1. An Analytic Comparison of RPS Video Repair
Yubing Wang, Mark Claypool, Robert Kinicki
Computer Science Department
WPI
Worcester, Massachusetts, USA
ACM/SPIE Multimedia Computing and Networking (MMCN)
San Jose, California, January 31, 2008

2. Introduction Packet loss degrades video quality
Reference Picture Selection (RPS) selects one of several previous frames as a reference frame
Stops error propagation more quickly than ARQ
Consume less network capacity than FEC and ARQ
Two modes: NACK (optimistic) and ACK (pessimistic)
 But … effectiveness and choice depends on packet loss, round-trip time, video content and GOP size
This work  analyze video quality with RPS
Two analytical models, RPS NACK and RPS ACK
Analysis of loss rate, round-trip time, video content, GOP
 Determine choice between NACK and ACK

3. Outline Introduction (done) Background (next) Analytical Models
Analysis
Conclusions

4. Reference Picture Selection - ACK1 2 3 4 5 6 7
ACK(1)
ACK(2)
ACK(3)
Decoder acknowledges all correctly received frames
Only acknowledged frames used as reference
Error propagation avoided entirely
Reference distance increases with round-trip time
Coding efficiency decreases with reference distance
Video quality degrades due to limited bit-rate

5. Reference Picture Selection - NACK1 2 3 4 5 6 7 8
NACK(3)
Previous frame used as reference during error-free transmission
Reference distance always 1 regardless of round-trip time
Better coding efficiency
Decoder sends NACK for erroneous frame and reference frame number
Suffers from error propagation
Loss propagation increases with round-trip time

6. Outline Introduction (done) Background (done) Analytical Models (next)
Analysis
Conclusions

7. Parameters for Analytic Models

8. Summary of Analytical Models
Average Quality for RPS ACK:
Average Quality for RPS NACK:

9. Outline Introduction (done) Background (done) Analytical Models (done)
Analysis (next)
Conclusions

10. Quality versus Round-Trip Time
RPS NACK
RPS ACK
Quality degrades with round-trip time increase
NACK resistant to degradation with round-trip time for low loss
ACK degrades uniformly with round-trip time

11. Quality versus Loss Rate
RPS NACK
RPS ACK
Quality degrades with loss rate increase
NACK degrades faster with high round trip times
ACK uniform degradation

12. Loss Crossover versus Round-Trip Time
Above trend line, ACK better. Below trend line, NACK better
Crossover points for low-motion are higher than for high-motion
Error propagation more harmful to quality than reference distance

13. Quality versus GOP Length
RPS NACK
RPS ACK
With RPS, quality increases with GOP length
NACK performs no worse than no repair
For small GOP, RPS ACK worse than no repair

14. Conclusions RPS ACK outperforms RPS NACK when round-trip times are low
RPS NACK outperforms RPS ACK when the loss rate is low
RPS NACK outperforms RPS ACK over a wider range for low motion videos than for high motion videos
RPS NACK always performs the same or better than no repair, but RPS ACK can perform worse than no repair for small GOP sizes

15. Future Work Use Video Quality Metric (VQM) to analyze RPS performance
Explore the impact of network capacity constraints on RPS performance

16. An Analytic Comparison of RPS Video Repair
Yubing Wang, Mark Claypool, Robert Kinicki
Computer Science Department
WPI
Worcester, Massachusetts, USA
ACM/SPIE Multimedia Computing and Networking (MMCN)
San Jose, California, January 31, 2008