  In this paper, we present the LSF parameters by a unit vector form, which has
directional characteristics. The underlying distribution of this unit vector
variable is modeled by a von Mises-Fisher mixture model (VMM). With the high
rate theory, the optimal inter-component bit allocation strategy is proposed
and the distortion-rate (D-R) relation is derived for the VMM based-VQ (VVQ).
Experimental results show that the VVQ outperforms our recently introduced DVQ
and the conventional GVQ.
