Subtitle File Formats:
A Comprehensive Overview

Subtitle file formats are essential for enhancing the accessibility and understanding of video content. Different formats offer unique features and compatibility options. In this article, we will provide a comprehensive overview of popular subtitle file types and discuss their characteristics.

Last modified on September 22, 2023
Frame from the film Lara - the gift of languages

In the dynamic world of video and media, subtitle file formats play a crucial role in ensuring accessibility and enhancing viewer experiences. With a myriad of formats available, each tailored for specific needs, this article delves into some of the most widely used subtitle formats. From the simplicity of SRT to the complex capabilities of TTML, these formats cater to diverse requirements such as compatibility with different players, support for detailed formatting, and synchronization precision. Understanding these formats is essential for anyone looking to navigate the intricacies of modern media consumption and creation.

There are two main types of formats:

Text-Based Formats: These formats, like SRT or SSA, store subtitles as plain text that you can read and even edit easily with a regular text editor. They’re commonly used in online videos and offline players, making them super accessible.

Binary Formats: Binary formats, such as IDX/SUB and STL, are a bit trickier. They store subtitles in a way that’s not human-readable without special software. You’ll find these in DVDs and Blu-ray discs, making them perfect for your movie nights.

Understanding these two categories is your ticket to mastering subtitles in today’s digital age, whether you’re a content creator or just a movie enthusiast.

Let’s explore some of the most widely used text-based subtitle formats.

Text-based subtitle formats

These text-based subtitle formats cater to various needs, from basic captioning to advanced styling and platform compatibility. Understanding the options available empowers content creators and viewers alike to make the most of subtitles in the digital age.

SubRipper (.srt)

The SubRipper format, commonly known as SRT, is widely supported by subtitle converters and players. It features a concise and easily understandable structure. When opened with a text editor, an SRT file displays the time when the text appears and the corresponding subtitles. This format is widely compatible and can be edited without difficulty.

The SRT Structure

Let’s take a closer look at how SRT works. Imagine a subtitle as a tiny snippet of text that appears on your screen when you’re watching a movie or video. SRT organizes these snippets neatly.

Each snippet in an SRT file has three main parts:

  • The Sequence Number: Think of it as a tag that says which subtitle comes first, second, third, and so on.

  • Timing Information: This part tells your video player when to display the subtitle and when to remove it. It’s like a stopwatch for your subtitles.

  • The Text: This is the actual subtitle, the words you see on the screen. It can be anything from spoken dialogue to captions.

An Example of SRT in Action:

0
00:00:00,000 --> 00:00:02,000
Subtitles created using Matesub

1
00:00:03,000 --> 00:00:08,000
Welcome to the world of subtitles.

2
00:00:10,500 --> 00:00:15,500
Matesub makes it easy to create and edit subtitles.

3
00:00:18,000 --> 00:00:23,000
With precise timing, your subtitles are seamless.

4
00:00:26,000 --> 00:00:31,000
Syncing subtitles with your video is a breeze.

5
00:00:34,000 --> 00:00:39,000
SRT, a universal format, works seamlessly with Matesub.

6
00:00:42,000 --> 00:00:48,000
Elevate your content with SRT subtitles.

7
00:00:50,000 --> 00:00:55,000
Experience the power of Matesub in enhancing your subtitles!

In this SRT example, each snippet has a sequence number, timing information, and text. When you play the video, your player reads the SRT file and displays each snippet at the right time.

The Pros of SRT Subtitles

Now that we’ve introduced you to SRT (SubRip Text) subtitles, let’s dive deeper into what makes them truly remarkable. SRT subtitles have captured the hearts of content creators and viewers alike for several compelling reasons:

Universal Compatibility: SRT subtitles are like the friendly neighborhood superhero of the subtitle world. They work seamlessly with a wide range of video players and platforms. Whether you’re watching a video on your favorite streaming service or a personal project on your computer, chances are, SRT has got you covered.

