ADAPTATION TO A SPEAKERS VOICE IN A SPEECH RECOGNITION SYSTEM BASED ON SYNTHETIC PHONEME REFERENCES
A speech recognition system based on synthetic generation of reference prototypes is described. The vocabulary and grammar are described in a finite-state phoneme network. In the transformation from symbolic to spectral representation, reduction rules modify the initial phoneme target values and a coarticulation module inserts interpolated transition states at phoneme boundaries. The phoneme templates are specified in terms of control parameters to a serial formant synthesiser. At each state, a 16-channel filter bank section is computed from the synthesis parameters. The recognition process uses a time-synchronous dynamic programming technique to find the path in the network that minimises the accumulated spectral distance to the input utterance. A technique for dynamic adaptation to the speaker's voice source spectrum is performed during recognition. Without adaptation, the average recognition for ten male speakers was 88% on an isolated-word task using a 26-word vocabulary. Adding voice source adaptation raised the performance to 96%. On a vocabulary of 3 connected digits, the adaptation technique improved the recognition rate for six male speakers from 87.7% to 92.8%. The improvement was largest for subjects with low initial recognition rate, indicating the benefit of the voice source adaptation technique for certain voices. Changing the voice source model and optimising the adaptation time constant raised the recognition rate further to 96.1%. Current work is directed towards speaker adaptation of phoneme parameters and modelling of the variability of the parameter dynamics at phoneme boundaries.
speech recognition voice source adaptation synthetic prototypes speech production oriented recognition