How to use it
Add the master file
MP4, MOV, and WebM are accepted. A file over 2 GB is transcribed from its audio track.
Check the source bitrate
The studio shows the bitrate it read from the file.
Export at that rate
WebCodecs encodes the captioned frames at that constant bitrate and copies the audio when the container allows it.
Or hand the words to an NLE
Download ASS, or a transparent WebM of the captions alone. ProRes 4444 is not created in the browser.
Questions
Is the export lossless?
Burned-in captions have to be encoded again. The bitrate target is the source bitrate, so the file is not run through a low-bitrate web preset. Audio is copied when the format allows.
What if the browser cannot encode?
MP4 export then tries a local ffmpeg fallback at the same bitrate target. If that fails, the error stays on screen and the download does not pretend to succeed.
Where does the 50 Mbps number come from?
Only when the file does not expose a bitrate and the duration cannot be used to estimate one. The export tells you when that happens.