Ijharul Islam
Audio goes out of sync with video in two ways: a fixed delay, where the sound is late or early by the same amount from start to finish, or drift, where it starts in sync and slides further out as the video plays. A fixed delay is fixed by shifting the audio; drift comes from a frame rate or sample rate mismatch and is fixed by converting the file or stretching the audio.
More on this topic: Video editing how-tos, every guide in one place.
Here's how to measure which you have, the usual causes, and step-by-step fixes in an editor and with FFmpeg.
The short answer
Check the offset at the start and near the end. Same offset both times: shift the audio by that amount (in an editor, or ffmpeg -i in.mp4 -itsoffset 0.3 -i in.mp4 -map 0:v -map 1:a -c copy out.mp4). Offset grows: the file has a variable frame rate (phone and screen recordings) or mismatched sample rates; convert it to constant frame rate (ffmpeg -i in.mp4 -fps_mode cfr -r 30 -c:a aac out.mp4) or resample the audio to 48 kHz, then edit. If it's only out of sync on your device, check Bluetooth headphones before touching the file.
Key takeaways
- Fixed delay = same offset throughout; drift = offset grows with time.
- Phone and screen recordings are often variable frame rate, a common cause of drift in editors.
- A 44.1 kHz file treated as 48 kHz runs 8.8% fast; small clock mismatches drift slowly.
- Bluetooth headphones add playback delay that isn't in the file at all.
- Fix the source before editing: convert to constant frame rate and 48 kHz audio first.
Is it a fixed delay or drift?
Find a sharp sound with a visible cause, such as a clap, a door or a hard consonant (p, b, t), near the start, then another near the end. Compare where the sound lands with the frame where it happens:
| What you see | Type | Typical causes |
|---|---|---|
| Same offset at start and end | Fixed delay | Separate audio recorder, webcam latency, audio interface, an editing slip |
| In sync at start, worse later | Drift | Variable frame rate, sample rate mismatch, two devices' clocks |
| In sync in one player, not another | Playback problem | Bluetooth headphones, a slow computer, a player bug |
| Out of sync only after export | Export problem | Frame rate mismatch between clip, timeline and export |
At 30 fps one frame is 33 ms; most people notice sound that's more than about 2 frames late, and notice early sound sooner. To measure precisely, note the timecode of the frame and of the sound's waveform peak and subtract; the free timecode calculator converts between timecode, frames and seconds.
How do you fix a fixed audio delay?
You fix a fixed audio delay by moving the audio track by the measured offset. In any editor, unlink the audio from the video, slide it by the offset (zoom in until you can move it frame by frame or by milliseconds), and line the clap or consonant up with its frame.
| Tool | How |
|---|---|
| Premiere Pro, DaVinci Resolve, Final Cut, CapCut | Unlink audio, nudge it, relink |
| FFmpeg, audio late by 0.3 s (move it earlier) | ffmpeg -i in.mp4 -itsoffset -0.3 -i in.mp4 -map 0:v -map 1:a -c copy out.mp4 |
| FFmpeg, audio early by 0.3 s (delay it) | ffmpeg -i in.mp4 -itsoffset 0.3 -i in.mp4 -map 0:v -map 1:a -c copy out.mp4 |
| VLC (viewing only) | Tools > Track Synchronization, or J and K keys |
The FFmpeg commands read the same file twice, take the picture from the first copy and the sound from the second, offset by the amount you give, and copy both without re-encoding. With two-device recordings (camera plus recorder), line up a clap at the start; most editors can also auto-sync by waveform.
Why does audio drift out of sync in long videos?
Audio drifts because the audio and video are running at slightly different rates. The three common reasons:
- Variable frame rate (VFR): phones and screen recorders vary the frame rate to save space. Many editors assume a constant rate, so the picture and sound slowly part company on the timeline.
- Sample rate mismatch: audio recorded at 44.1 kHz but read as 48 kHz (or the reverse) changes speed by 8.8%. Larry Jordan describes slow drift as a classic symptom of recording at one sample rate and capturing at another.
