1. A method of speech synthesis comprising the steps of: retrieving a first sequence of digital samples corresponding to a first desired speech waveform and first pitch data defining excitation instants of the waveform corresponding to glottal closures; retrieving a second sequence of digital samples corresponding to a second desired speech waveform and second pitch data defining excitation instants of the second waveform corresponding to glottal closures; forming an overlap region by: a) synthesising from at least one sequence an extension sequence, the extension sequence comprising a segment of said at least one sequence, which segment represents at least a substantial part of a pitch period of said first waveform that is expanded or compressed with respect to time so as to have synthesized excitation instants synchronous with respective excitation instants of the other sequence; and b) forming for the overlap region weighted sums of samples of the first and/or second sequence(s) and samples of the synthesized extension sequence(s).