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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.
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).
4:4:4 (Cb/Cr Same as Luma) Cb and Cr are sampled at the same full rate as the luma. MPEG-2 supports 4:4:4 coding, but having the same number of color difference samples as the luma is considered overkill and not worth the additional bandwidth to transmit it. When video is converted from one color space to another, it is often resampled to 4:4:4 first.