To make an FFmpeg loop stream output 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. Looping and pacing are separate: -stream_loop -1 repeats the file, while -re reads it at its native pace for a live-style output. Neither option sets CFR.
Use this FFmpeg command for a 30 fps loop
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 file, 30 with your chosen frame rate, and the RTMP address and STREAM_KEY placeholders with the ingest URL and stream key shown in your YouTube Live control room. Do not share or publish your real key. Adjust encoding parameters to suit your source, available encoding capacity, and YouTube’s current ingest settings.
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-stream_loop -1repeats the input indefinitely.-rereads the file at its native pace for live-style output.-vf "fps=30"creates a 30 fps cadence, dropping or duplicating frames as needed.
These functions are distinct: looping repeats content, pacing controls how quickly a file is read, and CFR conversion establishes the output cadence. FFmpeg documents input looping and real-time reading in its FFmpeg documentation, and describes the fps filter in its filters documentation.
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Choose between the fps filter and output CFR options
The filter-based command above is a straightforward way to normalize cadence in the video filter graph. Another option is to ask FFmpeg to synchronize the output stream to a constant rate:
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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"Here, -fps_mode:v cfr tells FFmpeg to duplicate or drop frames to reach the requested constant rate, and -r:v 30 supplies that rate. FFmpeg documents this per-stream mode as the modern explicit output-sync setting. The older global -vsync option is marked deprecated.
- Use
-vf "fps=30"when you want cadence conversion expressed in the filter graph, particularly when working with other video filters. - Use
-fps_mode:v cfr -r:v 30when you want to set output synchronization behavior explicitly.
Choose one deliberate conversion point. Stacking both approaches without a specific reason can add redundant processing. Test either form with your actual source and FFmpeg version; the suitable choice depends on where you want cadence normalized and whether your filter graph already handles frame timing. See FFmpeg’s documentation for frame-rate and synchronization options.
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Do not confuse input -r with output frame-rate settings
FFmpeg options generally apply to the next input or output, so position matters. For 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. Use a filter or output synchronization setting when the encoded output must be resampled to a new cadence.
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Stream copying with -c:v copy does not re-encode the video or run a video filter, so it cannot create a newly encoded CFR cadence. If the source already has the constant rate you want, you may not need frame-rate conversion at all; verify the output instead of adding a transformation automatically.
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Pick a frame rate that suits the source and stream
Choose a target cadence based on the source material and intended stream profile, rather than defaulting to the highest available rate. Conversion to a higher rate can duplicate frames; conversion to a lower rate can drop them. YouTube’s live encoder guidance says to specify resolution, frame rate, and bitrate in the encoder, lists frame rates up to 60 fps, and recommends testing with representative audio and movement and monitoring stream health. A 60 fps setting is not automatically preferable for every loop.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Check the output and test the full ingest path
- Inspect the source. Use FFprobe to review its stream metadata, including nominal and average frame rate. Differences may flag a mismatch worth investigating, but metadata alone does not prove that playback will look smooth.
- Run a short encode test. Confirm FFmpeg sustains real-time speed with the selected frame-rate conversion and encoding settings.
- Send a test stream through YouTube ingest. Use representative audio and motion, then check YouTube Live’s stream-health feedback and review the video visually.
- Watch for timing symptoms. Check whether FFmpeg is dropping or duplicating frames and whether the resulting stream remains healthy during the test.
YouTube recommends testing and monitoring stream health in its live encoder settings guidance. Treat metadata as one diagnostic, not a substitute for checking the encoded picture and the ingest path.
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Troubleshoot common frame-rate problems
- The stream does not loop: confirm
-stream_loop -1appears before the relevant-iinput. It controls input repetition, not frame rate. - The file runs too quickly or slowly for live output: for file-based live-style output, check that
-reis before-i. It paces input reading; it does not make the output CFR. - The output rate differs from the target: confirm that the chosen filter or output-sync options apply to the video stream and that the requested rate is set as intended. Do not rely on input
-rfor a timestamped file. - Video filters have no effect: check whether the command uses
-c:v copy. Stream copying bypasses video filtering and re-encoding. - Motion looks uneven or frames are being dropped: compare the target cadence with the source and inspect FFmpeg’s runtime behavior. Test a representative section; forcing a higher rate may duplicate frames rather than add motion detail.
- YouTube reports stream-health issues: verify the encoder’s resolution, frame rate, and bitrate against YouTube’s current ingest guidance, then test again while monitoring the Live health feedback.
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