Human-Friendly Editing: Have you ever wanted to make a quick change to a subtitle? With SRT, you don’t need to be a tech wizard. SRT files are plain text, which means you can edit them with a regular text editor. It’s as simple as updating a document on your computer. No need for complicated software or special skills.

Flexibility at Your Fingertips: SRT subtitles offer a level of flexibility that’s hard to beat. You have precise control over when each subtitle appears and disappears in your video. This precision ensures that your subtitles sync perfectly with the spoken dialogue or action on screen. It’s like having a personal conductor for your subtitles, making sure they harmonize with your video’s rhythm.

Accessible to All: One of the most beautiful things about SRT subtitles is their accessibility. They bridge language barriers, making it possible for people from different parts of the world to enjoy your content. Whether you’re sharing a heartwarming story, a tutorial, or a funny moment, SRT subtitles open the doors to a global audience.

User-Friendly for Everyone: Whether you’re a seasoned content creator or just someone who loves watching videos, SRT subtitles make the experience more enjoyable. They’re there to enhance your understanding, add context, and make sure you never miss a moment.

In a world where video content knows no boundaries, SRT subtitles are your trusted companions. They ensure that your message reaches far and wide, transcending languages and cultures. So, the next time you see those neat lines of text at the bottom of your screen, know that it’s not just text—it’s the magic of SRT subtitles, making your video experience exceptional.

That’s why SRT is one of the formats fully supported by Matesub for export. So, the next time you use Matesub to create or edit subtitles, rest assured that you can export them in the user-friendly and widely compatible SRT format, ready to enhance your videos and reach a global audience.

MicroDVD (.sub)

SUB subtitles are like the reliable Swiss Army knife of subtitles. They are named after MicroDVD, the software that popularized this format. What sets SUB apart is its straightforwardness and effectiveness.

The SUB Structure

To truly appreciate SUB subtitles, it’s essential to understand their structure. Imagine each subtitle as a small puzzle piece that fits perfectly into your video. SUB organizes these pieces neatly.

Each subtitle in a SUB file consists of three key elements:

  • The Sequence Number: This serves as an indicator, telling your video player the order in which the subtitles should appear.

  • Timing Information: Just like SRT, SUB subtitles come with precise timing details. These details dictate when a subtitle should begin and end. It’s like having a conductor directing each subtitle’s entrance and exit.

  • The Text: This is the heart of the subtitle—the actual words that appear on the screen. It can be dialogues, captions, or translations.

An Example of SUB in Action:


{3000}{8000}Step into the world of subtitles.

{10500}{15500}Matesub simplifies creating and enhancing subtitles.

{18000}{23000}With precise timing, your subtitles flow seamlessly.

{26000}{31000}Syncing subtitles with your video is a walk in the park.

{34000}{39000}SRT, a universal format, pairs flawlessly with Matesub.

{42000}{48000}Take your content to new heights with SRT subtitles.

{50000}{55000}Feel the impact of Matesub in elevating your subtitles!

In this example, each subtitle consists of three parts: the sequence number, timing information, and the text. When you play the video, your player reads the SUB file and showcases each subtitle at the precise moment.

The Pros of SUB Subtitles

Having familiarized ourselves with the SUB format, let’s now explore its key advantages and why it could be a valuable choice for subtitles:

  • Precise Timing: SUB subtitles shine in the realm of synchronization. Their timing information ensures that subtitles align precisely with spoken dialogue and on-screen actions. It’s like having a conductor orchestrating each subtitle’s entrance and exit, delivering a seamless viewing experience.

  • Efficiency and Compactness: SUB files are renowned for their efficiency. They are compact, making them an ideal choice when storage space is a concern. Despite their small size, SUB subtitles pack a punch in delivering clear and effective communication.

  • Universal Appeal: SUB subtitles enjoy widespread support across various video players and platforms. This universal compatibility makes them versatile and accessible to a broad and diverse audience, transcending language and geographical boundaries.

WebVTT (.vtt)

The WebVTT format, often referred to as VTT, holds a prominent place in the world of subtitles. It’s recognized as a W3C standard, making it a dependable choice for web-based content. VTT subtitles are prized for their simplicity and compatibility, ensuring accessibility across various platforms.

