Juan J. Ramos-Muñoz, Angel M. Gómez, Juan M. Lopez-Soler

1. Signal Theory, Telematics and Communications Department Intelligibility Evaluation of a VoIP Multi-flow Block Interleaver Juan J. Ramos-Muñoz, Angel M. Gómez,Juan M. Lopez-Soler

2. Outline • Introduction • Basic and Multi-flow Block Interleavers Algorithms • Intelligibility Estimation • Experimental Results • Conclusion

3. Introduction • Background • Bursts degrade the perceived quality in Voice over IP • VoIP services are extremely delay concerned • Enhanced VoIP services must be subjectively evaluated

4. Introduction • In this work • We contribute to demonstrate some perceptual benefits that can be obtained by using active routers in VoIP • We propose a new delay-aware interleaver to mitigate the bursty-error-prone nature of IP • We evaluate the service performance by using Automatic Speech Recognition

5. Basic and Multi-flow Block Interleavers Algorithms • Given and an interleaver is defined by • Basic TypeI(s): to face bursts of length equal to s packets, with the minimum incurred delay, an (s x s) matrix is required • Packets are written by rows, from left to right and from top to bottom. • Packets are read by columns, from bottom to top and from left to right. • Type I(s) maximum interleaver delay is given by • Type I(s) is limited to such that • For typical VoIP values and TypeI(s) is restricted to bursts with length s < 5

6. Basic and Multi-flow Block Interleavers Algorithms • We interleave packets from different flows to face consecutive losses by introducing a tolerable delay • We use the reference TypeII(nf) and propose TypeII(nf,s) where • nf is the number of available flows and • s is the maximum expected burst length • Round-robin interleaver (TypeII(nf)) is suited for nf≥ s • it requires one (nf x 1) interleaver matrix • to write the matrix each row will be assigned to a single flow. • the matrix will be read from bottom to top • if no switching delay is assumed then • Drawback: when nf < s, TypeII(nf) interleaver does not isolate all the packet losses

7. Basic and Multi-flow Block Interleavers Algorithms • The proposed TypeII(nf,s) assures the isolation of losses for a given (nf ,s) pair of values, even when nf< s, with tolerable (bounded) delay • Memory requirements • If s is a multiple of nf then one (s x s) matrix is required. • Otherwise, nf squared (s x s) matrices are needed.

8. Basic and Multi-flow Block Interleavers Algorithms • Let us define Rij as the number of consecutive rows that the flow i will be assigned for matrix j. rem(a,b) as the remainder of the integer division a/b. int(a) largest integral value not greater than a. • TypeII(nf,s) matrix writing procedure • First matrix: • Ri1 = int(s/nf), for i = { 1, 2, …, nf - rem(s,nf)}. • Rj1 = int(s/nf) + 1 for j = { n f -rem(s,nf), nf -rem(s,nf)+1, …, nf} • Next j = 2, …, nf matrices and flows i = 2, …, nf • If Ri(j-1) = int(s/nf)+1 and R(i-1)(j-1) = int(s/nf) then Rij = int(s/nf) and R(i-1)j = int(s/nf)+1 • Type II(nf,s) matrix reading procedure • Packets are read by columns, from bottom to top and from left to right.

9. s·(r·(d +1)-1-(r - 1)·d) if r  (nf - r) s·(r·(d +1)-1-((r - 1)·d+2·r - nf -1)) if r > (nf - r) Basic and Multi-flow Block Interleavers Algorithms • TypeII(nf ,s) maximum delay Dmax is given by Dmax =º where r = rem(s,nf ) d = int((s-r) / nf ) • For typical VoIP values and for the best case TypeII(nf ,s) interleaver scatters bursts up to s < 15 favorably compared to TypeI(s) limited to s < 5

10. Intelligibility Estimation • For performance evaluation, we propose to use a high level end-user intelligibility estimation: ASR rate • Compared to MOS, ASR has lower cost and is more reproducible • For end-user intelligibility estimation ASR rate can be more suitable than other measures like PESQ (P.862) or the E-model • The Word Error Rate is defined by

11. Experimental Results • Experimental results were obtained by simulation. • We adopt a single error model based on a Markov chain (Yajnik et al [9]).

12. Experimental Results • For ASR evaluation we use the connected digit Project Aurora 2 database • The speech recognizer is based on eleven 16-state continuous Hidden Markov Models (HMM) • The HMM models are trained from a set of 8440 noise-free sentences, while the out-of-train-test set comprises 4004 noise-free sentences • More details are explained in the printed version of the paper

13. Experimental Results • WER and Dmax (seconds) obtained values

14. Conclusion • Compared to a single-flow approach, our proposed interleaver reduces the packet delay and makes it applicable under conditions where the reference scheme is unfeasible. • Compared to the round-robin multi-flow interleaver, our proposed scheme increases the perceived end-user intelligibility (WER) • With a slight penalty on the introduced delay • We propose to consider ASR as a tool to measure VoIP services enhancements. • Future work: by setting up mapping functions • for ASR rate to human intelligibility • for ASR rate to MOS score and using it together with AN technology, enhanced VoIP services can be envisaged. Thank you for your attention