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 | Timed Text Markup Language Version 1 (TTML1) |
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| Description |
Timed Text Markup Language Version 1 (TTML1) is an XML-based format for the interchange of timed text media, but also can be used as an authoring and distribution format. TTML1 is defined by the World Wide Web Consortium (W3C) and is specified in the Timed Text Markup Language (TTML1) (Third Edition) W3C Recommendation 08 November 2018 (referenced throughout this document). TTML1 is the first version specification of the Timed Text Markup Language (TTML) W3C Recommendation standard, previously referred to as Distribution Format Exchange Profile (DFXP). TTML1 was specified by the Timed Text Working Group (TTWG) and the W3C Recommendation First Edition was released in November 2010. See History for more information. Per TTML1 specification, “In the first edition of this specification, the version number 1.0 was used to refer to this version of TTML. In this second edition, this version number is simplified to 1 as a result of a determination to call the next (future) version 2 instead of 1.1." W3C Timed Text documentation TTML and Derivative Captions Formats describes Time Text Markup Language (TTML1) as “textual information that may be used directly as a distribution format for online captioning and subtitles, and as interchange format among legacy distribution content formats. It is a superset that encompasses preceding captioning approaches. It supports the semantics of most of the closed caption files.” System Model Per specification, TTML timed text files are designed to be used as a bidirectional interchange format for authoring systems and as a unidirectional interchange format for distribution formats. TTML1 Document Instance: A Document Instance is a defined TTML document, per specification, “a concrete realization of a TTML document, where the concrete form is specific to the context of reference.” Specification Example Fragment: Consisting of a tt (root) document element containing:
The TTML1 specification defines the TTML Abstract Document Type as a set of constraints that defines a class of XML Information Sets and TTML Content as a profile intended to be used for interchange among distribution systems. TTML1 specification defines the XML namespaces, profiles, and vocabulary (element/attribute catalog). Namespaces specified in the TTML1 specification include TT, TT Parameter, TT Style, TT Metadata, TT Profile, TT Feature, and TT Extension. TTML1 Profiles and Extensions: W3C Timed Text documentation TTML and Derivative Captions Formats states, “TTML allows extensibility and profiling, therefore multiple caption formats have been developed by the W3C or other external organizations.” Per the TTML1 specification, TTML “employs a number of standard, predefined profiles of its vocabulary and associated semantics.”
External TTML Profiles (Restricting and/or Extending TTML caption formats):
The TTML1 specification defines the vocabulary, divided into two catalogs, core and extension. The core catalog defines the baseline vocabulary of TTML, in particular TTML Content. The extension catalog is a placeholder for vocabulary extensions defined by TTML. The core catalog baseline vocabulary includes Parameters, Content, Styling, Layout, Timing, Animation, and Metadata. Defined TTML1 elements include set, body, div, p, span, br, tt, head, layout, region, metadata, ttm:actor, ttm:agent, ttm:copyright, ttm:desc, ttm:name, ttm:title, ttp:profile, ttp:features, ttp:feature, ttp:extensions, ttp:extension, styling, and style. Parameters are either essential or of significant importance for interpreting TTML Content. Content specifies the structure and principal content. Uses of TTML1 TTML1 specification describes the TTML1 format as mainly used in the broadcast industry and by online applications for the purpose of authoring, transcoding, and exchanging timed text information. Typically used when adding captioning/subtitles to video content such as movies, films, or presentations. |
| Production phase | Primarily middle state for the delivery of captions and subtitles online, but also developed for the purpose of authoring, transcoding, and exchanging time text information. |
| Relationship to other formats | |
| Defined via | XML, Extensible Markup Language (XML) |
| Component of | HTML5,
HyperText Markup Language (HTML) 5. The TTML1 specification states “While TTML is not expressly designed for direct (embedded) integration into an HTML or a SMIL document instance, such integration is not precluded.” TTML may provide a "standard content format to reference from a <track> element in an HTML5 document. |
| Component of | SMIL,
Synchronized Multimedia Integration Language. The TTML1 specification states “While TTML is not expressly designed for direct (embedded) integration into an HTML or a SMIL document instance, such integration is not precluded.” TTML may provide a standard content format to reference from a <text> or <textstream> media element in a SMIL2.1 document. Vocabulary Derivation - TTML element vocabulary items are based on the syntactic and/or semantic model defined by the reference SMIL 2.1. |
| May contain | CSS,
Cascading Style Sheet (CSS) Markup. CSS style specification syntax is mapped to XML by use of attributes defined in the TT Style Namespace. Vocabulary Derivation - TTML element vocabulary items are based on the syntactic and/or semantic model defined by the reference CSS2, CSS3. |
| May contain | XHTML 1.0,
Extensible HyperText Markup Language (XHTML). Vocabulary Derivation - TTML element vocabulary items are based on the syntactic and/or semantic model defined by the reference XHTML 1.0. |
