FFmpeg Concat Demuxer with Different Subtitles

This article explains how to configure and use the FFmpeg concat demuxer when joining video files that contain different, mismatched, or missing subtitle tracks. Because the concat demuxer requires identical stream configurations across all input files, attempting to merge files with differing subtitle layouts will normally result in errors or dropped tracks. Below, you will find the precise steps and workarounds to successfully merge these files while preserving your subtitles.

The Core Problem with Concat and Subtitles

The FFmpeg concat demuxer is a physical-level demuxer. It reads a list of files and joins them consecutively. For this to work seamlessly, all input files must have the exact same stream layout. This means: * The same number of streams (e.g., one video, one audio, one subtitle). * The same codecs in the same order. * The same time bases.

If File A has an English subtitle track (SRT) and File B has no subtitle track, or if File B has a subtitle track in a different codec (e.g., ASS), the concat demuxer will fail or output a file with corrupted/missing subtitles.

Solution 1: Standardize Streams Prior to Concatenation (Recommended)

To use the fast concat demuxer, you must first preprocess your files so their stream structures match perfectly.

Step 1: Add a Dummy Subtitle Track to Files Lacking One

If some of your video files lack subtitles entirely, you must add an empty subtitle track to them.

First, create an empty subtitle file named empty.srt:

1
00:00:00,000 --> 00:00:00,100
 

Then, remux the subtitle-less video (e.g., input2.mp4) with this empty track:

ffmpeg -i input2.mp4 -i empty.srt -c copy -map 0 -map 1 output2.mp4

Step 2: Convert Subtitles to a Common Codec

If your files have subtitles but in different formats (e.g., one is ASS, the other is SRT), transcode them to a unified format like SubRip (SRT) or WebVTT:

ffmpeg -i input1.mp4 -c:v copy -c:a copy -c:s srt output1.mp4

Step 3: Run the Concat Demuxer

Once all files have the exact same stream mapping (Video, Audio, Subtitle), create your inputs.txt file:

file 'output1.mp4'
file 'output2.mp4'

Run the concat command, making sure to use the -map flag to include the subtitle stream in the output:

ffmpeg -f concat -safe 0 -i inputs.txt -map 0 -c copy final_output.mp4

Solution 2: Extract, Concat, and Re-Merge (The Safest Workaround)

If standardizing the streams inside the video containers is too difficult, the most reliable method is to extract the subtitles, concatenate the videos separately, concatenate the subtitles separately, and then merge them back together.

Step 1: Extract the Subtitles

Extract the subtitle tracks from each of your input files:

ffmpeg -i input1.mp4 -map 0:s:0 sub1.srt
ffmpeg -i input2.mp4 -map 0:s:0 sub2.srt

Step 2: Concatenate the Videos (Without Subtitles)

Concatenate the video and audio streams using the concat demuxer. Create an inputs.txt containing your original video files, then run:

ffmpeg -f concat -safe 0 -i inputs.txt -map 0:v -map 0:a -c copy video_only.mp4

Step 3: Merge the Subtitle Files

You can merge the .srt files using a subtitle editor, or manually adjust the timestamps of sub2.srt to start exactly where input1.mp4 ends, then paste them into a single file named merged.srt.

Step 4: Remux Video and Merged Subtitles

Finally, combine the concatenated video file and the merged subtitle file back into a single container:

ffmpeg -i video_only.mp4 -i merged.srt -c copy -map 0 -map 1 final_video.mp4