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

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

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

Reference Picture Selection - ACK 1 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

Reference Picture Selection - NACK 1 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

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

Parameters for Analytic Models

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

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

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

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

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

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

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

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

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