What is a SUB file?

SUB is a plain-text format you can open in any editor. It is used for subtitling.

The extension is .sub, and the full name is MicroDVD Subtitle. Both matter less than what the file can hold, which is what the rest of this page is about.

It dates from 1998.

A format that has been readable for that long is a format worth trusting with something you want back in ten years.

It is written down and implementable, though the vendor decides what the next version looks like. In practice that is enough for the file to keep opening; it is not enough to be sure of it.

It still opens everywhere and is still written by older tools, but nothing new is being built around it. Convert what you mean to keep, and do not choose it for something starting today.

VLC and Subtitle Edit read it, and so do most programs of the same kind.

If a file will not open, the format is rarely the problem — it is more often that the program predates it. Converting to something older is the reliable way past that, and it is what the rest of this site is for.

No browser reads it.

That is the single most common reason to convert it: not that the format is bad, but that the place you want to show the file cannot read it.

SUB is meant to be opened and changed. Keep the file in this format for as long as the work is going on, and export from it whenever a finished copy is needed.

The recurring complaints: support outside its home ground is uneven.

None of these is a reason to avoid the format. They are the things worth knowing before you are surprised by one, which is a different claim and a more useful one.

Every other subtitle format on this site says when a line appears: two minutes, fourteen seconds and 300 milliseconds. MicroDVD says which frame it appears on — {3210}{3455} and then the text.

That made sense when subtitles accompanied a specific video file at a specific frame rate, and it is the source of every problem the format has. A frame number only means a moment in time if you know how many frames there are per second. Divide by the wrong number and the whole file is wrong, progressively, from the first line to the last.

This is the diagnostic worth knowing. If subtitles are wrong by a constant amount from beginning to end, the file is time-based and needs an offset. If the error grows steadily — barely visible at the start, seconds out after ten minutes, minutes out by the end — the timing is being scaled wrongly, and with a SUB file that means a frame rate mismatch.

The classic case is a film mastered at 23.976 frames per second and a subtitle file made for the 25-frame European broadcast version, or the reverse. 25 divided by 23.976 is 1.0427, so the error is 4.3 per cent of however far into the film you are: about 26 seconds after ten minutes, and nearly four minutes by the end of a ninety-minute feature. Imperceptible in the first thirty seconds, unwatchable in the third act.

There is a convention: a first line with frame numbers of one and one whose text is the frame rate, which players read as a declaration rather than displaying. Many files have it and many do not, because it was a convention rather than part of any specification.

When it is absent the player guesses, usually by assuming the video’s own rate — which is right if the subtitles were made for that video and wrong otherwise. If a SUB file drifts, opening it in a text editor to see whether that first line exists is the fastest thing to check, and adding or correcting it is a one-line fix.

MicroDVD SUB is plain text, and it is what this page describes. There is also a VobSub .sub, which is something else entirely: the bitmap subtitles ripped from a DVD, stored as images, always accompanied by an .idx file beside it.

Telling them apart takes a second. Open the file in a text editor — if you see readable lines in curly braces, it is MicroDVD. If you see binary noise and there is an .idx next to it, it is VobSub, the subtitles are pictures rather than text, and getting text out of them requires optical character recognition rather than a conversion.

It has a small set of inline control codes: italic, bold, underline, colour, font and size, written in curly braces at the start of a line. More than SubRip has, and applied inconsistently across players.

What it has no concept of is positioning. A line appears where the player puts it, which is the bottom. There is also no styling section, no named styles, and nothing resembling the typesetting that ASS was built for. It is a lightweight format from an era when the alternative was nothing.

The conversion turns frame numbers into timestamps, which requires knowing the frame rate — and once it is done, the file is no longer tied to one. That is the real benefit: an SRT states times, so it stays correct against any copy of the video that has the same running length, whatever rate it was encoded at.

Text and line breaks carry across and the styling does not: every curly-brace code is stripped, so italic, bold, underline, colour, font and size all go together, even though SRT could have expressed the italics. If the source drifts, fix the frame rate before converting rather than after — converting with the wrong rate bakes the drift into timestamps, and correcting it then means stretching the whole file instead of changing one number.

Age, mostly. The format belongs to the era of MicroDVD and DivX players, and files with this extension are typically fifteen to twenty-five years old — subtitle archives, ripped collections, material that has been passed around since before SubRip won.

Nothing produces them now. Modern subtitle editors read them and write something else, which is the correct treatment: convert once, keep the SRT, and treat the SUB as an archive of where the text came from rather than a file to maintain.