I. Introduction
The estimation of fundamental frequency is an essential component in a variety of speech processing systems such as the speech analysis-synthesis system and speech coding system [1], [2]. The contour of fundamental-frequency (i.e., pitch contour) also plays an important role in language communication [3] [4]–[6]. There are some difficulties in the estimation of fundamental frequency, although it can be observed by eye inspection. First, the voiced speech is not a perfectly periodic waveform because of the variation of fundamental frequency and the movement of vocal tract. Second, it is difficult to estimate the fundamental frequency of low-level voiced speech at its beginning and ending. Third, the performance of estimation will degrade when the speech signal is corrupted by noise.