lasub

lasub is an open standard and reference implementation for live subtitles and captions in digital media. It provides a machine-readable data format and a set of tools intended to improve interoperability between streaming services, caption editors, and video players. The project aims to complement existing formats such as WebVTT and TTML by emphasizing streaming scenarios, multilingual tracks, and accessibility features.

lasub defines a stream of cue blocks, each representing a subtitle segment with a start time, end time, and text content. A cue can also include optional metadata such as speaker identity, style hints, and environment tags. The format is designed to be forward-compatible with legacy caption formats while enabling incremental updates for live broadcasts. In typical deployments, lasub data is delivered as a lightweight payload over WebSocket or HTTP, separate from the video bitstream, and is integrated by the player at runtime.

The LASUB project originated in open-source captioning communities in the mid-2010s and has since evolved through community governance and published specifications. Implementations are available in multiple languages, including JavaScript and Python, and several media players provide experimental LASUB support, often in conjunction with WebVTT or TTML workflows. Advocates cite improved streaming performance, easier localization workflows, and better accessibility tooling as key benefits.

See also: WebVTT, TTML, SubRip (SRT).