Impact of Packet Loss Location on Perceived Speech Quality

1. Impact of Packet Loss Location on Perceived Speech Quality Lingfen Sun Graham Wade, Benn Lines Emmanuel Ifeachor University of Plymouth, U.K. {L.F.Sun@jack.see.plym.ac.uk} {j.wade,B.Lines,E.Ifeachor@plym.ac.uk}

2. Outline • Introduction • Codec's internal concealment and convergence time • Perceptual speech quality measurement • Simulation system • Loss location with perceived quality • Loss location with convergence time • Conclusions and future work

3. Gateway Gateway IP Network SCN SCN Introduction • End-to-end speech transmission quality • IP network performance (e.g. packet loss and jitter) • Gateway/terminal (codec + loss/jitter compensation) • Impact of packet loss on perceived speech quality • Loss pattern (e.g. burst/random) • Loss location (codec's concealment)

4. Introduction (cont.) • Previous research on loss location • Concealment performance is speech content related (e.g. voiced/unvoiced) • Analysis based on MSE or SNR for limited codec • Perceptual objective methods only to assess overall quality under stochastic loss simulations • Questions: • How does a packet loss location affect perceived speech quality ? • How does a packet loss location affect codec's convergence time (for loss constraint)?

5. Codec's internal concealment • What is codec's concealment? • When a loss occurs, the decoder interpolates the parameters for the lost frame from parameters of previous frames. • Which codec has concealment algorithm? • G.729/G.723.1/AMR (main VoIP codecs) • CELP analysis-by-synthesis • What are the limitations of concealment algorithms? • During unvoiced(u) or voiced(v) • During u/v

6. Codec's convergence time • What is convergence time? • The time taken by decoder to resynchronize its state with encoder after a loss occurs. It is also called resynchronization time. • For set up loss constraint distance between two consecutive losses for new packet loss metrics • What is the relationship between convergence time with loss location, codec type and packet size?

7. System/network under test Perceptual quality measurement Reference signal Objective perceptual quality test Objective MOS Degraded signal • Transform the signal into the psychophysical representation approximating human perception • Calculating their perceptual difference • Mapping to objective MOS (Mean Opinion Score) • Algorithms: PSQM/PSQM+/MNB/EMBSD/PESQ

8. Simulation System Reference speech Degraded speech without loss Bitstream encoder decoder convengence time analysis Degraded speech with loss loss simulation decoder • Perceptual speech quality analysis with loss location • Convergence time analysis with loss location perceptual quality measure Reference speech

9. Speech test sentence • Speech test sentence is about 6 seconds. • First talkspurt (about 1.34 second, above waveform) is used for loss location analysis. • Four voiced segments, V(1) to V(4), which can be decided by pitch delay in G.729 codec

10. Pitch delay from G.729 codec V(2) V(1) V(3) V(4)

11. Loss location with perceived quality • Each time only one packet loss is created • Loss position moves from left to right one frame by one frame • Overall perceptual quality is measured from PSQM/PSQM+, MNB and EMBSD • Packet size: 1 to 4 frames/packet • Codec: G.729/G.723.1/AMR • How does a loss location affect perceived speech quality ?

12. Loss position with quality (1) Loss position reference speech PSQM+ degraded speech PSQM

13. Loss position with quality (2) Loss position reference speech PSQM+ degraded speech PSQM

14. Loss position with quality (3) Loss position reference speech PSQM+ degraded speech PSQM

15. Loss position with quality (4) reference speech Loss position degraded speech PSQM+ PSQM

16. Overall PSQM+ vs loss location (G.729) G.729

17. Overall MNB vs loss location (G.729) G.729

18. Overall EMBSD vs loss location (G.729) G.729

19. Overall PSQM+ vs loss location (G.723.1) G.723.1

20. Loss location with perceived quality • Loss location affects perceived quality. • The loss at unvoiced speech segment has no obvious impact on perceived quality. • The loss at the beginning of the voiced segment has the most severe impact on perceived quality. • PSQM+ yields the most detailed result comparing to MNB/EMBSD

21. Convergence time based on MSE G.729

22. Convergence time based on PSQM+

23. Convergence time based on PSQM+

24. Loss location with convergence time • Convergence time is almost the same for different packet size • Convergence time for a loss at unvoiced segments appears stable • Convergence time shows a good linear relationship for loss at the voiced segments • maximum at the beginning • linear descending • Up bound to the end of voiced segments

25. Conclusions and future work • Investigated the impact of loss locations on perceived speech quality • Investigated the impact of loss locations on convergence time • The results will be helpful to develop a perceptually relevant packet loss metric. • Future work will focus on more extensive analysis of the impact of packet loss on speech content