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When MP4 files have different audio codecs, sample rates, or channel layouts, do not concatenate them as-is with FFmpeg’s concat demuxer. First re-encode each file to the same stream profile, then put those matching files in an ffconcat playlist and loop that playlist into YouTube Live. If you want to avoid creating normalized copies, use FFmpeg’s concat filter, which is designed for concatenation that requires re-encoding.
Why mixed-audio MP4 files can break a concat playlist
An MP4 is a container; it does not guarantee that the video and audio streams inside it match those in another MP4. One file may contain AAC stereo audio at 44.1 kHz, while another contains a different codec, sample rate, or channel layout. Explicitly selecting streams with -map determines which tracks FFmpeg uses, but it does not make incompatible tracks compatible.
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The concat demuxer expects every input file to have the same streams, including the same codecs and time bases. It is useful for compatible files, including stream-copy workflows, but it is not a format converter. When inputs differ and need re-encoding, FFmpeg’s FAQ recommends the concat filter instead.
Inspect the files before choosing stream maps
Run this for each file and note its video and audio streams, dimensions, frame rate, channel layout, and duration:
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ffmpeg -i "part1.mp4"FFmpeg prints the stream information and exits when it reaches the end of the input. Look for lines beginning with Stream, and identify the intended video and audio tracks. A file can contain multiple audio tracks or no audio track at all, so do not assume that 0:a:0 is present or is the track you want.
- Use
-map 0:v:0to select the first video stream in the input. - Use
-map 0:a:0to select the first audio stream when that track exists and is the one you want. - If the desired track has a different stream index, adjust the map after inspecting the file. Explicit maps prevent FFmpeg’s automatic stream selection from choosing an unintended track.
Normalize the MP4 files to a common profile
For a simple, repeatable playlist, encode each source file to the same video dimensions, frame rate, video codec, audio codec, sample rate, and channel layout. The example below targets 1080p, 30 fps, H.264 video, and AAC stereo audio at 44.1 kHz. It assumes every input has a video stream and a usable first audio stream. Change the input map or profile to fit your material and intended output.
ffmpeg -i "part1.mp4"
-map 0:v:0 -map 0:a:0
-vf "scale=1920:1080:force_original_aspect_ratio=decrease,pad=1920:1080:(ow-iw)/2:(oh-ih)/2,fps=30"
-c:v libx264 -pix_fmt yuv420p -r 30 -g 60
-c:a aac -ar 44100 -ac 2 -b:a 128k
"part1-normalized.mp4"Repeat the command for every source, using the same output settings and a different output filename. Check that your installed FFmpeg build includes libx264. The scale-and-pad filter fits each video into a 1920×1080 frame without changing its aspect ratio; the resulting padding may be visible on sources with a different shape. If you want another resolution or frame rate, choose it consistently for all files.
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The example uses a 60-frame GOP (-g 60). At 30 fps, that is a two-second keyframe interval. YouTube’s current live-encoder guidance recommends a two-second interval and says it should not exceed four seconds. If you change the frame rate, adjust the GOP to keep the intended interval: for example, 60 frames at 60 fps is also two seconds.
Files without the selected audio track
If a file has no audio, the example’s -map 0:a:0 will fail. If every item should have audio, add a silent audio stream to that file before using it in the playlist, matching the same AAC, sample-rate, and channel settings. Alternatively, build the sequence with a concat filter graph that handles each input deliberately. Do not silently omit a wanted track or assume that unmatched stream layouts will work in the demuxer.
Create the ffconcat playlist
Save a plain-text file named playlist.ffconcat. Its first line must be exactly ffconcat version 1.0, with no blank line or other text before it. Add one file directive per normalized video:
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ffconcat version 1.0
file 'part1-normalized.mp4'
file 'part2-normalized.mp4'
file 'part3-normalized.mp4'Keep the paths valid from the directory where you will run FFmpeg. If paths contain characters that need quoting, follow FFmpeg’s ffconcat escaping rules. The command below uses -safe 0 so the playlist can include paths that the default safe-path check would reject; only use a playlist you trust.
