A teleprompter that scrolls as you speak sounds simple: recognize speech, find the words in the script, move the text. I built that for iOS and the recognizing part was the easy part. The hard parts were making the scroll feel calm and not trusting the recognizer too much. Here is what I learned. Disclosure: this is my app, Teleprompter Offline & Private.

1. Keep recognition on the device

I use Apple's speech recognition and require on-device mode. If the language has no on-device model, the app says so instead of silently falling back to a server. That choice makes the app work in Airplane Mode and means your voice never leaves the phone. The cost: some languages need the offline speech pack, downloaded once in iOS settings.

2. Match words, not text

The recognizer returns a stream of partial transcripts. I align them against the script with a fuzzy matcher that searches a lookahead window of 40 to 70 words past the current position.…