Reconstruction of missing packets for CELP-based speech coders

A common aspect of speech transmission through packetised networks is the need to consider the discarded (missing) packets as a result of error detection or network overload. The missing packets and the possible mistracking that results in the speech decoder lead to significant quality degradation. We introduce a packet recovery technique for CELP based speech coders. The proposed technique extrapolates independently the excitation signal and the short-term synthesis filter. A recovery strategy based on speech classification (voiced, unvoiced, transition, silence) is discussed. The extrapolation of the short-term filter uses a least-squares fading memory polynomial filter applied to the reflection coefficients. Objective and subjective quality evaluations of the recovery system applied to the LD-CELP G.728 standard and a variable rate CELP system for random and burst frame erasures are presented. The results indicate that the system is robust up to a frame erasure rate of 10%. Very little degradation in quality was observed at erasure rates up to 3%