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    Saturday, October 01, 2005

    Speech Recognition: Formants for Vowels Classification (II)

    In the previous post, the fundamental of formants has been explained. In this article, we will look into the vowel classification based on the formants which has been extracted from a speech signal.

    Fig 1 shows the formants location for six vowels and three diphthongs that contains in English digits. The data are extracted from four male speakers, and the first formants are plotted over second formants in a two dimensional graph.

    Fig 1

    There are overlaps between formant frequencies for different vowels/diphthongs by different speakers. The circles illustrate the location of each vowels/diphthongs which centred at the mean values of the same groups of data.

    From the graph, it can be easily observed that vowels and diphthongs which have similar pronunciations grouped closely together. For instants, vowels /i/, /I/ and diphthongs /ie/ , /ei/ circles overlap more than others.

    It is also possible to visualize first three formants of vowels and diphthongs in three dimensional graphs. Fig 2 illustrates the same formants for English digits vowels and diphthongs with the addition of the third formant.

    Fig 2

    The spheres indicated the location of each vowels/diphthongs which centred at the mean values of the same groups of data. With the addition of third formant, the overlapping of the vowels/diphthongs is reduced. An obvious example is vowel /u/ with vowel /ie/.

    However, due to the overlapping problem of the formants feature even in the three dimension space, and the difficulty of defining the formants for unvoiced regions, the formats for speech analysis are more for visualization rather than implementation for speech recognition.


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