11 - Differential Encoding

Abstract

This chapter highlights different approaches that use the structure in the source output in a different manner, resulting in a significantly less complex system. When one designs a quantizer for a given source, the size of the quantization interval depends on the variance of the input. If one assumes the input is uniformly distributed, the variance depends on the dynamic range of the input. The chapter discusses adaptive differential pulse code modulation (DPCM). As DPCM consists of two main components—the quantizer and the predictor— making DPCM adaptive means making the quantizer and the predictor adaptive. One can adapt a system based on its input or output. The former approach is called “forward adaptation;” the latter is called “backward adaptation.” In the case of forward adaptation, the parameters of the system are updated based on the input to the encoder, which is not available to the decoder. Therefore, the updated parameters have to be sent to the decoder as side information. In forward adaptive quantization, the input is divided into blocks. The quantizer parameters are estimated for each block. These parameters are transmitted to the receiver as side information. In DPCM, the quantizer is in a feedback loop, which means that the input to the quantizer is not conveniently available in a form that can be used for forward adaptive quantization. Therefore, most DPCM systems use backward adaptive quantization.