6. A method comprising: receiving a set of ordered lists of speech units from a single speaker, wherein the set of ordered lists of speech units is based on fundamental frequencies of the speech units; constructing a sublist of speech unit pairs which are suitable for concatenation based on a respective pitch of each speech unit in the set of ordered lists of speech units, the sublist of speech unit pairs comprising pairs having a pitch difference below 10 hertz; performing, via a processor, a cost analysis of paths through the set of ordered lists of speech units based on the sublist of speech unit pairs; selecting speech units from the set of ordered lists of speech units based on the cost analysis; concatenating the speech units, to yield concatenated speech units; and synthesizing the concatenated speech units.
10. The method of claim 6 , further comprising dynamically adjusting a threshold value which determines suitability for concatenation.