Type B (lost align.): For stationary voiced signals, the correction consists in a gain ( g ) and a shift (  )

1. Audio In old “good” past exc. Encoder Internal State Concealment Standard Encoder Correction D “Good” past excitation “Good” past excitation z-1 g new “good” past exc. “Bad” past excitation “Bad” past excitation T0 Bitstream Out Side Information T0 Improved Frame Loss Recovery Using Closed-Loop Estimation of Very Low Bit Rate Side Information VoiceAge Corporation 750 Chemin Lucerne, Suite 250 Montreal (Quebec) H3R 2H6 Canada University of Sherbrooke Faculté de Génie 2500, boul. de l’Université Sherbrooke (Québec) J1K 2R1 Canada Philippe Gournay Philippe.Gournay@USherbrooke.ca 1. Abstract 2. Modified Encoder 4. Performance evaluation 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). (a) no frame lost; (b) standard decoder; (c) modified decoder using side information; (d) and (e) error signals for the standard and modified decoders. Lost Frame Type A (lost onset) Error signals show the effect of a restored onset 3. Estimation of the Correction • 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. Type B (lost align.) Error signals show a faster reconvergence Type A (lost onsets): Side information describes the position and amplitude of the largest pulse Type B (lost align.): For stationary voiced signals, the correction consists in a gain (g) and a shift () 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. StandardModified Preference 5. Conclusions and Perspectives D • 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. Interspeech 2008, Brisbane, Australia