StillreelAuto captions on a Mac, without uploading your video
Most caption tools send your video to a server and charge by the minute. Your Mac can do the speech recognition itself. Here is how that works, and how accurate it is.
Why on-device?
Web caption tools upload the whole video, then sell transcription time as minutes or credits. That is slow on a big file and adds up if you post every day, and some people simply don't want unreleased footage, client work or family video on someone else's server.
An Apple silicon Mac is fast enough to run a speech model locally. Stillreel uses whisper.cpp, an open-source (MIT-licensed) engine for OpenAI's Whisper models, so the audio never leaves your Mac and there are no minutes to buy.
How to auto-caption a video with Stillreel
- Drop in an MP4, MOV or M4V file. Phone footage with a variable frame rate is fine; it is converted to a constant frame rate on export.
- Pick the spoken language, or Detect language, and click Transcribe.
- Captions appear as cues on their own track under the video. Click a cue, correct its text and click Apply text; the words are re-spaced across the cue for you.
- Choose a style and position, then export a captioned MP4, or an SRT and VTT file, or both.
How accurate is it?
We measured the bundled Whisper model against public recordings of real people with published word timings: 40 English audiobook passages (LibriSpeech test-clean, 1,009 words) and 30 German sentences (FLEURS).
| Test audio | Words right | Word timing |
|---|---|---|
| English, read speech (real voices) | about 96% (4.2% word error rate) | Median 55 ms off; 71% of words within 0.1 s; 90% within about 0.2 s |
| German, read speech (real voices) | about 84–90% (10–16% word error rate, depending on the sentences) | Not measured (no reference timings available) |
In practice: captions usually land on or just around the word, and a few in each clip may start a couple of tenths of a second early or late. That is why every caption can be nudged by one or five frames, or have its start or end moved, before you export. We haven't measured strong accents, noisy rooms or overlapping speakers, so check a short clip first.
Which languages?
The language menu offers English, Spanish, French, German, Italian, Portuguese and Japanese, plus Detect language. We have measured English and German with real voices, and checked French only with a synthetic voice; the others use the same Whisper multilingual model but we haven't measured them, so check a short clip before relying on them.
Other ways to caption on a Mac
- Final Cut Pro ($299.99) has Transcribe to Captions, which Apple documents as English-only.
- iMovie has no automatic captions; you type titles by hand.
- Transcription apps such as MacWhisper also run Whisper locally and write subtitle files; to burn captions into a video you need a second tool.
Stillreel does the whole loop in one place: transcribe, correct, style, frame for 9:16, and export.
Do you need an internet connection?
No. The model ships inside the app, so transcription works offline, on a plane or with Wi-Fi switched off.
Try Stillreel free for 14 days
Every feature, no watermark, nothing uploaded. Then $24 (launch price until 2026-11-30, then $29) once if you keep it.
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