A METHOD FOR SPEECH SYNTHESIS HARDWARE, AND SPEECH SYNTHESIS APPARATUS

Abstract: Speech synthesis is based on a human vocal chords model with two dynamically interlinked masses. Their positions determine glottis parameters. In particular, a first sub-model describes the dynamically interlinked masses acted upon by external forces. A second dynamical partial model describes dynamics of a contour of the glottis in dependence of geometry parameters produced by the first sub-model, and outputs a first part of the external forces in addition to transmitting the geometry parameters. A third hydrodynamic sub-model describes hydrodynamics in dependence of the transmitted parameters, is acted upon by air pressure parameters, and outputs a second part of the external forces and an actual volume velocity.

Description: A METHOD FOR SPEECH SYNTHESIS HARDWARE, AND SPEECH SYNTHESIS APPARATUS BACKGROUND TO THE INVENTION The invention relates to a method for modelling speech synthesis, based on a human vocal chord model that comprises two dynamically interlinked masses, which have positions determining glottis model parameters. The physical modelling of human speech synthesis is used to construct devices for producing synthesized human speech, such devices being industrially applicable on a wide scale, for example in human-machine dialog systems and announcement systems. Human vocal chords are the active elements for generating voiced sounds such as vowels. Due to the interaction between the air flowing and the human vocal chords, or folds, the instantaneous air flow through them is modulated periodically. The opening between the vocal chords is called the glottis, and the pulsating air flow is called the glottal pulse. Hereinafter, the terms vocal chords and glottis are generally used as equivalents. The shape of the glottal pulse is changed on passing the vocal tract and the lips. The resulting variations in atmospheric pressure at the lips are perceived as speech. From a signal-processing point of view, the vocal chords can be seen as a signal source, and the combination of vocal tract and lips as a filter. Source-filter models are often used in speech synthesizers. A method according to the preamble has been described in a publication by K. Ishizaka & J.L. Flanagan, Synthesis of Voiced Sounds From a Two-Mass Modal of the Vocal Cords, Bell Syst. Tech. J. 51 (1972) pp. 1233-1267. Present models that describe the interaction between vocal chords and air flow have proven to be computationally costly, which has precluded their broad use in speech synthesizers. SUMMARY TO THE INVENTION Accordingly, amongst other things it is an object of the present invention to provide a method for modelling speech synthesis hardware, that is open to straightforward calculating through inter alia using various submodels as building blocks, each of such building blocks by itself being of a relatively elementary nature. Now, according to one of its aspects, the invention is characterized in that in that said human vocal chord model comprises, in addition to a first sub-model describing said dynamically interlinked masses acted upon by external forces, a second dynamic partial model describing a contour of the glottis in terms of its geometry parameters and including its dynamics, in dependence of position and velocity parameters produced by said first sub¬ model, and outputting a first part of said external forces in addition to transmitting said geometry parameters, and also a third hydrodynamic sub-model, describing hydrodynamics of an air flow through the vocal chords in dependence of said transmitted geometry parameters, and being acted upon by air pressure parameters, for outputting a second part of said external forces and an actual volume velocity. The invention also relates to a speech synthesizer device modelled according to the method of to the invention. Various further advantageous aspects of the invention are recited in dependent Claims. BRIEF DESCRIPTION OF THE DRAWING These and other aspects and advantages of the invention will be described in detail hereinafter with reference to the disclosure of preferred embodiments, and in particular with reference to the appended Figures that show: Figure 1 an exemplary speech signal for reference; Figure 2 the glottal pulse for the same signal; Figure 3 an elementary set-up of the two-mass model; Figure 4 the model of Figure 3, built up from mechanical, geometrical, impact, and fluid dynamics sub-models; Figure 5 a state diagram description; Figure 6 a complete speech synthesizer system. Figures 7A,7B,7C give 17 equations used in the description and derivation of various quantitative entities for the modelling. DETAILED DISCLOSURE OF PREFERRED EMBODIMENTS For reference, Figure 1 gives an exemplary speech signal as a function of time, in this case representing a vowel 'a'. The vertical axis is in arbitrary units of intensity. For the same signal, Figure 2 gives the amount of air flow between the vocal chords or glottal pulse; clearly, both signals have the same basic frequency or pitch. Figure 3 gives an elementary set-up of a two-mass model used in the modelling. For approximating, the glottis is considered to be a two-dimensional obstruction with a plane of symmetry at y=0, and a uniform cross-section in a direction perpendicular to the plane of the Figure. In consequence, for the calculations only the upper half of the Figure needs to be considered. During generation of the speech, air is generally flowing from left to right in the Figure. The Figure shows masses in ! , m 2 , damping constants r,, r 2 , elastic coupling constants k,, k 2 , and the elastic coupling constant between the two masses k u . The positions of the masses are y,(t), and y 2 (t), respectively, whereas the x-positions may be left out of consideration. The hydrodynamic forces on the masses f,(t) and f 2 (t), are functions of the geometry of the vocal chords, and of the air pressure Pτ(t) in front of and py(t) behind the vocal chords, respectively. The geometry of the contour, indicated as a drawn line, can be given as a function y e (x,y,(t),y 2 (t)). The solution of the dynamic equations will be a self- oscillating system that modulates the velocity Uc(t) of the air mass flowing through the obstruction. The volume velocity Uc(t) itself also depends on the geometry of the vocal chords and on the air pressures in front of and behind the obstruction. The equations of motions of the system are given by EQ1 in the Fig. 7, f and !