Structures for SNR scalable speech coding

SNR scalable speech coding is desirable for a number of network multimedia applications, but relatively few SNR-scalable speech coders exist for operation at rates below 16 kb/s. We investigate several SNR scalable source coding structures and define the new concepts of dependent and independent SNR scalability, where independent SNR scalable coders depend on the core layer coder only through the core layer output. Independent SNR scalable structures offer the possibility of providing bit rate scalable functionality to existing nonscalable coders and standards. We show that the MPEG-4 scalable coders are examples of dependent SNR scalable coders, and we introduce a new independent SNR scalable coder called CELPTree, which has the additional advantage of being low delay. We compare the performance of the MPEG-4 coders and CELPTree for both clean and noisy speech, and we examine the effects of frequency-weighted distortion measures in the enhancement layers of SNR scalable speech coders.