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What you need to decide before building the command
This workflow assumes you already have an ordered list of local video files, permission to use and retransmit them, FFmpeg installed, and a YouTube Live stream configured in Live Control Room. It does not cover downloading a YouTube playlist: playlist access, authentication, DRM, and rights are separate issues. Commands below are patterns to adapt to your files and FFmpeg build, not universal drop-in commands.
Different source frame rates do not, by themselves, require changing each file before ingest. You do need to choose how the combined output should handle frame timing: use constant frame rate (CFR) for a fixed output cadence, or variable frame rate (VFR) when preserving source timestamps is the priority and the downstream pipeline supports it. FFmpeg’s current command-line documentation describes -fps_mode cfr as duplicating or dropping frames to reach the requested cadence; vfr passes timestamps while avoiding duplicate timestamps, and passthrough retains demuxer timestamps.
Inspect every input first
Use ffprobe or FFmpeg’s input summary to inventory each file’s video and audio streams, dimensions, pixel format, codec, duration, time base, and frame rate. Check whether every clip has audio and whether audio stream layouts match. Missing audio, differing channel layouts, or inaccurate duration metadata can complicate concatenation just as much as differing frame rates.
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Review the installed version and available options with ffmpeg -version and the local ffmpeg -h output. The online FFmpeg manuals use a rolling current version, so options can differ from an older installed build.
Choose a frame-rate policy and joining method
Recommended for mixed inputs: concat filter and re-encode
FFmpeg’s FAQ recommends the concat filter when re-encoding is needed: FFmpeg FAQ: concatenating video files. It decodes the inputs, lets you normalize them, and joins the resulting streams before encoding. This is generally the more controllable approach when clips differ in FPS or other stream properties.
For a constant-cadence output, choose a target FPS that suits the planned stream and your source material, then apply it deliberately. CFR can duplicate or drop frames; it does not make every source’s original motion cadence identical. If preserving the clips’ timing is more important, use a VFR policy and verify the encoded output’s timestamps and compatibility in a test. Do not treat -re as an FPS-normalization option: it paces reading, not frame conversion.
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Illustrative two-file filter pattern
This example assumes two clips, each with one video stream and one compatible audio stream, and normalizes picture geometry to 1920×1080. It uses a 30-fps CFR output as an example, not a universal recommendation. Change the dimensions and cadence to suit the material and your planned YouTube output. If any file lacks audio, use the missing-audio procedure below rather than mapping a nonexistent stream.
ffmpeg -re -i clip1.mp4 -re -i clip2.mp4
-filter_complex "[0:v]scale=1920:1080:force_original_aspect_ratio=decrease,pad=1920:1080:(ow-iw)/2:(oh-ih)/2,setsar=1,format=yuv420p[v0];[1:v]scale=1920:1080:force_original_aspect_ratio=decrease,pad=1920:1080:(ow-iw)/2:(oh-ih)/2,setsar=1,format=yuv420p[v1];[0:a]aformat=sample_rates=48000:channel_layouts=stereo[a0];[1:a]aformat=sample_rates=48000:channel_layouts=stereo[a1];[v0][a0][v1][a1]concat=n=2:v=1:a=1[v][a]"
-map "[v]" -map "[a]" -r 30 -fps_mode cfr
-c:v libx264 -pix_fmt yuv420p -preset veryfast
-b:v 8M -minrate 8M -maxrate 8M -bufsize 16M -g 60 -keyint_min 60 -sc_threshold 0
-c:a aac -b:a 128k -ar 48000
-f flv "rtmps://YOUR_INGEST_URL/YOUR_STREAM_KEY"The concat filter’s segment count and labels must match your actual playlist and streams. For more clips, add a normalized video/audio pair for each input and set concat=n= to the number of segments. If your FFmpeg build does not accept -fps_mode, check its local help for the supported frame-sync option. Keep the real stream key private; never paste it into a public command, screenshot, or shared log.
Handling a clip with no audio
The example maps an audio stream from every clip, so it will fail if any input has none. For such a file, create a silent audio source with the required duration and channel layout, then feed that stream into the concat filter in place of the missing track. Alternatively, prepare all clips so they have matching audio streams before joining. In either case, confirm that the silent segment has the right duration and that audio remains synchronized across transitions.
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When the concat demuxer and stream copy are suitable
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 use the manifest as input and copy the streams, for example: ffmpeg -re -f concat -safe 0 -i playlist.ffconcat -c copy -f flv "rtmps://YOUR_INGEST_URL/YOUR_STREAM_KEY". This is conditional: FFmpeg’s concat demuxer documentation requires the files to have the same streams, including matching codecs and time bases, and related stream properties. Confirm compatibility rather than assuming matching extensions or nominal FPS is enough.
