There are over a hundred legitimate subtitle file formats, each designed to support different types of applications and functional requirements. In this series of articles, we explain the main characteristics of each subtitle file format. This article provides further detail about the EBU STL subtitle file format.

The EBU STL (Standard Transmission Format for Subtitles) is a file format standardised by the European Broadcasting Union (EBU) for the exchange of subtitle data between systems. It has been widely adopted in the broadcast industry due to its support for rich formatting, positioning, and multi-language subtitles.


Full specification via the EBU website (https://tech.ebu.ch/docs/tech/tech3264.pdf).

TABLE OF CONTENTS

Structure of an STL File


EBU STL files are binary files containing both metadata and subtitle data. They are structured into a series of blocks, each serving a specific purpose. This structure ensures compatibility and efficient processing in professional broadcast environments.


Key Components of an STL File


  1. File Header
    Contains metadata about the file, such as the title, creation date, and the character code table used (e.g., Latin, Cyrillic).

  2. Timecode Information
    Specifies the frame rate and the start of program timecode (e.g., 25fps for PAL or 29.97fps for NTSC).

  3. Subtitle Blocks
    Each subtitle block contains:
    • Start Timecode: When the subtitle should appear.
    • End Timecode: When the subtitle should disappear.
    • Subtitle Text: The text to display, including formatting and control characters.
    • Positioning: Row and column information for precise on-screen placement.


Example STL File Content


Although STL files are binary, the structure can be visualized as follows:


Although STL files are binary, this infograph illustrates the structure of an STL file


Features and Limitations of EBU STL

Features:

  • Rich Formatting: Supports bold, italics, and underline text styles.

  • Positioning: Allows precise placement of subtitles on the screen using rows and columns.

  • Multi-Language Support: Facilitates subtitle data in multiple languages within the same file.

  • Frame Rate Flexibility: Accommodates various frame rates, making it suitable for global broadcast standards.

  • Timecode Precision: Frame-accurate timecode ensures precise timing.

Limitations:

  • Binary Format: Requires specialized software for editing and viewing.

  • Complexity: The structured format can be challenging to manipulate without professional tools.

  • No Multimedia Enhancements: Lacks support for modern multimedia features, such as animated or styled text beyond basic formatting.


Limecraft Support for EBU STL Files

Limecraft fully supports EBU STL files for both import and export, ensuring seamless integration into professional media workflows.

Import:

  • Limecraft parses STL files to extract subtitle data, timing, and metadata.

  • Formatting and positioning details are retained for accurate reproduction.

Export:

  • Limecraft generates STL files with precise timing, formatting, and metadata, ensuring compatibility with broadcast systems.

  • Multi-language subtitle blocks can be exported as a single STL file or split into separate files as needed.


Please note that due to the binary nature of STL files, editing these files directly is not recommended. Limecraft provides tools and interfaces to handle STL files efficiently and accurately.