| 摘要 |
In a digital coder for video signals, transform means (such as a discrete cosine transform coder) operates on blocks of input samples or of difference values formed as the difference between input sample values and reference values to provide sets of co-efficients in the spatial frequency domain. A quantizer quantises the co-efficients and means responsive to the output of the quantizer produces the reference values. Criticality control means responsive to the output of the transform means determines the criticality of the picture areas corresponding to the blocks of input samples or difference values and controls the quantizer so that critical areas are coded differently to less critical areas. The criticality control means determines criticality in dependence on the number of co-efficient values in each set of such values having a magnitude above a predetermined threshold. <IMAGE> |
