[gstreamer-bugs] [Bug 599292] New: Synchronizing two RTP streams from different devices using RTCP is inaccurate.

[GStreamer (bugzilla.gnome.org)]

https://bugzilla.gnome.org/show_bug.cgi?id=599292
  GStreamer | gst-plugins-good | git

           Summary: Synchronizing two RTP streams from different devices
                    using RTCP is inaccurate.
    Classification: Desktop
           Product: GStreamer
           Version: git
        OS/Version: Linux
            Status: UNCONFIRMED
          Severity: normal
          Priority: Normal
         Component: gst-plugins-good
        AssignedTo: gstreamer-bugs at lists.sourceforge.net
        ReportedBy: slow.racoon at gmail.com
         QAContact: gstreamer-bugs at lists.sourceforge.net
      GNOME target: ---
     GNOME version: ---


This example tries to demonstrate why synchronizing two devices using RTCP
is impossible to get accurate. This is done by look at the relation between
the RTP timeline and the NTP timeline.

RTP PACKAGE (#1)
RTP: 146893517
NTP: 39:14.947678 (minute:second.microseconds)

RTP PACKAGE (#2)
RTP: 147050080
NTP: 39:16.687274

RTCP PACKAGE (#3)
RTP: 147069882
NTP: 39:17.0959

DIFF (RTP2-RTP1): 156563 / 90000 = 1.739588888888888889
DIFF (NTP2-NTP1): 1.739596
Here the time difference between two RTP packages are the same.

DIFF (RTP3-RTP1): 176365 / 90000 = 1.959611111111111111
DIFF (NTP3-NTP1): 2.148222
When comparing the clocks between a RTP and RTCP package we see
that the difference in RTP time is not the same as the difference
in NTP time. Therefore when synchronizing using RTCP we will get
an incorrect RTP timestamp.

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