SRT / VTT / ASS Subtitle Formats

SRT (SubRip Subtitle), VTT (WebVTT), and ASS (Advanced SubStation Alpha) are three very common subtitle formats. This article provides a detailed introduction to each subtitle format, including its attributes and settings.

SRT Subtitle Format

SRT is a simple and widely used subtitle format with the extension .srt, especially popular in video players and subtitle editors. Its basic structure includes subtitle number, timestamp, and subtitle text. Subtitle attributes cannot be directly defined by SRT (e.g., color, font), and usually rely on the player's default settings or external style files for control.

SRT Format Structure

Each subtitle block in an SRT file is arranged in the following format:

1. Subtitle Number (incrementing line by line)
2. Timestamp (display start and end times, accurate to milliseconds)
3. Subtitle Content (can contain multiple lines of text)
4. A blank line (to separate subtitle blocks)

SRT Example

1
00:00:01,000 --> 00:00:04,000
Hello my friend!

2
00:00:05,000 --> 00:00:08,000
The weather is nice today, don't you think?

Detailed Explanation

• Subtitle Number: Each subtitle block has a unique number that increments sequentially. The number starts from 1 and must be an integer.

  • Example: 1

• Timestamp: The format is HH:MM:SS,mmm, where HH is hours, MM is minutes, SS is seconds, and mmm is milliseconds. The timestamp consists of two times separated by -->, with a space on each side of the symbol, indicating the start and end times of the subtitle.

  • Example: 00:00:01,000 --> 00:00:04,000

• Subtitle Content: The subtitle text can contain one or more lines, displayed on the video. SRT does not support formatting text such as color and font size. These must be defined through player settings or additional style files.

  • Example: Hello my friend!

SRT Format Limitations

• No Text Formatting: Cannot directly set colors, fonts, etc., requiring style adjustments through the player or other tools.

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VTT Subtitle Format

WebVTT (Web Video Text Tracks) is a subtitle format for HTML5 video elements, specifically designed for online video. It is more powerful than the SRT format, supporting attributes such as styles, annotations, multiple languages, and position information. The subtitle file format extension is .vtt. However, it cannot be directly embedded in the video, but must be referenced in the <video> tag in HTML.

VTT Format Structure

VTT files are similar to SRT but with more features. A VTT file starts with WEBVTT followed by a blank line, and uses a . dot symbol instead of , to separate seconds and milliseconds.

VTT Example

WEBVTT

1
00:00:01.000 --> 00:00:04.000
Hello, <b>friends!</b>

2
00:00:05.000 --> 00:00:08.000
The rain is <i>very, very heavy</i> today.

Detailed Explanation

• WEBVTT Declaration: All VTT files must start with WEBVTT to declare their file format.

  • Example: WEBVTT

• Subtitle Number: The subtitle number is optional, unlike the required number in SRT format. Its function is to distinguish the order of each subtitle, but it can be omitted in VTT.

• Timestamp: The format is HH:MM:SS.mmm, where HH is hours, MM is minutes, SS is seconds, and mmm is milliseconds. Use a . period to separate seconds and milliseconds instead of ,. The timestamp consists of two times separated by -->, with a space on each side as well.

  • Example: 00:00:01.000 --> 00:00:04.000

• Subtitle Content: The subtitle text can contain HTML tags for formatting, such as bold (<b>), italic (<i>), and underline (<u>).

  • Example:

    Hello, <b>friends!</b>

Other Features Supported by VTT

1. Styles (CSS):

   • VTT supports adjusting text styles through CSS, such as color, font size, and position. Styles can be defined in HTML through the <style> tag or an external CSS file.
   • Example:

     <c.red>Hello friends!</c>

     In HTML, define .red { color: red; }, then Hello, world! will be displayed in red.

2. Position Information:

   • VTT supports setting the specific position of the subtitle through attributes such as position and line.
   • Example:

     00:00:01.000 --> 00:00:04.000 position:90% line:10%

3. Annotations:

   • VTT supports adding comments to the file, starting with NOTE.
   • Example:

     NOTE This line is a comment and will not be displayed.

