The Tool Desk
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What the concat demuxer does—and what it does not
The concat demuxer reads a text list of files and presents their packets one after another as if they had been muxed together. It does not render transitions or automatically make dissimilar clips compatible. It is best suited to files with matching stream layouts, codecs, and time bases. If the files need normalization or re-encoding, FFmpeg’s concat filter is the better route; the FFmpeg FAQ distinguishes the demuxer approach from filter-based concatenation: FFmpeg FAQ.
Create the ffconcat playlist
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Put the clips and playlist in a working folder, or note the paths you will use. Relative paths from the working directory are simplest.
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Create a UTF-8 or ASCII-compatible plain-text file named
playlist.ffconcat. Its first line must be exactlyffconcat version 1.0: no blank line, extra spaces, or byte-order mark before it.Windows Errors? Fix Them Before They Spread
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Add one
filedirective per clip, in the playback order you want:ffconcat version 1.0 file 'clip1.mp4' file 'clip2.mp4' file 'clip3.mp4' -
Save the file. Paths with spaces or special characters must use concat-file quoting and escaping rules; this syntax is not a shell command line.
FFmpeg’s documentation describes the directive format, path safety, and compatibility requirements in its concat demuxer documentation. Safe-path checking is enabled by default and permits simple relative names. For trusted absolute paths or paths FFmpeg considers unsafe, use -safe 0 as an input option before -i. Do not use it with an untrusted playlist, because it relaxes path restrictions.
Prepare clips before looping them
Match streams and timing
The files need corresponding streams—for example, consistent audio and video streams—and matching codecs and time bases for the demuxer workflow. If one clip has audio and another does not, or the codecs, layouts, or time bases differ, first normalize or re-encode the material to a common format, or use a filter-based concat workflow.
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Check durations and file boundaries
The demuxer schedules each next file using the preceding file’s duration. Incorrect duration metadata, truncated files, or different audio and video lengths can cause gaps or artifacts at joins. If a file’s duration is wrong but you know the correct value, the concat format’s duration directive can override it. Consult the FFmpeg concat demuxer documentation for the directive’s syntax.
Decide whether to use the demuxer or filter
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Choose the concat demuxer when the clips already match and you want to join their packets without using a filter workflow.
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Choose the concat filter when clips need re-encoding, normalization, or other per-clip processing. A filter workflow requires an output encode.
Send the repeating playlist to YouTube Live
This representative RTMP(S) command shows the workflow, not a tested or universal preset. Replace the ingest address and key with the values provided for your YouTube stream. Keep the stream key private.
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ffmpeg -re -stream_loop -1 -f concat -safe 0 -i playlist.ffconcat
-c:v libx264 -preset veryfast -pix_fmt yuv420p
-r 30 -g 60 -b:v 4500k -maxrate 4500k -bufsize 9000k
-c:a aac -b:a 128k -ar 44100
-f flv "$INGEST_URL/$STREAM_KEY"-
-rereads file input at its native rate instead of sending it as fast as storage permits.-stream_loop -1repeats the input indefinitely. -
-f concatselects the concat demuxer.-safe 0relaxes its path checks for the trusted playlist; omit it when simple relative paths work. -
The example encodes video as H.264 and audio as AAC into an FLV output. Its 30 fps, 4,500 kbps video rate, and other encoding values are illustrative only. Choose resolution, frame rate, and bitrate for the source and the upload connection’s reliable capacity.
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YouTube recommends a two-second keyframe interval and says not to exceed four seconds. In this example,
-g 60at 30 fps sets a 60-frame GOP, or two seconds. At a different frame rate, adjust GOP length to match the interval.Special offer. See more information about Outbyte and uninstall instructions. Please review EULA and Privacy policy.Rank #4
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The command assumes usable audio in the input. If the clips contain no audio, plan an intentional silent-audio source or another compatible audio arrangement; this command does not create audio automatically.
Set YouTube Live encoder settings and verify the stream
YouTube’s current encoder guidance supports RTMP or RTMPS ingest, H.264, H.265, or AV1 video, frame rates up to 60 fps, AAC or MP3 audio, and constant bitrate (CBR). It recommends RTMPS for an encrypted connection, a two-second keyframe interval, and stereo audio at 44.1 kHz and 128 kbps in its advanced guidance. Video bitrate recommendations depend on codec, resolution, and frame rate, so check YouTube’s live encoder settings when setting up rather than treating the example command’s bitrate as a rule.
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In YouTube Live Control Room, copy the ingest URL and stream key for the intended broadcast and substitute them in the command.
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Before the broadcast, test with audio and motion representative of the actual stream. Monitor YouTube’s stream health and messages while it is running, as recommended in the YouTube encoder guidance.
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If you are diagnosing ingest, check the Live Control Room’s status and health messages. The YouTube Live Streaming API documents stream states including
active,ready,inactive, anderror, and health states includinggood,ok,bad, andnoData.
Troubleshoot common concat and ingest problems
| Symptom | Likely cause | What to check or change |
|---|---|---|
| FFmpeg does not recognize the playlist format | The signature is not the exact first line, or a blank line, spaces, or a byte-order mark precedes it. | Make ffconcat version 1.0 the exact first line of the plain-text file. |
| A file is rejected or cannot be opened | The path is wrong, unsafe under the default check, or incorrectly quoted. | Try a simple relative path from the working directory. For trusted paths only, use -safe 0 before -i; apply concat syntax quoting and escaping for special characters. |
| Clips fail to join cleanly or FFmpeg reports incompatible streams | Stream layouts, codecs, or time bases do not match. | Normalize or re-encode clips to a common format, or switch to a concat-filter workflow when processing is required. |
| There is a gap, abrupt shift, or artifact at a boundary | Duration metadata may be inaccurate, a file may be truncated, or audio and video lengths may differ. | Inspect the affected files and duration values; if the correct duration is known, use the concat duration directive. |
| YouTube reports poor or missing stream health | The ingest URL or key may be wrong, the connection may not sustain the chosen bitrate, or the encoder settings may not match YouTube’s current guidance. | Verify the URL and key in Live Control Room, check upload capacity and current encoder settings, then test and monitor the stream’s health messages. |
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Frequently Asked Questions
Does the concat demuxer add transitions between clips?
No. It presents the clips one after another; it does not render transitions.
Will this command make a playlist with no audio acceptable to YouTube?
No. The example assumes audio is available in the input. A no-audio playlist needs a deliberate silent-audio source or another compatible audio plan.
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