Here are the key features of WebVTT:

  1. Simple Text-Based Format: WebVTT is a plain text format, making it easy to create and edit using basic text editors. It uses a straightforward structure, making it human-readable and editable without specialized software.

  2. Timestamps: WebVTT supports precise timing of captions and subtitles. Each caption or subtitle line is associated with a specific timestamp in hours, minutes, seconds, and milliseconds (HH:MM:SS.sss).

  3. Cue Settings: You can specify various settings for individual cues (captions or subtitles) using the “::cue” selector. These settings include text styling (font, color, background), positioning, and voice differentiation.

  4. Cue Styles: WebVTT allows you to define global styles or styles specific to certain cues. This enables you to customize the appearance of captions and subtitles to match your design or to distinguish between different speakers or voices.

  5. Support for Multiple Languages: WebVTT supports multiple language tracks within a single file. You can include captions or subtitles in different languages, and viewers can select their preferred language if the video player supports it.

  6. Line Breaks and Positioning: You can control line breaks in subtitles, ensuring that text is displayed in a readable manner. Additionally, you can specify the positioning of captions on the video screen.

  7. Comments: You can include comments within a WebVTT file by using lines that begin with “NOTE.” These comments are ignored by the video player and can be used for documentation or annotations.

  8. Compatibility: WebVTT is widely supported by HTML5 video players and web browsers, making it a reliable choice for adding captions and subtitles to web-based video content.

  9. Accessibility: WebVTT supports accessibility features, allowing you to provide text descriptions for audio content (audio descriptions) and textual representations of non-speech sounds (sound descriptions) to ensure accessibility for users with disabilities.

  10. Error Handling: WebVTT includes error-checking mechanisms to help identify and resolve formatting issues in the file, making it more robust.

  11. Extensibility: While WebVTT provides a standardized format, it also allows for extensions and custom cues, which can be helpful in specific use cases.

Overall, WebVTT is a versatile and user-friendly format for adding captions, subtitles, and other text tracks to web videos, making them more inclusive and accessible to a wide range of viewers.

The WebVTT Structure

The WebVTT structure is relatively straightforward and consists of key components:

  1. Header: The header section of a WebVTT file contains metadata and settings for the entire subtitle track. It begins with the keyword “WEBVTT” on the first line, which indicates that the file is using the WebVTT format. The header may also include settings for styling, positioning, and language preferences.

    Example:

    WEBVTT
    
  2. Style Block: The style block is an optional section within the header where you can define styles for captions and subtitles. These styles include font properties, colors, background, text shadow, and more. You can use the “::cue” selector to apply these styles globally to all cues or use specific selectors to target cues with specific attributes (e.g., speakers).

    Example:

    STYLE
    ::cue {
        font-family: Arial;
        color: #fff;
    }
    
  3. Cues: Cues are the individual subtitle or caption segments in a WebVTT file. Each cue starts with a timestamp indicating the cue’s start and end times, followed by the text content of the cue. Cues are separated by empty lines.

    Example:

    00:00:05.000 --> 00:00:10.000
    This is the first subtitle.
    
    00:00:15.000 --> 00:00:20.000
    This is the second subtitle.
    
  4. Comments: You can include comments within a WebVTT file to provide annotations or additional information. Comments start with the keyword “NOTE” followed by the comment text. These comments are typically ignored by video players.

    Example:

    NOTE
    This is a comment providing additional context.
    
  5. Whitespace: WebVTT allows for some flexibility in terms of whitespace. You can have multiple spaces or tabs between elements, but line breaks are significant as they separate different components (e.g., header, cues).

  6. Encoding: WebVTT files are typically encoded in UTF-8 to support various character sets and languages.

  7. Language Support: WebVTT supports the inclusion of subtitles or captions in multiple languages within a single file, allowing viewers to select their preferred language if the video player supports it.

  8. Accessibility Features: WebVTT also supports features for accessibility, including text descriptions for audio content (audio descriptions) and textual representations of non-speech sounds (sound descriptions).

