Acoustic model adaptation based on coarse/fine training of transfer vectors using directional statistics

In this paper, we reformulate an adaptation scheme of Coarse/Fine Training (CFT) of transfer vectors in acoustic modeling by using directional statistics. In CFT, the transfer vector is decomposed into a unit direction vector and a scaling factor. By using coarse tied Gaussian class (coarse class) estimation for the unit direction vector, and by using fine tied Gaussian class (fine class) estimation for the scaling factor, we can obtain accurate transfer vectors with a small number of free parameters. Directional statistics is a method for analyzing geometric parameters (e.g. angle and unit vector) using directional data, and is suited for the analysis of the CFT representation. Using directional statistics as a basis, we construct expectation-maximization algorithms for CFT parameters an-alytically using the maximum likelihood and Bayesian (maximum a posteriori) approaches. In particular, with the Bayesian approach, prior and posterior distributions for unit direction vectors are represented with a von Mises distribution, a representative distribution in directional statistics. Speaker adaptation experiments show that our proposal improves the performance of large vocabulary continuous speech recognition due to the efficient coarse/fine representation of transfer vectors, compared with the conventional transfer vector adaptation.