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First find out whether the audio/video mismatch is already present in FFmpeg’s output, appears only in YouTube playback, stays roughly constant, or grows over time. A fixed offset may call for a measured timestamp shift; growing drift points instead to clocks or audio samples that are not keeping pace with timestamps. Changing bitrate or forcing a frame rate without identifying the pattern can leave the cause untouched—or introduce new artifacts.
Diagnose the sync problem before changing FFmpeg
Use the same clear event—such as a clap, a spoken syllable, or a visible impact—to compare audio and video near the beginning of a test and again later. Note which leads and approximately how far apart they are at each point. The gap between those observations matters as much as the initial mismatch.
- About the same gap both times: suspect a fixed offset between the relevant streams.
- A gap that increases or decreases over time: investigate source clocks and whether audio samples follow their timestamps.
- A sudden jump: look for an input interruption or timestamp discontinuity around the time it happens.
- Uneven motion or cadence: check the source frame rate and FFmpeg’s frame-synchronization policy.
These patterns are diagnostic clues, not proof by themselves. Establish where the problem first appears before applying a correction.
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Save the exact FFmpeg command, but redact the YouTube stream key before sharing it. Record the output of ffmpeg -version, relevant startup and runtime log lines, the type of each input, and whether audio and video come from one file or separate live sources. Where possible, inspect stream start times, time bases, frame rates, sample rates, and timestamps with ffprobe or FFmpeg logs. Record two sync observations separated in time.
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Compare FFmpeg output with YouTube playback
Check a local recording or capture of the FFmpeg output and compare it with YouTube’s Live Control Room preview and, if available, the resulting playback. If the local output is synchronized but the YouTube result is not, examine stream health and playback conditions before adding timestamp filters. YouTube Help recommends testing before a live stream and monitoring stream health and status messages.
Correct a measured, fixed audio/video offset
When the gap is nearly constant, measure it and identify which input needs to be delayed. FFmpeg’s -itsoffset shifts timestamps for an input; a positive value delays that input. Place the option before the -i belonging to the input you intend to shift. For example, if audio is ahead, apply a measured positive delay to the audio input; if video is ahead, determine the appropriate shift for the video input instead.
Do not choose a delay by guesswork. Make one measured change, repeat the same sync checks near the start and later in the test, and keep the known-good command so you can revert. FFmpeg’s command-line documentation notes that expected results depend on the timestamps of multiple inputs deriving from the same clock source; with separate live sources, their clock relationship may be part of the problem.
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Investigate drift, jumps, and irregular frame cadence
When the mismatch grows over time
Check whether the audio and video sources are independently captured and whether they share a clock. Inspect whether timestamp progression matches real-time capture, and whether audio samples are diverging from their timestamps. FFmpeg’s aresample filter can stretch or squeeze audio, or add or cut samples, to match timestamps. Consider compensation only after confirming a timestamp/sample mismatch, then test it in a controlled run: unsuitable or excessive correction can affect audio quality. The installed FFmpeg build’s filter documentation describes the options available for that version.
When sync jumps suddenly
Look in the logs for interruptions or timestamp discontinuities at the point of the jump. Establish which input changed timestamps and whether its source paused, restarted, or otherwise changed. Fix the source or capture interruption if possible; a broad timestamp reset can conceal the symptom without correcting its cause.
When video cadence is irregular
Review the input frame rate and the output fps_mode. FFmpeg’s documented modes behave differently: passthrough preserves demuxer timestamps; constant-frame-rate (CFR) output duplicates or drops frames to meet a requested rate; variable-frame-rate (VFR) output passes timestamps or drops frames with duplicate timestamps. Choose based on the actual input and output needs. Forcing CFR is not a general audio-sync repair and can change the frames viewers see. Option names and behavior can depend on the FFmpeg build installed on your Hetzner host.
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Separate timestamp sync from YouTube ingest and network issues
A healthy timestamp relationship and a healthy delivery path are different questions. Check YouTube’s stream-health messages, test with representative movement and audio, and confirm that the stream settings suit the resolution, frame rate, and codec you actually send. YouTube Help’s current encoder guidance recommends RTMP or RTMPS ingestion, up to 60 fps, a two-second keyframe interval (not over four seconds), AAC or MP3 audio, and constant bitrate (CBR). Its recommended bitrates vary by codec, resolution, and frame rate; for H.264, the guidance lists 10 Mbps for 1080p30, 17 Mbps for 1080p60, and 8 Mbps for both 720p30 and 720p60. These are platform recommendations, not universal requirements; check YouTube’s current table for your intended settings.
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Add the bitrate of the primary stream and any backup stream, then compare the total with measured outbound capacity. YouTube Help recommends leaving 20% headroom beyond the total streaming bitrate. Check the bandwidth allocation and observed throughput for your specific Hetzner server, account, and region; a provider name alone does not establish the capacity available to your instance. A bandwidth shortfall can disrupt delivery, but it does not by itself prove why a timestamp offset exists.
Use RTMPS for transport security, not sync correction
YouTube provides instructions in Live Control Room for obtaining an RTMPS URL. RTMPS encrypts the connection; changing to it is not a correction for timestamp drift or lip-sync offset.
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Run a controlled test and verify the result
- Use a short private or unlisted test where permitted. Include a visible sync event, such as a clap, and audio and movement representative of the real stream.
- Capture the local FFmpeg output and compare sync near the start and later, then check the Live Control Room preview and stream-health messages.
- Change one relevant timestamp, resampling, or frame-sync setting at a time. Keep the previous command and logs so you can compare results or revert.
- After each change, repeat the same observations. A correction is useful only if it improves the measured result without introducing unwanted audio or frame artifacts.
YouTube Help recommends testing before going live and monitoring stream health. If the defect remains, the useful next evidence is the redacted command, FFmpeg version and logs, source and clock details, sync measurements over time, and whether local output is already out of sync.
Common causes and fixes at a glance
| Observed symptom | What to check | Next step |
|---|---|---|
| Nearly constant gap | Which stream leads, and the measured offset | Test a measured -itsoffset on the affected input |
| Gap grows or shrinks | Whether separate sources share a clock; sample progression versus timestamps | Investigate the clock relationship; test aresample compensation only if the evidence supports it |
| Sudden sync jump | Logs and input interruptions or timestamp discontinuities | Find and address the input that changed timestamps |
| Uneven frame cadence | Source frame rate and selected fps_mode |
Choose passthrough, CFR, or VFR to suit the input and output; do not force CFR just to treat audio drift |
| Local output is in sync; YouTube playback is not | Live Control Room preview, stream-health messages, and delivery conditions | Investigate ingest and playback before altering FFmpeg timestamps |
| Connection or stream-health warnings | Total primary and backup bitrate versus measured outbound capacity | Check capacity and leave YouTube’s recommended 20% headroom |
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