In summary, a WebVTT file has a simple structure that includes a header with optional styling information, followed by individual cues that specify the timing and content of subtitles or captions. It is designed to be human-readable and easily editable with basic text editors, making it a popular choice for web-based video content.

Let’s consider a simple example thatillustrates the use of WebVTT to style and structure subtitles or captions for a video, including the differentiation of speakers (Speaker1 and Speaker2) and applying specific styles to their text:

STYLE
::cue {
    background: rgba(#000, 0.56);
    text-shadow: #000 0 0 7px;
    color: #fff;
    font-family: Arial;
    font-weight: bold;
}
::cue(v[voice="Speaker1"]) { color: #00FF00; }
::cue(v[voice="Speaker2"]) { color: #0000FF; }


1
00:00:01.785 --> 00:00:03.804 align:center line:90% position:50%
<v Speaker1>Hello, I am the Speaker1</v>

2
00:00:04.123 --> 00:00:10.602 align:center line:90% position:50%
<v Speaker2>and I am the almigthy Speaker2!</v>

3
00:00:11.102 --> 00:00:15.105 align:center line:10% position:50%
<v Speaker1>and again this is Speaker1 speaking</v>

Let’s have a look at The STYLE section (where you define the styling rules for subtitles or captions):

STYLE
::cue {
    background: rgba(#000, 0.56);
    text-shadow: #000 0 0 7px;
    color: #fff;
    font-family: Arial;
    font-weight: bold;
}
::cue(v[voice="Speaker1"]) { color: #00FF00; }
::cue(v[voice="Speaker2"]) { color: #0000FF; }
  • ::cue applies the specified styles to all cues (subtitles) by default.
  • In this example, all cues have a semi-transparent black background (rgba(#000, 0.56)), white text color (#fff), a bold Arial font (font-family: Arial; font-weight: bold), and a text shadow for readability.
  • Additionally, cues with the attribute voice="Speaker1" have green text color (color: #00FF00;), and cues with voice="Speaker2" have blue text color (color: #0000FF;).

Advanced SubStation Alpha (.ass) and SubStation Alpha (.ssa)

The Advanced SubStation Alpha (.ass) and SubStation Alpha (.ssa) subtitle formats are powerful and feature-rich options for adding subtitles and captions to video content. These formats are favored by experienced subtitlers and offer a wide range of capabilities. Both .ass and .ssa formats share similarities, and .ass is considered an enhanced version of .ssa. Here, we’ll explore the structure and advantages of these formats.

The .ass and .ssa Structure

Both .ass and .ssa formats follow a similar structure that includes key components:

  • Script Info: This section contains metadata and settings for the subtitle file, including the title, author, and various configuration options. It may specify the video resolution, default font, and more.

  • V4 Styles: .ass and .ssa formats offer extensive styling options. The V4 Styles section defines how subtitles are displayed, including font properties (typeface, size, color, bold, italic, underline), positioning, alignment, and more.

  • Events: The Events section contains the main body of the subtitles. Each subtitle event includes timing information (start and end times), layering information (important for complex formatting), and the actual text content.

An Example of .ass/.ssa in Action:

[Script Info]
; Script generated by Matesub
Title: Example Subtitles
ScriptType: v4.00+
WrapStyle: 0
ScaledBorderAndShadow: yes
YCbCr Matrix: None

[V4 Styles]
Format: Name, Fontname, Fontsize, PrimaryColour, SecondaryColour, OutlineColour, BackColour, Bold, Italic, Underline, StrikeOut, ScaleX, ScaleY, Spacing, Angle, BorderStyle, Outline, Shadow, Alignment, MarginL, MarginR, MarginV, Encoding
Style: Default, Arial, 24, &H00FFFFFF, &H000000FF, &H00000000, &H80000000, -1, 0, 0, 0, 100, 100, 0, 0, 1, 3, 2, 7, 10, 10, 10, 1

[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:00.00,0:00:03.00,Default,,0000,0000,0000,,Subtitle line 1
Dialogue: 0,0:00:04.00,0:00:08.00,Default,,0000,0000,0000,,Subtitle line 2
Dialogue: 0,0:00:09.00,0:00:12.00,Default,,0000,0000,0000,,Subtitle line 3

In this example, the .ass/.ssa format includes metadata in the Script Info section, styling information in the V4 Styles section, and the actual subtitle events in the Events section. Each event specifies timing, styling, and the text content to be displayed.

