Language-agnostic automatic synchronization of subtitles with video, so that subtitles are aligned to the correct starting point within the video.
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First, make sure ffmpeg is installed. On MacOS, this looks like:
brew install ffmpeg
(Windows users: make sure
ffmpeg is on your path and can be referenced
from the command line!)
Next, grab the script. It should work with both Python 2 and Python 3:
pip install ffsubsync
If you want to live dangerously, you can grab the latest version as follows:
pip install git+https://github.com/smacke/[email protected]
ffsubsync all work as entrypoints:
ffs video.mp4 -i unsynchronized.srt -o synchronized.srt
There may be occasions where you have a correctly synchronized srt file in a language you are unfamiliar with, as well as an unsynchronized srt file in your native language. In this case, you can use the correctly synchronized srt file directly as a reference for synchronization, instead of using the video as the reference:
ffsubsync reference.srt -i unsynchronized.srt -o synchronized.srt
ffsubsync uses the file extension to decide whether to perform voice activity
detection on the audio or to directly extract speech from an srt file.
If the sync fails, there are a few recourses available. The best one to try
first is to specify
--vad=auditok as a command line option, since sometimes
auditok works well with ffsubsync in the
case of of muffled or otherwise low-quality audio. Auditok does not
specifically detect voice, but instead detects all audio; this property can
yield suboptimal syncing behavior when a proper VAD can work
well, but can be effective in some cases.
The next step is to try different values for
--max-offset-seconds. By default
ffsubsync runs with
--max-offset-seconds=60, since subititles are unlikely
to be offset by more than 1 minute in practice, and enforcing this constraint
typically leads to a better outcome. There may be some rare cases in which
subtitles are more egregiously out of sync and where increasing this value can
If the sync still fails, consider trying one of the following similar tools:
ffsubsync usually finishes in 20 to 30 seconds, depending on the length of the
video. The most expensive step is actually extraction of raw audio. If you
already have a correctly synchronized "reference" srt file (in which case audio
extraction can be skipped),
ffsubsync typically runs in less than a second.
The synchronization algorithm operates in 3 steps:
The best-scoring alignment from step 3 determines how to offset the subtitles in time so that they are properly synced with the video. Because the binary strings are fairly long (millions of digits for video longer than an hour), the naive O(n^2) strategy for scoring all alignments is unacceptable. Instead, we use the fact that "scoring all alignments" is a convolution operation and can be implemented with the Fast Fourier Transform (FFT), bringing the complexity down to O(n log n).
(*) When say video, it is actually the audio part in video/audio muxed
In most cases, inconsistencies between video and subtitles occur when starting or ending segments present in video are not present in subtitles, or vice versa. This can occur, for example, when a TV episode recap in the subtitles was pruned from video. FFsubsync typically works well in these cases, and in my experience this covers >95% of use cases. Handling breaks and splits outside of the beginning and ending segments is left to future work (see below).
Besides general stability and usability improvements, one line of work aims to extend the synchronization algorithm to handle splits / breaks in the middle of video not present in subtitles (or vice versa). Developing a robust solution will take some time (assuming one is possible). See #10 for more details.
The implementation for this project was started during HackIllinois 2019, for which it received an Honorable Mention (ranked in the top 5 projects, excluding projects that won company-specific prizes).
This project would not be possible without the following libraries:
Code in this project is MIT licensed.