Module 1.2: Caption File Formats: SRT, VTT, SCC and When to Use Each

When you're working with video captions, you'll encounter three main file formats: SRT, VTT, and SCC. Understanding which format to use helps you meet accessibility requirements efficiently and avoid technical problems.

What caption files contain

Caption files include timing codes that tell the video player when each caption appears and disappears, the caption text, and a structure that video players can interpret. Different platforms prefer different formats, but they serve the same purpose.

SRT: the universal standard

SRT (SubRip Subtitle format) is the most widely supported caption format. Each caption includes a sequence number, time codes showing when the caption starts and ends (using commas like 00:00:01,000), caption text, and a blank line separator.

SRT works with almost every platform—YouTube, Vimeo, social media, and local video players. However, SRT doesn't support text formatting, color changes, or custom caption positioning. For basic accessibility compliance, these limitations don't matter.

VTT: the modern alternative

VTT (Web Video Text Tracks) is a newer format developed for HTML5 video. A VTT file must start with "WEBVTT" on the first line. Time codes use periods instead of commas (like 00:00:01.000), and sequence numbers are optional.

VTT supports text styling, caption positioning, and metadata that SRT doesn't. It works natively with HTML5 video without extra plugins. Some older platforms don't support VTT yet, which is why SRT remains more universal.

SCC: the broadcast standard

SCC (Scenarist Closed Caption format) was developed for broadcast television. While SRT and VTT are for web video, SCC was built for television broadcast systems. Many government entities use SCC for cable television distribution.

SCC files use hexadecimal code (computer-readable numbers and letters) representing Line 21 closed caption data. The format includes caption text, timing, positioning, text styling (color, italics, underline), and control codes. Time codes use SMPTE timecode format (hours:minutes:seconds:frames), making SCC files difficult to edit without specialized software.

Use SCC for broadcast television, cable access channels, or professional video workflows. SCC is also valuable for archives needing format flexibility.

SCC has limitations for web use—it doesn't work with HTML5 video players without conversion, and most social platforms don't accept SCC directly. For web-only distribution, use SRT or VTT.

Choosing the right format

Use SRT for web platforms, offline players, or distributing files to various devices. SRT ensures universal web compatibility.

Use VTT for HTML5 video on websites, caption styling, or positioning captions to indicate speakers.

Use SCC for broadcast television, cable access channels, or workflows requiring this format.

Maintain multiple formats based on distribution channels. SRT-to-VTT conversion is straightforward. SCC conversion requires specialized software.

Format comparison

Platform compatibility guide

YouTube and Vimeo accept SRT, VTT, and SCC, converting them automatically.

Social media platforms (Facebook, LinkedIn, Twitter) support SRT universally. Some accept VTT. Most require SCC conversion before upload.

HTML5 video players work best with VTT. SRT works with JavaScript caption libraries. SCC requires conversion for web playback.

Streaming platforms like Zoom and Microsoft Teams accept SRT. Some professional solutions support SCC.

Broadcast television and cable access typically require SCC. Check with your cable provider about format requirements.

Government accessibility requirements don't specify a format. All three meet WCAG 2.1 Level AA when captions are accurate and synchronized.

Working with multiple formats

Most caption editing tools support SRT and VTT. Professional video software supports SCC.

SRT-to-VTT conversion uses free online tools, preserving timing and text.

SCC conversion requires specialized software. Converting from SCC may lose styling information. Converting to SCC may require manually adding styling.

For video libraries, consider distribution channels. Web-only needs SRT and VTT. Broadcast or cable access requires SCC versions.

Technical considerations

SRT and VTT are plain text files editable with any text editor. SCC uses hexadecimal encoding and requires specialized software.

Save SRT and VTT files using UTF-8 encoding to ensure special characters display correctly. SCC has its own encoding system.

File extensions identify formats automatically: .srt, .vtt, and .scc. Never change these extensions.

For SCC files, frame rate matters. SCC files are tied to specific video frame rates (typically 29.97 or 30 fps). Mismatched frame rates cause caption sync issues.

MediaScribe integration

MediaScribe generates captions in SRT, VTT, and SCC formats automatically. When you capture live captions, the Gateway Appliance stores them in its native format, then allows export in any format based on your distribution needs.

The 'Export Options' screen lets you select your preferred format. Export the same caption file multiple times in different formats for web, social media, and cable television distribution.

For live streaming to websites, MediaScribe uses VTT for seamless HTML5 integration. For cable access channels, it generates SCC for broadcast equipment compatibility. For archival and maximum compatibility, it maintains SRT versions.

Key takeaways:

• SRT is the universal web standard with the widest platform compatibility

• VTT offers advanced features like styling and positioning for modern HTML5 video

• SCC is essential for broadcast television and cable access channels

• All three formats meet WCAG 2.1 Level AA accessibility requirements

• Web-only entities typically need SRT and VTT versions

• Organizations with cable television distribution should maintain SCC versions

• Format choice depends on distribution channels, not compliance requirements