There are over a hundred of legitimate subtitle file formats, designed to support different applications or enabling a different range of functional requirements. In this series of articles, we explain the main characteristics of each file format. This article provides further detail about the TTML and DFXP Subtitle File Formats.
DFXP (Distribution Format Exchange Profile) and TTML (Timed Text Markup Language) are closely related, as DFXP is a specific profile of TTML.
TTML (Timed Text Markup Language)
TTML or Timed Text Markup Language is a standard developed by the the World Wide Web Consortium (W3C) in 2018 for representing timed text in online and broadcast media. TTML is a content type that represents timed text media for the purpose of interchange among authoring systems. Timed text is textual information that is intrinsically or extrinsically associated with timing information. TTML provides a general framework for defining subtitles, captions, and other timed text information. It defines a set of requirements and capabilities that any other derived standard can incorporate, in part or in whole. See Timed Text Markup Language 1 (Third Edition) for more info.
TTML Technical Characteristics
Highly flexible and extensible format, allowing for a wide range of features including complex text styling, internationalisation, and precise timing, which serves as the basis for other timed text specifications including DFXP, EBU-TT and EBU-TT-D. Supports both streaming and file-based workflows, making it versatile for different use cases.
TTML Example Use Case
Any application requiring timed text, from web video players to broadcast television and digital cinema.
TTML Document Example
See https://www.w3.org/TR/2018/REC-ttml1-20181108/#example
DFXP (Distribution Format eXchange Profile)
DFXP or Distribution Format eXchange Profile is the de facto standard for XML captions developed by the World Wide Web Consortium (W3C) in 2005, developed to unify the increasingly divergent set of existing caption formats. It is derived from TTML (Timed Text Markup Language) format, which was intended to be a meta-standard. See Distribution Format eXchange Profile for more info.
Derived standards are called “profiles.” So, profiles are essentially groups of capabilities and requirements from the underlying TTML standard. DFXP is a set of three profiles: DFXP Presentation, DFXP transform, and DFXP full. The Presentation profile is for video players, and the Transform profile is used for video editing. The Full profile includes all the features defined in the base TTML standard. Hence, the terms TTML and DFXP are often used interchangeably.
DFXP Technical Characteristics
DFXP supports colour, position and timecode with HH:MM:SS:FF and HH:MM:SS:MMM. DFXP supports language and unicode characters.
When the positioning is configured to use percentages, the value of tts:fontSize is calculated as a percentage of the row height. This in turn is calculated as the vertical height of the display in pixels divided by 15 (the default number of screen rows defined by the DFXP specification). For example, a font size of 30 pixels in a 720x576 display equates to 78% (i.e. (30 / (576/15)) * 100)
DFXP Example Use Case
While Netflix recommends a limited DFXP output that 'hard codes' position, colour, font etc for their programmes, and text colour would be default, white, only. Position is top or bottom, size is always 100%. There is limited styling support, with only italics being supported. Limecraft does not support this export format yet. For further information, please find a link to the Netflix Format.
DFXP Document Example
Key Differences Between TTML and DFXP
| Aspect | TTML | DFXP |
|---|---|---|
| Purpose | A general-purpose framework for timed text applications, allowing for extensive customization. | A streamlined profile designed for interchange and distribution of timed text content. |
| Complexity | Broad and flexible, includes advanced features like multiple profiles, styles, and extensibility. | Simplified for interoperability, focusing on essential features for exchanging subtitles and captions. |
| Customization | Fully customizable, allowing users to create profiles tailored to specific needs. | Predefined profile with limited customization to ensure compatibility across systems. |
| Use Cases | Broad use cases, including advanced broadcasting systems, web applications, and multimedia presentations. | Focused on content exchange between broadcasters, OTT platforms, and subtitling systems. |
| Adoption | Can be too broad or complex for certain scenarios. | Widely adopted for content exchange due to its simplicity and compatibility. |
| Structure | Includes features like metadata, animations, and extended styling. | A subset of TTML features, excluding advanced elements like animations. |
Current Status
- DFXP has historically been a widely used profile of TTML but is often used interchangeably with TTML in informal contexts. Over time, TTML profiles have evolved, and newer profiles like SMPTE-TT,EBU-TT, or EBU-TT-D may be preferred depending on regional or industry standards.
Limecraft Support for TTML and DFXP File Format
Limecraft supports input of subtitle files in TTML and DFXP file formats. Limecraft does not import text styling or positioning.