4. Multilingual Support:

   • VTT can support multilingual subtitles through metadata or the HTML5 <track> tag.

Advantages of VTT Format

• Text Formatting: Supports HTML tags for simple text formatting, such as bold and italics.
• Styling and Positioning: CSS can be used to set the style and position of subtitles.
• Annotations and Metadata: Supports adding annotation information without affecting subtitle display.
• Web Compatibility: Specifically designed for HTML5 video, suitable for web environments.

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SRT vs. VTT

Feature	SRT	VTT
File Header	None	WEBVTT followed by a blank line
Timestamp Format	HH:MM:SS,mmm, comma separates seconds and milliseconds	HH:MM:SS.mmm, period separates seconds and milliseconds
Text Formatting Support	No	Yes, supports HTML tags like <b>, <i>
Subtitle Number	Required	Optional
Style and Position Support	Depends on player or external style files	Built-in CSS style support, supports position information
Annotations	No	Supports NOTE annotations
Advanced Features Supported	Basic subtitle features only	Supports Karaoke, annotations, styles, etc.
Use Cases	Local video files, simple subtitle display	HTML5 video, online subtitles, complex subtitle display
Embedding in Video	Embeddable in video files	Not embeddable; only for use within HTML <video> element

VTT (WebVTT) subtitle format cannot be directly embedded into MP4 files, but VTT files can be associated with MP4 videos via the HTML5 <track> tag. When the MP4 is opened in a browser, these associated subtitles can be displayed normally.

Using VTT Subtitles to Play MP4 in a Browser

In HTML5, you can load an MP4 video through the <video> element and use the <track> element to associate VTT subtitles with the video.

HTML Example:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title></title>
</head>
<body>
    <video controls width="600">
        <source src="video.mp4" type="video/mp4">
        <track src="subtitles.vtt" kind="subtitles" srclang="en" label="English">
        Your browser does not support the video tag.
    </video>
</body>
</html>

HTML Element Explanation

• <video>: Used to embed the video file. The controls attribute allows the user to control video playback (play/pause, etc.).
• <source>: Defines the path and type of the video file; here, it's MP4.
• <track>: Defines the subtitle file; the src attribute points to the path of the VTT file, kind="subtitles" indicates that it is a subtitle, srclang specifies the language of the subtitle (en represents English), and label gives the subtitle track a descriptive label.

Place the HTML file and associated video and subtitle files in the same directory. Then, open the HTML file (e.g., index.html) through a browser. You will see the video player, and the subtitles will be displayed automatically when you click play (if the player supports it and the user enables subtitles).

Most modern browsers and video players support subtitle switching. You can select different subtitles (if there are multiple subtitle tracks) through the subtitle button in the video control bar.

VTT Subtitle Notes

• Browser Compatibility: Almost all modern browsers (such as Chrome, Firefox, Edge, etc.) support the <video> element and WebVTT subtitles. As long as the VTT file and MP4 file are correctly associated, the subtitles should be displayed when the video is played in the browser.

• Cannot Be Directly Embedded in MP4 Files: VTT subtitle files cannot be directly embedded into MP4 files like SRT or other subtitle formats. The MP4 file itself does not contain VTT subtitle tracks. You need to use an external subtitle file and associate it through the HTML5 <track> tag.

• VTT Subtitle Styling: In the browser, WebVTT subtitles can be styled to a certain extent using CSS. If you need to customize the appearance of the subtitles, you can further modify the style through JavaScript and CSS.

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ASS Subtitle Format

ASS (Advanced SubStation Alpha) is a feature-rich subtitle format widely used for anime, Karaoke subtitles, and other scenarios that require complex subtitle effects. It supports rich style control, including font, color, position, shadow, and outline.

Below is an example of an ASS subtitle.

[Script Info]
; Script generated by FFmpeg/Lavc60.27.100
ScriptType: v4.00+
PlayResX: 384
PlayResY: 288
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,黑体,16,&hffffff,&HFFFFFF,&h000000,&H0,0,0,0,0,100,100,0,0,1,1,0,2,10,10,10,1
[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:01.95,0:00:04.93,Default,,0,0,0,,This is an ancient galaxy,
Dialogue: 0,0:00:05.42,0:00:08.92,Default,,0,0,0,,We have been observing it for several years,
Dialogue: 0,0:00:09.38,0:00:13.32,Default,,0,0,0,,The Webb Telescope recently sent back many previously undiscovered photos.

