When I first heard the synthetic voice that Stephen Hawking used as a communication aid, I said “I know that voice”. And indeed the voice was known to anyone who worked in speech research in the 1970’s and 1980’s, because it was the voice of Dennis Klatt.
Dennis Klatt, MIT research scientist from 1965 until 1988, specialised in speech synthesis. In the 1960’s it took hours of computing to form a single sentence. The process started with having data on the individual sounds of speech and then modifying that data to form a synthetic utterance. This was ‘phoneme synthesis’, and the starting point was enough knowledge of speech to write down, like Professor Henry Higgens in Pygmalion (My Fair Lady), an utterance in a phonetic script.
Dennis Klatt spent years making measurements on his own voice to produce, in the end, an advance on the state of the art in terms of natural sounding speech. Indeed, it was so natural that it sounded like Dennis himself: Klattalk. Then he added ‘text to speech’ rules, so the synthesiser could, at long last, be controlled by ordinary text rather than by phonetic symbols. The full system was described in an IEEE article in 1982, by which time Dennis was already suffering from thyroid cancer — and losing his voice!
I could recognise his voice because I saw him frequently at Acoustical Society of America meetings during 1967 to 1974 (when I left the USA to come to England). He took an interest in me, a very junior researcher, because we had both done graduate study in the department of computer and communication sciences at the University of Michigan, though he was there several years before me. I was struggling to learn as much as I could about speech, particularly synthesis, and so we had several discussions — certainly enough so that when I first heard Stephen Hawking’s synthetic voice in the 1990’s I knew that it was Klattalk, and indeed it was still recognisably the voice of Dennis Klatt.
I last spoke with Dennis in 1982, at a speech conference in Paris. His larynx was already affected and his voice was hoarse. He died in 1988, but Klattalk had by that time been marketed by Digital Equipment Corporation (DEC) as DecTalk. Stephen Hawking continued to use a version of DecTalk throughout his life, because DecTalk had become identified as the voice of Stephen Hawking, though to anyone who did speech research in the 1970’s and 80’s it will remain the voice of Dennis Klatt.
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Added 6 April 2018: Stephen Hawking’s daughter Lucy Hawking did a BBC Radio 4 programme on speech synthesis and her father’s voice, including a detailed discussion with Dr Laura Fine, the daughter of Dennis Klatt.