In this thesis, excitation of the LPC vocoder has been studied in three areas. First, a high speed pitch estimation algorithm that is based on peak detection and average magnitude difference function (AMDF) is developed. A few pitch candidates are first estimated from the low-pass filtered (800 Hz) speech by peak detection algorithm. AMDF values of the pitch candidate that yields the minimum AMDF value is chosen as the desired pitch period. The new method requires far less computation time than other pitch estimation algorithms, yet it yields reasonably accurate results.
Second, an improved 4800 bps LPC vocoder system that virtually eliminates the buzzy effect from synthetic speech is developed. Excitation signal in the new system is formed by adding pitch pulse or random noise to the baseband residual (0-600 Hz) that has been coded by pitch prediction DPCM. According to our informal listening tests the synthetic speech of the new system does not have the buzzy effect. As a result the vocoder speech quality is more natural than that of a conventional LPC vocoder.
Third, a pitch extraction algorithm based on LPC inverse filtering and AMDF is hardware implemented using a bit slice micro-computer. The pitch extraction method that is modified to reduce the computation, its hardware architecture, and the system development procedure are described.
음성분석 및 합성장치인 LPC vocoder의 excitation에 대한 연구가 세 부분으로 나뉘어져 행해졌다. 우선, peak 검출과 AMDF (Average Magnitude Difference Function)를 이용해서 연산을 매우 줄인 pitch 추출방법이 연구되었다.
두번째로, LPC vocoder의 음질을 개선하기 위해서 hybrid excitation을 사용한 4800 bps LPC vocoder에 대한 연구결과가 기술되었다. 이 hybrid excitation은 음성을 inverse filtering한 residual 신호의 baseband (0-600 HZ) 부분에 고역통과된 pitch pulse 또는 noise 를 섞어서 만들었다.
세번째로, inverse filtering과 AMDF를 이용한 pitch 추출방법의 hardware implementation 과정이 기술되었다. 빠른 계산이 요구되므로 bit-slice microcomputer가 사용되었다.
