Editing DECtalk Speech Synthesizer (1984) (section)

== Key Innovations ==

Dennis Klatt's pioneering work in formant-based synthesis, exemplified by DECtalk and later "KLSYN88," revolutionized text-to-speech systems. DECtalk's collection of standard voices, including the iconic "Perfect Paul," became famous. Notably used by Stephen Hawking, it served as a remarkable augmentative speaking device. DECtalk's innovation combined formant synthesis with articulatory methods, enhancing speech synthesis technology.<ref>Story B H. History of speech synthesis[J]. Teoksessa The Routledge Handbook of Phonetics, toimittaneet William F. Katz & Peter F. Assmann, 2019: 9-33.</ref>

DECtalk converts text from ASCII format to phonemic code using an internal dictionary and English pronunciation rules. Phonemic code is further processed by setting pitch and timbre. Finally, it generates speech with synthesizer control commands.<ref name=":1" />

The division in synthesis into two parts in DECTalk's BST system allows for easy configuration of synthesizers for various languages, including English, German, French, Spanish, and Japanese, by creating distinct sets of rules. The common feature is that both have used DECtalk as a formant synthesis system, providing dedicated processing modules which adapt their input according to the acoustic properties of the number of emotions.<ref>Balyan A, Agrawal S S, Dev A. Speech synthesis: a review[J]. International Journal of Engineering Research & Technology (IJERT), 2013, 2(6): 57-75.</ref>

DECtalk has made significant innovations in the field of speech synthesis. Traditional concatenation techniques often led to choppy, unnatural speech with fixed intonation. DECtalk, on the other hand, employs a digital formant synthesizer that takes phonemic symbols as input, enabling better intonation control based on clause structure and more natural coarticulation effects. This approach significantly reduces storage requirements while allowing for the generation of multiple voices.<ref name=":0" />

Earlier methods relied on word databases or formant patterns, making it difficult to adjust pitch and duration within words.<ref>D. Klatt, “Review of Text-to-Speech Conversion for

English,” ''Journal of the Acoustical Society of America,'' 

vol. 82, no. 3 (1987): 737–793. </ref> Diphone synthesis, a popular technique, used actual speech segments for improved coarticulation but suffered from inconsistent pitch and intonation. DECtalk overcame these limitations by combining phonetic rules and vocal tract parameter modifications. One point of contention was how speech samples should be returned. Some argued for in-memory buffers, placing the audio system responsibility on the application. The chosen approach, however, supports standard workstation audio and includes an API call to switch the system into a speech-to-memory mode. This decision simplifies most applications' usage, ensuring a seamless experience for users without requiring extensive code rewrites with updates.<ref name=":0" />

High intelligibility scores in standard tests have recognized DECtalk's advancements in speech synthesis. Among text-to-speech systems, DECtalk is the preferred choice for visually impaired individuals. Its compact data footprint, at less than 1.5 megabytes, exemplifies efficient innovation in delivering high-quality, intelligible speech. DECtalk Software offers three unique modes to cater to various user needs: the speech-to-wave file mode, log-file mode, and speech-to-memory mode. Each mode has paired entry and exit calls. In the speech-to-wave file mode, the system records all speech samples into a wave audio file, which is closed upon exiting. This feature is particularly handy for slower Intel systems that can't handle real-time speech synthesis. The log-file mode captures the phonemic symbol output of the LTS thread, saving it to a file. The speech-to-memory mode is another standout innovation. When in this mode, an application uses a special API call to provide memory buffers to the text-to-speech system. The system then generates synthesized speech, returning it to the application via the same mechanism used for index marks. This mode offers flexibility and efficient memory handling.<ref name=":0" /><ref>Olofsson A. Phonemic awareness and the use of computer speech in reading remediation: Theoretical background[J]. Fonetiks, 1988, 1(5): 15-27.</ref>

In formant synthesis field, Dennis Klatt introduced an advanced formant synthesizer, the Klatt Formant Synthesizer, which combined both cascade and parallel methods in 1980. It featured extra resonances and antiresonances for nasalized sounds, a sixth formant for high-frequency noise, and a bypass path for a flat transfer function. This complex excitation model, controlled by 39 parameters every 5 ms, improved sound modeling and was incorporated into DECtalk, enhancing its speech synthesis capabilities.<ref name=":2" /> This is also a major innovation of DECtalk.