What did Hawking use as his speech synthesizer?

Stephen Hawking used a DECtalk speech synthesizer to produce his famous computer-generated voice. DECtalk was a text-to-speech system developed by Digital Equipment Corporation and based heavily on research by speech scientist Dennis Klatt.

After Hawking lost the ability to speak following an emergency tracheotomy, he selected words and phrases with a switch connected to a computer. The system sent the selected text to the synthesizer, which spoke in the distinctive American-accented voice that became closely associated with him. He kept that voice even when newer alternatives became available because he identified with it.

The hardware changed over time. Hawking’s system was later adapted to use cheek movements, and predictive software helped him communicate more efficiently. His equipment was sometimes described generally as a speech-generating device, but DECtalk is the canonical answer to this trivia question. Another common mix-up is assuming Siri, Cortana, or IBM ViaVoice supplied the iconic voice; none of those was the original system.