ries the medium was sampled. The reference clock is shared and stop error propagation. If one thread is damaged due by all media to be synchronized. to packet loss, the remaining threads can still be used to Synchronizing audio and video can be achieved by predict the Sync-frame. VRC provides improved error playing out audio and video according to their original resilience, but at the cost of a much higher data rate. sampled time. By doing so, the receiver can play back audio Dynamic control of the prediction dependency can also and video at a proper pace by mapping their original sam- be used by employing long-term memory prediction to pled time to the receiver's local time. RTCP SR control achieve improved R-D performance [33,45,46]. packets provide useful information to help the receiver Typically a channel coding module in a robust video calculate the sampled time of the audio and the video at the communication system may involve FEC and automatic sender. Figure 3.6-10 illustrates an example of audio and retransmission on request (ARQ). Similar to their video synchronization by using RTCP SR control packets. applications in speech communication, when FEC is When an RTCP SR control packet is generated, it will carry employed across packets, missing packets can be re- the wallclock time (NTP) and the RTP timestamp using its covered at the receiver as long as a sufficient number of corresponding media reference time. In Figure 3.6-10, the packets is received [47-50]. FEC is widely used as an RTCP SR control packet for the audio RTP session is gen- unequal error protection scheme to protect prioritized erated at time 10:20:30.730 and the corresponding time- transmissions. In addition to FEC codes, data randomi- stamp is 200. When receiving this SR packet, the receiver is zation and interleaving are also employed for enhanced able to calculate when all the audio RTP packets are sam- protection [51-55]. pled at the sender. For the example in Figure 3.6-10, the ARQ techniques incorporate channel feedback and audio p