Cleaning, muxing, and extracting subtitles using ffmpeg and the Python srt library

I became a huge subtitle user when I met my wife. We both like to watch a lot of non-English/non-Chinese movies, and while I use English subtitles, she prefers to have subtitles in Chinese most of the time since she can read it faster than English.

Over the years of doing this I’ve acquired quite a lot of knowledge in this area, and built quite a few tools to help. This post is a way of introducing them to the world, and hopefully it will help anyone in a similar predicament to mine.

Things you will need:

Conversion from other formats to SRT

The SRT format is by far my favourite subtitle format. Its spec has its oddities (not least that there is no widely accepted formal spec), but in general if you stick to the accepted commonalities of the format between media players, you’ll find it’s not only simple, but easy to modify and script around.

If you have another format, like SSA, for example, you’ll probably find that ffmpeg does a pretty good job converting it with ffmpeg -i foo.ssa foo.srt.

Acquiring subtitles

I won’t go into too much detail on this, since you probably will have good enough luck Googling “[movie] [language] subtitle”, but here are some recommendations:

Fixing encoding problems

All of the SRT tools take UTF-8 as input, since it’s a sane, reasonable encoding across the board. You may find that your subtitles are not encoded as UTF-8 and require conversion.

Let’s take Chinese subtitles as an example, as they often use country-preferred encoding schemes. Chinese subtitles usually come encoded as Big5 or GB18030.

I personally find that enca is pretty good at detecting the encoding and converting it appropriately. You can call it as enca -c -x UTF8 -L <language iso code> <sub> to convert subtitles to UTF-8 based on encoding detection heuristics, regardless of their source encoding.

Extracting subtitles from a video file

I’ll assume you’re using a Matroska file, since they’re so popular nowadays, but much of this will also apply elsewhere.

Inside an MKV file are multiple streams. They contain things like the video data, the audio data, and subtitles. You can list them with ffprobe:

% ffprobe ES.mkv |& grep Stream
    Stream #0:0: Video: h264 (High), yuv420p(tv, bt709), 1024x554, SAR 1:1 DAR 512:277, 23.98 fps, 23.98 tbr, 1k tbn, 47.95 tbc (default)
    Stream #0:1(eng): Audio: ac3, 48000 Hz, 5.1(side), fltp, 448 kb/s (default)
    Stream #0:2(eng): Audio: aac (HE-AAC), 48000 Hz, stereo, fltp
    Stream #0:3(eng): Subtitle: subrip
    Stream #0:4(spa): Subtitle: subrip
    Stream #0:5(fre): Subtitle: subrip

Looking at the three streams marked “Subtitle”, you can see that we have English, Spanish, and French subtitles available in this MKV.

Say you want to extract the Spanish subtitle to an SRT file. When converting, ffmpeg will pick the first suitable stream that it finds – by default, then, you will get the English subtitle. To avoid this, you can use -map to select the Spanish subtitle for output.

In this case we know that the Spanish subtitle is stream 0:4, so we run this command:

% ffmpeg -i ES.mkv -map 0:4 ES-spanish.srt

We can see that the right subtitle has been selected:

% cat ES-spanish.srt
1
00:01:34,579 --> 00:01:37,099
<i>Tren a Montauk en la vía B.</i>

2
00:01:37,182 --> 00:01:41,522
<i>Pensamientos al azar</i>
<i>para el Día de San Valentín, 2004.</i>

We will use this subtitle for most of the subsequent examples.

Stripping HTML-like entities from subtitles

As you can see in the subtitle above, sometimes subtitles contain HTML entities, like <b>, <color>, etc. These are not part of the SRT spec, they remain to be interpreted by the media player. Since not all media players support this sometimes they are just shown raw, which looks quite bad.

The srt project contains a tool to deal with this called process, which can perform arbitrary operations on files:

% cat ES-spanish.srt
1
00:01:34,579 --> 00:01:37,099
<i>Tren a Montauk en la vía B.</i>

2
00:01:37,182 --> 00:01:41,522
<i>Pensamientos al azar</i>
<i>para el Día de San Valentín, 2004.</i>

% srt process -m re -f 'lambda sub: re.sub("<[^<]+?>", "", sub)' < ES-spanish.srt
1
00:01:34,579 --> 00:01:37,099
Tren a Montauk en la vía B.

2
00:01:37,182 --> 00:01:41,522
Pensamientos al azar
para el Día de San Valentín, 2004.

Correcting time shifts

Getting subtitles from the internet is an imperfect business. There are often a few different packagings of a movie in different markets, some with different intros, some from different original sources, etc. This can result in the subtitles requiring some correction prior to use.

