There is no universal FFmpeg flag that fixes every audio/video sync problem. First determine whether the stream starts with a fixed offset or drifts progressively, then check input timestamps, clock sources, and video frame-sync behavior. Test the corrected setup with a representative stream before relying on it live.
Diagnose the kind of sync problem first
Audio and video can fall out of sync because of unreliable input timestamps, separate clocks for audio and video, frame-rate conversion, buffering, or the playback path. The symptom helps narrow the cause:
- Constant offset: audio is consistently ahead of or behind video. A measured timestamp adjustment may help.
- Progressive drift: the gap grows over time. A fixed offset will not correct a clock-rate mismatch.
- Sudden jumps or intermittent stutter: investigate timestamp discontinuities, reconnects, buffering, or packet loss rather than assuming a simple offset.
Before changing options, record your FFmpeg version and build, input types, current command, and whether audio and video come from the same file or device or from independent sources. These details determine which timing assumptions are safe.
Inspect the input and FFmpeg timing
Review the input stream metadata and FFmpeg logs for missing, irregular, discontinuous, or non-monotonic timestamps. Keep the original command and logs so you can compare each change. The FFmpeg documentation describes timestamp and muxer options, but their effects depend on the source and output chain.
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Do not add -copyts, wall-clock timestamping, timestamp offsets, or asynchronous audio resampling indiscriminately. Each changes timing assumptions; without evidence about the inputs, it can leave the fault unchanged or introduce another one. An offset is appropriate only when measurement shows a stable lead or lag, not when the error grows during the stream.
Use -re only to pace file input
When sending a media file as a live source, -re can make FFmpeg read it at its native rate instead of consuming it as fast as possible. Put it on the file input it is meant to pace, for example:
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ffmpeg -re -i input.mp4 ...This controls how quickly a file is read; it does not synchronize two independently clocked live sources. FFmpeg warns that applying a low read rate to an actual capture device or live stream can cause packet loss. See the FFmpeg real-time input documentation before using read-rate options on anything other than file input.
Choose video frame-sync behavior deliberately
The output -fps_mode choice determines how FFmpeg handles video frames and their timestamps. It is a video-timing decision, not a general correction for an audio clock that is drifting independently.
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| Mode | What it does | When to consider it |
|---|---|---|
passthrough |
Passes demuxer timestamps through to the output. | When preserving source timing is appropriate. |
cfr |
Duplicates or drops frames to produce the requested constant frame rate. | When the output needs a constant frame rate and that conversion suits the source. |
vfr |
Passes timestamps through or drops frames to avoid duplicate timestamps. | When variable frame timing is acceptable and duplicate timestamps should be avoided. |
The muxer can subsequently modify timestamps, so inspect the resulting output rather than assuming the selected mode guarantees a particular end-to-end timing result. Consult the FFmpeg -fps_mode documentation for the behavior relevant to your output.
Measure and test before going live
FFplay can display audio/video synchronization drift and the selected master clock. Use it to help distinguish a fixed offset from a gap that changes over time; its documentation explains the available synchronization statistics.
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- Reproduce the issue with the same input chain and a duration long enough to reveal progressive drift.
- Use material with representative speech and movement so sync errors are easier to notice.
- Change one timing choice at a time, then compare playback and FFmpeg logs.
- Run a preflight YouTube stream and check stream health and event messages before relying on the setup for a live event.
YouTube recommends testing with similar audio and movement and monitoring stream health. Its live encoder guidance is useful for delivery checks, but a clean platform health indication does not by itself prove that two source clocks are synchronized.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Meet YouTube’s encoder guidance separately
Use YouTube’s encoder recommendations as platform-delivery settings, not as a cure for source-clock drift. Its current help page recommends constant bitrate (CBR), lists AAC or MP3 for audio, and recommends a 2-second keyframe frequency without exceeding 4 seconds. It gives sample-rate guidance of 44.1 kHz for stereo and 48 kHz for 5.1 surround. Check the YouTube settings page for the applicable resolution, frame-rate, and bitrate guidance for your stream.
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When audio and video use independent clocks
If the gap grows progressively and audio and video are captured from different devices or clock domains, a fixed delay is the wrong tool: it can move the starting point but cannot stop the timing error from accumulating. Investigate whether the sources can share a clock, and whether the audio input needs source-specific resampling or rate correction. The right FFmpeg configuration depends on the devices, timestamps, and capture path; without those details, a universal command would be misleading.
Or let it run in the cloud
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