Sampling Systems and Ratios
The subsampling in a video system is usually expressed as a three part ratio. The three terms of the ratio are: the number of brightness ("luminance" "luma" or Y) samples, followed by the number of samples of the two color ("chroma") components: U/Cb then V/Cr, for each complete sample area. For quality comparison, only the ratio between those values is important, so 4:4:4 could easily be called 1:1:1; however, traditionally the value for brightness is always 4, with the rest of the values scaled accordingly.
Sometimes, four part relations are written, like 4:2:2:4. In these cases, the fourth number means the sampling frequency ratio of a key channel. In virtually all cases, that number will be 4, since high quality is very desirable in keying applications.
The sampling principles above apply to both digital and analog television.
MUSE implements a variable sampling system of ~4:2:1 ... ~4:0.5:0.25 depending on the amount of motion on the screen.
Read more about this topic: Multiple Sub-Nyquist Sampling Encoding
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