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Normalize each clip to the same frame size, frame rate, pixel format and audio format, then re-encode and join them with FFmpeg’s concat filter. Finally, loop the joined file into YouTube Live. Clips with different resolutions generally are not suitable for a simple stream-copy join.
Why mixed-resolution MP4s need preparation
MP4 is a container, not a guarantee that two files have matching video and audio streams. Different dimensions—or differences in frame rate, codec, time base, pixel format, sample rate or channel layout—can make a direct join fail or produce glitches. FFmpeg’s concat demuxer is intended for inputs with matching streams; FFmpeg’s FAQ recommends its concat filter when re-encoding is needed (FFmpeg FAQ; concat demuxer documentation).
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Before joining, inspect the clips and choose a common output format. When aspect ratios differ, fit each video inside a shared canvas and pad the remaining area; do not stretch the picture. FFmpeg’s scale filter supports preserving the source aspect ratio with force_original_aspect_ratio=decrease and even dimensions with force_divisible_by=2 (FFmpeg scale filter documentation).
Inspect the clips and choose an output
Check each file
Use ffprobe to examine dimensions, frame rate, codecs, pixel format, audio presence, sample rate and channel layout. For example:
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ffprobe -v error -show_streams -of json first.mp4Repeat for every input. The example reports stream details in JSON; inspect both the video and audio streams. If a clip has no audio, note that before using an audio concat chain.
Set one canvas and frame rate
Choose the output dimensions for your intended YouTube stream, guided by the source footage and the connection that will send the stream. Avoid upscaling low-resolution footage on the assumption that it will gain detail. The example below uses 1920×1080 at 30 fps; replace those values if another common format better suits your inputs and delivery.
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Fitting and padding preserve the whole image, but leave bars where the source aspect ratio does not match the canvas. That is preferable to distorting people, text or other content.
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This example assumes both MP4s contain audio. It scales and pads each video to the same canvas, sets a common frame rate, sample aspect ratio and pixel format, converts audio to 48 kHz stereo, then re-encodes the joined output.
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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 starting pattern, not a tested command for your files. Confirm that your FFmpeg build includes the selected encoder and filters, and adjust the canvas, frame rate and audio handling to match the actual inputs. Re-encoding takes processing time and can reduce quality compared with the source.
Handle silent or mixed-audio clips deliberately
The example’s audio chain requires an audio stream in both inputs. For two silent clips, use a video-only concat filter by changing the concat settings to concat=n=2:v=1:a=0 and omit the audio inputs and output mapping; decide separately whether your live stream should have no audio. If only some clips have audio, first give every segment a compatible audio stream—for example, by supplying silence for segments that need it—or use a video-only join. Do not pass a missing audio stream into the example’s audio chain.
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Check the joined file
Play joined.mp4 from beginning to end before streaming. Check that every transition is clean, the audio remains continuous where expected, and the end-to-start boundary works for a loop. If a transition has a gap, repeated sound or abrupt level change, correct the source or filter workflow and render again.
Why not just concatenate or copy the files?
| Method | Works with mismatched resolutions? | Re-encoding | Speed and quality trade-off | Timestamp concern |
|---|---|---|---|---|
| Concat filter | Suitable when each input is normalized to compatible properties. | Used for the recommended workflow when re-encoding is needed. | Requires processing and can introduce generation loss. | Produces a consistent output stream, but transitions still need checking. |
| Concat demuxer with stream copy | No general shortcut for different resolutions or incompatible stream properties; input streams must match, including codecs and time base. | No, when stream copy is viable. | Faster and avoids another encode when inputs meet compatibility requirements. | FFmpeg adjusts timestamps using file durations; inaccurate durations can cause artifacts. |
| File-level concatenation | Not a general MP4 joining method. | Depends on container-specific support. | Only a few containers permit simple file concatenation, according to FFmpeg’s FAQ. | Not a general solution for MP4 inputs. |
Loop the result into YouTube Live
FFmpeg documents -stream_loop -1 as infinite looping for an input (FFmpeg main options). The following command is an example pattern for sending the prepared file; it is not a universal configuration. Use the ingest endpoint and protocol shown in YouTube for your account, and keep the stream key private.
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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'The encoder settings need to suit the selected resolution, frame rate, machine and reliable upload capacity. In particular, the example’s 5 Mbps video rate is not a universal recommendation. YouTube’s encoder guidance, checked 2026-10-03, recommends RTMP/RTMPS, CBR and a two-second keyframe interval that does not exceed four seconds. Its published H.264 recommendations include 8 Mbps for 720p at 30 fps, 14 Mbps for 1080p at 30 fps and 17 Mbps for 1080p at 60 fps; its listed minimum for 1080p at 30 fps is 5 Mbps. These are platform figures, not guarantees of quality or proof that your connection can sustain the rate. See YouTube’s live encoder settings for guidance by resolution and frame rate.
YouTube lists H.264, H.265 and AV1 video options and AAC or MP3 audio for RTMP/RTMPS. Its guidance recommends 44.1 kHz stereo audio and 128 kbps stereo audio. The command above uses H.264 and AAC. YouTube states, “YouTube will automatically detect which encoder settings you chose.”
Test and monitor before relying on the loop
- Use a test that has movement and audio like the intended stream, as YouTube recommends.
- Watch the stream in YouTube’s live control room and confirm that incoming stream health is healthy.
- Check the joined file’s transitions and loop boundary independently of the live stream.
- Choose a sustainable quality based on your connection, and test before going live; bitrate recommendations do not guarantee available upload capacity.
Common problems and fixes
- The concat filter reports mismatched parameters: Recheck each input’s video and audio properties. Normalize frame size, frame rate, pixel format, sample rate and channel layout consistently before concatenating.
- The audio chain fails on a silent clip: The two-audio example cannot map a nonexistent stream. Use video-only concatenation or add compatible silence for the silent segment before building the audio chain.
- The picture looks stretched or has unexpected bars: The fit-and-pad approach preserves aspect ratio. Bars are expected when source and canvas aspect ratios differ; do not remove them by stretching.
- The direct stream-copy join fails or glitches: The inputs may not have matching streams, codecs or time bases. Normalize and re-encode with the concat filter instead of assuming
-c copycan bridge different resolutions. - Artifacts appear around a demuxer join: The concat demuxer bases timestamp adjustment on file durations, so inaccurate durations can cause artifacts. For incompatible clips, use the filter workflow.
- YouTube reports poor stream health: Check the selected resolution and bitrate against the current YouTube guidance and your reliable upload capacity; test with representative motion and sound.
- The stream stops at the file’s end: Confirm
-stream_loop -1is applied to the input and review FFmpeg output for errors. The option requests indefinite looping, but does not itself guarantee an uninterrupted live stream.
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