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To make a looped video stream at a constant frame rate, choose a target cadence and have FFmpeg convert the video to it. For a straightforward transcode, use the fps video filter; alternatively, use output synchronization with -fps_mode:v cfr and output -r:v. Looping with -stream_loop -1 and pacing with -re are separate jobs: neither sets the frame rate.
Use the fps filter for a straightforward loop
This example loops a file, reads it at a live-style pace, converts its video to 30 fps, and sends the encoded output 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 file and 30 with the intended frame rate. Replace the RTMP destination and placeholder key with the values for your YouTube stream; never share a real stream key. This is a command shape, not a complete encoding profile: set bitrate and other encoder options for your source, encoding capacity, and YouTube’s current ingest guidance.
-stream_loop -1repeats the input indefinitely.-rereads the file at its native pace for live-style output.-vf "fps=30"makes the filter output 30 frames per second by dropping or duplicating frames as needed.-map 0:v:0selects the first video stream.-map 0:a?selects audio if present; the question mark makes the audio mapping optional.
FFmpeg documents input looping and real-time input reading and its frame-rate conversion filter.
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Alternative: set CFR at output synchronization
If you want to express constant-frame-rate output synchronization rather than put cadence conversion in the filter graph, use this form:
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 output rate, and -r:v 30 supplies that rate. FFmpeg documents this per-stream mode in its main documentation. Use one deliberate frame-rate conversion point; stacking the fps filter and output CFR settings without a specific reason can add needless complexity.
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Choose between fps and -fps_mode:v cfr
| Approach | Where cadence is set | Useful when |
|---|---|---|
-vf "fps=30" |
In the video filter graph | You are transcoding and want the filter chain to normalize the cadence. |
-fps_mode:v cfr -r:v 30 |
At output synchronization | You need explicit output timestamp synchronization, or prefer to specify CFR as an output behavior. |
Both options can produce CFR by dropping or duplicating frames. Which is appropriate depends on the processing chain, existing filters, source, and FFmpeg version. Test the actual command and input rather than combining both methods by habit.
Understand input and output frame-rate options
FFmpeg options apply to the next input or output, so their position matters. Input -r and output -r do different things. For raw input, placing -r before -i tells FFmpeg to ignore stored timestamps and assume a constant input rate. Output -r requests an output rate. For ordinary timestamped video, do not put -r before -i as a shortcut for output CFR.
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A filter or re-encoding is needed to resample video to a new cadence. Stream copying with -c:v copy does not run a video filter and cannot create a newly encoded CFR cadence. If the source already has the intended constant rate, you may not need conversion; inspect and test the output first.
The older global -vsync option is marked deprecated in current FFmpeg documentation. Prefer the per-stream -fps_mode:v cfr setting when you need output synchronization explicitly. See the FFmpeg options documentation.
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Select a target and check the stream
Choose a rate that fits the source and content, not simply the highest available number. YouTube’s live encoder guidance says to specify resolution, frame rate, and bitrate in the encoder, lists support for frame rates up to 60 fps, and recommends testing with representative audio and movement and monitoring stream health. That upper limit is not a recommendation to convert every loop to 60 fps: a lower-rate source may not benefit from duplicated frames.
- Inspect the input. Use FFprobe to review its video and audio streams and frame-rate metadata. Nominal and average frame rates can reveal obvious mismatches, but metadata alone does not prove playback will look smooth.
- Run a short end-to-end test. Test the actual encode and YouTube ingest path before relying on the loop.
- Review both picture and health feedback. Watch representative motion and audio, check YouTube Live health feedback, and confirm FFmpeg sustains real-time speed without problematic dropped or duplicated frames.
- Adjust the profile if needed. Set resolution, frame rate, and bitrate to suit the source and YouTube’s current ingest settings; the example commands do not prescribe a universal bitrate.
Troubleshooting common CFR problems
- The stream is not actually being converted. If the command uses
-c:v copy, FFmpeg is not re-encoding the video through the filter. Remove stream copying when conversion is required and use a video encoder. - The input is treated as the wrong cadence. Check whether
-ris before or after-i. For ordinary timestamped files, do not use input-ras an output-rate setting. - The stream is paced incorrectly.
-stream_loop -1repeats the file, but does not pace it;-rereads it at native pace, but does not set CFR. Use each option for its separate purpose. - Playback looks uneven despite CFR settings. Verify the chosen rate against the source, inspect representative movement, and check whether the encoder keeps up in real time. CFR describes the output cadence; it does not by itself guarantee smooth-looking motion or healthy ingest.
- The chosen rate creates needless duplication. Match the target to the source and content. A higher target can duplicate frames without adding captured motion.
- YouTube reports stream-health issues. Check the encoder’s resolution, frame rate, and bitrate against YouTube’s current guidance, then repeat a representative test and monitor Live health feedback.
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