4:4:4

You can see details about this for example on wikipedia here : https://en.wikipedia.org/wiki/HDMI#Refresh_frequency_limits_for_standard_video