1. Impact of Packet Loss Location on Perceived Speech Quality
Lingfen Sun
Graham Wade, Benn Lines
Emmanuel Ifeachor
University of Plymouth, U.K.
IPTEL'2001, New York, USA

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
IPTEL'2001, New York, USA

3. Introduction End-to-end speech transmission quality
SCN
IP Network
Gateway
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)
IPTEL'2001, New York, USA

4. Introduction (cont.) Previous research on loss location Questions:
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)?
IPTEL'2001, New York, USA

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
IPTEL'2001, New York, USA

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?
IPTEL'2001, New York, USA

7. Perceptual quality measurement
Reference signal
System/network under test
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
IPTEL'2001, New York, USA

8. Simulation System encoder decoder convengence time analysis loss
Reference speech
Degraded speech
without loss
Bitstream
encoder
decoder
convengence
time analysis
Degraded speech
with loss
loss
simulation
decoder
perceptual
quality measure
Reference speech
Perceptual speech quality analysis with loss location
Convergence time analysis with loss location
IPTEL'2001, New York, USA

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
IPTEL'2001, New York, USA

10. Pitch delay from G.729 codec
V(1)
V(2)
V(3)
V(4)
IPTEL'2001, New York, USA

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 ?
IPTEL'2001, New York, USA

12. Loss position with quality (1)
reference speech
PSQM+
degraded speech
PSQM
IPTEL'2001, New York, USA

13. Loss position with quality (2)
reference speech
PSQM+
degraded speech
PSQM
IPTEL'2001, New York, USA

14. Loss position with quality (3)
reference speech
PSQM+
degraded speech
PSQM
IPTEL'2001, New York, USA

15. Loss position with quality (4)
reference speech
Loss position
degraded speech
PSQM+
PSQM
IPTEL'2001, New York, USA

16. Overall PSQM+ vs loss location (G.729)
IPTEL'2001, New York, USA

17. Overall MNB vs loss location (G.729)
IPTEL'2001, New York, USA

18. Overall EMBSD vs loss location (G.729)
IPTEL'2001, New York, USA

19. Overall PSQM+ vs loss location (G.723.1)
IPTEL'2001, New York, USA

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
IPTEL'2001, New York, USA

21. Convergence time based on MSE
IPTEL'2001, New York, USA

22. Convergence time based on PSQM+
IPTEL'2001, New York, USA

23. Convergence time based on PSQM+
IPTEL'2001, New York, USA

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
IPTEL'2001, New York, USA

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
IPTEL'2001, New York, USA