Engine

A TTS Engine is software that actually performs conversion of text to audio.  Engines are supplied by various vendors to be plugged into programs like TextAloud, and other applications that utilize Text-To-Speech.  The developer of an engine will typically offer many voices to choose from, but all the voices will use the vendor's engine for the text to audio conversion.

Converting text to audio is a complex process, and engines use different techniques to accomplish it.  For this reason, engines vary widely in the quality of audio produced and system resource (cpu and memory) requirements.