http://en.wikipedia.org/wiki/Chroma_subsampling: Sampling systems and ratios The subsampling scheme is commonly expressed as a three part ratio (e.g. 4:2:2), although sometimes expressed as four parts (e.g. 4:2:2:4). The parts are (in their respective order): Luma horizontal sampling reference (originally, as a multiple of 3.579 MHz in the NTSC television system) Cr horizontal factor (relative to first digit) Cb horizontal factor (relative to first digit), except when zero. Zero indicates that Cb horizontal factor is equal to second digit, and, in addition, both Cr and Cb are subsampled 2:1 vertically . Zero is chosen for the bandwidth calculation formula (see below) to remain correct. Alpha horizontal factor (relative to first digit). May be omitted if alpha component is not present. To calculate required bandwidth factor relative to 4:4:4 (or 4:4:4:4), one needs to sum all the factors and divide the result by 12 (or 16, if alpha is present). From Microsoft Help 4:4:4 means
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