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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchTo join MP4 videos with different resolutions for a YouTube live loop, normalize each clip to the same canvas, frame rate, pixel format, and audio layout, then use FFmpeg’s concat filter to re-encode them into one file. Loop that prepared file with FFmpeg’s -stream_loop -1 option when sending it to YouTube Live. Do not use stream-copy concatenation as a shortcut unless the inputs have compatible streams.
1. Inspect the MP4 files and choose an output format
Before joining clips, check their dimensions, frame rates, codecs, pixel formats, and audio streams. The output must use compatible settings throughout; otherwise transitions can fail or produce playback problems. Use ffprobe to inspect each file:
ffprobe -v error -show_streams -show_format first.mp4Note the video width and height, frame rate, pixel format, and whether audio is present. For audio, note the sample rate and channel layout. Repeat for each input. Choose an output size and frame rate that suit the intended stream and the quality of the source footage. Upscaling a small source does not restore lost detail.
When aspect ratios differ, fit each clip inside the chosen output canvas while preserving its proportions, then pad the remaining space. This avoids stretching people, text, or other content. FFmpeg’s scale filter documentation describes force_original_aspect_ratio=decrease for fitting within a target size and force_divisible_by=2 for dimensions divisible by two.
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2. Join clips with different resolutions using the concat filter
For inputs with different resolutions or other incompatible stream properties, use the concat filter and re-encode. The following is an illustrative two-clip command for files that both contain audio. It fits both videos into a 1920×1080 canvas, outputs 30 fps video, and normalizes audio to 48 kHz stereo:
ffmpeg -i first.mp4 -i second.mp4
-filter_complex "[0:v]scale=1920:1080:force_original_aspect_ratio=decrease:force_divisible_by=2,pad=1920:1080:(ow-iw)/2:(oh-ih)/2,fps=30,setsar=1,format=yuv420p[v0];[1:v]scale=1920:1080:force_original_aspect_ratio=decrease:force_divisible_by=2,pad=1920:1080:(ow-iw)/2:(oh-ih)/2,fps=30,setsar=1,format=yuv420p[v1];[0:a]aresample=48000,aformat=sample_fmts=fltp:channel_layouts=stereo[a0];[1:a]aresample=48000,aformat=sample_fmts=fltp:channel_layouts=stereo[a1];[v0][a0][v1][a1]concat=n=2:v=1:a=1[outv][outa]"
-map "[outv]" -map "[outa]"
-c:v libx264 -c:a aac -b:a 128k -movflags +faststart joined.mp4This is a pattern to adapt, not a guaranteed command for every input or FFmpeg build. Change the target resolution, frame rate, and audio handling to match your clips and installed encoders. The concat filter takes each segment’s video and audio in sequence; every segment in the chain must be made compatible.
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If one or more clips have no audio
The example assumes every input has an audio stream. If all clips are silent, concatenate video only by changing the filter’s concat setting to concat=n=2:v=1:a=0 and removing the audio filter chains and audio map. If audio is present in only some clips, explicitly create or handle audio for the silent segments before building an audio concat chain; do not reference a missing [a] stream. Then listen across each join to check that the result is continuous.
Check the joined file before streaming
Play joined.mp4 from start to finish. Inspect every transition for black frames, unexpected crops, jumps, or aspect-ratio changes, and listen for gaps, sudden volume changes, or unwanted silence. Confirm the end behaves acceptably before it loops back to the beginning.
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3. Choose the right FFmpeg concatenation method
| Method | Works with different resolutions? | Re-encodes? | Speed and quality trade-off | Timestamp considerations |
|---|---|---|---|---|
| Concat filter | Yes, after normalizing the inputs to compatible output properties | Yes, in the usual workflow | More processing time; re-encoding can add generation loss | Builds a consistent output stream from the normalized segments |
| Concat demuxer with stream copy | Not as a general shortcut; the streams must match, including codecs and time base | No | Faster and avoids re-encoding when inputs meet compatibility requirements | Uses file durations to adjust timestamps; inaccurate durations can cause artifacts |
| File-level concatenation | No general MP4 solution | Not applicable as a general method | Only a few container formats allow simple file concatenation | Not a general-purpose approach for MP4 files |
FFmpeg’s FAQ recommends the concat filter when re-encoding is needed. The concat demuxer documentation describes its matching-stream requirements and timestamp behavior. For mixed resolutions, use the filter workflow unless you have verified that the inputs satisfy the demuxer’s requirements.