The Pros of .ass/.ssa Subtitles

1. Advanced Styling: .ass/.ssa formats provide extensive control over subtitle styling. You can specify fonts, colors, sizes, bold, italic, underline, and more for precise subtitle appearance.

2. Complex Formatting: These formats support complex formatting options, such as multiple text layers, rotation, and advanced positioning. This flexibility is particularly useful for typesetting and stylized subtitles.

3. Rich Metadata: The Script Info section allows you to include detailed metadata about the subtitle file, enhancing its documentation and organization.

4. Script Type: .ass/.ssa formats support advanced script types, making them suitable for various applications, including karaoke, complex animations, and typesetting.

5. Versatility: .ass/.ssa subtitles work well with multimedia players that support them, making them suitable for a wide range of video content.

6. Multilingual Support: These formats can handle subtitles in multiple languages within a single file, making them versatile for international audiences.

7. Precise Timing: .ass/.ssa formats enable precise control over subtitle timing, ensuring synchronization with video dialogues and actions.

8. Editability: While they are more complex than some other formats, .ass/.ssa files are still human-readable and can be edited using text editors, providing flexibility for subtitlers and content creators.

9. Community and Tool Support: There is an active community of subtitlers and tools like Aegisub that facilitate the creation and editing of .ass/.ssa subtitles.

10. Compatibility: .ass/.ssa formats are supported by various media players and multimedia applications, making them suitable for both professional and amateur subtitlers.

In conclusion, the .ass and .ssa subtitle formats offer powerful styling, formatting, and timing capabilities, making them a preferred choice for subtitlers who require advanced features and precise control over subtitle appearance. These formats are widely compatible and versatile for a range of multimedia content.

Timed Text Markup Language (TTML)

Timed Text Markup Language (TTML) is a comprehensive and standardized format for representing subtitles, captions, and other timed text in multimedia content. TTML is widely used in broadcasting, streaming, and web-based video services. It offers a rich set of features for creating accessible and styled text tracks. Here, we’ll explore the structure and advantages of TTML for subtitles and captions.

The TTML Structure

TTML documents consist of XML markup that represents timed text content. The structure of a TTML document typically includes the following components:

  • Head: The head section contains metadata and styling information for the entire TTML document. It may include details such as the language of the subtitles, styling preferences, and metadata about the content.

  • Body: The body section contains the main content of the subtitles or captions. It includes individual text elements that are associated with specific timing cues.

  • Styling: TTML allows for detailed styling of subtitles, including font family, size, color, background, positioning, and more. Styling is typically defined within the head section and can be applied to individual text elements within the body.

  • Timing Information: Each text element within the body of the TTML document is associated with timing information, specifying when the text should be displayed and when it should disappear.

An Example of TTML in Action:

<?xml version="1.0" encoding="UTF-8"?>
<tt xmlns="http://www.w3.org/ns/ttml">
  <head>
    <styling>
      <style xml:id="defaultStyle" tts:fontFamily="Arial" tts:fontSize="24px" tts:color="white"/>
    </styling>
  </head>
  <body>
    <div xml:id="subtitle1" begin="00:00:01.000" end="00:00:03.000" style="defaultStyle">
      Hello, I am the speaker.
    </div>
    <div xml:id="subtitle2" begin="00:00:04.000" end="00:00:08.000" style="defaultStyle">
      Welcome to the world of TTML.
    </div>
  </body>
</tt>

In this TTML example:

  • The <tt> element defines the TTML document.
  • The <head> section contains metadata and styling information.
  • The <body> section contains the main content, with each subtitle enclosed in a <div> element.
  • Timing information (begin and end attributes) specifies when each subtitle is displayed.
  • Styling information is defined in the <styling> section and applied to each <div> element.

