Convert SUB to SRT

Two entirely different things share this extension. MicroDVD SUB is plain text: frame numbers in curly braces followed by the words, one entry per line. VobSub SUB is bitmap subtitles ripped from a DVD, stored as pictures, and it always has an .idx index file sitting beside it.

Telling them apart takes a second in any text editor. Readable lines in curly braces mean MicroDVD, and this converter handles it. Binary noise with an .idx alongside means VobSub, the subtitles are images rather than text, and no conversion can help — the words have to be recovered with optical character recognition first, which is a different tool and a different kind of accuracy.

MicroDVD records which frame a line appears on. A frame number becomes a moment in time only when divided by a frame rate, and the file usually does not state one, so the rate is the input the converter cannot guess. The control accepts anything from 1 to 120 in thousandths of a frame, which exists because the two rates that matter most are not whole numbers.

The three worth trying, in order: 25, which is European broadcast and the default; 23.976, which is what a film transfer runs at and the most common alternative; and 29.97 for North American broadcast material. Getting it right is not a matter of taste — the wrong rate does not shift the subtitles, it stretches them, and a file stretched by four per cent is right at the start and minutes out at the end.

The fastest source is the video itself. Any player that shows media information reports the frame rate of the file it is playing, and if the subtitles were made for the copy you have, that number is the answer. It is also the answer a player assumes when it loads a MicroDVD file directly, which is why some files work in a player and produce drift when converted with a different guess.

When the subtitles were made for a different copy, the number has to be inferred. Films distributed on disc and downloaded from film sources are usually 23.976; anything captured from European television is 25; North American broadcast captures are 29.97. If two of those produce drift in opposite directions, the correct rate is between them, and the file was probably retimed by hand at some point in its life.

There is a long-standing convention for recording the rate inside a MicroDVD file: a first entry spanning frame one to frame one whose text is the frame rate, which some players read as a declaration rather than displaying. It was a convention rather than part of a specification, and support for it was never universal.

This converter does not treat it specially. The line matches the ordinary entry pattern, so it becomes an ordinary cue — the first subtitle in your SRT will be a number such as 23.976, on screen for a fraction of a second at the very start. It is unmistakable and the fix is to delete the first cue. Worth knowing before you see it in a player and conclude the conversion went wrong.

Load the SUB file directly in a player alongside the video and watch two moments: the first subtitle and one near the end. Both correct means the player guessed the rate right, and that guess is the number to use here. Both wrong by the same amount means an offset, which is a different fault with a different fix and has nothing to do with the frame rate.

A growing error is the frame-rate case. If the subtitles start almost right and are minutes out by the end, the rate is wrong by a small percentage: 25 against 23.976 is 4.3 per cent, which is close to four minutes over a ninety-minute film. Converting at the correct rate fixes it completely, and it is worth spending two minutes on this diagnosis rather than converting three times and comparing.

The arithmetic is a division, and seeing it done makes the choice concrete. A line beginning at frame 3210 starts at 128.400 seconds if the rate is 25, which is written 00:02:08,400. The same frame number at 23.976 starts at 133.875 seconds, written 00:02:13,875.

Five and a half seconds apart at the two-minute mark, from one setting. That is the difference the control on this page decides, and it is why a converter that silently assumed a rate would be doing something dishonest. The output timestamps carry full millisecond precision because the division rarely lands on a round number, which is a small tell that a file began life as MicroDVD.

A MicroDVD entry occupies one physical line, so a forced break inside a caption is written as a vertical bar. Those become real line breaks in the SRT, which is what they always meant. Any pipe that was in the text for another reason becomes a break too, because the format never offered a way to escape one.

Inline control codes — italic, bold, colour, font, size, written in curly braces — are removed rather than translated. That is a small loss and mostly a mercy: player support for them was inconsistent for the format’s entire life, so a file relying on them looked different on every machine. What arrives in the SRT is the words, which is what SubRip is for.

This is the real reason to convert rather than to keep fixing the SUB. A timestamp states a moment; it means the same thing against any encode of the same cut, at any frame rate, on any device. The dependency that made the file fragile is gone, permanently, after one correct conversion.

It also opens the file to tooling. Every subtitle editor reads SubRip well, most read MicroDVD grudgingly, and anything you might want to do next — shifting, retiming, translating, merging two languages — is better supported on the other side of this conversion. Old archives are worth converting once, in bulk, rather than each time somebody wants to watch something.

The one thing worth recording while you still have both files is which rate you used. Nothing in the SRT remembers, and a collection converted over several evenings at whichever number seemed right that night is a collection nobody can audit later. A note in the folder, or the rate in the filename, turns a guess into a fact that survives the next person to open it.

It is worth being explicit about the order of operations. If you convert with the wrong frame rate, the drift stops being a setting and becomes the timestamps themselves. Nothing in the SRT records that it was ever frame-based, so no later tool can undo the division.

Fixing it then means a proportional stretch across every timestamp in the file, which subtitle editors can do and which requires knowing both the rate you used and the rate you should have used. Reconverting from the original SUB is faster and exact. Keep the SUB until the SRT has been checked against the video from beginning to end — it costs a few kilobytes and it is the only copy of the original frame numbers.

Once converted, the SRT loads automatically when it sits beside the video with the same base name. That convention has not changed in twenty years and it accounts for most reports of a subtitle file not working when the file is fine.

Encoding is the other artefact of age. MicroDVD files predate UTF-8 being the default and were commonly saved in a regional single-byte encoding, which is why an old Polish, Czech or Spanish subtitle file arrives full of question marks. The output here is written as UTF-8; if the accented characters were already wrong before conversion, they were misread on the way in, and re-saving the source in the correct encoding first is the only thing that recovers them.