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Structural representation of pronunciation and its use in pronunciation training

1 Introduction No two students are the same. Pronunciation teaching should be started after teachers know exactly how individual students are in their development. In one-to-fifty situations such as classrooms, it is difficult for a teacher to know their pronunciations precisely. Computers have provided us with tools to visualize the pronunciations based on acoustics. In this case, however, good knowledge of acoustics is needed. Further, the acoustic representation inevitably shows many things irrelevant to the proficiency, such as speaker individuality, gender, age, microphone differences and so on. The first author already proposed a unique method of representing speech acoustics, where dimensions of the non-linguistic factors are well removed, Minematsu (2004a). This representation can be regarded as physical implementation of structural phonology, where only the interrelations among speech sounds are focused. In this paper, the non-native pronunciations are described based on the new method and the development of a student’s pronunciation is traced. Further, automatic assessment of the pronunciation is investigated experimentally. 2 Structural representation of speech acoustics Spectrogram is noisy representation in that it inevitably shows things completely irrelevant to the pronunciation proficiency. The first author proposed a method to represent speech acoustics where the static non-linguistic factors can hardly be seen, Minematsu (2004a). Since explanation of this method requires good knowledge of mathematics, only its short introduction is done here. This new method was inspired by Jakobson’s phonological structure in Figure 1, where French vowels and semi-vowels are structurally represented and it was claimed that this structure is invariant with speakers. In acoustic phonetics, the vowel structure is often represented as F1-F2 formant chart. It is known that this representation clearly shows gender and age difference. With the proposed method, these non-linguistic factors can be effectively suppressed. What is geometrical definition of a structure? A triangle is determined by fixing length of all the three segments. An n-point structure, in turn, is determined by fixing length of all the segments including its diagonal lines. This means that an n-point structure is fully represented by its distance matrix among the n points. If cepstrum parameters are used to represent envelopes of the spectrogram and if a speech sound is represented as point in a cepstrum space, an acoustic structure of n speech sounds is an n-point structure in the space. The acoustic structure can become invariant if the non-linguistic factors cannot change distance between any two points. Mathematically speaking, however, the structure has to be variant due to the non-linguistic factors. How to make inevitably variant structures invariant? The invariant structure can be obtained by applying theorem of the invariant structure, which was proposed by the first author Minematsu et al (2005). Here, every speech sound is represented as cepstrum distribution, not point. Distance between two distributions is calculated by distorting space so that the structure can be invariant. The space distortion can easily make variant structures invariant.