Package: ffsubs
FFsubsLanguage-agnostic automatic synchronization of subtitles with video, so that
subtitles are aligned to the correct starting point within the video.Turn this:Into this:InstallFirst, make sure ffmpeg is installed. On MacOS, this looks like:brew install ffmpegNext, grab the script. It should work with both Python 2 and Python 3:pip install git+https://github.com/smacke/subsync@stableIf you want to live dangerously, you can grab the latest version as follows:pip install git+https://github.com/smacke/subsync@latestUsagesubsync video.mp4 -i unsynchronized.srt > synchronized.srtorsubsync video.mp4 -i unsynchronized.srt -o synchronized.srtThere 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:subsync reference.srt -i unsynchronized.srt -o synchronized.srtsubsyncuses the file extension to decide whether to perform voice activity
detection on the audio or to directly extract speech from an srt file.Speedsubsyncusually 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),subsynctypically runs in less than a second.How It WorksThe synchronization algorithm operates in 3 steps:Discretize video and subtitles by time into 10ms windows.For each 10ms window, determine whether that window contains speech.  This
is trivial to do for subtitles (we just determine whether any subtitle is
"on" during each time window); for video, use an off-the-shelf voice
activity detector (VAD) like
the one built intowebrtc.Now we have two binary strings: one for the subtitles, and one for the
video.  Try to align these strings by matching 0's with 0's and 1's with
1's. We score these alignments as (# video 1's matched w/ subtitle 1's) - (#
video 1's matched with subtitle 0's).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).LimitationsIn 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. Subsync 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).VLC IntegrationTo demonstrate how one might usesubsyncseamlessly with real video software,
we developed a prototype integration into the popularVLCmedia player, which was demoed
during the HackIllinois 2019 project expo. The resulting patch can be found in
the filesubsync-vlc.patch.
Here are instructions for how to use it.First clone the 3.0 maintenance branch of VLC and checkout 3.0.6:git clone git://git.videolan.org/vlc/vlc-3.0.git
cd vlc-3.0
git checkout 3.0.6Next, apply the patch:wget https://github.com/smacke/subsync/raw/master/subsync-vlc.patch
git apply subsync-vlc.patchFollow the normal instructions on theVideoLAN wikifor building VLC from source.Warning: this is not easy.You should now be able to autosynchronize subtitles using the hotkeyCtrl+Shift+S(only enabled while subtitles are present).Future WorkI am currently working to extend the synchronization algorithm to handle
splits / breaks in the middle of video not present in subtitles (or vice
versa). It will take some time before I have a robust solution (assuming one
is possible). See#10for
more details.The prototype VLC patch is very experimental -- it was developed under
pressure and just barely works. I would love to see this project more
robustly integrated with VLC, either directly in the VLC core, or as a
plugin.  If you or anyone you know has ideas for how to accomplish this,
please let me know!HistoryThe implementation for this project was started during HackIllinois 2019, for
which it received anHonorable Mention(ranked in the top 5 projects,
excluding projects that won company-specific prizes).CreditsThis project would not be possible without the following libraries:ffmpegand theffmpeg-pythonwrapper, for extracting raw audio from videoVAD fromwebrtcand thepy-webrtcvadwrapper, for speech detectionsrtfor operating onSRT filesnumpyand, indirectly,FFTPACK, which powers the FFT-based algorithm for fast scoring of alignments between subtitles (or subtitles and video)sklearnfor its data pipeline APIOther excellent Python libraries likeargparseandtqdm, not related to the core functionality, but which enable much better experiences for developers and users.LicenseCode in this project isMIT licensed.