In OBS, add the MP4 as a Media Source and turn on Loop in its properties. That makes OBS replay the file when it reaches the end; it does not fix timing problems already inside the file. Check synchronization at several points and across the loop seam before relying on a long recording.
Choose the right OBS source
One MP4: Media Source
Add a Media Source to your scene, choose the MP4, and enable Loop in the source properties. OBS lists MP4 as a supported format and describes Loop as replaying the file after playback finishes. For one file, this built-in source avoids a separate VLC installation. OBS Media Sources documentation
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A playlist: VLC Video
Use the VLC Video source when you need a playlist; OBS documents a Loop Playlist control for it. The source requires VLC, and 64-bit OBS requires 64-bit VLC. Use Media Source instead when you only need one MP4 and do not need VLC-specific playlist behavior. OBS Media Sources documentation
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Check the MP4 before looping
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Play the file outside OBS. Listen and watch near the beginning, around the middle, near the end, and through the transition from the end back to the beginning. This helps establish whether synchronization is already wrong in the file or whether the problem appears during OBS playback or at the loop boundary.
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Add the MP4 as a Media Source, enable Loop, and test it in the actual scene through repeated transitions. Note whether the audio offset steadily grows during playback or whether you hear a skip or click only at the seam. These are different symptoms and do not necessarily share a fix.
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Keep the original file. If you make a converted copy, repeat the checks at multiple points and across the seam before replacing or relying on it.
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OBS documents what its loop control does; it does not claim the control rewrites timestamps or repairs synchronization. A loop setting can repeat a file, but it cannot by itself guarantee a seamless transition.
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CBR is not CFR
Bitrate and frame timing are separate properties. CBR means constant bitrate; CFR means constant frame rate. An MP4 marked CBR is not thereby known to have constant frame timing. OBS forum administrator, moderator, and developer RytoEX states: “CBR (Constant BitRate) vs. VBR (Variable BitRate) has nothing to do with CFR (Constant Frame Rate) vs. VFR (Variable Frame Rate).” OBS Project forum discussion
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Remuxing versus transcoding
A remux changes the container without re-encoding the audio or video streams. In FFmpeg, stream copy uses -c copy. It avoids transcoding, but does not perform the same operation as converting frame timing to CFR. FFmpeg’s CFR output mode can duplicate or drop frames to reach the requested frame rate, so it changes the video stream. Choose a target rate only after inspecting the source; forcing an arbitrary rate can alter the video without addressing the actual cause of the sync issue.
FFmpeg’s documentation explains stream copy, frame-rate modes, and timestamp behavior. Timestamp and synchronization behavior depends on the selected modes and streams, so there is no single conversion command that can be assumed to repair every long MP4.
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Investigate gradual drift separately
If the offset grows over time rather than appearing all at once, check whether OBS and the relevant audio devices use compatible sample-rate settings. In one OBS support case, a volunteer pointed to inconsistent device sample rates and recommended 48 kHz for that setup. That is a diagnostic clue from a particular case, not proof that every drifting MP4 has a sample-rate mismatch or that changing the setting will repair an existing file. OBS Project support discussion
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Troubleshoot by symptom
| What you notice | What to check | Practical next step |
|---|---|---|
| Audio and video are out of sync outside OBS from the start | The source file itself may already have an offset. | Keep the original and investigate or convert a copy; then verify it outside OBS and in the scene. |
| Sync is correct at first but drifts progressively during playback | Distinguish a timing issue in the file from a playback or audio-device configuration issue. CBR does not establish CFR. | Check compatible OBS and audio-device sample-rate settings, then test a converted copy only if the source warrants it. |
| Playback is fine until the file repeats | This points to a loop-boundary discontinuity rather than necessarily to gradual drift. | Inspect the end-to-start transition in the media and test repeated playback in the actual scene. |
| The playlist source is unavailable or does not behave as expected | VLC Video depends on VLC; 64-bit OBS requires 64-bit VLC. | Install the matching VLC version or use Media Source for a single file. |
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