Critical Theory:

However, a computer expert in California, called Walt Woltosz, heard of my plight. He sent me a computer program he had written, called Equalizer. This allowed me to select words from a series of menus on the screen, by pressing a switch in my hand. The program could also be controlled by a switch, operated by head or eye movement. When I have built up what I want to say, I can send it to a speech synthesizer. At first, I just ran the Equalizer program on a desk top computer. However David Mason, of Cambridge Adaptive Communication, fitted a small portable computer and a speech synthesizer to my wheel chair. This system allowed me to communicate much better than I could before. I can manage up to 15 words a minute. I can either speak what I have written, or save it on disk. I can then print it out, or call it back, and speak it sentence by sentence. Using this system, I have written a book, and dozens of scientific papers. I have also given many scientific and popular talks. They have all been well received. I think that is in a large part due to the quality of the speech synthesizer, which is made by Speech Plus. One's voice is very important. If you have a slurred voice, people are likely to treat you as mentally deficient: Does he take sugar? This synthesizer is by far the best I have heard, because it varies the intonation, and doesn't speak like a Dalek. The only trouble is that it gives me an American accent. However, the company is working on a British version.

From Steven Hawking's web-page*.