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    Saturday, October 08, 2005

    Speech Recognition: End-Point Detection

    The end point detection technique is applied to extract the region of interest from the raw speech signal. In other words, it removes the silent region from speech signals. The basic technique of end point detection is to find the energy level of a signal. Signal energy level is calculated in frames, where each frame consists of N samples. The frames are usually overlapped with the adjacent frames to produce a smooth energy line. Fig 1 shows the energy plot of “One”.

    Fig 1: (a) Amplitude vs time plot of “One” (b) energy level of the signal

    Accurate end point detection is important to reduce processing load and increase the accuracy of a speech recognition system. Basically there are two famous endpoint detection algorithms. First algorithm uses signal features based on energy levels and second algorithm uses signal features based on the rate of zero crossings. The combination of both gives good result, but nevertheless increases the complexity of the program and also the processing time.

    Fig 2 shows the signal of “one” sampled at 8000Hz for 10650 samples or 1.33 seconds. Before the speech begins, the waveform started as silence for about 5000 samples. After the utterance, the signal remains in silence state again for about 2000 samples. Throwing the unwanted silence region, the processing time can be improved to 3650/10650 * 100 = 34.3% by assuming all the frames in the region of interest have been processed. The energy level of the signal is inspected and a threshold value is determined from the energy plot. Fig 3 shows the cropped signal, where the silence region has been eliminated, and the remaining region of interest are used for further processing.

    Fig 2: (a) Original signal, (b) End-point detection by using the energy level of the speech signal

    Fig 3: (a) Detected end point, (b) Cropped signal/region of interest

    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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