Multiple description quantization is a signal compression technique for robust networked multimedia communication. In this paper we consider the problem of optimally quantizing a random variable into two descriptions, with each description being produced by a side quantizer of convex codecells. The optimization objective is to minimize the expected distortion given the probabilities of receiving either and both descriptions. The problem is formulated as one of shortest path in a weighted directed acyclic graph with constraints on the number and types of edges. An O(K 1 K 2 N 3 ) time algorithm for designing the optimal twodescription quantizer is presented, where N is the cardinality of the source alphabet, and K 1 , K 2 are the number of codewords of the two quantizers, respectively. This complexity is reduced to O(K 1 K 2 N 2 ) by exploiting the Monge property of the objective function. Furthermore, if K 1 = K 2 = K and the two descriptions are transmitted through two channels of the same statistics, then the optimal two-description quantizer design problem can be solved in O(K N 2 ) time.
Introduction.Quantization is a common technique of compressing multimedia signals such as images, video and audio [5]. Data compression is achieved by quantizing signal samples from a representation of higher resolution to lower resolution so that fewer bits suffice to code each sample of the quantized signal. Optimal quantization falls into the class of resource allocation problems in operational research. The central issue is how to describe a random variable X or a random vector X to the maximum precision (or minimum distortion) possible using a given number of bits. The problem is called optimal scalar or vector quantizer design depending on whether the input is a random variable or a random vector. This paper is restricted to the treatment of scalar quantization.In conventional single-description quantization, a quantized signal is coded and transmitted in a single bit stream through a communication channel. If the channel fails then the reconstruction of the signal will be necessarily interrupted or abandoned at the receiver. Modern IP communication networks, however, offer multiple routes between any two nodes. This design of distributed communication can be utilized to improve the error resilience of conventional quantizers. Multiple description or networked quantization is such a technique [4], [8]-[10] of robust data communications.