First determine whether the audio is out of sync by roughly the same amount throughout the stream or whether the mismatch grows over time. A steady offset usually calls for aligning input start times; increasing drift calls for investigating timestamps and clock differences, and may require timestamp-based audio resampling. There is no reliable one-command fix without knowing your inputs and how they are timed.
Classify the sync problem before changing FFmpeg settings
Compare audio and video near the beginning of the stream and at several later points. Use a visible event with a clear sound, such as a clap or a spoken word, and note whether the sound leads or trails the picture.
- About the same mismatch throughout: suspect an initial offset or different input start times.
- The mismatch gets larger: investigate clock drift, inaccurate or irregular timestamps, or separate real-time sources whose clocks do not track each other.
This distinction is a diagnostic guide, not proof of a particular cause. Avoid changing timing options until you have measured the symptom.
Capture the information needed to diagnose it
- Record the FFmpeg version/build and save the complete command line. Remove the YouTube stream key before sharing logs or commands.
- Inspect the input and output stream information with your local FFmpeg tools. Note audio and video start times, time bases, audio sample rate, video frame rate, and any warnings about timestamps.
- Compare the sync at the start and later points of the recording or test stream. Record whether audio leads or trails and by approximately how much at each point.
- Identify whether audio and video come from the same media file or device, or from separate real-time inputs. For separate sources, determine whether their timestamps come from a shared clock.
FFmpeg’s online documentation is regenerated nightly and corresponds to its newest revision, so the options described there may differ from an older installed build. Check the documentation for the FFmpeg version you actually run: FFmpeg command-line documentation.
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Correct a fixed offset with input alignment
If the offset is approximately constant, test an adjustment to the timestamps of the input that starts early. FFmpeg’s -itsoffset adds an offset to timestamps for an input; a positive value delays that input. Apply it to the relevant input and verify that sync improves across the whole stream. The position of an input-specific option matters, so consult the command-line documentation for your installed version rather than inserting it arbitrarily.
When the inputs have meaningful start times and derive their timestamps from the same clock source, -isync can offset a target input by the start-time difference from a reference input. FFmpeg documents the shared-clock assumption for expected results; it is not a general correction for two independent clocks that drift apart. See FFmpeg’s option documentation.
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Investigate growing drift with timestamps and audio compensation
A mismatch that increases over time is different from a simple start offset. Check whether the source timestamps are continuous and track real time, and whether independently captured audio and video use clocks that can diverge. Shifting an input’s start time may make one point line up while leaving the later drift unresolved.
FFmpeg’s audio resampler can use timestamps to stretch, squeeze, fill, or trim audio. Its async option is disabled by default; async=1 enables filling and trimming, while larger values set the maximum sample compensation per second. The right setting depends on the actual source timing, so do not treat a guessed value as a universal fix. Review the installed build’s resampler options, test a change, and compare sync at multiple points. The documentation also describes first_pts for padding or trimming at stream start and min_comp as the threshold for compensation: FFmpeg resampler documentation.
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Check video frame timing without treating it as an audio fix
Video timestamp handling can affect the apparent relationship between picture and sound. FFmpeg’s -fps_mode passthrough forwards frame timestamps; -fps_mode cfr duplicates or drops frames to reach a requested constant frame rate. Muxers can further modify timestamps. Changing frame mode or imposing an arbitrary frame rate can alter video timing, but it is not automatically a remedy for audio drift. Diagnose the timestamps before testing a video timing change. See FFmpeg command-line documentation.
Avoid timing changes that cannot fix relative sync
- Do not shift all output timestamps uniformly and expect audio/video sync to improve. A uniform shift preserves the relative timing difference already present between the streams.
- Do not treat a larger output queue or muxing buffer as a general drift correction. Interleaving and buffering affect packet output behavior; they do not establish that the source clocks or timestamps are correct.
- Do not change several unrelated timing flags at once. You will not know which change affected the result.
FFmpeg’s format documentation describes timestamp shifting and muxing behavior: FFmpeg format documentation.
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Test one change at a time and verify the result
- Use a private or otherwise suitable test stream and retain the exact command and logs.
- Change only the option that matches the diagnosis: input alignment for a fixed offset, or timestamp/source-clock investigation and carefully tested resampler compensation for accumulating drift.
- Check sync at the beginning and at multiple later points. A change that fixes only the start has not fixed growing drift.
- If the mismatch remains, restore the prior command and revisit the input timing, timestamp warnings, and clock relationship before trying another setting.
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