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Configure the live output for YouTube
For file-based live output, put -re before each relevant input so FFmpeg reads at native rate. The FFmpeg command-line documentation equates -re with -readrate 1 and explains its real-time pacing use. Without pacing, a local file can be read faster than real time; pacing does not correct mismatched frame cadence or repair timestamps.
YouTube’s live encoder settings guidance lists RTMP/RTMPS ingest, H.264, H.265, and AV1 video codecs, a maximum frame rate of 60 fps, and AAC or MP3 audio. For H.264, YouTube recommends a 2-second keyframe frequency and says not to exceed 4 seconds. The example’s -g 60 at 30 fps yields a 2-second GOP; at a different target FPS, calculate the GOP accordingly. YouTube also specifies CBR encoding. The example’s bitrate controls are illustrative; select bitrate for the codec, resolution, and frame rate you actually use, and ensure your upload connection can sustain it.
| H.264 live output listed by YouTube | Recommended video bitrate |
|---|---|
| 1080p at 30 fps | 14 Mbps |
| 1080p at 60 fps | 17 Mbps |
| 720p at 30 fps | 8 Mbps |
| 720p at 60 fps | 8 Mbps |
These are YouTube’s recommendations for the specific H.264 modes shown, not universal targets for every codec, resolution, or connection. Check the current table in YouTube’s live encoder guidance before going live; the page does not display a publication or last-updated date. YouTube says it transcodes live streams into multiple output formats.
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Set the destination and protect your key
Choose the ingest protocol and destination shown for your stream in YouTube Live Control Room, then substitute its ingest URL and stream key in the output URL or the appropriate FFmpeg option for your setup. The example uses placeholders intentionally. Do not share a command containing a real key; anyone who obtains it may be able to broadcast to your stream. If you suspect exposure, replace or reset the key in Live Control Room before the next broadcast.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Test transitions and monitor stream health
- Run a short local validation. Encode a sample that crosses each important mixed-FPS transition. Confirm timestamps move forward, audio remains synchronized, and motion is not unintentionally sped up, slowed down, or visibly juddering.
- Check edge cases. Test clips with missing or shorter audio, different dimensions, and durations that may not match metadata. Look for black frames, freezes, gaps, clicks, abrupt audio changes, and filter or mapping errors.
- Test on YouTube before the intended broadcast. YouTube recommends testing with representative audio and movement. Check the preview and stream health in Live Control Room and confirm the output resolution, cadence, audio, and ingest are what you intended.
- Monitor during the stream. YouTube advises: “During the event, monitor the stream health and review messages.” Treat warnings as a reason to inspect the encoder output and upload connection rather than assuming the source playlist is at fault.
Troubleshooting common mixed-FPS failures
| Symptom | Likely cause | What to check or change |
|---|---|---|
| Concat filter reports mismatched inputs or fails to initialize | Inputs have incompatible dimensions, pixel formats, audio layouts, or missing streams before they reach the filter. | Inspect streams with ffprobe; normalize video geometry, sample format, rate, and channel layout. Provide silence where audio is absent and adjust filter labels to the actual inputs. |
| Video judders or cadence changes at a cut | The output policy is unspecified or the chosen CFR target requires many duplicated or dropped frames. | Set an intentional output cadence and test the transition. If source timing matters more, test VFR instead and verify platform compatibility. |
| Audio gap, drift, or click at a transition | Audio durations differ, streams are missing, or concat timing follows inaccurate duration metadata. | Normalize audio, synthesize silence for missing segments, and inspect timestamps and durations around the join. |
| Stream runs too fast or ingest becomes unstable | File inputs are not being paced in real time, or the upload cannot sustain the selected output bitrate. | Place -re before file inputs; choose a bitrate appropriate to the output and available upload bandwidth. |
| YouTube reports unsupported settings or poor stream health | Codec, keyframe interval, frame rate, bitrate, audio, or destination settings do not match the selected ingest configuration. | Compare the actual encoded output and Live Control Room settings with YouTube’s current encoder guidance, then test again. |
| Stream-copy output has glitches or gaps between files | Streams or time bases differ, or file duration metadata is inaccurate. | Verify concat-demuxer compatibility and every transition. Use concat filtering and re-encoding if the files are not structurally compatible. |
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Copyright and YouTube policy still apply
FFmpeg changes how media is processed and transmitted; it does not grant permission to rebroadcast it. Use videos and music you own or are licensed to stream, and check applicable rights and YouTube rules before building a playlist. A stream that runs technically can still receive a copyright claim, restriction, or removal. YouTube’s upload guidance says content should be encoded and uploaded at its recorded frame rate, but that instruction concerns uploads; it should not be mistaken for a rule requiring every source in a mixed-FPS live playlist to retain its original cadence. For background checks before streaming, use the copyright safety checklist.
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