The effect of an intermediate articulatory layer on the performance of a segmental HMM

We present a novel multi-level HMM in which an intermediate ‘articulatory’ representation is included between the state and surface-acoustic levels. A potential difficulty with such a model is that advantages gained by the introduction of an articulatory layer might be compromised by limitations due to an insufficiently rich articulatory representation, or by compromises made for mathematical or computational expediency. This paper decribes a simple model in which speech dynamics are modelled as linear trajectories in a formant-based ‘articulatory’ layer, and the articulatory-to-acoustic mappings are linear. Phone classification results for TIMIT are presented for monophone and triphone systems with a phone-level syntax. The results demonstrate that provided the intermediate representation is sufficiently rich, or a sufficiently large number of phone-class-dependent articulatory-to-acoustic mapping are employed, classification performance is not compromised.