Edit, validate, synchronize, compare, and convert subtitle files locally in the browser. Subtitle Tools is an independently versioned application in the add·ideas Toolbox.

Version 0.1 scope

Bounded SRT, WebVTT, ASS and legacy SSA-compatible dialogue parsing
Cue timeline, direct text/timestamp editing and configurable diagnostics for overlaps, order, duration, reading speed, line count and line length
Shift and anchor-based stretch operations, negative-time protection and explicit source/target frame-rate conversion
SRT, WebVTT and ASS serialization with format-specific preservation where possible and a visible cross-format loss warning
Deterministic index-aligned revision comparison
Immediate browser-native audio/video playback plus bounded local waveform peak extraction and subtitle-start markers
UTF-8, UTF-16 LE/BE and ISO-8859-1 file decoding

All source, media and generated output stays in the current browser. The app makes no external request. Validation is configurable editorial guidance, not a broadcast, accessibility, or legal compliance certificate. Version 0.1 does not perform speech recognition, automatic alignment, semantic revision matching, or pixel-perfect ASS rendering.

Development

Requires Node.js 22 and npm 11.

npm ci
npm run check
npm run test:browser

Release

npm run release:artifact creates the deterministic release/subtitle-tools-0.1.0.zip and checksum sidecar.

Licence

GPL-3.0-or-later. Runtime dependencies retain their own licences; see THIRD_PARTY_NOTICES.md.