Context adaptive training with factorized decision trees for HMM-based statistical parametric speech synthesis

To achieve natural high quality synthesized speech in HMM-based speech synthesis, the effective modelling of complex acoustic and linguistic contexts is critical. Traditional approaches use context-dependent HMMs with decision tree based parameter clustering to model the full combinatorial of contexts. However, weak contexts, such as word-level emphasis in natural speech, are difficult to capture using this approach. Also, due to combinatorial explosion, incorporating new contexts within the traditional framework may easily lead to the problem of insufficient data coverage. To effectively model weak contexts and reduce the data sparsity problem, different types of contexts should be treated independently. Context adaptive training provides a structured framework for this whereby standard HMMs represent normal contexts and transforms represent the additional effects of weak contexts. In contrast to speaker adaptive training in speech recognition, separate decision trees have to be built for different types of context factors. This paper describes the general framework of context adaptive training and investigates three concrete forms: MLLR, CMLLR and CAT based systems. Experiments on a word-level emphasis synthesis task show that all context adaptive training approaches can outperform the standard full-context-dependent HMM approach. However, the MLLR based system achieved the best performance.