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To join MP4 clips with different resolutions for a YouTube live loop, normalize each clip to the same frame size, frame rate, pixel format, and audio format, then join them with FFmpeg’s concat filter and re-encode the result. The concat demuxer with -c copy is not a general shortcut for mismatched clips: it requires compatible streams, including matching codecs and time bases.
1. Inspect the clips and choose an output format
Before joining anything, check each MP4’s width and height, frame rate, video codec and pixel format, and whether it has audio. For audio, note the sample rate and channel layout. ffprobe can report these properties; for example:
ffprobe -v error -show_streams -of json first.mp4Choose one output canvas and frame rate for the finished loop based on the footage and your intended YouTube stream. Do not upscale a low-resolution source just to make its numbers larger. When clips have different aspect ratios, fit them inside the common canvas and pad the unused area rather than stretching the image. FFmpeg’s scale filter supports force_original_aspect_ratio=decrease for fitting while preserving aspect ratio, and force_divisible_by=2 for dimensions divisible by two (FFmpeg scale filter documentation).
2. Normalize and join the clips with FFmpeg
For two clips that both contain audio, this example fits each into a 1920×1080 canvas, converts video to 30 fps and yuv420p, standardizes audio to 48 kHz stereo, and concatenates the normalized segments:
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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 tested command for every file or FFmpeg build. Change the canvas, frame rate, and audio handling to suit your inputs. If a clip is silent, give every segment a compatible audio stream before using an audio concat chain, or use a video-only concat with a=0. If only some clips have audio, explicitly handle the missing audio rather than assuming FFmpeg can concatenate unlike segment layouts. Play the output and inspect every transition, including whether audio cuts, gaps, or changes unexpectedly.
3. Choose the right concatenation method
| Method | When it fits | Trade-off |
|---|---|---|
| Concat filter with re-encoding | Clips differ in resolution or other stream properties and need normalization. | Slower and adds a generation of encoding, but produces a consistent output stream. FFmpeg’s FAQ recommends this operation when re-encoding is needed (FFmpeg FAQ). |
| Concat demuxer with stream copy | Inputs already meet FFmpeg’s stream compatibility requirements. | Fast and avoids re-encoding, but the streams must match, including codecs and time base. Incorrect file durations can also cause timestamp artifacts (FFmpeg concat demuxer documentation). |
| Simple file-level concatenation | Not a general-purpose way to join MP4 files. | Only a few container formats permit simple file concatenation, according to the FFmpeg FAQ. |
4. Loop the joined video to YouTube Live
FFmpeg’s -stream_loop -1 option loops an input indefinitely (FFmpeg input options). A starting command for sending a prepared file to a YouTube ingest endpoint is:
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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 shown for your YouTube stream, and keep the stream key private. This is only a starting pattern: choose bitrate, resolution, frame rate, and encoder settings for your content, machine, connection, and current YouTube guidance. YouTube’s live encoder page, checked October 3, 2026, recommends RTMP/RTMPS, CBR, and a two-second keyframe interval (not exceeding four seconds); it lists H.264, H.265, and AV1 video options and AAC or MP3 audio for RTMP/RTMPS. For stereo audio it recommends 44.1 kHz and 128 kbps (YouTube Live encoder settings).
For H.264, YouTube’s published bitrate figures include 8 Mbps for 720p at 30 fps, 14 Mbps for 1080p at 30 fps, and 17 Mbps for 1080p at 60 fps; its published minimum for 1080p at 30 fps is 5 Mbps. These are platform recommendations, not guarantees of picture quality or proof that your upload connection can sustain a stream. Settings depend on resolution and frame rate. YouTube says, “YouTube will automatically detect which encoder settings you chose.”
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5. Test the loop and monitor the live stream
Before relying on a long-running loop, play the complete joined file and check the transitions and loop boundary. Then test the live stream using movement and audio similar to the intended program. Watch YouTube’s live control room for stream health and resolve any warnings before treating the setup as ready. YouTube recommends testing and monitoring stream health (YouTube Live encoder settings).
6. Troubleshoot common problems
- The concat command reports mismatched streams or fails at a transition: Check that every segment has the same stream layout. Normalize video properties and audio, and make sure silent clips are handled consistently; re-encode through the concat filter when inputs differ.
- The picture stretches or has unexpected bars: Use aspect-ratio-preserving scale and pad values for your chosen canvas. Do not force a source of one aspect ratio to fill a differently shaped frame.
- The joined file has a pause, timestamp jump, or broken transition: Review the segment boundaries and output. The concat demuxer can show timestamp artifacts when file durations are inaccurate; it is not the preferred path for incompatible inputs.
- The video plays, but YouTube reports poor stream health: Compare the selected resolution and frame rate with YouTube’s guidance, check that the upload connection can sustain the bitrate, and test again. The published recommendation is not a connection-capacity guarantee.
- The video or audio looks different after joining: Re-encoding can introduce generation loss. Check the output format and quality settings, and avoid unnecessary upscaling or repeated encoding.
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