Chroma subsampling

Chroma subsampling is the practice of encoding images by implementing less resolution for chroma information than for luma information. It is used in many video encoding schemes — both analog and digital — and also in JPEG encoding.
Because of storage and transmission limitations, there is always a desire to reduce or compress the signal. Since the human visual system is much more sensitive to variations in brightness than color, a video system can be optimized by devoting more bandwidth to the luma component (usually denoted Y'), than to the color difference components Cb and Cr. The 4:2:2 Y’CbCr scheme for example requires two-thirds the bandwidth of (4:4:4) RGB which has a chroma information for every luma information.