How to convert YouTube SBV to SRT

Simply like this:

ffmpeg -i captions.sbv captions.srt

​YouTube supports only the basic features of SBV (SuBView) and SRT (SubRipText) closed captions. Notably users can download existing closed captions (including auto-generated subtitles) as SBV, convert them to SRT supported by, e.g., MPlayer, edit the subtitles in a text editor such as vim or notepad (hint: YouTube wants UTF-8), and upload the fixed version to YouTube, overwriting the old subtitles for the given language.

As far as YouTube supports these two formats (among others) they are very similar. The SRT captions start with a counter (increasing numbers), followed by the from --> to timestamps, text (one or two lines), and an empty line:

1
00:00:05,000 --> 00:00:13,700
you know where's the girl who hasn't

2
00:00:10,100 --> 00:00:16,400
gotten to college or even maybe finished

The SBV captions have no counter and use periods as decimal points in their from,to timestamps:

0:00:01.400,0:00:06.770
you know where's the girl who hasn't

0:00:04.610,0:00:08.840
gotten to college or even maybe finished

SRT timestamps have the form hh:mm:ss,nnn, SBV timestamps use h:mm:ss.nnn.

YouTube is smart enough to detect and report simple errors, but you should make sure that timestamps do not overlap, i.e., the earliest from in the next caption can be the same as the to end of the last caption. Obviously the duration should be not too short, and it cannot be "negative" with a from later than its corresponding to.

The auto-generated SBV example shown above (downloaded from a YouTube movie trailer) actually uses overlapping timestamps, and FFmpeg kept them as is in its SRT conversion. For the theory see the WebM Project ​General WebVTT Notes, in practice input option -fix_sub_duration can get rid of this:

ffmpeg -fix_sub_duration -i captions.sbv captions.srt

Fixed SRT output sample:

1
00:00:05,000 --> 00:00:10,100
you know where's the girl who hasn't

2
00:00:10,100 --> 00:00:13,700
gotten to college or even maybe finished

SMplayer using MPlayer is unsuited as a syntax check, e.g., it doesn't insist on file extension .srt, anything including .txt goes, it accepts missing counters, and simply ignores timestamps greater than the duration of the clip. As of early 2019 YouTube could not handle the latter, the upload failed without reporting an error.

Hint for Windows users, if your Chrome uploads fail with an unclear JSON-error use IE11, or try the YouTube studio beta UI instead of classic studio.