In CELP coders, the past excitation signal used to build the adaptive codebook is the main source of error propagation when a frame is lost. We presents a novel resynchronization technique using very low bit rate side information to correct the past excitation signal after a frame erasure. The novelty of this technique is that the correction is computed at the encoder in a closed loop fashion, based on the actual error introduced by the concealment. Objective and subjective test results show that this approach is a promising area for future research on frame loss recovery. A frame loss is simulated at the encoder (concealment) in order to determine the correction that should be applied to the past excitation signal (adaptive codebook). T YPE B ( LOST ALIGN.): For stationary voiced signals, the correction consists in a gain (g) and a shift (  ) T YPE A ( LOST ONSETS ): Side information describes the position and amplitude of the largest pulse We have demonstrated a concept which can be applied to any CELP codec:  Very efficient for single frame loss  Very limited bit rate (13 bits per frame)  Minimum complexity overhead at the encoder, no overhead at the decoder Various improvements for errors of type A and B, and various solutions for errors of type C, are proposed in the paper. Analysis of actual “good” and “bad” past excitation signals shows that typical CELP concealment introduce 3 types of errors which are characterized by strong error propagation:  Type A: Lost onsets  Type B: Lost alignments  Type C: Waveform mismatch The determination of the correction is done in the LPC excitation domain. The correction information depends on a signal classification step. To demonstrate the concept, we have chosen to concentrate on errors of types A and B. (a) no frame lost; (b) standard decoder; (c) modified decoder using side information; (d) and (e) error signals for the standard and modified decoders. AB comparison test between the standard and a modified AMR-WB codec; one lost frame every 10 frames; 32 sentence pairs (4 speakers); 6 experienced listeners. S TANDARD M ODIFIED P REFERENCE University of Sherbrooke Faculté de Génie 2500, boul. de l’Université Sherbrooke (Québec) J1K 2R1 Canada IMPROVED FRAME LOSS RECOVERY USING CLOSED-LOOP ESTIMATION OF VERY LOW BIT RATE SIDE INFORMATION Philippe Gournay VoiceAge Corporation 750 Chemin Lucerne, Suite 250 Montreal (Quebec) H3R 2H6 Canada 1. Abstract Bitstream OutSide Information z -1 Audio In Encoder Internal State old “good” past exc. Standard Encoder Concealment Correction new “good” past exc. “Good” past excitation “Bad” past excitation g  T0T0 “Good” past excitation “Bad” past excitation T0T0  5. Conclusions and Perspectives 2. Modified Encoder 3. Estimation of the Correction 4. Performance evaluation T YPE A ( LOST ONSET ) E RROR SIGNALS SHOW THE EFFECT OF A RESTORED ONSET T YPE B ( LOST ALIGN.) E RROR SIGNALS SHOW A FASTER RECONVERGENCE L OST F RAME Interspeech 2008, Brisbane, Australia