Chrominance format
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In digital video encoding, chrominance format describes the ratio between the horizontal spatial sampling frequencies of the luminance and chrominance components. The chrominance format is expressed as three numbers. The first number represents the luminance sampling frequency, the second and third the red and blue chrominance sampling frequencies respectively. In digital video, the first number is by convention always 4.
Common chrominance formats are:
| 4:4:4 | Luminance and chrominance have equal sampling frequencies. There is one sample in each chrominance channel for each luminance sample. |
| 4:2:2 | Chrominance is sampled at half the horizontal rate of luminance, but at the same vertical rate. There is one sample in each chrominance channel for each pair of luminance samples. |
| 4:1:1 | Chrominance is sampled at one-quarter the horizontal rate of luminance, but at the same vertical rate. There is one sample in each chrominance channel for every four luminance samples. |
| 4:2:0 | Technically an abuse of the notation, this indicates that chrominance is sampled at half the rate of luminance in both the vertical and horizontal directions. There is one sample in each chrominance channel for every four luminance samples. |
Although 4:1:1 and 4:2:0 have the same overall bandwidth requirements, they differ in the relative positions of the chrominance and luminance samples. 4:1:1 is preferred in NTSC environments, and 4:2:0 in PAL.