The Pros of TTML Subtitles

1. Standardization: TTML is an industry-standard format with well-defined specifications, making it suitable for professional and broadcast applications.

2. Rich Styling: TTML supports extensive styling options, allowing for precise control over the appearance of subtitles, including font properties, colors, backgrounds, and positioning.

3. Internationalization: TTML provides excellent support for multilingual content and ensures accurate text rendering for various languages and scripts.

4. Accessibility: TTML supports accessibility features, such as text descriptions, ensuring that content is accessible to individuals with disabilities.

5. Timing Precision: TTML allows for precise timing control, ensuring synchronization with video dialogues and actions.

6. Versatility: TTML can be used in various scenarios, including broadcasting, streaming services, and web-based multimedia content.

7. Compatibility: TTML is supported by a wide range of multimedia players and platforms, making it a reliable choice for content distribution.

8. Community and Tool Support: There are numerous authoring tools and software applications available for creating and editing TTML subtitles, facilitating content production.

9. XML Format: Being based on XML, TTML documents are structured and machine-readable, enabling automated processing and integration with other systems.

10. Global Adoption: TTML is widely adopted by broadcasters and streaming platforms worldwide, making it a global standard for timed text representation.

In summary, TTML is a robust and versatile format for creating subtitles and captions in multimedia content. Its standardization, rich styling options, internationalization support, and accessibility features make it a preferred choice for professional and accessible video content distribution.

SAMI Format (.smi)

The SAMI (Synchronized Accessible Media Interchange) format, often referred to as SMI, is a versatile format for creating subtitles and captions in multimedia content. It is widely supported and offers a structured and easy-to-understand framework for displaying text alongside video or audio content. When viewed in a text editor, an SMI file displays the time when text appears and corresponding subtitles, similar to the SRT format.

The SMI Structure

Let’s take a closer look at how SMI works. Think of a subtitle as a brief snippet of text that appears on your screen while you’re watching a video or listening to audio. SMI organizes these snippets in a clear and structured manner.

Each snippet in an SMI file typically consists of three main parts:

  • Sequence Number: Similar to SRT, the sequence number tags each subtitle, indicating the order in which they should appear.

  • Timing Information: This section specifies when the subtitle should be displayed and when it should disappear. It serves as a time reference for your media player, ensuring precise synchronization.

  • Text Content: The text content is the actual subtitle or caption. It represents the spoken dialogue, captions, or any relevant text that accompanies the media.

Here’s an example of SMI in action:

<SYNC Start=0>
<P>In this SMI example, we'll explore the SAMI format, also known as SMI (Synchronized Accessible Media Interchange).</P>
</SYNC>

<SYNC Start=5000>
<font color="#FFFFFF"><P>SMI provides a structured and accessible way to display subtitles and captions alongside multimedia content.</P></font>
</SYNC>

<SYNC Start=10000>
<font color="#FFFF00"><P>Each SYNC element defines when a subtitle should be displayed, ensuring perfect synchronization with the media.</P></font>
</SYNC>

<SYNC Start=15000>
<font color="#FF00FF"><P>With SMI, you can convey spoken dialogues, provide translations, or add context to your videos or audio clips.</P></font>
</SYNC>

<SYNC Start=20000>
<font color="#00FFFF"><P>SMI is widely supported, making your multimedia content accessible to a global audience.</P></font>
</SYNC>

<SYNC Start=25000>
<font color="#FF0000"><P>Whether you're creating educational videos, entertainment content, or informative clips, SMI has you covered.</P></font>
</SYNC>

In this SMI example, each snippet is enclosed within <SYNC> tags, specifying the timing for when the text should be displayed. The text content is styled with font color attributes to enhance readability.

The Pros of SMI Subtitles

1. Versatile Format: SMI is a versatile format suitable for both video and audio content, making it a valuable tool for multimedia creators.

