Comparison and Interconversion Guide for SRT, VTT, ASS Subtitle Formats

SRT, short for SubRip Text, was developed by developer TUBRI in 1999 as the output format for the SubRip subtitle extraction tool. It has no official standardization body and is a commonly used community-developed format.

VTT, short for Web Video Text Tracks, was launched by the W3C HTML5 Working Group in 2010. It is the native subtitle format specified in the HTML5 standard and has official format specifications.

ASS, short for Advanced SubStation Alpha, is an upgraded version of the SubStation Alpha format, maintained by community developers. It is currently one of the open subtitle formats with the most complete special effect capabilities.

The core difference between the timestamps of the three formats lies in the usage rules for decimal separators. SRT uses a comma to separate seconds and milliseconds, formatted as hours:minutes:seconds,milliseconds; both VTT and ASS use a period to separate seconds and milliseconds. The VTT format requires the first line to be a WEBVTT declaration, while SRT and ASS have no mandatory requirement for a leading declaration.

Styling capability presents a three-level hierarchical structure. SRT does not support any styling, and only stores plain text content and timestamps. VTT supports basic style tags that can set bold, italic, color, and positioning of text. ASS supports a complete special effect system that can implement complex effects such as karaoke gradients, scrolling subtitles, and custom borders and shadows.

Different software has varying degrees of support for the three formats. We have compiled a compatibility matrix for common software for reference. In ordinary playback scenarios, most software supports SRT, while playback of special effect subtitles requires software to support ASS functionality.

For online web playback scenarios, HTML5 natively supports only VTT format. Other formats require JavaScript transcoding before they can be loaded and displayed.

When converting from an advanced format to a lower-level format, irreversible information loss occurs. When converting ASS to SRT, all style, effect, and positioning information is cleared, and only plain text content and timestamps are retained. When converting ASS to VTT, only basic colors and bold/italic styling can be retained, and complex dynamic effects are completely lost.

When converting from a lower-level format to an advanced format, the original text and timestamp information can be fully preserved. Converting SRT to ASS or SRT to VTT does not lose the original content, and the converter can automatically complete the structural fields required by the target format. For interconversion between ASS formats, conversion between the same version can retain all effect information.

The SRT to VTT converter can complete the format structure conversion. It only needs to modify the timestamp separator and add the WEBVTT leading declaration, and will not modify the original content.