Embedded CELP with adaptive codebooks in enhancement layers and multi-layer gain optimization

The paper describes an embedded CELP coder in which an adaptive codebook is included in every enhancement layer and the lower-layer codebook gains are re-optimized in the higher layers to further improve speech quality. Each layer maintains its own filter memories to generate required target vectors, adds adaptive and fixed codebook contributions, and re-optimizes all codebook gains to improve coder performance (multi-layer gain optimization). The common elements across the embedded layers include the lower-layer adaptive and fixed codebook entries. The pitch-lag used in the core layer is also re-used in the enhancement layers to maintain time synchronization between layers. Estimation and encoding of selected lower-layer parameters may take into account their estimated impact on the higher layers. The described embedded CELP coder has been implemented in the embedded variable bit-rate (EV-VBR) codec standardized by ITU-T as recommendation G.718. The characterization test results of the G.718 embedded CELP are summarized.