In OBS, add a single video as a Media Source and enable Loop in its properties. That makes OBS replay the file when playback ends, but it does not repair timing problems already inside the MP4. To avoid carrying a sync problem into a loop, check the file at several points and across the loop boundary, then investigate the specific symptom before converting anything.
Choose the right OBS source
For one local MP4, use OBS’s built-in Media Source. OBS lists MP4 as a supported format, and its Loop option replays the file after playback finishes. For a playlist, use VLC Video; OBS documents a Loop Playlist option for that source. VLC Video requires VLC, and 64-bit OBS requires 64-bit VLC. OBS’s media-source documentation describes these source types and controls.
| Need | OBS source | What to know |
|---|---|---|
| Repeat one MP4 | Media Source | Built into OBS; enable Loop in the source properties. |
| Play a playlist | VLC Video | Requires a matching VLC installation; offers playlist controls including Loop Playlist. |
For a single file, Media Source keeps the setup simpler. Neither source’s loop control claims to correct timestamps or audio/video synchronization in the media.
Check the MP4 before relying on a loop
First play the file outside OBS. Check whether its audio and video are synchronized near the beginning, middle, and end, and listen and watch through the intended transition from the end back to the beginning. This is a diagnostic procedure, not a guarantee: a file can play correctly in one player and still behave differently in an OBS scene.
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- In OBS, add a Media Source to the scene and select the MP4.
- Open the source properties and enable Loop.
- Play the scene long enough to reach the end and let it repeat. Check sync at several points and at the transition.
- Note whether the offset is present from the start, grows over time, or appears only at the loop boundary. These are different symptoms and do not imply the same cause.
Keep the original file. If you make a converted copy, compare that copy in the same checks before replacing anything.
Tell drift apart from a loop-seam glitch
Audio is offset from the beginning
If the same offset is audible or visible early and remains roughly consistent, the problem may already be present in the file or playback setup. Looping repeats it; it does not synchronize the tracks.
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The offset grows during playback
A steadily increasing mismatch is different from a fixed offset. Check the file’s timing and the relevant OBS audio-device settings rather than assuming the Loop option is responsible. In one OBS support discussion, a volunteer identified inconsistent device sample rates as a diagnostic possibility and recommended 48 kHz for that particular setup. That example does not establish a universal cause or fix. The OBS forum discussion also addresses frame-rate terminology and timing questions.
The problem happens only when the file repeats
A click, pause, skipped frame, or brief sync jump at the seam is a boundary issue to inspect separately from drift across the full file. Check the final and first frames and the audio at the transition. The media may simply not have been authored for a seamless repeat; OBS’s Loop control does not promise to smooth or repair a seam.
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Understand what a conversion can and cannot fix
Before changing a file, identify whether you need to change its container, its frame timing, or something else. These operations are not interchangeable.
- CBR and VBR describe bitrate. CFR and VFR describe frame timing. A constant-bitrate label does not mean a file has constant frame timing. OBS forum administrator, moderator, and developer RytoEX put it plainly: “CBR (Constant BitRate) vs. VBR (Variable BitRate) has nothing to do with CFR (Constant Frame Rate) vs. VFR (Variable Frame Rate).” Read the OBS Project community thread.
- Remuxing changes the container without re-encoding the audio or video. FFmpeg’s stream-copy mode,
-c copy, copies streams rather than transcoding them. A remux is not the same as converting variable frame timing to constant frame timing. - CFR output changes video timing. FFmpeg’s constant-frame-rate output mode duplicates or drops frames to meet the requested rate. That can alter the video stream; it is not a no-reencode remux. FFmpeg’s documentation also notes that timestamp and synchronization behavior depends on the chosen modes and streams. See the FFmpeg documentation.
Do not blindly force an assumed frame rate or run a generic “drift fix” command. Inspect the source and choose a conversion only when its purpose is clear. The available documentation does not establish one command that repairs every long MP4.
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Troubleshoot with the symptom in mind
| What you notice | What to check | Next step |
|---|---|---|
| Audio and video are already out of sync outside OBS | Check the original file at multiple points in another player. | Do not expect OBS Loop to fix it. Preserve the source and investigate the file’s timing before trying a conversion. |
| Sync is fine at first, then gradually worsens | Distinguish growing drift from a fixed offset; check relevant audio-device sample-rate settings. | Treat sample-rate mismatch as one possible clue, not a diagnosis. Re-test after any setting change. |
| Sync changes only at the repeat point | Inspect the end-to-start transition in the media. | Investigate the seam separately; a loop setting does not create a seamless edit. |
| A remux did not change the drift | Confirm whether you changed only the container using stream copy. | Container changes do not by themselves convert VFR timing to CFR. |
| A CFR conversion looks different | Check for duplicated or dropped frames and confirm the chosen target rate. | Compare sync and visual continuity at multiple points and across the loop seam before using the converted copy. |
| VLC Video or its playlist controls are unavailable | Check that VLC is installed and matches OBS’s bitness. | For one MP4, use the built-in Media Source instead. |
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