Some properties of Bayesian sensing hidden Markov models

In Bayesian sensing hidden Markov models (BSHMMs) the acoustic feature vectors are represented by a set of state-dependent basis vectors and by time-dependent sensing weights. The Bayesian formulation comes from assuming state-dependent zero mean Gaussian priors for the weights and from using marginal likelihood functions obtained by integrating out the weights. Here, we discuss two properties of BSHMMs. The first property is that the marginal likelihood is Gaussian with a factor analyzed covariance matrix with the basis providing a low-rank correction to the diagonal covariance of the reconstruction errors. The second property, termed automatic relevance determination, provides a method for discarding basis vectors that are not relevant for encoding feature vectors. This allows model complexity control where one can initially train a large model and then prune it to a smaller size by removing the basis vectors which correspond to the largest precision values of the sensing weights. The last property turned out to be useful in successfully deploying models trained on 1800 hours of data during the 2011 DARPA GALE Arabic broadcast news transcription evaluation.