Amazon Polly
Developer Guide

Speech Marks

Speech marks are metadata that describe the speech that you synthesize, such as where a sentence or word starts and ends in the audio stream. When you request speech marks for your text, Amazon Polly returns this metadata instead of synthesized speech. By using speech marks in conjunction with the synthesized speech audio stream, you can provide your applications with an enhanced visual experience.

For example, combining the metadata with the audio stream from your text can enable to you synchronize speech with facial animation (lip-syncing) or to highlight written words as they're spoken.