Entropy-constrained tree-structured vector quantizer design by the minimum cross entropy principle
The authors address the variable rate tree-structured vector quantizer design problem, wherein the rate is measured by the quantizer's entropy. For this problem, tree pruning via the generalized Breiman-Friedman-Olshen-Stone (1980) algorithm obtains solutions which are optimal over the restricted solution space consisting of all pruned trees derivable from an initial tree. However, the restrictions imposed on such solutions have several implications. In addition to depending on the tree initialization, growing and pruning solutions result in tree-structured vector quantizers which use a sub-optimal encoding rule. To remedy the latter problem, they consider a "tree-constrained" version of entropy-constrained vector quantizer design. This leads to an optimal tree-structured encoding rule for the leaves. In practice, though, improvements obtained in this fashion are limited by the tree initialization, as well as by the sub-optimal encoding performed at non-leaf nodes. To address these problems, they develop a joint optimization method which is inspired by the deterministic annealing algorithm for data clustering, and which extends their previous work on tree-structured vector quantization. The method is based on the principle of minimum cross entropy, using informative priors to approximate the unstructured solution while imposing the structural constraint. As in the original deterministic annealing method, the number of distinct codevectors (and hence the tree) grows by a sequence of bifurcations in the process, which occur as solutions of a free energy minimization. Their method obtains performance gains over growing and pruning methods for variable rate quantization of Gauss-Markov and Gaussian mixture sources.<<ETX>>