(n.) A process applied in digitizing the color portion of video that compresses the chrominance to various degrees. Three numbers are used to represent ratios between sample rates and color clock rates in digital video. A:B:C: notation is used to represent the ratios as follows: (A) The basic sample rate compared with the color clock rate. (B) The color horizontal downsampling rate compared with the basic sampling rate. (C) The color vertical downsampling rate plus 1. Some of the common sub-sampling formats are shown below:
- 4:1:1 Color (n.) A moderately compressed method of video color sub-sampling in which the luminance channel is not subsampled, but the chrominance channel has one fourth of the original resolution. Most of the DV formats use 4:1:1 color.
- 4:2:0 Color (n.) A moderately compressed method of video color sub-sampling with results similar to 4:1:1. This is the standard method of encoding color for MPEG.
- 4:2:2 Color (n.) These ratios indicate that the sampling is done at four times the color clock rate (3.58 MHz), that two horizontal chroma samples are taken for every four luminance samples, and that there is no reduction in vertical color resolution (1+1). It is a slightly compressed method of video color sub-sampling in which the luminance channel is not subsampled, but the chrominance channel has half the original resolution. This is the ratio defined in the CCIR-601 standard for professional videotape equipment and the D-1 format. It is used in most professional video formats, such as BetaCam SP.
- 4:4:4 Color (n.) This designation represents totally uncompressed video color that has no sub-sampling.