On relaxing the strict hierarchical constraints in layered coding of audio signals

Scalable coders generate hierarchically layered bitstreams to serve content at different quality levels, wherein the base layer provides a coarse quality reconstruction and successive layers incrementally refine the quality. However, it is widely recognized that there is an inherent performance penalty due to the scalable coding structure, when compared to independently encoded copies. To mitigate this loss we propose a layered compression framework, having roots in information theoretic concepts, which relaxes the strict hierarchical constraints, wherein only a “subset” of the information of a lower quality level is shared with higher quality levels. In other words, there is flexibility to also have “private” information at each quality level, beside information that is common to multiple levels. We employ this framework within the MPEG Scalable AAC and propose an optimization scheme to jointly select parameters of all the layers. Experimental evaluation results demonstrate the utility of the flexibility provided by the proposed framework.