Voice Simulation: Factors Affecting Quality and Naturalness

VOICE SIMULATION: FACTORS AFFECTING QUALITY AND NATURALNESS B. Yegnanarayana Department of Computer Science and Engineering Indian Institute of Technology, Madras-60O 036, India J.M. Naik and D.G. Childers Department of Electrical Engineering University of Florida, Galnesville, FL 32611, U.S.A. ABSTRACT In this paper we describe a flexible analysls-synthesls system which can be used for a number of studies In speech research. The maln objective Is to have a synthesis system whose characteristics can be controlled through a set of parameters to realize any desired voice characteristics. The basic synthesis scheme consists of two steps: Generation of an excita- tion signal from pitch and galn contours and excitation of the linear system model described by linear prediction coefficients, We show that a number of basic studies such as time expansion/ compression, pitch modifications and spectral expansion/compression can be made to study the effect of these parameters on the quality of synthetic speech. A systematic study is made to determine factors responsible for unnaturalness tn synthetic speech. It is found that the shape of the glottal pulse determines the quality to a large extent. We have also made some studies to determine factors responsible for loss of Intel- ligibility tn some segments of speech. A signal dependent analysts-synthesis scheme ts proposed to improve the intelligibility of dynamic sounds such as stops. A simple implementation of the signal dependent analysis is proposed. I. INTRODUCTION The maln objective of this paper is to develop an analysis-synthesls system whose parameters can be varied at will to realize any desired voice characteristics. Thls wlll enable us to determine factors responsible for the unnatural quality of synthetic speech. It is also possible to determine parameters of speech that contribute to intelligibility. The key ideas In our basic system are similar to the usual linear predictive (LP) coding vocoder [I], [2]. Our main contributions to the design of the basic system are: (1) the flexibility incorpor- ated in the system for changing the parameters of excitation and system independently and (2) a means for combining the excitation and system through convolution without further interpolation of the system parameters during synthesis. Atal and Hanauer [1] demonstrated the feasl- billty of modifying voice characteristics through an LPC vocoder. There have been some attempts to modify some characteristics (llke pitch, speaking rate) of speech without explicitly extracting the source parameters. One such attempt is with the phase vocoder [3]. A recent attempt to independently modify the excitation and vocal tract system characteristics is due to Senef [4]. Unlike the LPC method, Senef's method performs the desired transformations in the frequency domain without explicitly extracting pitch. However, it Is difficult to adjust the intonation patterns while modifying the voice characteristics. In order to transform voice from one type (e.g., masculine) to another (e.g., feminine), it is necessary to change not only the pitch and vocal tract system but also the pitch contour as well as the glottal waveshape independently. It is known that glottal pulse shapes differ from person to person and also …

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