- Clock differences between devices: a camera and a separate audio recorder each have their own clock, and over a long take they disagree slightly.
| Rate error | Drift after 10 minutes | After 1 hour |
|---|---|---|
| 0.01% (tiny clock difference) | 0.06 s | 0.36 s |
| 0.1% (29.97 vs 30 fps) | 0.6 s | 3.6 s |
| 4.3% (23.976 vs 25 fps) | 26 s | 2.6 min |
| 8.8% (44.1 vs 48 kHz) | 49 s | 4.9 min |
Errors over 1% are obvious within seconds; the slow, hard-to-pin-down drift that appears 10 or 20 minutes in is usually VFR or device clocks. 44.1 kHz vs 48 kHz explains the sample rate case.
How do you fix audio drift?
You fix audio drift by converting the source to a constant frame rate and a standard sample rate before you edit, or by stretching the audio to match.
- Convert variable frame rate to constant:
ffmpeg -i in.mp4 -fps_mode cfr -r 30 -c:v libx264 -crf 18 -c:a aac -ar 48000 out.mp4(use your recording's nominal rate instead of 30). HandBrake's Constant Framerate option does the same. Then edit the converted file. - Resample mismatched audio:
ffmpeg -i in.wav -ar 48000 out.wav, and set your project, recorder and system audio device to the same rate. - Stretch to match (device clocks): line up the start, find how far off the end is, and change the audio's speed by that ratio. If a 60-minute take is 0.36 s long at the end, the audio needs to play 0.01% faster:
atempo=1.0001in FFmpeg, or the rate-stretch tool in your editor.
Check lengths quickly with the free video duration calculator, which reads the duration of each file in your browser: if the separately recorded audio is longer or shorter than the video by more than the gap at the start, you have drift.
Why is audio out of sync only after exporting?
Audio is out of sync only after exporting when the export settings don't match the clips or the timeline. Common culprits:
| Cause | Fix |
|---|---|
| Timeline frame rate differs from the clips | Set the sequence to the clips' frame rate, or conform the clips |
| Variable frame rate clips | Convert to constant frame rate before editing |
| Export frame rate differs from the timeline | Export at the timeline's rate, constant |
| Hardware encoder bug | Try the software encoder or update graphics drivers |
| Player issue | Play the export in VLC on another device before re-exporting |
Export settings that work for YouTube and social uploads are in the best export settings for YouTube; frame rate choices are in 24 vs 30 vs 60 fps.
What if it's only out of sync on your device?
If the file is fine elsewhere, the delay is in playback. Bluetooth headphones and speakers add latency, often enough to see, because audio is buffered before it's sent. Try wired headphones or the device's speakers; many players and TVs have an audio delay setting to compensate. A slow computer can also drop video frames while audio keeps going, which looks like the sound is ahead; close other apps or play a lower-resolution copy.
If the problem is captions rather than audio, that's a separate fix: how to fix out-of-sync subtitles. And for clean recordings next time, how to record audio on a computer covers settings that avoid mismatches.
Frequently asked questions
Why is my audio out of sync with my video?
Either a fixed delay (shift the audio) or drift caused by variable frame rate, mismatched sample rates or device clocks (convert or stretch).
How do I fix audio delay in a video for free?
Use FFmpeg's -itsoffset to shift the audio without re-encoding, or nudge the audio track in a free editor such as DaVinci Resolve or CapCut.
Why does audio drift out of sync over time?
The audio and video run at slightly different rates, often because of variable frame rate recordings or 44.1/48 kHz mismatches.
How do I fix variable frame rate video?
Convert it to constant frame rate with HandBrake or FFmpeg (-fps_mode cfr -r 30) before editing.
Why is my video out of sync only with Bluetooth headphones?
Bluetooth adds playback latency. The file is fine; use wired audio or the player's audio delay setting.
How much delay is noticeable?
Most people notice audio that is more than about 2 frames late (around 65 ms at 30 fps), and early audio even sooner.
Try it with your own video
Caption your videos with subtitles timed to the audio, then translate them into 32 languages. Upload a video, check the timed transcript, and export SRT or VTT. QuickVeed plans start at $18/month, and new accounts get a 14-day trial with 250 credits, no card needed. Sign up or see pricing.
Sources
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