Subtitle Formats Explained

Subtitles may be the least understood aspect of digital video. Even they're found on nearly every DVD, with the complexity of dealing with video, audio, and menus it's common for people to leave them for last, and often they get ignored completely. Ironically, for most people subtitles are much easier to grasp than other multimedia elements. With a little bit of reading it becomes easy to figure them out.

Subtitle Categories

There are many ways to categorize subtitles. They can be mandatory or selectable, depending on whether they can be turned off. They can be hard coded or separate streams, depending on whether they're encoded as part of the video or simply muxed into the same container. For our purposes we'll be focusing on subtitles that exist as separate streams in a video container, or even separate files entirely. But we'll still be considering two categories. First we'll be considering the difference between image and text based subtitles. Then we'll take a look at three specific subtitle formats; VobSub, SubRip, and SubStation Alpha.

Text vs. Images

Most subtitles consist purely of text characters. Since text is also some of the easiest data to store and compress it makes sense to store subtitles as simple text files or a text stream within a video file. Although it's normal for all subtitles to start out this way, that doesn't mean that's how they're stored.

As a matter of fact subtitles on DVDs aren't actually text. They're actually encoded as raster graphics. Much like the way characters on older text-based computer interfaces, they're actually just a collection of dots on a grid. These images are put over the top of the video frame when displayed. It's not necessarily important to understand how either type of subtitles work, or why a particular application uses one or the other. Still, there are times you need to understand the difference and know which you have or need.

The Big Advantage to Text

The important thing about any text-based subtitle format is that you do have the ability to edit subtitles easily. Since editing a text-based subtitle can generally be done with even a simple text editor like Notepad they're the easiest to modify and by far the easiest to create yourself. Creating subtitles isn't exactly something most people have the inclination (or time) to do. But if you happen to belong to the small minority with a need to make subtitles you'll have to at least start with a text-based format.

Perhaps the biggest reason for the widespread development of text-based subtitles is their use in AVI files. While AVI files can't contain graphic subtitles, they can have text subtitles. AVI was/is the most common container for MPEG-4 ASP video encoded with codecs like DivX and XviD, and was also the format first added to DVD players for MPEG- support.

VobSub Subtitles

VobSub subtitles have become very common because it's easy to get them from DVDs. In fact, VobSub basically just re-packages the images from the DVD into a file that has the extension of .SUB and additional information in another file with an extension of .IDX. They're generally referred to as either VobSub or IDX + SUB (IDX/SUB) subtitles. Information in the IDX file tells media player software the color of the subtitles, their position on the screen, when they appear and disappear, and a number of other important pieces of information.

Since they're stored in a separate file VobSub subtitles are extremely portable. They're also not generally supported by DVD players, even when they're capable of playing formats like MPEG-4 (DivX, XviD, Nero Digital, etc,...). However, in recent years the growing popularity of the Matroska multimedia container has been a bonus for people who prefer this subtitle format because it can be muxed into MKV files so it's no longer necessary to have a separate file for subtitles.

Why Use VobSub?

The real advantage to VobSub is its similarity to DVD subtitles. In order to create text-based subtitles from an image-based format a process called OCR, or Optical Character Recognition, is used. OCR software essentially attempts to 'read' the text represented by the images. The problem is that there can be big differences between two different images of the same character. Differences in fonts and spaciing make it nearly impossible for even the most sophisticated OCR software to identify every character correctly. And that's assuming you had the best possible OCR engine at your disposal; something that would cost thousands of dollars. By sticking with the original images VobSub maintains perfect accuracy.

SubRip

SubRip is one of the more popular text-based subtitle formats. Since most subtitles are still ripped from DVDs they must be created using OCR. Although SubRip was the first program with the ability to create SRT subtitle files, VobSub can also make them. Whatever application you use, you should expect to spend at least a little bit of time tweaking the settings to get the most accurate results possible. You may also need to go through the text to manually correct areas where the OCR engine simply made the wrong choice.

SRT subtitles are most commonly found in DivX or XviD encoded AVI files.

SubStation Alpha

SubStation Alpha is one of the older text-based subtitle formats. It became popular for adding subtitles to anime, which before the birth of DVD was generally only available in the original Japanes without subtitles, or dubbed into another language. Although you may occasionally find SSA subtitles now, they've generally been replaced by SubRip's SRT format.

Working With Subtitles

Most people do their ripping and encoding with tools that don't require you to know anything more than whether you want to keep the subtitles and which ones. However, if you have to add your own subtitles to a video file or fix the subtitles already there you may need to know a little more. Here's a selection of our subtitle related guides along with a short description.

Rip Subtitles with VobSub	Instructions for ripping subtitles in VobSub format from DVD files.
Convert Subtitles from SUB / IDX to SRT	Read this guide for help converting subtitles in VobSub's IDX / SUB format to SubRip's SRT format.
Convert Subtitles from SUB / IDX to SSA	Read this guide for help converting subtitles in VobSub's IDX / SUB format to SubStation Alpha's SSA format.
Convert Subtitles from SRT to SSA	Read this guide for help converting subtitles in SubRip's SRT format to SubStation Alpha's SSA format.
Convert Subtitles from SSA to SRT	Read this guide for help converting subtitles in SubStation Alpha's SSA format to SubRip's SRT format.
Convert Subtitles from SRT to SUB / IDX	Read this guide for help converting subtitles in SubRip's text-based SRT format to VobSub's image-based IDX / SUB format.
How to Join Subtitles	This guide will show you how to join two subtitle files.
How to Sync Subtitles	If you have a movie with subtitles that aren't in sync with the video and audio read this guide for help.

Version History
v1.0 Original version by Rich "Vurbal" Fiscus