SAPI vs 3rd Party Engines

[Welle@discussions.microsoft.com James Welle]

Hello,

I am researching text to speech technologies and I have a general question.
How are 3rd party technologies like Wizzard Speech
(http://www.wizzardsoftware.com/speech_overview.php) and NeoSpeech
(http://www.neospeech.com/default.aspx) related to the Microsoft Speech API?
Are they technologies built on top of the SAPI or are they completely
unrelated?

I downloaded two of the ATT Natural Voices and have been comparing how they
sound when I generate speech using the SAPI directly vs. using Wizzard
Speech. The output audio sounds much more natural in Wizzard Speech because
the emphasis on certain syllables is much better.

I am new to TTS technology, so I am just trying to figure out how the entire
technology stack works at this point.

Thanks.