1. Apparatus for processing received samples of a modulated data signal representing a stream of data symbols, comprising means responsive to said received samples for forming a first set of decisions as to the values of said data symbols, means for forming a replica of non-linear distortion in said data signal in response to a weighted sum of the products of the values of decisions in said first set, and means responsive to signals including said replica for forming final decisions as to the values of said data symbols.