| May contain | SVG 1.1,
Scalable Vector Graphics (SVG) File Format Family. Vocabulary Derivation - TTML element vocabulary items are based on the syntactic and/or semantic model defined by the reference SVG 1.1 |
| May contain | MPEG 7-5,
Multimedia Content Description Interface (MPEG 7-5). Not described separately on this website at this time. Vocabulary Derivation - TTML element vocabulary items are based on the syntactic and/or semantic model defined by the reference MPEG 7-5. |
| May contain | XSL 1.1,
Extensible Stylesheet Language (XSL) Version 1.1. Not described separately on this website at this time. Vocabulary Derivation - TTML element vocabulary items are based on the syntactic and/or semantic model defined by the reference XSL 1.1. |
| Has later version | TTML2,
Timed Text Markup Language (TTML), Version 2 (TTML2). Timed Text Markup Language (TTML), Version 2, also referred to as TTML2. |
| Component of | DVB-SUB, Digital Video Broadcasting; Subtitling systems (DVB-SUB). Per standard, the document "specifies the transport of TTML subtitle streams in DVB MPEG-2 transport streams, based on the MPEG-2 system described in ISO/IEC 13818-1...The present document provides syntax for delivery of TTML subtitle streams over MPEG-2 transport stream, and is based on EBU-TT-D compatible with the IMSC1 Text Profile of W3C TTML." See TTML1 for more information on the W3C's IMSC1 Text Profile. |
| LC experience or existing holdings | Likely exists in LC collections but not separately documented. |
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| LC preference | The Library of Congress has not defined format preferences for caption or subtitle files for moving image works in the Recommended Formats Statement. |
| Disclosure | Open standard and fully documented. Timed Text Markup Language (TTML1) (Third Edition) is a W3C Recommendation which is considered Web standard. |
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| Documentation | Timed Text Markup Language 1 (TTML1) (Third Edition) | W3C Recommendation 08 November 2018, edited in place 20 August 2020 is the most current specification and is maintained by the W3C’s Timed Text Working Group (TTWG). The mission of TTWG is “to develop W3C Recommendations for media online captioning by developing and maintaining new versions of the Timed Text Markup Language (TTML).” |
| Adoption |
Widely adopted format in the television industry and supported by most modern media players, caption software, and video recording software. YouTube, Netflix, Amazon Prime Video (link to content provided but can't be made active this website's XML schema due to forbidden punctuation: https://videodirect.amazon.com/home/help?topicId=G201979140&ref_=avd_sup_G201979140), and Telestream are some supporters of the TTML format. According to Andrea Tai in Balisage Paper: XML in the Air – How TTML can change the Workflows for Broadcast Subtitles from August 2016, “The adoption of TTML started with DFXP as pre-final candidate recommendation and quickly gained popularity. Unfortunately, the final TTML 1 version specification changed not only the name of the format from ‘Timed Text (TT) Authoring Format 1.0 – Distribution Format Exchange Profile (DFXP)’ to ‘Timed Text Markup Language 1 (TTML1)’ but also replaced XML namespaces (the local names of nearly all elements and attributes remained the same). This led not only to confusion (the older version is often not associated with TTML because of the different spec title) but is also an interoperability problem. Although two simple XSLT scripts exist that replace namespaces in both directions out of the box, an XML-compliant TTML 1 player is not able to process DFXP files and an XML-compliant DFXP player is not able to process TTML 1 files.” In a press release from W3C in January 2016, World Wide Web Consortium (W3C) to Receive Emmy ® Award for Standards Work on Accessible Video Captioning and Subtitles. The category recognized TTML’s standard and how it “helps ensure that the needs of people with disabilities, particularly people who are deaf or hard of hearing, are addressed.” Liam Moran in the featured article on StreamingMedia.com titled, The Complete Guide to Closed Captions, April 2022, “But TTML is on a major comeback campaign. The international ATSC 3.0 broadcast standard adopted TTML as its mandatory captioning standard, specifically the IMSC1 profile that defines the subset of the TTML specification required for captions and subtitles. ATSC 3.0 went nationwide in South Korea in 2016, and the transition is occurring in the U.S. under the NextGen TV moniker, which is currently on the air on more than 150 TV stations in 43 cities. Apple enthusiastically added support for IMSC1/TTML captions to the HTTP Live Streaming (HLS) specification in 2017...That TTML is both the captioning standard for broadcast television and the adopted standard used by streaming industry leaders like Netflix presents an obvious benefit to content producers and points the way to a bright future for TTML.” |
| Licensing and patents |
None. The W3C Patent Policy has the goal of assuring that all W3C Recommendations can be implemented on a royalty-free basis. |
| Transparency |
Based on XML and hence readable with a basic viewer or editor, although editing software is recommended, especially for users not familiar with XML. Per TTML1 specification, M Concrete Encoding (Non-Normative): In the absence of other requirements, it is recommended that a Document Instance be concretely encoded as a well-formed XML 1.0 [XML 1.0] document using the UTF-8 character encoding. Note: When using XML 1.0 as the concrete encoding of TTML, only the following named character entities are defined: &, ', <, >, and ". |
