A View from Dresden onto the History of Speech Communication
Part 7: Early electronic demonstrators for speech synthesis
The development of early vocoders had a large impact on speech research, starting with the patents of Karl Otto Schmidt in Berlin and with Homer Dudley’s vocoder at Bell Labs in the 1930s. In some other places, vocoder prototypes had been designed during and after World War II. In Dresden, the development of a prototype was performed in the framework of the Dr.-Ing thesis of Eberhard Krocker in the 1950s.
The first channel vocoders had been large and expensive due to the application of electronic valves. There was some doubt whether they could be widely used in commercial applications. Krocker summarized: “The importance of the vocoder is less the frequency band compression than the potential for essential investigations on speech. The analyzer can be combined with registration equipment for the analysis of sounds, whereas the synthesizer can be combined with a control mechanism for the synthetic production of speech.”
This was an exact prognosis. The analysis-synthesis technology proved to be a very powerful tool in speech research. The synthesizer part of a channel vocoder could be used for early attempts in electronic speech synthesis. The analyzer part of the vocoder had to be replaced by a control unit. This was a manual/pedal control in the case of Dudley’s famous voder. Another way for controlling the synthesizer was the optoelectronical reading of a spectrogram. The first of these so-called pattern playback devices were developed at the Bell Labs and the Haskins Labs.
It became clear that there are more effective kinds of parameterization of the speech signal, and other vocoder types than the channel vocoder arose. Formant coding proved to be a very effective approach. Consequently, the early types of speech synthesis terminals followed the principle of formant synthesis. This development was strongly influenced by the work of Gunnar Fant, who developed the vowel synthesizer OVE, which was controlled by moving a pointer across the formant plane.
Both devices, the vowel synthesizer and the playback device, showed huge didactic value. When Walter Tscheschner (1927–2004) was appointed the chair for Electronic Speech Communication at the former Technische Hochschule (later Technische Universität) Dresden, he wanted to have own versions of these devices as demonstrators for his lectures. The vowel synthesizer (Figure 1) was built in 1962. It applies circuitry with electron valves to establish three formants. The two lower formants can be adjusted by a pointer (Figure 2).
The playback device was constructed in 1973 as an optical spectrogram reader, which controlled the 19 channels of the abovementioned vocoder. Because the device was very attractive as a demonstrator, a portable version (PBG 2, Figure 3) was implemented in 1982.
Both devices are now exhibits of the historic acoustic-phonetic collection (HAPS) of the TU Dresden. The most remarkable fact is that they are still working.
Rüdiger Hoffmann and Ulrich Kordon
Photographs Copyright TU Dresden / HAPS