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Intermedia Synchronization

Intermedia Synchronization. Ooi Wei Tsang. Overview. Why we need to sync? How close must we sync? How to sync? How to measure sync? Applications. Synchronization. Intrastream. Interstream. Local playback. Networked. Problem.

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Intermedia Synchronization

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  1. Intermedia Synchronization Ooi Wei Tsang nus.soc.cs5248Ooi Wei Tsang

  2. Overview • Why we need to sync? • How close must we sync? • How to sync? • How to measure sync? • Applications

  3. Synchronization Intrastream Interstream Local playback Networked

  4. Problem • Ensure that audio/video capturedat the same instance at the source, are displayedat the same instance at the receivers.

  5. Why we need Synchronization? nus.soc.cs5248Ooi Wei Tsang

  6. V A V A Why not Pre-sync? Sender Receiver

  7. V V A A Sender Sender Receiver Receiver Seperating A/V Flows • Can receive audio without video • Can prioritize audio over video

  8. May Come from Different Sources • UNLV Access Grid V V A A Sender Sender Receiver Receiver

  9. or Displayed at Different Sources V A V A Sender Sender Receiver Receiver

  10. More Complications • Different decoding time for video and audio • Network delay and jitter • Different clocks

  11. Bottom Line • Must synchronize at the receiver

  12. Synchronization Requirements nus.soc.cs5248Ooi Wei Tsang

  13. Reference • Human Perception of Jitter and Media Synchronization by Ralf Steinmetz IEEE JSAC 14(1) 1996

  14. Results “Badness” A/V offset -80ms +80ms

  15. What matters • Video ahead of audio vs. audio ahead of video • Size of head • Occupancy

  16. What does not matter • Language • Speaking or Singing • Hobby • Speed

  17. Conclusion • -80ms to +80ms acceptable • -160ms to +240ms noticable but not annoying • - audio > video • + video > audio

  18. How to Synchronize? nus.soc.cs5248Ooi Wei Tsang

  19. Receiver needs • Exact time a sample is generated • Each packet needs a timestamp

  20. RTP Protocol RTP Hdr Media Timestamp

  21. But.. • Media timestamp has a random offset

  22. RTCP Protocol : Media Timestamp RTCP Hdr NTP Timestamp :

  23. NTP • Network Time Protocol • Synchronize clock up to few milliseconds

  24. Audio RTCP Audio RTP Sender Audio Video RTCP Video RTP Sender Video Receiver Video Receiver Audio

  25. Audio RTCP Audio RTP Sender Audio Video RTCP Video RTP Sender Video Receiver Video Receiver Audio Synchronization Delay

  26. Multiple Receivers • Everyone must have the same synchronization delay

  27. How to set Sync Delay? • Receivers tell each other their network delay • Synchronization Delay = max {Network Delays} + Buffer

  28. Buffer Control

  29. No Delay No Jitter

  30. Delay, No Jitter

  31. Delay, Jitter

  32. If Buffer too Full/Empty • Send message “Play faster!” or “Play slower!” to other receivers

  33. Must Synchronize! • Send message “Play faster at time T!” or “Play slower at time T!” to other receivers

  34. How do we Measure Synchronization? nus.soc.cs5248Ooi Wei Tsang

  35. Reference • A Method and Apparatus for Measuring Media Synchronization • by Brian Smhmidt etc. • NOSSDAV 1995

  36. Generator System under Test Measurement System

  37. Beyond Video Conferencing nus.soc.cs5248Ooi Wei Tsang

  38. HQ Video and Audio • Multiple Channels • Multiple Displays

  39. Network Musical Performance BOS BOS SF SF BOS ACM Multimedia ‘94

  40. NASDAQ • Synchronize display of stock quotes

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