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constant frame rate

How to Keep Frame Rate Constant When Looping Videos to YouTube with FFmpeg

Use FFmpeg’s input loop option to repeat a video, then re-encode at the intended rate for constant-frame-rate output. Learn the command and key YouTube settings.

By DocumentaryTube Team 5 min read

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Use FFmpeg’s -stream_loop to repeat a whole video, and re-encode with an output frame rate to make the result constant-frame-rate (CFR). For example, set -r 30 only when 30 fps is the intended rate. YouTube recommends encoding and uploading at the rate the video was recorded, so preserving the source rate is usually the right choice.

Loop the whole video and encode at a constant frame rate

This example repeats an MP4 indefinitely, writes a one-hour output, and encodes at 30 fps. It assumes an input with video and may or may not have audio:

ffmpeg -stream_loop -1 -i input.mp4 -map 0:v:0 -map '0:a?' -t 3600 
  -c:v libx264 -r 30 -pix_fmt yuv420p 
  -c:a aac -movflags +faststart output.mp4

Replace 30 with the source’s intended frame rate. If the actual rate is fractional, use its rational form—for example, 30000/1001—rather than rounding it to 30. Replace 3600 with the desired duration in seconds. The optional audio map (0:a?) lets FFmpeg continue when the input has no audio.

-stream_loop is an input option, so it must appear before -i. The value -1 means repeat indefinitely. The command re-encodes the video; it is an illustrative starting point, not a tested result. Check the input streams and play the output before uploading, particularly if the source has unusual timestamps, variable frame rate, multiple tracks, or an audible or visible seam where the loop restarts. FFmpeg documents the stream-loop option.

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Choose an output duration

The example uses -t 3600 to stop after 3,600 seconds (one hour). Set a different duration to suit the intended file. With infinite input looping, the duration limit is what makes this example output finite.

Choose the rate deliberately

Use the source’s intended rate unless you have a reason to convert it. YouTube’s upload guidance lists 24, 25, 30, 48, 50, and 60 fps as common rates, says other rates are acceptable, and recommends matching the recording rate. YouTube’s frame-rate guidance does not make 30 fps a universal requirement.

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What makes the output frame rate constant?

Looping and frame-rate conversion are separate operations. -stream_loop repeats the input; it does not itself make the output CFR. When encoding, FFmpeg’s output -r duplicates or drops frames as needed to reach the requested rate. You can also express constant-frame-rate synchronization with -fps_mode cfr, for example:

ffmpeg -stream_loop -1 -i input.mp4 -map 0:v:0 -map '0:a?' -t 3600 
  -c:v libx264 -r 30 -fps_mode cfr -pix_fmt yuv420p 
  -c:a aac -movflags +faststart output.mp4

Use a suitable output rate with -fps_mode cfr; it is not a reason to select an arbitrary rate. FFmpeg defines CFR mode as duplicating or dropping frames to attain the requested constant rate. See the FFmpeg video options documentation.

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Preserve the source rate or convert?

  • Preserve it: Choose the source’s actual intended rate when the goal is to match the recording and avoid an unnecessary cadence change.
  • Convert it: Choose a different target only when the delivery or editing plan calls for it. CFR conversion can duplicate or drop frames; it does not create genuinely new motion.

Why stream copy is different

With -c:v copy, FFmpeg does not decode and re-encode frames, so it cannot duplicate or drop them to establish a new cadence. In this mode, output -r only signals a rate to the muxer; a mismatch with packet timestamps may produce an invalid file. If a changed CFR is required, use encoding rather than stream copy. FFmpeg’s documentation for output -r explains this distinction.

Whole-video loops versus frame loops

Use -stream_loop to repeat the entire input. A finite repetition count can replace -1; FFmpeg defines 0 as no repetition and -1 as infinite repetition. The separate video loop filter repeats selected frames, which is useful for a frame-level effect or a short section rather than replaying the whole source. Consult the FFmpeg loop-filter documentation when using that filter.

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Other YouTube upload settings to check

The loop and CFR options do not determine every encoding choice. YouTube’s recommended upload settings for video include MP4, H.264, progressive scan, High Profile, two consecutive B frames, closed GOP with a GOP length of half the frame rate, CABAC, and 4:2:0 chroma subsampling. For standard dynamic-range video, the page recommends BT.709. These are platform recommendations, not mandatory command-line settings for every workflow.

YouTube’s SDR bitrate guidance is organized by resolution and standard- or high-frame-rate groups. For example, it lists 8 Mbps for 1080p in the standard-rate group and 12 Mbps in the high-rate group; for 2160p/4K, it lists 35–45 Mbps and 53–68 Mbps respectively. Treat these as YouTube recommendations, not guarantees of quality or fixed requirements. Check the current YouTube recommended upload encoding settings for the full table and context.

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Check the output before uploading

  • Confirm that the output plays through the intended duration and that the restart seam does not create an unwanted pause, jump, or audio discontinuity.
  • Check that the encoded frame rate is the intended one, especially when the source is variable-frame-rate or uses a fractional rate.
  • Listen for audio dropouts or discontinuities at the loop boundary. The optional audio map only handles the absence of an audio stream; it does not guarantee a seamless audio loop.
  • If the input contains multiple video or audio tracks, review the selected mappings and adjust them to match the track you want.

Troubleshooting

The file is not actually repeating

Place -stream_loop -1 before -i input.mp4. It is an input option. If you used a finite loop count instead, verify that the value matches the intended number of additional repetitions.

The output rate differs from the source

Check the output -r value and any -fps_mode setting. Use the source’s intended rate if no conversion is needed. A target rate that differs from the source may require frame duplication or dropping.

The file has timestamp or playback problems

Do not rely on stream copy plus an output -r to repair cadence. Stream copy does not change frame timing by duplicating or dropping frames. Re-encode when a new constant rate is required, then check the output—especially with unusual input timestamps or variable-frame-rate sources.

The output has no audio or the wrong tracks

-map '0:a?' makes audio optional; it does not add audio where none exists. For files with multiple tracks, inspect the input and revise the maps so the intended streams are selected.

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