1. A method comprising: selecting candidate speech units for converting text to speech; ordering the candidate speech units according to a respective fundamental frequency of each candidate speech unit in the candidate speech units relative to all other fundamental frequencies in the candidate speech units, to yield a linear list of ordered candidate speech units; constructing a sublist of the ordered candidate speech units, wherein a respective fundamental frequency of each candidate speech unit in the sublist is within a threshold distance of a respective proximate fundamental frequency associated with at least one candidate speech unit in a next linear list of ordered candidate speech units; concatenating a proposed speech unit in the sublist with a chosen speech unit outside of the candidate speech units, to yield a concatenated speech unit; and synthesizing the speech using the concatenated speech unit.