1. A data transmitter for generating and sequentially applying to a transmission channel during each of a succession of signalling intervals at least a pair of signal pulses representing a respective one of a succession of (k+n)-bit input words, said channel being such that at least one of the signal pulses applied thereto during each one of said signalling intervals is modified as a function of the value of at least one signal pulse applied to the channel in a prior signalling interval, said transmitter comprising, means responsive to the values of n bits of said each input word and the value of at least one bit of a previous input word in said sequence for identifying one of 2.sup.m predetermined cosets in a predetermined lattice, m being an integer greater than n, means responsive to the value of said at least one previous signal pulse for identifying a predetermined alphabet of seed signal points, each of said seed signal points being such that, when it is transmitted through said channel in said one signalling interval, the resulting signal point at the output of said channel is a point in the identified one coset, means for selecting an individual one of the seed signal points of the identified alphabet as a function of the values of the other k bits of said each input word, and means for applying to said channel as said at least pair of said pulses a plurality of signal pulses representing said selected seed signal point.