ASS Subtitle Structure

A standard ASS subtitle file contains several parts:

1. [Script Info]: Basic information about the script, such as title, original subtitle author, etc.
2. [V4+ Styles]: Subtitle style definitions, where each style can be referenced by different subtitle lines.
3. [Events]: Actual subtitle events, defining the appearance time, disappearance time, and specific content of the subtitles.

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1. [Script Info] Section

This section contains the metadata of the subtitle file, defining some basic information about the subtitle.

[Script Info]
Title: Subtitle Title
date: 2024-01-22 14:33:00
description: 
Original Script: Subtitle Author
ScriptType: v4.00+
PlayDepth: 0
PlayResX: 1920
PlayResY: 1080
ScaledBorderAndShadow: yes
YCbCr Matrix: None

• Title: The title of the subtitle file.
• Original Script: The author information of the original subtitle.
• ScriptType: Defines the script version, usually v4.00+.
• PlayResX and PlayResY: Define the video resolution, indicating the display effect of the subtitle at that resolution.
• PlayDepth: The color depth of the video, generally 0.
• ScaledBorderAndShadow: Specifies whether to scale the subtitle's border (Outline) and shadow (Shadow) according to the screen resolution. yes for scaling, no for no scaling.
• YCbCr Matrix: Specifies the YCbCr matrix used for color conversion. In video processing and subtitle rendering, YCbCr is a color space commonly used for video encoding and decoding. This setting may affect the display effect of subtitles in different color spaces.

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2. [V4+ Styles] Section

This section defines the style of the subtitles, and each style can control the font, color, shadow, etc., of the subtitles through fields. The format is as follows:

[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,20,&H00FFFFFF,&H0000FFFF,&H00000000,&H00000000,-1,0,0,0,100,100,0,0,1,1,0,2,10,10,20,1

Field Explanation:

1. Name: The name of the style, used for referencing.

   • Example: Default, indicating this is the default style.

2. Fontname: The font name.

   • Example: Arial, the subtitle will use the Arial font.

3. Fontsize: The font size.

   • Example: 20, the font size is 20.

4. PrimaryColour: The primary subtitle color, representing the main color of the subtitle (usually the color of the displayed text).

   • Example: &H00FFFFFF, white font. The color value format is &HAABBGGRR, where AA is the transparency.

5. SecondaryColour: The secondary subtitle color, usually used for the transition color of Karaoke subtitles.

   • Example: &H0000FFFF, blue.

6. OutlineColour: The outline color.

   • Example: &H00000000, black outline.

7. BackColour: The background color, usually used in the case of BorderStyle=3 (subtitles with a background box).

   • Example: &H00000000, black background.

8. Bold: Bold setting.

   • Example: -1 means bold, 0 means non-bold.

9. Italic: Italic setting.

   • Example: 0 means non-italic, -1 means italic.

10. Underline: Underline setting.

    • Example: 0 means no underline.

11. StrikeOut: Strikethrough setting.

    • Example: 0 means no strikethrough.

12. ScaleX: Horizontal scaling ratio, 100 means normal ratio.

    • Example: 100, means no scaling.

13. ScaleY: Vertical scaling ratio.

    • Example: 100, means no scaling.

14. Spacing: Character spacing.

    • Example: 0, means no extra spacing.

15. Angle: Subtitle rotation angle.

    • Example: 0, means no rotation.

16. BorderStyle: Border style, defines whether the subtitle has an outline or background box.

    • Example: 1 means there is an outline but no background box, 3 means there is a background box.

17. Outline: Outline thickness.

    • Example: 1, means the outline thickness is 1.

18. Shadow: Shadow depth.

    • Example: 0, means no shadow.

19. Alignment: Subtitle alignment, uses numbers 1-9 to define different alignment positions.

    • Example: 2, means the subtitle is center-aligned.

