1. A computer readable medium storing computer-executable instructions that, when executed, cause one or more processors to perform acts comprising: obtaining trained decision tree-tied hidden Markov Models (HMMs) for a speech corpus; estimating mean parameters of a plurality of rich context models based on the trained decision tree-tied HMMs by performing a single pass re-estimation; setting variance parameters of the plurality of rich context models equal to the variance parameters of the trained decision tree-tied HMMs to produce a plurality of refined rich context models; and generating synthesized speech for an input text based at least on some of the plurality of refined rich context models.