How to add captions to a video on your Mac without uploading it

What are on-device captions?

Captions made entirely on your own computer: the transcription, the timing of every word, the styling and the render. Most caption tools work online instead. You upload the video, their servers transcribe it, and you pay by the minute or by the video, with a cap on length.

On-device captions skip all of that. The video stays on your Mac, a two-hour podcast costs the same as a 30-second short, and there is no queue.

How accurate are captions made on a Mac?

As accurate as the transcription underneath them, which is why Sharpy uses two. Both engines run on your Mac and vote on every word, so where one mishears, the other usually has it. Then an aligner places each word at the moment it is spoken, quickly enough to time 405 words in under four seconds, so a karaoke caption lights up on the syllable, not near it.

Names, brands and jargon are where any transcription slips. Sharpy lists the words its two engines disagree on, so you check a handful instead of reading the whole transcript, and a word you correct is correct in every caption format.

What kinds of captions can you make?

Every kind a video needs, from the same transcript:

Burned-in captions come in a library of styles, from a clean classic line to bold word-by-word karaoke, and they add sound descriptions such as [MUSIC] and [LAUGHTER] where the audio carries meaning. If you gave Sharpy a reference video, it matches where the reference places its captions.

How do you add captions to a video on your Mac?

With Sharpy, as part of an edit or on their own:

1. Add the video. A finished video, or the raw footage Sharpy is editing.
2. Let it transcribe. Two engines on your Mac vote on every word, and each word is timed to where it is spoken.
3. Confirm the doubtful words. Sharpy shows you the few words its engines disagreed on; your answer carries into every format.
4. Choose the captions. Pick a style for burned-in captions, and choose files, tracks or closed captions as well.
5. Add languages if you need them. Sharpy translates on your Mac and adds a subtitle track for each language.
6. Render. Sharpy reads the captions back from the finished file, so what you deliver is what it checked.

Captions are one part of what Sharpy does to a recording; removing filler words and dead air usually comes first, and the captions follow the cleaned-up timing.

Can you translate captions on your Mac?

Yes. Sharpy translates with Apple's translation on the Mac, so the words never leave it, and it labels the result as machine translation, which is what it is. Each language becomes its own subtitle track inside the video, or its own SRT and WebVTT files beside it.

Do the captions follow broadcast and streaming rules?

They can be checked against them. Sharpy checks captions against the published caption guidelines of Netflix, Prime Video, the BBC, DCMP and ESIST, and tells you where a caption breaks a rule before you deliver it.