First check whether the audio is already out of sync near the start or whether the gap grows as the YouTube stream continues. A steady offset usually calls for aligning input start times; growing drift points instead to timestamps or clocks that do not track one another. Measure before changing FFmpeg options—there is no single safe command for every input setup.
Tell a fixed offset from growing drift
Compare audio and video near the beginning, then at several later points. Note whether the audio leads or trails and estimate the gap each time. Use the same identifiable sound and picture event where possible.
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- About the same gap throughout: suspect an initial offset or different input start times.
- The gap gets larger or smaller: suspect accumulating timestamp or sample-rate clock drift, or source timestamps that do not follow real time.
- The gap jumps suddenly: look for missing, irregular, or discontinuous timestamps and changes in the input or capture process.
This classification narrows the investigation; it does not prove a particular root cause. Avoid changing several timing options at once, or you will not know which change helped.
Collect the information needed to choose a fix
- Record the exact build and command. Capture the FFmpeg version/build and full command line, but remove the private YouTube stream key before sharing logs or saving examples.
- Inspect both streams. Use local FFmpeg tools, such as
ffprobe, to examine audio and video start times, time bases, audio sample rate, video frame rate, and any timestamp warnings. Compare those details with the sync measurements. - Identify the input arrangement. Determine whether audio and video come from one media file or device, or from separate real-time inputs. For separate inputs, find out whether their timestamps derive from the same clock source.
- Repeat the check later. Test with a private or otherwise controlled stream, then compare the beginning and later portions. Keep the command and logs for each attempt.
FFmpeg’s online documentation is regenerated nightly and corresponds to its newest revision, so options may differ from an installed build. Check documentation for the FFmpeg version you actually run before applying a change: FFmpeg command-line documentation and FFmpeg resampler documentation.
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Fix a consistent offset between audio and video
When the mismatch is approximately constant, test an input start-time adjustment rather than an audio drift correction. FFmpeg’s -itsoffset adds an offset to timestamps for an input; a positive value delays that input. Apply it to the input that needs to move, and verify the result across the stream. The correct direction and amount depend on whether audio or video leads and by how much.
FFmpeg’s -isync can offset a target input by the start-time difference from a reference input. Its expected behavior assumes the inputs’ timestamps come from the same clock source. It is therefore a plausible alignment tool for suitable inputs, not a general fix for independent clocks or a mismatch that keeps growing.
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Do not copy a guessed offset into a live command: measure the actual gap, make one controlled change, and check whether the entire stream improves.
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A growing gap is not normally corrected by simply delaying one input once. Check whether audio timestamps and the source clock remain consistent over time. When timestamp evidence supports it, FFmpeg’s resampler can stretch, squeeze, fill, or trim audio based on timestamps.
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The resampler’s async option is disabled by default. async=1 enables filling and trimming; larger values set the maximum sample compensation per second. The suitable setting depends on the actual source timing, so do not treat a particular value as universally correct. Check the installed FFmpeg documentation and test carefully. The resampler also documents first_pts for padding or trimming at stream start and min_comp as a threshold for compensation; these options address particular timing conditions, not every form of drift.
Before enabling compensation, establish that timestamps are usable and that the issue is accumulating audio mismatch against them. If the timestamps themselves are missing, irregular, or discontinuous, investigate that source behavior rather than assuming resampling alone will solve it.
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Check video timing without changing it blindly
Video frame handling can affect the timing you observe. FFmpeg documents -fps_mode passthrough as forwarding frame timestamps, while -fps_mode cfr duplicates or drops frames to reach a constant requested rate. Muxers may modify timestamps further.
Do not change the frame rate or select a different frame mode as a generic audio-drift cure. First determine whether the video timestamps are the problem and what behavior the output needs. Arbitrary frame-rate changes can alter video timing without correcting an audio clock mismatch.
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Avoid fixes that preserve the same sync error
- Shifting every output timestamp uniformly: this preserves relative audio/video timing differences, so it does not by itself repair an existing mismatch.
- Increasing a queue or buffer as a drift correction: muxer interleaving and buffering affect packet output behavior; they are not established here as a general correction for accumulating sync drift.
- Changing multiple timing flags together: this obscures the cause and makes results harder to verify.
Troubleshoot by symptom
| What you observe | What to inspect | Next step |
|---|---|---|
| The same audio lead or lag near the start and later | Input start times and whether audio/video begin together | Test an appropriate -itsoffset adjustment, or -isync if the inputs meet its shared-clock assumption. |
| The audio/video gap grows over time | Timestamp behavior, sample rate, and whether separate inputs share a clock | Investigate the source timing; test timestamp-based resampler compensation only when appropriate. |
| Sync changes abruptly or timestamp warnings appear | Missing, irregular, or discontinuous input timestamps and source changes | Address the timestamp/source issue before selecting an offset or compensation amount. |
| A frame-mode or frame-rate change appears to affect sync | Video frame timestamps and the selected -fps_mode |
Confirm that video timing is the actual cause; do not assume a frame-mode change repairs audio drift. |
| A uniform shift changes the stream’s start time but not the mismatch | Whether audio and video were shifted equally | Measure relative timing again; use an input-specific remedy rather than a uniform shift. |
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