The Script for Closed Captions
A Caption File is the crucial sidecar file that brings closed captions and subtitles to life. While the video file contains the picture and sound, the caption file contains all the textual information and, most importantly, the precise timing instructions that tell the media player what to display and when to display it. These files are plain text, meaning they can be opened and edited in a simple text editor, but they follow a strict syntax that allows a player to parse them correctly.
The Anatomy of a Caption File
Regardless of the specific format, all caption files are built on the same fundamental structure: a sequential list of caption ‘cues’ or ‘events.’ Each cue contains three essential pieces of information:
- A Start Timecode: The exact time (usually in hours:minutes:seconds,milliseconds) when the caption should appear on screen.
- An End Timecode: The exact time when the caption should disappear from the screen.
- The Caption Text: The line or lines of dialogue and descriptive text to be displayed.
For example, an entry in the common SRT format looks like this:
1
00:00:15,500 --> 00:00:18,000
[tense music playing]
This is where we make our stand.
Common Caption and Subtitle Formats
Numerous caption file formats have been developed over the years for different platforms and purposes. A professional captioning service will typically deliver files in several of these formats to ensure broad compatibility.
| Format | File Extension | Key Characteristics |
|---|---|---|
| SRT (SubRip Text) | .srt | The simplest and most widely compatible format. Contains only basic timing and text. |
| VTT (WebVTT) | .vtt | The modern standard for HTML5 web video. An extension of SRT that allows for text styling (bold, italics), color, and positioning. |
| SCC (Scenarist) | .scc | A professional broadcast format that preserves the rich styling of traditional TV captions (CEA-608), including pop-on/paint-on styles and precise screen positioning. |
| TTML / DFXP | .ttml, .xml | Timed Text Markup Language. A powerful, XML-based format used by many major broadcasters and streaming services like Netflix. It is highly customizable and can contain multiple languages in a single file. |
In a typical post-production workflow, the final edited video is sent to a captioning vendor, who creates these files. The video and caption files are then uploaded together to the streaming platform or broadcast server, where the media player syncs them during playback for the end user.