Backwards-compatible error propagation recovery for the amr codec over erasure channels

This paper presents a recovery scheme for the error-propagation distortion which frequently appears after a frame erasure in CELP-based speech coders, in particular the AMR codec. The extensive use of predictive filters and parameter encoding allow a high-quality speech synthesis in these codecs, but makes them more vulnerable to frame erasures. Thus, when a frame is lost, an additional distortion appears in the subsequent frame, although that was correctly received, further degrading the speech quality. This degradation can also propagate over several frames, being even more damaging than the loss itself. This well known fact has motivated the development of techniques which prevent or mitigate the error propagation. Nevertheless, the previously proposed methods in some respect modify the transmission scheme (by including additional frames, FEC codes, etc.) making them incompatible with the original decoder. In this work, we apply a steganographic technique to embed recovery data to assist the decoder after a frame loss. This data mainly consist of resynchronization pulses and correction vectors for the excitation signal and the spectral envelope, respectively. PESQ results confirm that our proposal achieves a higher robustness against error propagation while the full backwards-compatibility with the AMR standard is retained.