In 1986, when Hawking first needed a voice synthesizer, they were commercially available. He first used a software package by Words Plus, Equalizer on a Apple II as the control system.
Sources are vague about the first synthesizer, but by 1988 he was definitely using a Speech Plus CallText 5010. Speech Plus used DECTalk 'Perfect Paul' as single voice for this unit, but had apparently been refining it for 10 years.
So, as far as I can tell, the story of Klatt and Hawking that inspired this thread is largely fiction. I've included links for further reading.
Server run by the main developers of the project It is not focused on any particular niche interest - everyone is welcome as long as you follow our code of conduct!