Factored Semi-Tied Covariance Matrices

A new form of covariance modelling for Gaussian mixture models and hidden Markov models is presented. This is an extension to an efficient form of covariance modelling used in speech recognition, semi-tied co(cid:173) variance matrices. In the standard form of semi-tied covariance matrices the covariance matrix is decomposed into a highly shared decorrelating transform and a component-specific diagonal covariance matrix. The use of a factored decorrelating transform is presented in this paper. This fac(cid:173) toring effectively increases the number of possible transforms without in(cid:173) creasing the number of free parameters.