RESYNCHRONIZATION OF THE ADAPTIVE CODEBOOK IN A CONSTRAINED CELP CODEC AFTER A FRAME ERASURE

1. RESYNCHRONIZATION OF THE ADAPTIVE CODEBOOK IN A CONSTRAINED CELP CODECAFTER A FRAME ERASURE Mohamed Chibani, Roch Lefebvre and Philippe Gournay Université de Sherbrooke, Sherbrooke, Québec, Canada

2. Outline • Basic CELP model • Constrained optimization • Resynchronization at the decoder • Open-loop search of the shift (drift) of the ACB • Closed-loop search of the shift • Pitch contour modification • Experimental results • Conclusions

3. Excitation Model in CELP Coding

4. Excitation Search in CELP Coding

5. At the Encoder…

6. Constrained Search of the Excitation Parameters

7. At the Decoder…

8. Prelude to the Resynchronization Algorithm • After a frame erasure, both the waveform and the position of the pitch pulses in the ACB memory are erroneous. • For voiced speech, the pitch pulse waveform evolves slowly. • If the expected position of the last pitch pulse in the ACB memory can be determined, the ACB memory can be corrected. • Due to the constraint, a good approximation of the pitch pulse can be obtained using only the parameters of the current frame.

9. The Excitation Signal Obtained After Setting to Zero the ACB Memory

10. Block Diagram of the Resynchronization Algorithm

11. ACB delays P ( 0 ) P ( 1 ) P(-1) The last pulse in the ACB memory Determination of the Expected Pitch Pulse Position in the Erroneous ACB Memory The excitation e0(n) The correct excitation

12. Estimation of the Shift 0 P(0) : The expected position of the last pitch pulse in the ACB memory P(-1) : The actual position of the last pitch pulse in the ACB memory

13. Closed-loop Search for the Optimal Shift e is the excitation signal built after correcting the ACB for every shift candidate L_FRM=256 L=max(2*L_SBFR,T(3)) T(3) is the ACB delay of the 4th subframe

14. Example of a Resynchronized Excitation The correct excitation The excitation e0(n) The excitation signal built using the erroneous ACB memory The excitation signal built after correcting the ACB memory

15. Modification of the Pitch Contour After the Resynchronization The correct excitation The excitation after the resynchronization The excitation after the modification of the pitch contour iis the shift of each interval Np is the number of pitch periods

16. The Effect of the Resynchronization Algorithm when Applied on Voiced Speech Segment Error-free signal Standard codec Constrained codec Constr. + resynchro.

17. Standard codec 90 73.67 73.67 74.88 Constrained codec 80 Constr. + Resynchro. 53.90 70 49.18 60 42.36 40.40 36.70 50 29.99 40 30 20 10 0 Experimental Results Test features: • AMR-WB at mode 2 (12.65 kb/s) • 10 listeners • 14 pairs of sentences for each condition • Listening using binaural headphones MUSHRA Score 0% 5% 10% Frame erasure rate

18. Conclusions • The resynchronization allows to speed up the recovery of the decoder after a frame erasure. • The method (constraint + resynchronization) needs neither extra bits nor extra delay. • The modified codec is completely interoperable with the standard (the bitstream is not modified). • Only 10 to 15% of the frames following an erased frame are resynchronized. • The only drawback is a minor loss of quality in error-free channels.

19. Thank you