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OpenSubs: I built a subtitle generator that runs Whisper in the browser, so the video never gets uploaded (opensubs.app)

submitted 3 weeks ago by opensubs@lemmy.world to c/opensource@lemmy.ml

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I work on OpenSubs, a free, open source (AGPL-3.0) subtitle tool that runs entirely in the browser tab.

You drop in a video file, and Whisper transcribes it on your own machine, using transformers.js with WebGPU where available and WebAssembly otherwise. The model (40–250 MB) downloads once and is cached. There is no upload endpoint in the product, so the video has nowhere to go.

After that you can:

fix lines by typing over them (click a timestamp to jump to that moment)
translate into 20 languages with Chrome's built-in on-device translator
pick one of 12 caption styles, including word-by-word highlighting
export SRT / VTT / ASS, or burn the subtitles into an MP4 (libass compiled to WebAssembly, encoded with WebCodecs)

A few things I learned building it:

Whisper hallucinates on silence and music ("Thanks for watching!", or the Japanese equivalent). A Silero VAD pass runs before Whisper, and a cleanup step drops the known stock phrases.
Singing doesn't count as speech for the VAD, so a music video gets a "no speech found" warning. You can still force it.

Honest limits: it only takes video files, not audio-only files. Cue timings can't be edited yet. Builds are release candidates. Everything that runs locally is free with no account; the only paid part is optional cloud translation on our backend (US$5 for 1000 credits), and you can bring your own Claude / OpenAI / DeepL key instead.

Site: https://opensubs.app/ Code: https://github.com/open-subs/opensubs

Feedback welcome, especially on languages where the transcription goes wrong.

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[-] opensubs@lemmy.world 1 points 2 weeks ago

Not system-wide audio, but per-tab: there's a browser extension that puts subtitles over whatever is playing in the tab, transcribed as it plays. It's in the Chrome and Firefox stores now, free and AGPL like the rest. It won't replace OBS + localvocal for anything outside the browser, but there's no filter chain to set up — install it, open the tab, hit Start.

Two things to set expectations. It runs behind the audio: it transcribes in windows, and on integrated graphics that's tens of seconds, so the overlay prints how far behind it currently is rather than pretending it's live. And it only works where the page itself plays the media — a site embedding someone else's player in an iframe, or a file served without CORS headers, can't be captured at all, and it tells you so instead of sitting there.

On permissions: it asks for no site access when you install it. When you hit Start it requests that one site, and nothing else.

this post was submitted on 21 Sep 2026

185 points (89.4% liked)

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