| Self-documentation |
TTML1 specifications since Timed Text (TT) Authoring Format 1.0 define a metadata element within the header section of the TTML1 document. Document level metadata may specify a document title, description, copyright, agent, name, and actor information. Additionally, arbitrary metadata can be drawn from other specified namespaces. Per specification, the metadata element functions as a generic container and is understood to be a separable layer of information that applies to parameters, content, style, layout, timing, and metadata itself (two forms: documents specified and arbitrary). |
| External dependencies |
None beyond availability of supporting software. TTML1 specification Interoperability Considerations: |
| Technical protection considerations |
TTML1 specification Security Considerations: “As with other XML types and as noted in IETF RFC 7303, XML Media Types, Section 10, repeated expansion of maliciously constructed XML entities can be used to consume large amounts of memory, which may cause XML processors in constrained
environments to fail. |
| Text | |
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| Normal rendering |
Good support. Per the specification, TTML1 text files have a minimum character representability of Unicode 4.0, using UTF-8 character encoding. W3C Timed Text documentation TTML and Derivative Captions Formats describes TTML as content elements are HTML-like elements (such as <div>, <p>, <span>) and style elements are specified via a simple XML-based styling system. |
| Integrity of document structure |
Some support. XML-based which is ideal for document structure. SpeechPad.com, a captioning video service, in their TTML information page, state “TTML is very similar to HTML, in that it consists of a collection of nested structural elements, with tags to mark the beginning and end of each element.” |
| Integrity of layout and display |
Some support. TTML1 files support optional styling and layout layers. As described in the specification, the layers are considered independently separable; layout specifies where the author intends for the content to be presented; and style specifies how the content will be presented in the layout, referenced by the layout and content information. Styling and layout elements may rely on supportive software, where one rendering program supports more options than another. W3C Timed Text documentation TTML and Derivative Captions Formats states, “There is partial and full support of TTML in components used by several Web browsers agents, and in a number of caption authoring tools.” The TTML1 specification states, “Styling attributes are included in TTML to enable authorial intent of presentation to be included within a self-contained document...TTML Processors are not required to present Document Instances in any particular way; but an implementation of this model by a TTML Presentation Processor that provides externally observable results that are consistent with this model is likely to lead to a user experience that closely resembles the experience intended by the documents' authors.” |
| Support for mathematics, formulae, etc. |
Low Support. Little to no information on TTML1’s support of mathematics, chemical formulae, diagrams, etc. |
| Functionality beyond normal rendering |
Some support. The TTML specification mentions that certain properties of TTML support streamability of content, providing some useful streaming scenarios, but it is also non-normative. TTML specification notes the set element specified for animation aspects, such as expressing a discrete change in some style parameters applied over a time interval. Semantics for the set element are based on the defined SVG 1.1 specification. |
| Tag | Value | Note |
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| Filename extension | ttml |
Per specification. See https://www.w3.org/TR/ttml1/#media-types-registration. |
| Filename extension | dfxp |
Per specification, previous drafts referred to TTML as DFXP (Distribution Format Exchange Profile), the term is retained for historical reasons in certain contexts, such as profiles and designators. See https://www.w3.org/TR/ttml1/#intro. TTML's Wikipedia page lists this as an extension. According to Rudolf Boogerman in How to Create Captions/Subtitles for Video and Audio in WebVTT, SRT, DFXP Format, Updated March 2019, users can “save the file with an .dfxp extension, like mycaption.dfxp, but you also may use mycaption.xml since this basically is a XML file.” See Notes for more information found on file extensions. Comments welcome. |
| Filename extension | xml |
TTML's Wikipedia page lists this as an extension. According to Rudolf Boogerman in How to Create Captions/Subtitles for Video and Audio in WebVTT, SRT, DFXP Format, Updated March 2019, users can “save the file with an .dfxp extension, like mycaption.dfxp, but you also may use mycaption.xml since this basically is a XML file.” See Notes for more information found on file extensions. Comments welcome. |
| Internet Media Type | application/ttml+xml |
Per specification. See https://www.w3.org/TR/ttml1/#media-types-registration |
| Mac OS file type | TTML |
See https://www.w3.org/TR/2018/REC-ttml1-20181108/ |
| Pronom PUID | See note. | PRONOM has no corresponding entry as of April 2023. |
| Wikidata Title ID | Q25339304 |
Wikidata entry for Timed Text Markup Language. See https://www.wikidata.org/wiki/Q25339304. Wikidata does not specify a version of TTML. |
| General |
Timed Text Working Group The TTWG maintains the Time Text wiki that includes TTWG’s list of known implementations, as well as publications and other resources.
