Improvement of the post-processing algorithm in AMR-NB speech codec

Narrow band adaptive multi-rate (AMR-NB) is a speech codec that supports multiple bit-rate modes and is widely used in mobile communication systems. However, the synthesized speech quality of its lower bit-rate modes is not very satisfying. This paper aims to improve the synthesized speech quality of the lower bit-rate modes of AMR-NB (e.g., 4.75kb/s) by optimizing its post-processing algorithm. The optimization mainly focuses on four modules including the formant filter with adjusted correction factor, the fixed codebook gain smoothing, the adaptive gain control and the anti-sparse processing. The experimental results show that the AMR-NB codec with the optimized post-processing algorithm achieves an obvious improvement of the synthesized speech quality, and the lower bit-rate mode achieves the bigger improvement.