NTP (Network Time Protocol) gives a common, absolute wall-clock timestamp that independent media streams can be referenced against. In RTP systems it isn't used so much to set the machine's clock as to provide the shared timeline inside RTCP sender reports: each report maps a stream's media-clock RTP timestamp to a 64-bit NTP-format time, and the receiver uses those mappings to align audio and video that arrived on separate, unsynchronized RTP clocks. The NTP timestamp's job here is purely relative — a common reference both streams point at — so absolute clock accuracy matters less than sender and reports agreeing on the same time base for synchronization.
The book · Volume 5 of 7
Audio for Video: A Complete Guide to Sound in Video Products: From Jitter Buffers and Lip Sync to Atmos, Quality Metrics, and Neural Codecs
NTP timestamps in RTCP sender reports are what let a receiver line up separate audio and video RTP streams, and a bad mapping shows up as drifting lip sync. This volume covers NTP in detail with RTP timestamps, clock domains, lip sync and drift correction, so you can debug real-time sync from the packets up. Written by Nikolay Sapunov, CEO at Fora Soft.
Audio and video drifting in your RTP calls because NTP and RTCP mapping is off?
Share packet captures from an affected call and we will trace the NTP and RTP timestamp mapping and give you a sync fix plan. 250+ video products shipped since 2005.
