RGB and YCbCr encode the image differently.
RGB must use the full resolution for each channel (red, green, and blue) because anything less looks broken. This is equivalent to 4:4:4.
YCbCr splits the image into black & white (Luma, or Y) and color channels (Chroma difference, or CbCr).
By separating the two, it means that the resolution of the color signal can be reduced without having a significant impact on the image sharpness, since it overlays a sharp black and white image.
- 4:4:4 uses full resolution Luma and Chroma channels.
- 4:2:2 uses full resolution Luma and half-resolution Chroma channels.
- 4:2:0 uses full resolution Luma and quarter-resolution Chroma channels.
This saves bandwidth because our eyes do not have the same ability to see color detail as the do black and white detail.
Nearly all video files are encoded as 4:2:0 for example - even UHD Blu-rays.
Most televisions today work internally in YCC, and process the image in 4:2:2 by default - so even if you send them an RGB input, it is converted to YCC 422.
Some older TVs might even process the image in 4:2:0 (Pioneer Kuro Plasma TVs, for example).
Switching them to PC mode processes the image in 4:4:4, preserving the full resolution of an RGB or YCC 444 input; but it often disables many processing options that may be too taxing at that resolution, or reduces the precision of other processing.
Chroma resolution played a significant factor in image quality when gaming at lower resolutions.
When your image is 1080p, and the image is processed in 4:2:0, you only have 540p color resolution.
I remember games like
Rez HD looking awful on the Xbox 360 at anything less than 4:4:4/RGB, since it was 720p.
But once you get to 4K, even 4:2:0 only lowers the chroma resolution to 1080p, so the impact is far less visible now.
Here's an old example from a Pioneer Kuro plasma in one of the
Gran Turismo games on the PS3:
Note how fine details are blurred, and colors are dulled.
And with some 1px wide text:
Green is based on the Y channel (Luma) so it's sharper than red/blue in YCC signals - which matches how our vision works.
I believe LG's OLEDs process the image in 4:2:2 at all times, unless you switch them into PC mode.
If you're actually using the display with a computer, text should look much better in PC mode.
If you're using it for games, it may not make much difference most of the time.
I believe there was an issue where HDR (or was it 120Hz?) had more color banding outside of PC mode, but I think it was fixed in a recent update.
PC mode and Game mode are not mutually exclusive on these displays.
You can set the input label to "PC" and still switch to the Game Optimizer mode.