First find out whether the audio is consistently early or late, or whether the gap grows as the stream runs. Use a measured -itsoffset for a fixed offset; investigate input timestamps and clock sources when the mismatch changes over time. A fixed delay is not a general fix for ongoing drift.
Determine whether the offset is fixed or growing
Observe a local preview or the stream and note which arrives first, the approximate gap, and whether that gap stays about the same or increases. FFmpeg’s ffplay can show audio/video synchronization drift with -stats. Its documented default master clock is audio. The statistic describes playback behavior; by itself, it does not identify which device, clock, or timestamp caused the problem. See the FFmpeg Project’s ffplay documentation.
Compare the beginning of playback with a later point. A steady gap suggests a fixed offset may be suitable; a gap that changes calls for investigating ongoing timing differences rather than adding a one-time delay.
Collect the details that affect synchronization
Before changing the command, keep a copy of the complete command and uncut console output. Record the FFmpeg version, operating system, input order and formats, stream mapping, reported input start times, and relevant warnings. These details help distinguish an input-specific offset from timestamp or capture-clock behavior.
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- For webcam video and a separate audio capture source, check whether the devices derive timestamps from the same clock.
- Note whether either input has a reported start timestamp.
- Change one setting at a time, then measure the stream again.
Do not assume that audio or video input order is the cause. A 2020 FFmpeg-user mailing-list report described roughly half a second of audio delay in one webcam-and-PulseAudio setup after input order was changed. That is a clue worth reproducing in your own setup, not a general rule to reorder every command: the 2020 mailing-list case.
Correct a measured, constant offset with -itsoffset
The FFmpeg Project describes -itsoffset as “Set the input time offset.” It adds the specified duration to timestamps for the input that follows the option. A positive duration delays that input. Put the option immediately before the input it is intended to affect, and use a value based on what you measured. See the FFmpeg documentation.
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ffmpeg -i video_source -itsoffset OFFSET -i audio_source
... outputThis is an illustrative command shape, not a tested, ready-to-run command. Replace the input names and OFFSET with the values and options appropriate to your capture setup. If the audio is early, delaying the audio may help; if it is already late, a positive audio offset makes that relationship worse. Check which input the option precedes, confirm the sign, and measure again in the actual streaming path.
Consider -isync only when the inputs meet its conditions
-isync adjusts timestamps for one input by the difference between its start time and the start time of a reference input. The FFmpeg documentation cautions: “The source timestamps of the two inputs should derive from the same clock source for expected results.” If a starting timestamp is unavailable, FFmpeg makes no adjustment. Consult the documentation for the installed version before adding the option.
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There is also a documented interaction with -copyts: when using -isync with -copyts, -start_at_zero must also be set. Timestamp-preservation options change how FFmpeg processes timestamps, so do not add them casually. See FFmpeg’s input-options documentation for the option details.
Choose the next step based on what you observe
| Observation | Next step | Important limitation |
|---|---|---|
| A steady, measured offset | Try -itsoffset on the input that needs to be delayed. |
A wrong sign or target input can increase the mismatch; a fixed offset does not correct continuing clock drift. |
| Separate inputs with usable start timestamps and a shared clock | Consider -isync with the appropriate reference input. |
Expected results depend on the shared clock and available timestamps; account for the documented -copyts interaction. |
| You need to observe playback drift | Use ffplay -stats and compare the start with a later point. |
The statistic does not diagnose the root cause; verify the actual streaming path too. |
| The gap changes over time | Inspect capture clocks and timestamp progression; test any audio-processing change against the specific setup. | A historical mailing-list example using aresample=async=1 does not establish that value as a universal live-stream fix. |
Troubleshoot common sync problems
Audio is early or late by about the same amount throughout
Measure the gap and try a fixed offset on the input that needs delaying. Confirm the option is placed before that input. Recheck both the direction and magnitude after the change rather than assuming the intended setting took effect.
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The mismatch becomes larger as the stream runs
A starting offset alone does not show that two capture clocks remain aligned. Compare drift at different times, review timestamp continuity and capture behavior, and test one change at a time. Do not copy a filter value from a different setup without checking its behavior and side effects with your FFmpeg version.
Changing input order appears to affect the result
Reproduce the behavior while preserving the full command and output, and compare reported start times and stream mapping. The reported webcam/PulseAudio delay was specific to one 2020 setup; input order by itself is not established as a universal cause.
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A timestamp option seems to have no effect
Check whether the inputs have the start timestamps required by -isync and whether they share a clock source. If using -copyts, verify the required -start_at_zero interaction. Confirm option placement and behavior against the documentation for your installed FFmpeg version.
Check YouTube settings against current guidance
Use current official YouTube guidance for ingest and stream configuration. Historical forum advice about adding silent audio is not enough to establish a current YouTube requirement. No YouTube-specific encoding prescription is included here because the available evidence does not establish current platform settings.
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