Your media player may contain some rudimentary controls to correct this at runtime, which may suffice for fixed timeshifts, but for linear timeshifts and cases where you need two sync two subtitles exactly prior to muxing, modifying the SRT file directly is a good idea.

The srt project contains two tools to deal with this:

Muxing subtitles together

The srt project contains a tool, mux, that takes multiple streams of SRTs and muxes them into one. It also attempts to clamp multiple subtitles to use the same start/end times if they are similar (by default, if they are within 600ms of each other), in order to stop subtitles jumping around the screen when displayed.

Say we wanted to create Spanish/French dual language subs for this movie (having already retrieved a suitable French subtitle in ES-french.srt).

% cat ES-french.srt
1
00:01:34,579 --> 00:01:37,099
<i>Le train pour Montauk</i>
<i>sur la voie "B."</i>

2
00:01:37,182 --> 00:01:41,522
<i>Idée diverses</i>
<i>pour la Saint-Valentin, 2004.</i>

In that case, we’d run something like this:

% srt mux --input ES-spanish.srt --input ES-french.srt 
1
00:01:34,579 --> 00:01:37,099
<i>Tren a Montauk en la vía B.</i>

2
00:01:34,579 --> 00:01:37,099
<i>Le train pour Montauk</i>
<i>sur la voie "B."</i>

3
00:01:37,182 --> 00:01:41,522
<i>Pensamientos al azar</i>
<i>para el Día de San Valentín, 2004.</i>

4
00:01:37,182 --> 00:01:41,522
<i>Idée diverses</i>
<i>pour la Saint-Valentin, 2004.</i>

Removing other languages from dual-language subtitles

This is easier for some languages than others. For example, it’s easy to detect and isolate lines containing CJK characters from lines containing (say) English, since their range of characters tends not to intersect.

It’s more difficult (and more error prone) to try to detect languages using more advanced heuristics, but there are a few ways that you can do it using srt.

srt has a program called lines-matching, to which you can pass an arbitrary Python function that returns True if the line is to be kept, and False otherwise. This means you can easily build your own heuristics for language based detection, or anything else you want to isolate.

As an example, this is how you would isolate to Chinese lines using hanzidentifier (must be installed):

% srt lines-matching -m hanzidentifier -f hanzidentifier.has_chinese

You can pass -m multiple times for multiple imports. -f is a function that takes one argument, line. In this case, hanzidentifier.has_chinese already takes one argument, so we don’t need to do anything complicated.

As a more general solution, there is also langdetect, but since this is heuristic, you may find it gets it wrong some of the time. For example (langdetect must be installed):

% srt lines-matching -m langdetect -f 'lambda line: langdetect.detect(line) == "fr"'

Notice that we have to use double quotes instead of single quotes inside the syntax block, since we’re already quoting the expression itself with single quotes.

Using the muxed Spanish and French output we generated earlier as input, this outputs the following:

% srt lines-matching -m langdetect -f 'lambda line: langdetect.detect(line) == "fr"' < ES-spanish-french-muxed.srt
Skipped subtitle at index 1: No content
Skipped subtitle at index 3: No content
1
00:01:34,579 --> 00:01:37,099
<i>Le train pour Montauk</i>

2
00:01:37,182 --> 00:01:41,522
<i>Idée diverses</i>
<i>pour la Saint-Valentin, 2004.</i>

Notice that one line — <i>sur la voie "B."</i> — is completely gone. Language detection is not an absolute science, and sometimes langdetect gets it completely wrong, particularly on short sentences without much context and with language-ambiguous words. For example, in this case, it’s very unsure what the language is because the content is quite short. Notice that its certainties vary wildly between runs, sometimes even completely omitting French:

>>> langdetect.detect_langs('<i>sur la voie "B."</i>')
[ca:0.5714300788248391, it:0.2857133303787093, ro:0.14285632981661017]
>>> langdetect.detect_langs('<i>sur la voie "B."</i>')
[ca:0.7142825704732017, fr:0.28571320094169683]
>>> langdetect.detect_langs('<i>sur la voie "B."</i>')
[ca:0.571427410717758, fr:0.4285707742943302]
>>> langdetect.detect_langs('<i>sur la voie "B."</i>')
[ca:0.7142823512721495, fr:0.14285771416102766, ro:0.14285723363575129]

One thing you can do if you want to match per-subtitle rather than per-line (which only makes sense if your different languages are actually in different SRT blocks) is use -s/--per-subtitle, which may help to give better context to langdetect. This fixes the problem above:

>>> langdetect.detect_langs('<i>Le train pour Montauk</i>\n<i>sur la voie "B."</i>')
[fr:0.8571379079913274, ca:0.14285786337770048]