Lipsync drift

Symptom: audio and video on the same call are noticeably out of sync, often a few hundred milliseconds, sometimes worse over a long call.

PJSIP includes an inter-media synchronizer (pjmedia_av_sync) that PJSUA-LIB sets up automatically for every call with two or more media streams. If lipsync is visibly drifting, the synchronizer is either disabled or unable to do its job. Things to check:

  1. The call is not opted out of synchronization. Confirm the call setting does not include PJSUA_CALL_NO_MEDIA_SYNC. With that flag set, PJSUA-LIB skips synchronizer creation entirely. See Audio/Video Synchronization.

  2. The peer is sending RTCP Sender Reports. The synchronizer anchors its per-media timeline to wall-clock using the NTP and RTP timestamps the peer publishes in RTCP SR. If the peer doesn’t emit SRs (e.g. an old SBC strips RTCP, or RTCP-mux is misconfigured), the synchronizer has nothing to anchor on and cannot align the streams. Verify SRs are being received by inspecting RTCP traffic or pjsua_call_get_stream_stat() output.

  3. The lag tolerance is set unreasonably tight or loose. Two compile-time tunables in pjmedia/config.h shape the synchronizer:

    • PJMEDIA_AVSYNC_MAX_TOLERABLE_LAG_MSEC (default 45 ms) — once lag exceeds this and speed-up requests have not closed the gap, the synchronizer slows down the leading media. Raise this if your deployment legitimately runs with larger jitter (e.g. a flaky mobile link with a generous jitter buffer) and the synchronizer is fighting it.

    • PJMEDIA_AVSYNC_MAX_SPEEDUP_REQ_CNT (default 10) — number of speed-up requests tried before switching to slow-down on the leading media. Lower if you want it to fall back to slow-down faster.

  4. Audio jitter buffer is unusually large. A long audio jitter buffer can outpace the video pipeline’s ability to catch up; the sync algorithm tries speed-up first, but there’s a limit. Consider whether the audio buffer setting is excessive for your network conditions.

If the call is two-way video but no audio (uncommon), the synchronizer is still created (≥2 media streams) but it has nothing meaningful to align — that’s a degenerate case and you can disable it with PJSUA_CALL_NO_MEDIA_SYNC.