Increasing the robustness of CELP-based coders by constrained optimization

The adaptive codebook used in CELP-like speech coders is extremely effective on voiced signals. Unfortunately, it is also the main source of error propagation at the decoder when a frame is lost. In this paper, we study several ways of limiting the energy contribution of the adaptive codebook to the synthesized speech signal. We show that a constrained search of the adaptive and innovative codebooks significantly improves the recovery time of the decoder after a lost frame, at the cost of only minor quality degradation in a clear channel. When applied to a standard codec such as the AMR-WB, this constraint only affects the encoder, and the modified codec remains fully interoperable with the standard codec.