12. A method according to claim 1 , further comprising: repeating: the extracting of a partial time series of first speech parameters {x i } p . . . q ; the extracting of a partial time series of second speech parameter vectors {Δ i } p . . . q ; and the converting of the partial time series of first speech parameter vectors and the partial series of second speech parameter vectors into a partial time series of third speech parameter vectors {y i } p . . . q ; wherein each repetition is performed using a successive value of p, thereby producing a plurality of successive partial time series of third speech parameter vectors; and combining the plurality of successive partial time series of third speech parameter vectors to form a time series of output speech parameter vectors {ŷ i } 1 . . . m , wherein each output speech parameter vector ŷ i corresponds to an identically indexed one of the synchronisation points; wherein the synthesizing of the speech utterance comprises synthesizing the speech utterance from the time series of output speech parameter vectors {ŷ i } 1 . . . m .
16. A method according to claim 12 , wherein: for each repletion, p and q are such that the partial time series of first speech parameter vectors {x i } p . . . q , the partial time series of second speech parameter vectors {Δ i } p . . . q and the partial time series of corresponding third speech parameter vectors {y i } p . . . q overlap each other by a number of vectors; and a ratio of the overlap to a length of any one of the partial time series of speech parameter vectors is in a range of about 0.03 to about 0.20.