2. Standard Structure: Similar to SRT, SMI uses a standardized structure that is easy to understand and edit, even with basic text editors.

3. Precise Timing: SMI provides precise timing control, ensuring that subtitles are displayed and removed at the right moments, enhancing the viewer’s experience.

4. Accessibility: SMI supports accessibility features, making it suitable for creating content that is inclusive and accessible to individuals with disabilities.

5. Styling Options: Like the example, you can style SMI subtitles using HTML and CSS attributes, allowing for customization and improved visual appeal.

6. Global Compatibility: SMI is supported by various media players and platforms, ensuring compatibility with a wide range of devices and applications.

7. Multilingual Support: SMI can accommodate subtitles and captions in multiple languages within a single file, making it ideal for reaching a diverse audience.

8. User-Friendly Editing: SMI files are human-readable and can be edited with standard text editors, simplifying the editing process for content creators.

9. Educational Applications: SMI is commonly used in educational contexts to provide transcripts, translations, and additional context for audio and video materials.

In summary, the SAMI format (SMI) offers a structured and accessible way to display subtitles and captions alongside multimedia content. Its versatility, standardized structure, and compatibility make it a valuable choice for multimedia creators aiming to enhance the accessibility and comprehension of their videos and audio clips.

Binary subtitles formats

While text-based formats like SRT and WebVTT are popular choices for their simplicity and universality, there exists a fascinating realm of binary subtitle formats. These formats take a different approach. In this chapter, we’ll delve into the world of binary subtitle formats, exploring their unique features and applications in the world of video content.

STL (Spruce Subtitle File)

STL (Spruce Subtitle File) is a widely used binary subtitle format primarily employed in the broadcasting and video production industries. STL files contain graphical representations of subtitles in the form of bitmaps, allowing for precise styling, positioning, and timing. Here’s more information about STL:

1. Bitmap-Based Subtitles: STL subtitles are image-based, which means that each subtitle is represented as a bitmap image. These bitmap images can include text, symbols, and other graphical elements that make up the subtitles.

2. Precise Timing: STL files include timing information that specifies when each subtitle should appear and disappear during video playback. This timing information is crucial for synchronizing subtitles with the corresponding video or audio content.

3. Positioning and Styling: STL subtitles offer flexibility in terms of positioning and styling. Subtitle images can be placed at specific locations on the screen, and various font styles, colors, and sizes can be applied to enhance readability and visual appeal.

4. Compatibility: STL is a widely recognized and supported format in the broadcasting industry. It is compatible with professional broadcast equipment and video editing software used by broadcasters and post-production professionals.

5. Standardization: STL adheres to specific standards, making it suitable for television broadcast and professional video production. The format ensures consistent rendering of subtitles across different broadcasting systems.

6. Multilingual Support: STL supports multiple languages and character sets, making it suitable for international broadcasting and providing subtitles in various languages.

7. Editing: While STL files are primarily used for broadcast purposes, they can be edited using specialized software to adjust timing, positioning, and styling of subtitles when necessary.

8. Subtitle Placement: STL subtitles can be positioned at different locations on the screen, such as the top, bottom, or sides, depending on the requirements of the content and broadcasting standards.

9. Legacy Format: STL has been in use for many years and is considered a legacy format in the broadcasting industry. However, it continues to be a reliable choice for adding subtitles to television programs, documentaries, and other broadcasted content.

10. European Variant (EBU STL): The European Broadcasting Union (EBU) has its variant of STL, known as EBU STL. EBU STL adheres to European broadcasting standards and includes support for international character sets and symbols commonly used in European languages.

STL subtitles are essential for professional video production and broadcasting, ensuring that subtitles are accurately timed, visually appealing, and compliant with industry standards. While text-based subtitle formats are more common for consumer video content, STL remains a trusted format for delivering high-quality subtitles in the broadcast industry, where precision and consistency are paramount.

