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You can stream local videos with different frame rates to YouTube Live, but choose how FFmpeg should handle their timing and normalize the output deliberately. For mixed or structurally different files, the dependable route is to decode them, join them with FFmpeg’s concat filter, and re-encode to one output format and frame-rate policy. Use the concat demuxer with stream copy only when the files’ stream properties are compatible. For a live stream, pace file input with -re; it does not normalize frame rates.
What you need before building the FFmpeg command
This workflow assumes you already have permission to use and retransmit the video and audio, and that your playlist is an ordered set of local files. It does not cover obtaining a YouTube playlist, authentication, or DRM. Examples below use two files to make the filter graph readable; adapt the inputs, stream selection, and filter graph to your files and installed FFmpeg build.
Inspect every file
Use ffprobe or FFmpeg’s input summary to check each file’s video and audio streams, dimensions, pixel format, codecs, duration, frame rate, and time base. Note whether every clip has audio and whether the audio streams have compatible properties. Mixed frame rates are not the only potential incompatibility: differing dimensions, stream layouts, missing audio, or inaccurate duration metadata can also affect the join.
FFmpeg’s online manuals track the current documentation, while options can vary with the version installed on your system. Check ffmpeg -version and the local ffmpeg -h output if an option is unavailable or behaves differently.
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Choose how to handle frame rate and concatenation
There are two separate decisions: how to join the files and what timing policy to use for the output. A mixed-fps source playlist does not, by itself, mean every input must first be converted to the same recorded frame rate. The output still needs a deliberate timing policy.
| Method or policy | Best fit | Tradeoff |
|---|---|---|
| Concat filter plus re-encode | Inputs differ in stream properties, or you need to normalize geometry, pixel format, audio, or output cadence. | Uses more processing and creates another encode generation, but offers flexible output handling. FFmpeg recommends the concat filter when re-encoding is needed: FFmpeg FAQ. |
| Concat demuxer plus stream copy | Inputs have matching streams and compatible codecs, time bases, and related properties, and avoiding re-encoding matters. | Has strict compatibility and duration requirements; timestamp or transition artifacts and gaps are possible. See the FFmpeg concat demuxer documentation. |
Constant frame rate (cfr) |
You need one exact output cadence for a predictable live pipeline. | FFmpeg duplicates or drops frames to reach the chosen rate. See FFmpeg video options. |
Variable frame rate (vfr) |
Preserving frame timestamps is the priority and the output path supports variable timing. | FFmpeg passes frames with their timestamps, subject to dropping frames that would duplicate a timestamp; verify target-platform behavior. |
passthrough |
You want to retain demuxer timestamps. | It does not convert the output to a fixed cadence; inspect the resulting timestamps. |
For a straightforward live setup, choose a constant output cadence supported by your chosen YouTube settings. A 30 fps output is a common illustrative choice, not a requirement. If you instead choose VFR, test the actual output path rather than assuming every player or live workflow will handle it identically.
Build a normalized playlist with the concat filter
Use the concat filter when you are re-encoding. The following illustrative command assumes both clips have one video stream and one compatible audio stream, and that you want a 1920×1080, 30 fps H.264 output with AAC audio. It scales each video to fit within 1920×1080, pads the remainder, converts to a compatible pixel format, joins the segments, then encodes the result.
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ffmpeg -i clip1.mp4 -i clip2.mp4 -filter_complex "[0:v]scale=1920:1080:force_original_aspect_ratio=decrease,pad=1920:1080:(ow-iw)/2:(oh-ih)/2,fps=30,format=yuv420p[v0];[1:v]scale=1920:1080:force_original_aspect_ratio=decrease,pad=1920:1080:(ow-iw)/2:(oh-ih)/2,fps=30,format=yuv420p[v1];[0:a]aresample=48000[a0];[1:a]aresample=48000[a1];[v0][a0][v1][a1]concat=n=2:v=1:a=1[v][a]" -map "[v]" -map "[a]" -c:v libx264 -pix_fmt yuv420p -r 30 -fps_mode cfr -c:a aac -ar 48000 output.mp4Adjust the output dimensions and cadence to your intended stream settings. In this graph, the fps=30 filter and -fps_mode cfr establish a 30 fps constant-cadence output; frames can be duplicated or dropped to achieve it. The concat filter’s segment count and labels must match your actual files.
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The example assumes every clip has an audio stream. If one is silent or lacks audio entirely, do not use the same audio labels unchanged: provide a consistently formatted silent audio stream for that segment or build a graph that handles the differing streams. Confirm stream mapping with ffprobe before running the command. A mismatch in audio stream availability can make an otherwise valid video filter graph fail.
Generate the graph for a longer playlist
For more than two files, repeat the per-input normalization filters and assign each input unique labels, then list each video/audio pair in order in the concat filter. Keep the segment count (n) equal to the number of inputs. Since stream layouts vary, there is no single filter graph that can safely be pasted for every playlist.
