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This chapter covered several fundamental elements of a system that accurately achieves lip sync. The most important concept is this: The lip sync algorithm must depend on an absolute timebase instead of compensating for individual delays in the end-to-end path. To effectively use the NTP timebase as an absolute reference, the sender must establish accurate mappings between the NTP time and RTP media time stamps by sending RTCP packets for each media stream. The operation of the receiver consists of two phases: first, establishing buffer-level management for audio and video streams using only RTP time stamps, and then using NTP time to achieve synchronization. By maintaining absolute time references at both sender and receiver, audio and video remain in sync, even in the presence of variable delays in the end-to-end path.

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