Sustainability of Digital Formats: Planning for Library of Congress Collections
Sustainability of Digital Formats: Planning for Library of Congress Collections |
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| Introduction | Sustainability Factors | Content Categories | Format Descriptions | Contact | |
| Full name | VobSub Subtitle File (SUB) |
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| Description |
The VobSub Subtitle format contains extracted subtitle files found on DVDs, a pair of files, a SUB binary file and IDX subtitle index file. This document will cover information about the SUB file used by the VobSub subtitle software (See History.). The VobSub program extracts subtitles stored on DVDs in VOB files, subtitles are stored as Bitmap images, creating two files, a SUB file and an IDX file. The SUB files contain images, usually transparent images containing just text, that are shown as subtitled text while the IDX file’s metadata instructs the media player supporting the SUB/IDX format when, where, and what order to show the images, as well as text color that are laid over the movie. Bitmap-based subtitle formats are the most common form used in DVDs. According to Geza Kovacs in Multimedia for Language Learning, June 2013, “Bitmap formats do not include the machine-readable representation of the subtitle text. Rather, they are pre-rendered versions of the text which are overlayed onto the video when playing. Thus, they consist of a time-range associated with a bitmap image. Bitmap formats are the standard format used in DVDs, because it allows DVD producers to provide subtitles with arbitrary formatting in any language, while allowing the DVD player manufacturers to avoid providing fonts covering every possible character used in every possible writing system, in every possible font style that a DVD producer might require...The particular bitmap-based subtitle format used in DVDs is called VobSub. It consists of an index file that indicates the timestamp at which each new subtitle should be displayed, as well as the offset in the subtitle-bitmap file at which the bitmap that should be displayed at that time is located.” Example VobSub index file extracted from a DVD
# VobSub index file, v7 (do not modify this line!) Structure of a VobSub SUB: Limited information found on the binary structure of a VobSub SUB file. Docs.rs Documentation states “VobSub subtitles consists of a simple textual .idx file, and a binary .sub file. The binary .sub file is essentially an MPEG-2 Program Stream (PS) containing Packetized Elementary Stream (PES) data, but only for a single subtitle track.” MPEG-2 was initially developed for compressed television program transmissions but was adopted for DVD production as well. The MPEG-2 format description states: “Picture, sound, and data elements consist of streams...Elementary streams are the basic element; these are broken into packets of variable length, forming a packetized elementary stream (PES).” The VobSub program extracts the PES packets that represent subtitles on the DVD. SourcForge.net provides information on PES Headers, stating that “depending on the Stream ID, a PES header may contain various components.” Uses of a VobSub SUB: When using SUB/IDX supporting media players or the VobSub program, users can extract and play back DVD-format subtitles alongside the movie meant for those subtitles. The VobSub IDX/SUB format also has support for multiple tracks and can be embedded in MP4 (Nero) and Matroska (MKV) files. There are many programs that will convert SUB/IDX subtitle formats to various other subtitle formats, like SRT, depending on the need of the user’s software, including SubTitleEdit and GoTranscript. |
| Production phase | Primarily final state, with proper software users can exact subtitles to be viewed alongside the movie they were meant to be used with. |
| Relationship to other formats | |
| Has modified version | MPEG-2, MPEG-2 Encoding Family |
| Contains | IDX, Subtitle Index Files. As stated on FileInfo.com, “IDX files are always paired with accompanying SUB files.” |
| LC experience or existing holdings | None documented. |
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| LC preference | The Library of Congress has not defined format preferences for caption or subtitle files. |
| Disclosure |
Little documentation. A variety of websites describe the VobSub SUB subtitle format and give a description of how it is used, but no unified format standard was found. VideoHelp.com has a link to the latest software version, VobSub 2.23. VideoHelp.com states that VobSub as ‘No Longer Developed.” See History. |
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| Documentation |
No information publicly available. |
| Adoption |
Limited information. Although VobSub is no longer being developed, users can still download the latest version of the software, VobSub 2.23, which contains a subtitle filter and subtitle utilities that can extract, covert, cut, and join IDX/SUB files from DVDs. An archived guide on Afterdawn.com, titled Subtitle Formats Explained (date not found), stated that “VobSub subtitles have become very common because it’s easy to get them from DVDs...However, in recent years the growing popularity of the Matroska multimedia container has been a bonus for people who prefer subtitle format because it can be muxed into MKV files so it’s no longer necessary to have a separate file for subtitles.” |
