Subsmith

A privacy-first subtitle workbench that runs entirely in your browser.

Fix out-of-sync subtitles in seconds — shift timing, correct progressive drift with two-point sync or frame-rate conversion, repair garbled encoding, find & replace, merge split files, lint for problems, and convert between SRT, WebVTT, and SubStation Alpha (ASS). Nothing is uploaded. Everything happens on your device.

Why Subsmith?

If you watch downloaded media (Plex, Jellyfin, Kodi, VLC…), learn a language with subtitles, or do light fansubbing, you've hit this: the subtitles don't line up with the video.

• They start a few seconds early or late (constant offset).
• They slowly drift out of sync over a whole film because the subtitle was timed for a different frame rate — the classic 23.976 vs 25 fps / PAL↔NTSC problem (up to ~7 seconds of drift in a 2-hour movie).
• The text is garbled (é instead of é) because of the wrong character encoding.
• You have two files (CD1/CD2) that need merging.
• Your player wants .vtt but you have .srt or .ass.

The powerful desktop tools are Windows-centric downloads, and the web tools each fix only one of these. Subsmith does all of it in one polished page that works on macOS, Linux, Chromebook, Windows, and mobile — with nothing uploaded to any server.

Features

• Load .srt, .vtt, .ass/.ssa by drag-and-drop or file picker, with automatic format detection.
• Encoding detection & repair — auto-detects the text encoding (UTF-8/16, Windows-125x, ISO-8859-x, Big5, Shift_JIS, …), with a one-click manual override and a "fix mojibake" cleaner.
• Shift all or selected cues by any offset.
• Two-point sync — give the correct time for an early line and a late line; Subsmith solves the linear transform that fixes constant offset and progressive drift at once.
• Frame-rate conversion — presets (23.976→25, 25→23.976, 29.97→25, …) plus custom from/to fps.
• Scale timing by a percentage for fine corrections.
• Find & replace with regex (run safely off the main thread) and capture-group support.
• Clean up — strip formatting tags, trim whitespace, sort, remove empty cues.
• Merge a second subtitle file with an offset (combine CD1 + CD2).
• Validate — flags overlaps, negative/zero durations, out-of-order cues, too-short/long display times, and fast reading speed, with one-click fixes.
• Live preview — scrub a timeline and watch the active cue, or load a local video and see your edited subtitles rendered over it to verify the sync.
• Export to SRT, VTT, or ASS, with optional UTF-8 BOM.
• Undo/redo, multi-select, inline editing, and a fast virtualized list that stays smooth on files with thousands of cues.

Editor	Preview	Mobile

Privacy

Subsmith is 100% client-side. Your subtitle and video files never leave your browser — there is no backend, no upload, no account, and no analytics. Fonts are self-hosted, so the app makes no third-party network requests and works fully offline once loaded.

Getting started

Use it

Open the live app (linked from the repository's About / Releases), or run it locally:

git clone https://github.com/Skytuhua/subsmith.git
cd subsmith
npm install
npm run dev      # open the printed http://localhost:5173

Build it

npm run build    # outputs a static site to dist/
npm run preview  # serve the production build locally

The contents of dist/ are a static bundle — host them anywhere, or open them via a static server.

Run from a release artifact

Download subsmith-dist.zip from the latest release, unzip it, and serve the folder (e.g. npx serve subsmith-dist) or open it via any static server.

Usage examples

Fix a constant offset: load your .srt, open Shift timing, type -2.5 seconds (or use the quick buttons), Apply, then Export.

Fix progressive drift: open Two-point sync, type where the first line and the last line should appear (Subsmith shows their current times), Apply sync. Or, if you know the frame rates, use Frame-rate conversion (e.g. 23.976 → 25).

Repair garbled text: if a file looks wrong, change Encoding in the top bar (e.g. to Windows-1251) to re-decode it, then export as UTF-8.

Convert formats: load an .ass, choose WebVTT in the Export menu, download.

Scripts

Command	What it does
npm run dev	Start the dev server
npm run build	Type-check and build the static site to dist/
npm run preview	Serve the built site
npm test / npm run test:run	Run the unit test suite (Vitest)
npm run lint	Lint with ESLint
npm run format	Format with Prettier

Tech

Vite + React + TypeScript + Tailwind CSS. Encoding detection via jschardet; cue-list virtualization via @tanstack/react-virtual; icons from lucide-react. The subtitle engine (parsers, serializers, timing math, lint) is a pure, framework-free, unit-tested core in src/core/. See ARCHITECTURE.md.

Limitations

• No automatic audio/AI sync — by design (it would require server-side ML/ffmpeg and break the client-side guarantee). Subsmith focuses on fast, exact manual/two-point/frame-rate correction.
• Over-shifting into negative time is clamped to 00:00:00,000 on export (no subtitle format can store negative timestamps).
• Text-only formats — no image-based subtitles (PGS/VobSub) or OCR.

Contributing

Issues and pull requests are welcome. Run npm run lint && npm run test:run before submitting.

License

MIT © Skytuhua. Third-party dependency licenses are listed in THIRD_PARTY_NOTICES.md.