The synthesis engine is Cepstral's Theta system. As the Speechalator runs on very small hardware devices (at least small compared to standard desktops), it was important that the synthesis footprint remained as small as possible.
The speechalator is to be used for people with little exposure to synthetic speech, and the output quality must be very high. Cepstral's unit selection voices, tailored to the domain, meet the requirements for both quality and size. Normal unit selection voices may take hundreds of megabytes, but the 11KHz voices developed by Cepstral were around 9 megabytes each.