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4:4:4 is uncompressed and therefore provides the best image quality whereas 4:2:2 and 4:2:0 sacrifice color quality for lower data rate.
Chroma subsampling is a type of color compression that reduces data rate and file size.
Most content nowadays, including TV shows and movies, use 4:2:0 chroma subsampling because the loss in graphical quality is practically invisible, especially at 4K, while the bandwidth compression allows for easier data transfer including seamless streaming via services such as Netflix.
The only case where chroma subsampling becomes apparent is when you are looking at small text that’s on the colored background. The text will look blurry and fuzzy if you are too close to the TV. This is why if you want to use a TV as a monitor, it should have a 4:4:4 mode.
How Does Chroma Subsampling Work?
In the 4×2 grid sample above, 4:4:4 represents no chroma subsampling used whereas 4:2:2 has every other pixel is duplicated and 4:2:0 has bottom pixels duplicating top pixels.
So, 4:4:4 will have the best image quality but highest data rate while 4:2:0 has the lowest quality picture but also the smallest bandwidth requirements.
In the picture below, you can see how it would affect a complete picture.
How To Activate 4:4:4?
If your TV supports 4:4:4 chroma, you can enable it by going to the settings menu and finding an option typically called HDMI UHD Color, HDMI Enhanced Format, or something along those lines depending on the TV model.
Keep in mind that enabling 4:4:4 for 4:2:0 or 4:2:2 native content won’t improve the image quality.
So, the only time you would actually need it is when you are playing console games or using your TV as a PC monitor.