SRT / VTT / ASS Subtitle Formats

SRT (SubRip Subtitle), VTT (WebVTT), and ASS (Advanced SubStation Alpha) are three very common subtitle formats. Below is a detailed introduction to each subtitle format, including their properties and settings.

SRT Subtitle Format

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

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, incrementing sequentially. The numbering 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 either side of the symbol, indicating the start and end times of the subtitle.

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

• Subtitle Content: Subtitle text can contain one or more lines, displayed on the video. SRT does not support formatting text, such as color, font size, etc. 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 by 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 videos and must be referenced in the <video> tag in HTML.

VTT Format Structure

VTT files are similar to SRT, but with more features. VTT files begin with WEBVTT followed by a blank line and use 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 today is <i>very, very heavy</i>.

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 role 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 -->, again with a space on either side.

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

• Subtitle Content: Subtitle text can contain HTML tags to format the text, such as bold (<b>), italics (<i>), underline (<u>), etc.

  • 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, position, etc. Styles can be defined in HTML through the <style> tag or in an external CSS file.
   • Example:

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

     Defining .red { color: red; } in HTML will display Hello, world! in red.

2. Position Information:

   • VTT supports setting the specific position of subtitles through attributes like position and line.
   • Example:

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

3. Annotations:

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

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

4. Multiple Language Support:

   • VTT can support multiple language 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.
• Styles and Positioning: Styles and positions of subtitles can be set through CSS.
• 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 Comparison

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, HTML tags like <b>, <i>
Subtitle Number	Required	Optional
Style & Position Support	Relies on player or external style files	Built-in CSS style support, position info supported
Annotations	No	Yes, NOTE annotations
Advanced Features	Basic subtitle features only	Supports karaoke, annotations, styles, etc.
Use Cases	Local video files, simple subtitle display	HTML5 video, web subtitles, complex subtitle display
Embeddable in Video	Yes	No, only usable within the <video> element in a webpage

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

Playing MP4 Videos with VTT Subtitles in a Browser

In HTML5, you can load MP4 videos using the <video> element and associate VTT subtitles with the video using the <track> element.

HTML Example:

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>VTT Subtitle Example</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 users to control video playback (play/pause, etc.).
• <source>: Defines the path and type of the video file, using MP4 here.
• <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 for English), and label gives the subtitle track a descriptive label.

Save the HTML file and the associated video and subtitle files in the same directory. Then, open the HTML file (such as index.html) in a browser to see the video player. When you click play, the subtitles will be displayed automatically (if the player supports it and the user has enabled 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, subtitles should be displayed when playing the video 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. MP4 files themselves do not contain VTT subtitle tracks. You need to use external subtitle files and associate them via the HTML5 <track> tag.

• VTT Subtitle Styles: In browsers, WebVTT subtitles can be styled to some extent using CSS. If you need to customize the appearance of subtitles, you can further modify the styles using JavaScript and CSS.

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ASS Subtitle Format

ASS (Advanced SubStation Alpha) is a feature-rich subtitle format widely used in 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 and original subtitle author.
2. [V4+ Styles]: Subtitle style definitions, 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
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 resolution of the video, indicating the display effect of the subtitles at this resolution.
• PlayDepth: The color depth of the video, generally 0.
• ScaledBorderAndShadow: Specifies whether to scale the border (Outline) and shadow (Shadow) of the subtitles according to the screen resolution. yes is yes, no is no.
• 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 reference.

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

2. Fontname: Font name.

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

3. Fontsize: Font size.

   • Example: 20, the font size is 20.

4. PrimaryColour: The main subtitle color, which represents 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 not bold.

9. Italic: Italic setting.

   • Example: 0 means not 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, using numbers 1-9 to define different alignment positions.

    • Example: 2, means the subtitle is centered.

    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 timestamps, subtitle content, and the used styles.

[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: The layer, controls the stacking order of subtitles, the larger the number, the higher the layer.

   • Example: 0, means the default layer.

2. Start: The subtitle start time, in the format hours:minutes:seconds.milliseconds.

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

3. End: The 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, using the style named Default.

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

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

7. Effect: Subtitle effect, usually used for karaoke subtitles, etc.

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

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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 meaning is 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

• Fully 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).

• Fully 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. Fully Transparent Red:

   &H00FF0000

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

2. Fully Opaque Green:

   &HFF00FF00

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

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