    Alignment Explanation:

    • 1: Bottom left
    • 2: Bottom center
    • 3: Bottom right
    • 4: Middle left
    • 5: Center
    • 6: Middle right
    • 7: Top left
    • 8: Top center
    • 9: Top right

20. MarginL, MarginR, MarginV: Left, right, and vertical margins, in pixels.

    • Example: 10, 10, 20, means the left and right margins are 10 pixels, and the vertical margin is 20 pixels.

21. Encoding: Encoding format, 1 means ANSI encoding, 0 means default encoding.

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3. [Events] Section

This section defines the actual subtitle events, including the timestamp, subtitle content, and the style used.

[Events]
Format: Layer, Start, End, Style, Name, MarginL, MarginR, MarginV, Effect, Text
Dialogue: 0,0:00:01.00,0:00:05.00,Default,,0,0,0,,This is the first subtitle
Dialogue: 0,0:00:06.00,0:00:10.00,Default,,0,0,0,,This is the second subtitle

Field Explanation:

1. Layer: Layer, controls the stacking order of subtitles, the larger the number, the higher the layer.

   • Example: 0, means the default layer.

2. Start: Subtitle start time, the format is hours:minutes:seconds.milliseconds.

   • Example: 0:00:01.00, means the subtitle starts at 1 second.

3. End: Subtitle end time.

   • Example: 0:00:05.00, means the subtitle ends at 5 seconds.

4. Style: The name of the subtitle style used, referencing the style defined in [V4+ Styles].

   • Example: Default, uses the style named Default.

5. Name: Optional field, usually used for character name annotation.

6. MarginL, MarginR, MarginV: The left, right, and vertical margins of the subtitle, overriding the values defined in the style.

7. Effect: Subtitle effects, usually used for Karaoke subtitles, etc.

8. Text: The actual content of the subtitle, you can use ASS format control characters to achieve line breaks, special styles, and positioning, etc.

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Example Subtitle Event

Dialogue: 0,0:00:01.00,0:00:05.00,Default,,0,0,0,,{\pos(960,540)}This is the first subtitle

• {\pos(960,540)}: Controls the subtitle to be displayed at a specific position on the screen (960 pixels horizontally, 540 pixels vertically).
• This is the first subtitle: The actual subtitle text displayed.

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Color Settings in ASS

Taking &HAABBGGRR as an example, &HAABBGGRR is a hexadecimal format used to represent colors, which includes the transparency of the color and the value of the color itself. This format is used to define the color attributes of subtitles, such as PrimaryColour, OutlineColour, and BackColour.

The meanings are as follows:

• AA: Transparency (Alpha channel), indicating the transparency of the color.
• BB: Blue component.
• GG: Green component.
• RR: Red component.

The specific byte order is: Alpha (transparency) - Blue - Green - Red.

If you don't want to use transparency, you can directly ignore the value in the AA position, for example, &HBBGGRR.

Transparency and Color Values

• Completely Transparent: The color is completely transparent, that is, invisible. The representation is &H00BBGGRR, where the AA part is 00 (completely transparent).

  Example:

  &H00FFFFFF

  • Here, &H00FFFFFF represents completely transparent white. The transparency is 00 (completely transparent), and the color is FFFFFF (white).

• Completely Opaque: The color is completely opaque, that is, the color display effect is the most obvious. The representation is &HFFBBGGRR, where the AA part is FF (completely opaque).

  Example:

  &HFF000000

  • Here, &HFF000000 represents completely opaque black. The transparency is FF (completely opaque), and the color is 000000 (black).

Actual Color Examples

1. Completely Transparent Red:

   &H00FF0000

   • Transparency 00 (completely transparent), color FF0000 (red).

2. Completely Opaque Green:

   &HFF00FF00

   • Transparency FF (completely opaque), color 00FF00 (green).

• The AA part in &HAABBGGRR controls transparency, and the BB, GG, RR parts control color.
• Completely Transparent: Transparency 00, for example, &H00FF0000 represents completely transparent red.
• Completely Opaque: Transparency FF, for example, &HFFFF0000 represents completely opaque red.