To make a looped FFmpeg stream output a constant frame rate (CFR), choose a target cadence and use either the fps video filter or FFmpeg’s output-sync options. For example, -vf "fps=30" converts the video to 30 frames per second by dropping or duplicating frames as needed. Looping with -stream_loop -1 and pacing file playback with -re do not set the frame rate.
Use this command for a 30 fps loop
This example loops a local file, reads it at its native pace for live-style output, converts video to 30 fps, and sends the result to YouTube over RTMP:
ffmpeg -stream_loop -1 -re -i loop.mp4
-map 0:v:0 -map 0:a?
-vf "fps=30"
-c:v libx264 -pix_fmt yuv420p
-c:a aac
-f flv "rtmp://a.rtmp.youtube.com/live2/STREAM_KEY"Replace loop.mp4 with your input file and 30 with the frame rate you intend to stream. Replace the RTMP destination and STREAM_KEY with the server URL and stream key shown in your YouTube Live setup. The values in the example are placeholders; do not publish or share a real stream key.
-stream_loop -1repeats the input indefinitely.-rereads the file at its native rate, pacing it for a live-style output.-vf "fps=30"makes the filter output 30 frames per second, dropping or duplicating frames to meet that cadence.
These options have separate jobs: loop, pace, and convert frame rate. FFmpeg describes -stream_loop and real-time reading in its documentation.
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Choose one way to enforce CFR
Use the fps filter
The first command uses -vf "fps=30". This sets the cadence in the video filter graph, which is a clear choice when you want frame-rate conversion to happen as part of video processing. FFmpeg’s filters documentation describes the fps filter.
Use explicit output synchronization instead
You can use output frame-rate synchronization rather than the filter:
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ffmpeg -stream_loop -1 -re -i loop.mp4
-map 0:v:0 -map 0:a?
-fps_mode:v cfr -r:v 30
-c:v libx264 -pix_fmt yuv420p
-c:a aac -f flv "rtmp://a.rtmp.youtube.com/live2/STREAM_KEY"-fps_mode:v cfr tells FFmpeg to duplicate or drop video frames to achieve the requested constant rate, and -r:v 30 supplies that rate. The current FFmpeg documentation marks the older global -vsync option as deprecated; use the per-stream -fps_mode:v cfr form when you need explicit output-sync behavior. See the FFmpeg documentation.
Pick one deliberate cadence-conversion point. Do not add the fps filter and -fps_mode:v cfr -r:v together without a specific reason: redundant conversions can make it harder to understand where frames are being adjusted. Both approaches should be checked with your actual source and FFmpeg version.
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Do not confuse input -r with output CFR
FFmpeg options apply to the next input or output, so both the option and its position matter. For a raw input, -r before -i tells FFmpeg to ignore stored timestamps and assume an input rate. Output -r, placed after the input, requests an output frame rate. For an ordinary timestamped video file, putting -r before -i is not a shortcut for output CFR. The distinction is documented in the FFmpeg command-line documentation.
Likewise, stream-copying video with -c:v copy cannot apply a video filter or create a newly encoded cadence. If the source already has the intended constant rate, you may not need frame-rate conversion at all; inspect and test the output before adding transformations by habit.
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Select a frame rate that fits the source and stream
Set the resolution, frame rate, and bitrate in your encoder to match the intended YouTube stream profile. YouTube’s live encoder guidance supports frame rates up to 60 fps, but that is not a reason to set every loop to 60 fps. Match the target to the source material and the stream you intend to run; a higher target can duplicate frames without adding motion detail when the source cadence is lower.
Use YouTube’s current live encoder settings, bitrates, and resolutions guidance for the ingest profile appropriate to your resolution and frame rate. Test with representative audio and movement, then monitor YouTube Live stream health rather than assuming a successful FFmpeg launch means playback is healthy.
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Validate the output before relying on a long loop
- Inspect the input. Use FFprobe to review the file’s video and audio streams, including nominal and average frame-rate metadata. These values can reveal obvious mismatches, but do not by themselves prove smooth playback.
- Run a short encode and ingest test. Use the intended command, source, target resolution, frame rate, bitrate, and YouTube ingest path before leaving a loop running.
- Review stream health and playback. Check YouTube Live’s health feedback and watch representative video and audio. Look for dropped or duplicated frames and confirm that FFmpeg sustains real-time speed.
- Adjust only the relevant setting. If cadence is wrong, confirm the target and the selected conversion method. If the encoder cannot keep up, inspect encoding load and stream settings instead of adding a second frame-rate conversion.
Troubleshoot common frame-rate problems
The stream is not actually CFR
Confirm that you used either -vf "fps=TARGET" or -fps_mode:v cfr -r:v TARGET in the output path, with the target set as intended. -stream_loop -1 and -re alone only loop and pace the file. If you used -c:v copy, FFmpeg is not filtering and re-encoding the video to a new cadence.
The result is choppy or has repeated frames
CFR conversion necessarily drops or duplicates frames when the source cadence does not match the target. Choose a target appropriate to the source rather than raising it automatically. Review actual playback and stream-health feedback; nominal and average frame-rate metadata alone cannot establish smoothness.
FFmpeg finishes or the file does not repeat
Check that -stream_loop -1 appears before the relevant input’s -i option and that the input path is correct. The loop option repeats an input; it does not change frame rate or guarantee that the encode can run in real time.
YouTube reports stream-health issues
Verify that resolution, frame rate, and bitrate in FFmpeg match the intended YouTube ingest profile, then test with representative movement and audio. Monitor YouTube’s feedback and confirm the encoder sustains real-time speed. A correct CFR setting alone does not establish that the full stream configuration is healthy.
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