Text to speech & highlight text on iOS - karaoke program

I have a problem with my iPhone app. I have a text file and I use text-to-speech engine to read and highlight the word on screen.

The TTS engine can read sentences like human but I don't know how to highlight the word that is reading.

If I read the text w开发者_C百科ord-by-word and highlight it, it will not natural.

(This program like karaoke, but I use human's voice instead of background music.)

Thanks so much!

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I used Karafun editor to synchronize voice and highlight text to video file but It's ugly way