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First, identify how the command joins files
FFmpeg has three concat approaches, and they are not interchangeable. The concat demuxer reads a playlist script; the concat filter joins streams as part of a re-encoding workflow; the concat protocol joins files at the file level only when their format supports that operation. The FFmpeg FAQ explains when to use the demuxer or filter.
| Method | Use it when | Important limitation |
|---|---|---|
| Concat demuxer | You want to join files without re-encoding and their container does not support file-level concatenation. | The playlist must be valid, and inputs need compatible stream structure, codecs, and time bases. |
| Concat filter | You need to re-encode as part of joining the files. | It requires a filtering and transcoding workflow. |
| Concat protocol | The underlying format supports file-level concatenation. | It is not a general-purpose playlist reader. |
Check the concat demuxer playlist
If your command uses the concat demuxer, its input is a text script with one file directive for each story video. For example:
ffconcat version 1.0
file 'story-1.mp4'
file 'story-2.mp4'
file 'story-3.mp4'For FFmpeg to recognize the script automatically, ffconcat version 1.0 must be exactly at the beginning of the file. Check that there is no preceding text or blank line. Quote or escape filenames containing spaces or special characters; see the concat demuxer documentation for the script syntax.
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Verify every entry and path
- Confirm there is one correctly spelled
fileentry for every input, in the intended playback order. - Check that each referenced file exists relative to the working directory from which you run FFmpeg, or use a valid path.
- Look for a typo, missing quote, or malformed line around the entry after the first story; that is where a playlist may fail to continue.
The demuxer defaults to safe=1, which accepts relative paths made from a limited portable character set. It can therefore reject absolute paths or names with other characters. For a trusted playlist, -safe 0 permits those paths, but inspect every entry first: this setting relaxes path restrictions and should not be used with an untrusted playlist. The FFmpeg documentation describes the safe-path behavior.
Check whether the story files are compatible
The concat demuxer expects the files to contain the same streams, codecs, and time base. For example, a sequence may not be suitable for direct demuxer concatenation if one story has a different stream layout or encoding parameters. Such differences can prevent a valid join or produce playback and timestamp problems.
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Stored or incorrect file durations can also affect how FFmpeg adjusts timestamps between files. If the playlist parses but playback ends, freezes, or jumps at a boundary, check the input properties and durations as well as the text script. The concat demuxer documentation covers these requirements.
What to collect if it still stops
The title does not include enough diagnostic information to identify the cause for a particular playlist. To narrow it down, gather:
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- The exact FFmpeg command, including the concat method and options.
- The complete playlist text, including its first line and all file entries.
- Your FFmpeg version.
- The formats and stream details of each input file.
- The full error output or log, not just the last line.
With those details, you can distinguish a playlist syntax or path problem from incompatible inputs, a method mismatch, or a timestamp issue.
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