The demuxer places each file after the previous one by adjusting timestamps. If the video and audio streams in a file have different actual lengths, a boundary can contain a gap. Incorrect duration metadata can also cause timing artifacts. Inspect durations and playback at the joins before relying on the playlist for a long broadcast.
Send the looping playlist to YouTube Live
In YouTube Live Control Room, copy the ingest server address and stream key for the broadcast. Use the RTMPS address when YouTube provides it; YouTube recommends RTMPS because it encrypts the connection in transit. Treat the stream key as a password: do not publish it, commit it to a shared script, or show it in a screenshot.
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This is a template, not a universal tested command. Replace the bitrate placeholder with a bitrate appropriate for the selected resolution and YouTube’s current encoder guidance. Replace the output address with the ingest address and key YouTube gives you. Keep output options before the output URL.
ffmpeg -re -stream_loop -1
-f concat -safe 0 -i playlist.ffconcat
-map 0:v:0 -map 0:a:0
-c:v libx264 -pix_fmt yuv420p -r 30 -g 60 -b:v <chosen_video_bitrate>
-c:a aac -ar 44100 -ac 2 -b:a 128k
-f flv "<YouTube_RTMP_or_RTMPS_ingest_address_and_stream_key>"-re reads the file-based input at its natural rate rather than sending it as fast as possible; -stream_loop -1 repeats the input indefinitely. The -map options state which video and audio streams to send. Because the playlist was normalized, the final encode should see a consistent profile. Check that your FFmpeg build supports the requested encoders and that the output settings match the resolution and frame rate you actually intend to stream.
Do these 3 things before closing this tab:
1Fix the driver behind crashes, sound loss and screen glitches2Repair Windows errors before they cause bigger problems3Scan for outdated or missing drivers - takes under a minuteYouTube’s current RTMP/RTMPS guidance lists H.264, H.265, or AV1 video; AAC or MP3 audio; constant bitrate (CBR); and frame rates up to 60 fps. For stereo audio, it recommends 44.1 kHz and 128 kbps; for 5.1 audio, 48 kHz and 384 kbps. The sample command is an H.264/AAC stereo example, not a requirement to use those settings for every stream. Test with representative video movement and audio before going live, then monitor YouTube’s stream health.
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Choose between the concat demuxer and concat filter
| Approach | When it fits | What to expect |
|---|---|---|
| Concat demuxer | Inputs already have matching streams, codecs, and time bases, or you normalize them first. | Can avoid re-encoding when inputs are compatible. The playlist uses an ffconcat script and the demuxer adjusts timestamps; mismatched stream lengths can leave gaps. |
| Concat filter | Inputs differ and you want one filter-and-encode workflow. | Suitable when concatenation requires re-encoding. Explicitly map and normalize the resulting output streams for the YouTube encoder profile you choose. |
Use the demuxer workflow above when you prefer to normalize files once and reuse a playlist. Choose a concat filter graph when it is more convenient to process the inputs together in a single re-encoding command. Neither choice removes the need to inspect the streams and select the intended tracks.
Troubleshooting common failures
- Concat reports mismatched parameters or the output glitches at a join: the demuxer is seeing incompatible streams. Re-encode the inputs to a common profile, or switch to a concat-filter workflow that re-encodes.
- There are audio gaps or audio/video drift at boundaries: compare actual stream lengths and file durations. The demuxer adjusts timestamps globally, and unequal stream lengths can leave gaps. Verify the files’ duration metadata; if a stored duration is wrong, the demuxer supports a
durationdirective in the playlist. - The wrong audio track or video appears: inspect the input streams and set explicit
-mapoptions for the intended tracks. - FFmpeg says an encoder is unavailable: check the installed build for the requested encoder, such as
libx264, or choose a supported encoder and confirm its output is accepted by YouTube. - YouTube rejects the stream or reports poor stream health: compare the outgoing codec, bitrate mode, frame rate, keyframe interval, and audio settings with YouTube’s current ingest guidance. Run a test with representative motion and audio, then check stream health.
- The stream stops when the command exits or the computer sleeps: this file-based FFmpeg method depends on the machine running FFmpeg and maintaining its network connection. Keep the host and process available for the full broadcast, or use a cloud service instead.
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