sentences:  The transform itself is often of a small block rather than the entire macroblock.For example, in x264, a series of 4×4 blocks are transformed rather than doing the more processor-intensive 16×16 transform.SATD is slower than the sum of absolute differences (SAD), both due to its increased complexity and the fact that SAD-specific MMX and SSE2 instructions exist, while there are no such instructions for SATD.