4. Loop the finished file to YouTube Live
FFmpeg documents -stream_loop -1 as infinite looping for an input. This example reads the prepared file in real time, loops it, and sends an H.264/AAC RTMP stream:
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ffmpeg -re -stream_loop -1 -i joined.mp4
-c:v libx264 -pix_fmt yuv420p -r 30 -g 60
-b:v 5M -maxrate 5M -bufsize 10M
-c:a aac -b:a 128k -ar 44100 -ac 2
-f flv 'rtmps://YOUR_INGEST_ENDPOINT/YOUR_STREAM_KEY'Replace the endpoint and key with the values YouTube provides for your account. Keep the stream key private. The bitrate and encoder settings here are only a starting pattern: choose settings for your output resolution, frame rate, available upload capacity, and YouTube’s current guidance. Your FFmpeg build and stream configuration must support the output protocol you select.
Match encoder settings to YouTube’s guidance
YouTube’s live encoder help page, checked October 3, 2026, recommends RTMP/RTMPS, constant bitrate (CBR), and a two-second keyframe interval that does not exceed four seconds. It lists H.264, H.265, and AV1 video options, and AAC or MP3 audio for RTMP/RTMPS; its audio guidance recommends 44.1 kHz stereo and 128 kbps stereo audio. Recommendations depend on the ingestion resolution and frame rate. YouTube also states, “YouTube will automatically detect which encoder settings you chose.”
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| 1080p at 30 fps | 14 Mbps recommended; 5 Mbps minimum | YouTube live encoder help page checked October 3, 2026 |
| 1080p at 60 fps | 17 Mbps recommended | YouTube live encoder help page checked October 3, 2026 |
These are YouTube’s published encoder recommendations, not guarantees of picture quality or proof that an internet connection can sustain the selected bitrate. Use a quality your connection can reliably upload and test before relying on the stream.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.5. Test the stream and monitor its health
- Start FFmpeg with a private stream key and the ingest endpoint shown in YouTube’s live setup.
- Check YouTube Live Control Room for an incoming stream and a healthy stream status.
- Use audio and movement similar to what the intended loop will contain, then watch and listen through representative content and transitions.
- Monitor the loop boundary as well as clip joins, and stop or correct the stream if the incoming feed or playback is unhealthy.
YouTube’s live encoder help advises testing with similar audio and movement and monitoring stream health. Exact results depend on the source files, encoder, and connection; the command examples above are instructional patterns, not guarantees of uninterrupted streaming.
Common problems and fixes
- The concat command reports mismatched parameters: Normalize each segment’s video dimensions, frame rate, pixel format, and audio layout before concatenating. Confirm every input actually has the audio stream your filter graph references.
- The image is stretched or has black bars: Use aspect-ratio-preserving scaling followed by padding for a fixed canvas. Bars are expected where aspect ratios differ; stretching to fill the canvas distorts the image.
- The join has a glitch or timestamp artifact: Check the transition in the re-encoded output. If using the concat demuxer, verify that stream properties match and file durations are accurate; inaccurate duration information can cause timestamp artifacts.
- Audio drops out or changes unexpectedly: Check audio presence in every source, normalize sample rate and channel layout, and listen to each join. For silent sources, use a video-only concat chain or deliberately provide compatible audio for each segment.
- YouTube does not show a healthy incoming feed: Verify the endpoint and stream key, check that the FFmpeg process is still running, and use encoder settings and upload bitrate appropriate for the selected resolution and frame rate.
- The stream stalls or loses connection: Test with a lower, reliable quality if the upload connection cannot sustain the chosen bitrate, then monitor stream health before leaving the loop unattended.
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