- According to the Vantage’s (media processing platform) Timed Text Flip Application Note, “Essentially, all TTML files are XML's. However, packaging of video programming with side car TTML caption/subtitle files need to meet the required specifications to be accepted by a video programming distributor. Therefore, Timed Text Flip has the following bindable options:
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| History |
W3C announced the creation of the Timed Text Working Group (TTWG) in early 2003, as part of the Synchronized Multimedia Activity (SYMM), designers of the Synchronized Multimedia Integration Language (SMIL) (fdd000572), a markup language used for authoring multimedia presentations. The TTWG’s mission was to develop an XML-based format used to represent streamable text synchronized with other time media, i.e., audio or video. At the time there was no standard method of displaying text synchronized to another element on the Web. The three most popular media players, Apple’s QuickTime Player, Microsoft’s Media Player, and RealNetwork’s RealPlayer, all supported their own proprietary text formats, meaning media authors had to create multiple files to support more than one player. Multiple incompatible captioning and subtitle formats created issues for interoperable presentations, but also increased the cost of captioning Web content. It was determined that a standardized timed-text format could reduce duplicate work, but also simplify creating and distributing synchronized text across multiple devices, such as media players, but also caption encoders, character generators, and other text-display devices. The TTWG included vendors from the streaming multimedia technologies, web browser companies, and representatives from the accessibility communities, working on the goal to harmonize one or more of the existing formats, but their focus was on the development of an XML-based representation, incorporating all the functionality of the existing formats to become the base standard interchange format. The base standard was called Timed Text Authoring Format (TT AF) Distribution Format Exchange Profile (DFXP), released in 2004, and was intended to cover all aspects of timed text on the Web and to become the standard captioning interchange format. The TT AF defined timed text representation for interchanging between systems, while the DFXP document specified vocabulary, semantics, and styling for TT AF, with its purpose for exchanging timed text information among legacy distribution formats. The Time Text (TT) Authoring Format – Distribution Format Exchange Profile (DFXP) W3C Recommendation was released in 2009. In February 2010, the TTWG changed the name of TTAF DFXP and released a new specification called Time Text Markup Language (TTML) 1.0 W3C Candidate Recommendation. The TTML standard framework addressed requirements from three major groups, web developers, movie producers, and live broadcasters. One standard would not satisfy all of the needs of each group, leading to TTML being developed as a base standard, defining features needed for captioning and new Profiles could be defined that incorporated groups of these features. Profiles are based on the main specification, but are developed with certain constraints, extensions, or both depending on the application. TTML is a complex but flexible caption format used for authoring, interchange, and distribution cases. TTML has two versions of the TTML specification, TTML1 (described here) and TTML2 (fdd000588).
- Timed Text Markup Language 1 (TTML1) (Third Edition) | W3C Recommendation 08 November 2018, edited in place 20 August 2020 is the most current specification and is maintained by the W3C’s Timed Text Working Group (TTWG). - Participating Organizations of the Timed Text Working Group (TTWG) include: Adobe, Apple Inc., Brightcove, British Broadcasting Corporation, ERICSSON, Google LLC, Igalia, Kodansha, Publishers, Ltd., MovieLabs, Mux, and Netflix. |
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