| Licensing and patents |
No known license or patent concerns generation and use of the format as VobSub SUB. |
| Transparency |
VobSub SUB files are image-based subtitles (binary) so not easily read or edited with a text-editor. Users will need a converter program to create text-based subtitles. The process used is called Optical Character Recognition (OCR) and the software attempts to read the text represented on the DVD subtitle images, but this can be limited due to the many different characters and spacing styles. |
| Self-documentation |
No information about VobSub SUD files including metadata, other than MPEG-2 PES Headers including Stream ID that indicates the type of packet. |
| External dependencies |
None beyond availability of supporting software. |
| Technical protection considerations |
None. |
| Still Image | |
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| Normal rendering |
Low support. Based on limited information. VobSub SUB image files are not easily resized or edited, including zooming in. VobSub SUB files are images with their size determined when created for the DVD they are associated with, and the media player determines if the file should be scaled. After the VobSub software has indexed the subtitles, the VobSub configure tool prompts users to select subtitle language, adjust text colors, transparency, and position of the subtitles. (Filter: VobSub configure tool image shows “Scale” but limited information, see picture.) According to Elena Opris in Easily Gain the Ability to Load Subtitles with Media Player While Also Customizing Font and Adjust Timing With the Help of this Practical Utility", July 2014, “When configuring VobSub, you can modify the font type and size used for displaying the subtitles, as well as subtitle timing, the picture padding and the colors. In other words, you have the liberty to customize the way your subtitles look like in the slightest detail.” VideoHelp.com forum user Skiller wrote, October 2013, in response to questioning how to edit IDX/SUB subtitles, “You would have to extract the subtitles to image files (as that's what they really are, idx/sub stores bitmaps, not text) and edit them in an image editing software and then make a new set of idx/sub from that. The editing part can be a good deal of tedious work and you would need to find out the original font, outline, spacing, etc. for it to look like it's original.” |
| Clarity (high image resolution) |
Limited information found. The resolution size of the images is defined in the IDX file, under size. |
| Color maintenance |
Limited information found. The SubVob SUB/IDX format is limited to supporting four color types, including the background color, shadow, outline, and text, with 16 palette entries used by the subtitles. |
| Support for vector graphics, including graphic effects and typography |
Little to no information on VobSub SUB file’s support of graphic effects and typography. |
| Support for multispectral bands |
Little to no information on VobSub SUB file’s support of multispectral bands. |
| Functionality beyond normal rendering |
None known. |
| Tag | Value | Note |
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| Filename extension | sub |
See https://www.wikidata.org/wiki/Q27979377. |
| Filename extension | See related format. | See IDX. |
| Pronom PUID | See note. | PRONOM has no corresponding entry as of April 2023. |
| Wikidata Title ID | Q27979377 |
File format. See https://www.wikidata.org/wiki/Q27979377. |
| General |
DVD VOB files store, separately, the main video and audio streams, but also subtitles as streams of image files, with each stream being a different language (up to 32). DVD subtitles are 4 color bitmaps which are overlayed over the video stream, appearing one after another. FileInfo.com states the SUB file extension is used in 4 file types, including MircroDVD Subtitle file, CloneCD Subchannel file, SubTerra Level file, and VobSub Subtitles file. |
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| History |
SUB files, when alongside IDX subtitle index files, are often called VobSub Subtitles, as they are created and used by the VobSub software to extract subtitles from DVDs for users to view alongside the movie they are watching. Limited online information about the VobSub software history. According to Wikipedia DirectVobSub’s page, after the last release of VobSub 2.23, development stopped. DirectVobSub, also known as VSFilter, is a software add-on for Microsoft Windows that can read external subtitle files. It was formerly a part of the VobSub application. VSFilter was part of the guliverkli project on SourceForge’s website, but development of that project ended with version 2.37 in 2005. Guliverkl2 project, known as DirectVobSub started in 2007, and then sometime after MPC-HC sources took over the project, but that shutdown in 2017 due to lack of developers. |
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