There is a sequence of samples having values representative of a waveform. Samples corresponding to extrema including maximums and minimums in the waveform are detected. A decision is made as to whether values of successive samples between every two extremum-corresponding samples are in an upward slope or a downward slope. The number of these successive samples is detected. One is selected from coefficients in response to the detected sample number. A first group has one or more samples adjacently preceding a maximum-corresponding sample. A second group has one or more samples adjacently following the maximum-corresponding sample. A third group has one or more samples adjacently preceding a minimum-corresponding sample. A fourth group has one or more samples adjacently following the minimum-corresponding sample. Each of specified samples among the samples in the first to fourth groups is corrected in response to the slope decision result and the selected coefficient.