CHROMA SUBSAMPLING

NTSC was devised to provide color on suitable televisions while offering black and white on cheaper televisions. Color televisions took decades to come down in price.

NTSC 1953 calls for Luminance and Chrominance is Y = Luminance or Light (Grayscale), Cb = Bluescale (0 is Green, 1 is Blue), Cr = Redscale (0 is Green, 1 is Red). Early CRT televisions typically had controls for color control that could be adjusted.

cie-1931-xy color graph

Separating RGB color signals into luma and chrominance allows the bandwidth of each to be determined separately. Typically, the chrominance bandwidth is reduced in analog composite video by reducing the bandwidth of a modulated color subcarrier, and in digital systems by chroma subsampling.

There is effectively no visual difference between RGB and YCbCr444. In the HDMI specification YCbCr is the only way to transmit 10- and 12-bit color, which is why RGB is limited to 8-bit on televisions. YCbCr doesn’t require as much bandwidth as RGB. HDMI 2.0 adds RGB 10-bit and 12-bit.

Chroma sampling is not often discussed when purchasing a LCD display. The Blueray disk uses 4:2:0 which is a compromise due to limitations of H.264 encoding, the space available with a dual layer BD disk and the capability of televisions.

4:4:4 is the best and means that there is no subsampling happening, meaning each pixel has its own color information. 4:2:2 is very common and its shortcut is by halving the horizontal resolution while maintaining the full vertical resolution. 4:2:0 is, perhaps, what most people see when they record internally to a mirrorless camera or DSLR—this halves both vertical and horizontal resolution. Remember, this is referring only to color resolution, not luminance.

A PC generally uses 4:4:4 which HDMI cannot handle above 1920×1080 while DisplayPort can handle at 2560×1440 and 3820×2160. Revisions to the respective standards have improved the bandwidth but there are still issues with high dynamic range displays using more than 8-bit per color.

The image above shows 4:4:4 chromanince. This image will not look correct on most lower cost televisions and monitors which still largely use 4:2:0 chrominance. It also happens to be good for detecting color blindness. The color quality of IPS and rival technologies cannot be disputed when it comes to 4:4:4 vs 4:2:0. 

The color quality of 10-bit RGB is clearly superior compared to 8-bit RGB. HDR10 governs 10-bit RGB but it does not consider brightness which is related to contrast. Adequate brightness is vital to be able to appreciate the abilities of 10-bit RGB along with the 1.07 billion possible colors.