1. Department of Communication Technology A Subvector-Based Error Concealment Algorithm for Speech Recognition over Mobile Networks - ICASSP 2004, Montreal, Quebec, Canada Zheng-Hua Tan, Paul Dalsgaard and Børge Lindberg {zt, pd, bli}@kom.aau.dk Aalborg University, Denmark

2. Department of Communication Technology May 19, 2004 Page 2 of 12 Outline Motivation The ETSI-DSR standard Subvector-based error concealment Performance evaluation Conclusion

3. Department of Communication Technology May 19, 2004 Page 3 of 12 Distributed speech recognition (DSR) Error concealment (EC) techniques for DSR –Feature-domain: splicing, substitution, repetition, interpolation Motivation Server back-end Error-prone Channel Front-end Split VQ Coder Channel encoder Speech Client front-end Channel decoder Split VQ Decoder Words Recogniser

4. Department of Communication Technology May 19, 2004 Page 4 of 12 Motivation – cont. Observation1: conventional EC schemes share a common characteristic - conducting EC at the vector (or frame) level Subvector based EC Observation 2: within erroneous vectors, a substantial number of subvectors are often error-free Error Rates of Vectors and Subvectors vs. BER – calculated based on ETSI-DSR standard It may be advantageous to exploit error-free subvectors remaining in erroneous vectors

5. Department of Communication Technology May 19, 2004 Page 5 of 12 Feature-extraction Encoding scheme – Feature-pair and SVQ: The n’th vector V n is – Frame-pair: two consecutive frames are grouped and protected by 4-bit CRC 44 + 44 + 4 = 92 bits The ETSI-DSR standard Feature-pair Subvector2 8 bits Subvector1 6 bits 44 bits

6. Department of Communication Technology May 19, 2004 Page 6 of 12 The ETSI-DSR standard – cont. Error detection and buffering Assuming 2N erroneous vectors have been detected, they are buffered together with two error-free vectors: Error concealment –A repetition scheme only a single bit error the replacement of 92-bit frame-pair Why not at subvector level ?

7. Department of Communication Technology May 19, 2004 Page 7 of 12 Subvector-based EC algorithm A data consistency test at the subvector level Frame-pair, for j=0,1,…,6 The test generates a Boolean consistency matrix C ?>?> Subvector (Feature-pair) > Even though one “>”, inconsistent; otherwise, consistent.

8. Department of Communication Technology May 19, 2004 Page 8 of 12 An EC algorithm at the subvector level –An example consistency matrix where transmission data are corrupted by the GSM EP3 Subvector-based EC algorithm – cont. 0 for inconsistent 1 for consistent C = – Inconsistent subvectors are replaced by their nearest neighbouring consistent subvectors whereas consistent ones are kept unchanged

9. Department of Communication Technology May 19, 2004 Page 9 of 12 Performance evaluation 2 recognition tasks and 3 error distributions Experimental results –AWGN channels Danish digits City names %WER across the EC techniques for varying values of the BER

10. Department of Communication Technology May 19, 2004 Page 10 of 12 Performance evaluation – cont. – Rayleigh Fading Channels Danish digits City names %WER across the EC techniques for varying values of the burst-like BER

11. Department of Communication Technology May 19, 2004 Page 11 of 12 Performance evaluation – cont. – GSM Error Patterns Danish digits City names %WER across the EC techniques for the GSM error patterns The improvement for EP3 is significant.

12. Department of Communication Technology May 19, 2004 Page 12 of 12 Conclusion An EC technique at the subvector level – is superior to commonly used EC method having almost the same complexity. – has neither requirements for modification in the recogniser nor requirements for extra bandwidth. THANKS !

13. Department of Communication Technology May 19, 2004 Page 13 of 12

14. Department of Communication Technology May 19, 2004 Page 14 of 12 %WER across different threshold settings for the GSM EP3 for Danish digits No. 12345678 0.01.5236930 ETSI-DSR SVQ No EC

15. Department of Communication Technology May 19, 2004 Page 15 of 12 Vector and subvector error rates calculated across a number of bit-error-rates (BER) according to the following formula