PAC (Presentation Audio/Video Coding)

PAC (Presentation Audio/Video Coding) is a binary subtitle format primarily used for storing and displaying subtitles in the context of DVDs (Digital Versatile Discs) and DVD-Video. PAC files contain graphical representations of subtitles in the form of bitmap images and are used to provide high-quality, visually appealing subtitles for DVD content. Here’s more information about PAC:

1. Bitmap-Based Subtitles: PAC subtitles are image-based, meaning that each subtitle is represented as a bitmap image. These bitmap images can include text, symbols, and other graphical elements that make up the subtitles.

2. High Quality: One of the key features of PAC is its ability to provide high-quality and visually appealing subtitles. The use of bitmap images allows for detailed and well-styled subtitles, including various font styles, sizes, and colors.

3. Precise Timing: PAC files include timing information that specifies when each subtitle should appear and disappear during DVD playback. This timing information ensures that subtitles are synchronized with the corresponding video or audio content.

4. Versatility: PAC subtitles are versatile and can support subtitles in multiple languages, making them suitable for international DVDs. Different language tracks can be included, allowing viewers to select their preferred language.

5. DVD Compatibility: PAC is commonly used in the context of DVDs, including DVD movies, TV series, and other video content. It is recognized by DVD players and is part of the DVD-Video standard.

6. Editing: While PAC files are primarily used for DVD production, they can be edited using specialized software to adjust timing, positioning, and styling of subtitles when necessary. This is particularly important for DVD authoring and post-production.

7. Overlaying Capability: PAC subtitles can be overlaid on the video content, ensuring that they appear seamlessly during DVD playback. This overlaying capability contributes to the overall viewing experience.

8. Advanced Styling: PAC allows for advanced styling options, including the use of different fonts, font sizes, colors, and special effects. This flexibility enables DVD producers to create subtitles that match the visual style of the content.

9. Legacy Format: PAC is considered a legacy format, primarily used for DVDs and DVD-Video. While newer formats may offer more features and flexibility, PAC continues to be used in the DVD industry.

PAC subtitles play a crucial role in enhancing the viewer’s experience when watching DVDs, ensuring that subtitles are not only informative but also visually engaging. While this format is less common in modern streaming platforms and digital media, it remains essential for DVD production, where high-quality subtitles are a key component of the content.

Other binary formats

Aside from STL and PAC, there are several other binary subtitle formats that you may consider for various purposes. These formats may vary in terms of importance or adoption depending on your specific requirements and industry. Here is a list of some binary subtitle formats in approximate order of their importance and adoption:

  1. CineCanvas: Specifically designed for digital cinema, CineCanvas provides high-quality, image-based subtitles for cinematic presentations. It’s crucial in the film industry.

  2. DVB-Subtitles: Used in digital television broadcasting, including both image-based and text-based subtitle options. DVB subtitles are prevalent in Europe and other regions.

  3. IMX Subtitles: Supported in the IMX professional video format, used in broadcast and post-production environments. It allows for image-based subtitles.

  4. OP-47 and OP-42: Formats for encoding and decoding bitmap subtitles within MPEG-2 (OP-47) and MPEG-4 (OP-42) video streams, commonly used in digital broadcasting.

  5. HDMV PGS (Presentation Graphic Stream): A format used in Blu-ray Discs, providing graphic-based subtitles. Important for authoring Blu-ray content.

  6. Teletext: A binary subtitle format used for analog and digital television broadcasting, commonly found in some regions.

  7. ATSC A/53 Part 4 Captions: Used for closed captions in ATSC digital television broadcasts in North America.

  8. CPCM: Used for bitmap-based captions in digital television broadcasting, with various regional implementations.

  9. XDS (Extended Data Services): Provides a way to deliver additional data, including captions and subtitles, in the context of television broadcasting.

The importance and adoption of these formats can vary widely based on geographic location, industry, and the specific use case. For example, STL and PAC are crucial in broadcasting and DVD production, while formats like CineCanvas are vital in digital cinema. It’s essential to consider your specific needs and the standards prevalent in your region or industry when choosing a binary subtitle format.


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This activity has received funding from the European Institute of Innovation and Technology (EIT). This body of the European Union receives support from the European Union's Horizon 2020 research and innovation programme.