chroma subsampling - Computer Definition
A reduced color resolution in digital component video signals. To accommodate storage and bandwidth limitations, the two color components (Cb, Cr) in digital video signals are compressed by sampling them at a lower rate than the brightness (Y). Color information is actually discarded. YCbCr is the YUV color space recorded digitally. Y is brightness (luma), and Cb and Cr are the U and V color difference signals (see YUV for details). In chroma subsampling, only the colors are compressed, not the luma because the eye is more sensitive to brightness than to the color components. The 4:4:4, 4:2:2, 4:2:0... YCbCr is designated as 4:n:n. The 4 represents a sampling rate of 13.5 MHz, which is the standard frequency (ITU-R BT.601) for digitizing analog NTSC, PAL and SECAM. The next two digits represent the Cb and Cr rate. Review the illustrations below for details. Each 8x8 matrix represents a "macroblock" of 64 pixels in a video frame. The pink squares are the pixel locations where the sample is taken. Sony's HDCAM uses a different notation because it compresses both the luma and the colors (see 3:1:1).
Analog TooEngineers use the 4:n:n nomenclature loosely to refer to relative bandwidths of analog signals. For example, if digital RGB signals (computer graphics) are converted to analog color difference signals (YPbPr), it starts off at 4:4:4 YPbPr. When appropriately filtered prior to sampling, it might be characterized as 4:2:2 YPbPr, which is not an accurate use of the terminology because it is not digital, but it is used. See YCbCr, ITU-R BT.601 and YUV.
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