Captions are great for making videos (especially technical ones!) easier to understand and search.

If you see a talk that you'd like to caption, feel free to download it and start working on it with your favourite subtitle editor. Let me know what you pick by e-mailing me at so that I can update the index and try to avoid duplication of work. Find talks that need captions here. You can also help by adding chapter markers to Q&A sessions.

You're welcome to work with captions using your favourite tool. We've been using to caption things as VTT or SRT in Emacs, often starting with autogenerated captions from OpenAI Whisper (the .vtt).

We'll be posting VTT files so that they can be included by the HTML5 video player (demo:, so if you use a different tool that produces another format, any format that can be converted into that one (like SRT or ASS) is fine. subed has a subed-convert command that might be useful for turning WebVTT files into tab-separated values (TSV) and back again, if you prefer a more concise format.

You can e-mail me the subtitles when you're done, and then I can merge it into the video.

You might find it easier to start with the autogenerated captions and then refer to any resources provided by the speaker in order to figure out spelling. Sometimes speakers provide pretty complete scripts, which is great, but they also tend to add extra words.

Reflowing the text

First, let's start with reflowing. We like to have one line of captions about 60 characters long so that they'll display nicely in the stream. If the captions haven't been reflowed yet, you can reflow the captions at natural pausing points (ex: phrases) so that they're displayed nicely. You don't have to worry too much about getting the timestamps precisely.

For example, instead of:

  • so i'm going to talk today about a
  • fun rewrite i did of uh of the bindat
  • package

you can edit it to be more like:

  • So I'm going to talk today
  • about a fun rewrite I did
  • of the bindat package.

You probably don't need to do this step if you're working with the VTT files in the backstage area, since we try to reflow things before people edit them, but we thought we'd demonstrate it in case people are curious.

We start with the text file that OpenAI Whisper generates. We set my fill-column to 50 and use display-fill-column-indicator-mode to give myself a goal column. A little over is fine too. Then we use emacsconf-reflow from the emacsconf-el repository to quickly split up the text into captions by looking for where we want to add newlines and then typing the word or words. We type in ' to join lines. Sometimes, if it splits at the wrong one, we just undo it and edit it normally.

It took about 4 minutes to reflow John Wiegley's 5-minute presentation.

The next step is to align it with aeneas to get the timestamps for each line of text. subed-align from the subed package helps with that.

Edit the VTT to fix misrecognized words

The next step is to edit these subtitles. VTT files are plain text, so you can edit them with regular text-mode if you want to. If you're editing subtitles within Emacs, subed can conveniently synchronize video playback with subtitle editing, which makes it easier to figure out technical words. subed tries to load the video based on the filename, but if it can't find it, you can use C-c C-v (subed-mpv-find-media) to play a file or C-c C-u to play a URL.

Look for misrecognized words and edit them. We also like to change things to follow Emacs keybinding conventions. We sometimes spell out acronyms on first use or add extra information in brackets. The captions will be used in a transcript as well, so you can add punctuation, remove filler words, and try to make it read better.

Sometimes you may want to tweak how the captions are split. You can use M-j (subed-jump-to-current-subtitle) to jump to the caption if I'm not already on it, listen for the right spot, and maybe use M-SPC to toggle playback. Use M-. (subed-split-subtitle) to split a caption at the current MPV playing position and M-m (subed-merge-with-next) to merge a subtitle with the next one. Times don't need to be very precise. If you don't understand a word or phrase, add two question marks ([??]) and move on. We'll ask the speakers to review the subtitles and can sort that out then.

If there are multiple speakers, indicate switches between speakers with a [speaker-name]: tag.

Once you've gotten the hang of things, it might take between 1x to 4x the video time to edit captions.

Playing your subtitles together with the video

To load a specific subtitle file in MPV, use the --sub-file= or --sub-files= command-line argument.

If you're using subed, the video should autoplay if it's named the same as your subtitle file. If not, you can use C-c C-v (subed-mpv-play-from-file) to load the video file. You can toggle looping over the current subtitle with C-c C-l (subed-toggle-loop-over-current-subtitle), synchronizing player to point with C-c , (subed-toggle-sync-player-to-point), and synchronizing point to player with C-c . (subed-toggle-sync-point-to-player).

Using word-level timing data

If there is a .srv2 file with word-level timing data, you can load it with subed-word-data-load-from-file from subed-word-data.el in the subed package. You can then split with the usual M-. (subed-split-subtitle), and it should use word-level timestamps when available.

Starting from a script

Some talks don't have autogenerated captions, or you may prefer to start from scratch. Whenever the speaker has provided a script, you can use that as a starting point. One way is to start by making a VTT file with one subtitle spanning the whole video, like this:


00:00:00.000 -> 00:39:07.000
If the speaker provided a script, I usually put the script under this heading.

If you're using subed, you can move to the point to a good stopping point for a phrase, toggle playing with M-SPC, and then M-. (subed-split-subtitle) when the player reaches that point. If it's too fast, use M-j to repeat the current subtitle.

Starting from scratch

You can send us a text file with just the text transcript in it and not worry about the timestamps. We can figure out the timing using aeneas for forced alignment.

If you want to try timing as you go, you might find it easier to start by making a VTT file with one subtitle spanning the whole video, like this:


00:00:00.000 -> 00:39:07.000

Then start playback and type, using M-. (subed-split-subtitle) to split after a reasonable length for a subtitle. If it's too fast, use M-j to repeat the current subtitle.

Chapter markers

In addition to the captions, you may also want to add chapter markers. An easy way to do that is to add a =NOTE Chapter heading= before the subtitle that starts the chapter. For example:

00:05:13.880 --> 00:05:20.119
So yeah, like that's currently the problem.

NOTE Embeddings

00:05:20.120 --> 00:05:23.399
So I want to talk about embeddings.

We can then extract those with emacsconf-subed-make-chapter-file-based-on-comments.

For an example of how chapter markers allow people to quickly navigate videos, see .

Please let us know if you need any help!