At synthesis time we have a stream of target segments that we wish to
synthesize. For each target we use the CART for that unit type, and ask
the questions to find the appropriate cluster which provides a set of
candidate units. The function Tdist(U) is defined as the distance of
a unit U to its cluster center, and the function
as the join cost of the optimal coupling point
between a candidate unit 
and the previous candidate unit
it is to be joined to. We then use a Viterbi search to find
the optimal path through the candidate units that minimizes the
following expression:
W allows a weight to be set optimizing join cost over target cost. Given that clusters typically contain units that are very close, the join cost is usually the more important measure and hence is weighted accordingly.