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Use the concat demuxer only for compatible files
The concat demuxer can join compatible files without re-encoding. Create a text manifest such as:
ffconcat version 1.0
file 'clip1.mp4'
file 'clip2.mp4'Then try stream copy only after confirming that the files have matching streams, including codec and time-base properties:
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Stream the playlist to YouTube Live in real time
For a live output, FFmpeg must read local files at real-time pace. Put -re before the relevant input. FFmpeg documents it as equivalent to -readrate 1; it controls reading speed, not frame-rate conversion. See the FFmpeg main options.
For a simple one-file input, an illustrative H.264/AAC RTMP(S) pattern is:
ffmpeg -re -i input.mp4 -c:v libx264 -preset veryfast -r 30 -fps_mode cfr -g 60 -keyint_min 60 -sc_threshold 0 -b:v 8M -minrate 8M -maxrate 8M -bufsize 16M -c:a aac -b:a 128k -ar 44100 -f flv "YOUTUBE_INGEST_URL/YOUR_STREAM_KEY"This is a pattern to adapt, not a universal bitrate prescription. The example targets 30 fps with a 60-frame GOP, or two seconds, and uses CBR-oriented bitrate controls. Replace the destination with the ingest URL and stream key shown for your broadcast in YouTube Live Control Room. Do not publish or share a real stream key; treat it as a password. For the multi-file concat-filter workflow, apply real-time pacing to the file inputs and send the normalized/concatenated output through the same kind of encoder and ingest configuration. Test your installed FFmpeg command and actual stream layout before the event.
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Match the output to YouTube’s current ingest guidance
YouTube’s live encoder settings page lists RTMP/RTMPS, H.264, H.265, and AV1 video, a maximum of 60 fps, a recommended two-second keyframe frequency that should not exceed four seconds, AAC or MP3 audio, and CBR encoding. Its bitrate table depends on codec, resolution, and frame rate; choose a value for the exact output you plan to send and the upload bandwidth available to the sending machine. YouTube’s stated H.264 examples include 14 Mbps for 1080p at 30 fps, 17 Mbps for 1080p at 60 fps, and 8 Mbps for 720p at either 30 or 60 fps. These are YouTube recommendations for those specific settings, not general targets for every stream. Check the current YouTube live encoder settings, bitrates, and resolutions before going live because the guidance can change.
The output stream’s cadence is what matters for these live settings. YouTube’s separate upload guidance says uploaded content should keep its recorded frame rate and lists common upload rates of 24, 25, 30, 48, 50, and 60 fps. That upload recommendation is not a rule that every source clip in a mixed-fps live playlist must retain its original cadence. See YouTube recommended upload encoding settings.
Test transitions and monitor the broadcast
- Run the finished command against representative clips, including a transition between different frame rates and any segment without audio.
- Check the output for forward-moving timestamps, audio/video synchronization, and unintended speed changes or visible judder.
- Start a private or unlisted test in YouTube Live Control Room with audio and movement representative of the intended stream.
- Confirm the ingest destination, stream key, selected resolution, frame rate, and YouTube stream-health messages before relying on the setup.
- During the broadcast, watch stream health and review messages so you can respond to a connection or encoding problem.
YouTube Help states: “Make sure to test before you start your live stream.” It also advises: “During the event, monitor the stream health and review messages.” See the YouTube encoder settings page.
Troubleshooting common failures
| Symptom | Likely cause | What to check or change |
|---|---|---|
| Concat filter reports that inputs do not match | Different dimensions, pixel formats, audio layouts, or stream availability. | Inspect each file’s streams, normalize video geometry and format, and make audio consistent or synthesize silence where needed. |
| Stream copy fails or produces glitches at a frame-rate transition | The concat demuxer inputs do not meet its stream/time-base compatibility requirements, or timestamps/durations are problematic. | Try the concat filter with re-encoding, or normalize files first. Check duration metadata and inspect the transition. |
| Black/silent gap or audio drift between clips | Audio and video durations differ, audio is missing in a clip, or duration metadata creates a bad timestamp offset. | Check the audio streams and durations with ffprobe; provide silence consistently if appropriate and test the join again. |
| Playback feels too fast, slow, or uneven | Input pacing, timestamps, or output cadence were not handled as intended. | Use -re before local file inputs for live pacing, then choose and validate an explicit cfr, vfr, or passthrough policy. |
| YouTube reports poor stream health or drops frames | The selected output bitrate may exceed available upload capacity, or the encoder settings do not match the intended resolution/cadence. | Use YouTube’s current settings for the chosen codec, resolution, and fps; test the network and monitor stream health. |
| FFmpeg rejects an option | The installed version/build differs from the rolling manual or lacks an encoder. | Check ffmpeg -version, ffmpeg -h, and available encoders, then adapt the command to that build. |
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