This conversion performs decimation by 2 both vertically and horizontally. This is equivalent to a 2D separable filter implemented by cascading the 4:4:4 to 4:2:2 block and the 4:2:2 to 4:2:0 block. Quantized horizontal filter results are filtered by the vertical filter, which in turn quantizes results back to the [0 .. 2DW-1] range. (DW is the Data Width or number of bits per video component.)
Intermediate 4:2:2 chroma values are computed using This Equation. The resulting computation is shown in This Equation.
Next, these values are filtered according to This Equation. The resulting computation is shown in This Equation.
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X-Ref Target - Figure 3-17 
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The predefined filter coefficients are the same as defined in Convert 4:4:4 to 4:2:2 and Convert 4:2:2 to 4:2:0. In the horizontal direction, decimation is performed with the filter [0.25 0.5 0.25]. The same then happens in the vertical direction.