Video caption generator

Captions come out in short lines of up to about 32 characters, each on screen for no more than about 3 seconds, which is roughly what fits in two lines on a vertical video. A new line starts at the end of a sentence or at a pause, and a long sentence is split after a comma or between phrases rather than straight after “the” or “to”. The speech model is Whisper, released by OpenAI under the MIT licence; Fast is its tiny version (39 million parameters) and Better is base (74 million).

Clear speech in a quiet room mostly comes out right, with the odd slip. Small models stumble on brand names, usernames, slang, numbers said quickly and words sung over music, so read every line once before you post. OpenAI’s model card says Whisper performs unevenly across languages, with lower accuracy where it had less training data, and these small models show it. In a quick check with computer-generated voices, Better got all 28 words of a Spanish sentence right and Fast got about a third wrong. Hindi went worse: Fast turned it into rough English, and Better managed a few words in Urdu and Latin letters before getting stuck. For Hindi, treat the captions as a draft to retype.

Tip: If your Reel ends with “comment GUIDE”, make sure that word sits in its own caption line. People watching with the sound off then know exactly what to type.

Burned-in captions are part of the picture. They show for everyone, wherever the video ends up: on the platform you post to, in a downloaded copy, in another app. Nobody can switch them off, and nobody can edit them later. That makes them the dependable choice when people may watch with the sound off, which is the reason Meta’s Business Help Centre article “Add captions to your video ad” (facebook.com/business/help/1675722002698686) gives for adding captions at all.

An SRT or VTT file keeps the words separate from the video, as numbered lines with start and end times. Many video editors can import one and restyle it. Meta Ads Manager takes an SRT file for the captions of a video ad on Facebook and Instagram, and wants the file name to end in a language and country code, such as ice_cream_ad.en_US.srt, or the captions won’t upload. VTT is the version web players use.

• Burning records the video again in real time, so a 90-second Reel takes about 90 seconds and plays with sound while it records. Keep the tab in front: browsers stop drawing video in background tabs.
• Chrome (version 126 and later) and browsers built on the same engine, such as Edge, save an MP4 with H.264 video and AAC sound, which Meta’s Reels specifications for Instagram list. A browser that can only record WebM saves WebM, which you may need to convert before posting.
• Safari can’t burn captions in: the recording relies on capturing the playing video as a stream, which Safari doesn’t support, and almost every browser on iPhone and iPad runs on Safari’s engine. The SRT and VTT downloads work everywhere.

• Videos up to 3 minutes and 600 MB can be captioned, and up to 2 minutes can be burned in. For longer talks, the video to text tool transcribes up to 20 minutes.
• The sound is read by your browser’s own decoder, so MP4, MOV and WebM all work as long as your browser can play them. iPhone videos in HEVC need a browser that plays HEVC.
• Your video stays on your device. The only download is the speech model, which comes from Hugging Face once and is kept by your browser for next time.
• Burned videos are capped at 1920 pixels on the long side and 30 frames per second, which keeps the recording smooth on ordinary laptops.