_,, being associated with the rest positions of the two masses. The two equations may be solved by numerical methods for finding the air flow. In the above, the relationships between the geometry and the hydrodynamic forces are non-linear, which renders the solving of EQ1, and the modelling based thereon an intricate and time-consuming task. The present invention decomposes the overall model into sub-models, and in a particularly preferred embodiment into four such sub-models. Figure 4 gives the model of Figure 1, broken down into mechanical-, geometrical-, impact-, and fluid dynamics sub-models, these models being furthermore connected by interrelations that are shown as arrows. The four sub-models are as follows: a. The mechanical sub-model describes the behaviour of the mass-elastic constant system under arbitrary forces on the masses. This system is linear and can be realized with elementary time-discrete signal processing operators. The inputs are the forces f,, f 2 ; the outputs are the positions y and velocities (dy/dt) of the two masses in the y-direction. b. The geometry sub-model describes the geometry of the vocal chords. Instead of by the formal function y e (x,y,(t),y 2 (t)), the geometry is expressed in a set of parameters that are determined by the positions of the masses, such as through parameters that define finite straight lines. Also these relations can be expressed in a straightforward manner. The inputs to the geometry sub-model are the outputs of the mechanic sub-model. The outputs are the positions and velocities of the two masses in the y-direction, combined with the descriptions of the instantaneous geometry, as dependent on the model equations used. c. The impact sub-model describes the influence of impacts between the two vocal chord halves on the geometry, and calculates the ensuing forces on the two masses. The inputs to the sub-model are the outputs from the geometry sub- model. The outputs of the sub-model are in the first place the reactive forces _ι,c * f^c on the glottis itself, that are fed back into the mechanical sub-model, and in the second place, the corrected instantaneous geometry parameters. d. The hydrodynamic sub-model describes the hydrodynamic forces on the two masses as a consequence of the geometry and of the air pressures ...rτ[n], py[n] in front of and behind the vocal chords, respectively, and also calculates the volume velocity ιic[n] of the air through the glottis. The inputs are the instantaneous geometry parameters outputted by the impact sub-model, and furthermore the instantaneous upstream and downstream pressures. The outputs are further forces operating on the glottis itself, and also, the instantaneous mass flow. In Figure 4, all pressures and volume velocities through the glottis have been indicated as time discrete quantities. The geometry sub-model, the impact sub-model, and the hydrodynamic sub-model give instantaneous relations between the inputs thereto and the outputs therefrom. DESCRIPTION OF THE MATHEMAΗCS In the mechanical sub-model, the equations of state are given by EQ2 of the Fig. 7, wherein x(t) is a state vector with as entries the positions and velocities of the two masses and u(t) the input vector with components f,(t) and f 2 (t). The output vector is given in similar way as the input vector. The state vector x(t) itself is given by EQ3 as having four components. The input vector is given by EQ4 as having two components only. The output vector is given by EQ5 as having four components. The four matrices A,B,C,D from EQ2 are given by EQ6-9, respectively. Herein, matrix C is the unitary matrix, and matrix D consists exclusively of zeroes. In a more complex model, a non-zero version of this matrix may be used. Now, the system has two input quantities, which leads to the definition of two impulse responses. The first one, h,(t) is the response to a unitary delta change in f,(t), the second one, h 2 (t) is the response to a unitary delta change in f 2 (t), given by EQ10,11, respectively. A delta change has a pulse shape of finite area and infinitely small width. The matrix A can in a conventional manner be diagonalized as expressed by EQ12, wherein λl- λ4 are the respective Eigenvalues of A. Consequently, the two impulse responses h,(t), h 2 (t) can be expressed as shown by EQ13,14 respectively. Now, the object is to generate a time-discrete system with two inputs and four outputs, with impulse responses g,[n], g 2 [n], such that g i [n]=h 1 (nT), i= l,2, and wherein T is the sample period. The state description of such a time-discrete system is given by EQ16. Herein, u is the input given in EQ11, F=BxT is given by EQ7, G=C is given by EQ8, and H=0. The requirement of EQ15 in that the two impulse responses should be equal is fulfilled if EQ17 is met, which also yield E. Here, indeed holds that D=0. Figure 5 gives a state diagram description realized as a set of interconnected blocks, of which three form a linear chain, combined with one feedback block. The contents of the various blocks 40, 42, 44 have been indicated by the associated capital letters from the above equations, in particular the set of equations EQ16. The block T implements a unitary delay. The realization of the state diagram of Figure 5 in hardware has been shown in Figure 4 already. Technical applications of the modelling can be found in speech synthesis for traffic-, flight-, or telephone information systems. In this respect, Figure 6 gives a complete speech synthesizer system, with blocks 30, 32, 34, 36. The input quantity u[n] at block 36 is the simulated volume speed from the lungs. Block 36 itself is a tube-model of the human trachea as governed by its cross-sectional parameters inputted at left (38). The quantities u v + [n], u v ' [n], u τ + [n], u τ " [n] represent the forward and backward volume velocities (+ ,-), respectively, at the upstream and downstream side (T,V), respectively, of the glottis represented by block 32. Upstream is the trachea model, downstream the vocal tract model; both of these are represented by means of a tube model, that produces the forward and reflected airstreams. The volume velocities are equivalent to the pressures Pvln]. Pτ- n L ^d tne glottal volume velocity u G [n], respectively. The relationship between the total glottal volume velocity U G and p-r(k) and Pv(k) and x(k-l) are known. The latter is the 'state vector' of the glottis as determined by the positions and velocities of the two masses. In a formula, this is: " G = G (Pτ(k),Pv(k) ,x(k-l)) This depends on the model actually used and will therefore not be specified in detail. Furthermore, the following holds: (l) u G (p- r ,P v ,x)=u τ + -u τ - (2) Pτ=(p.c/A τ )(_ τ + +u τ ' ) (3) u G (p τ ,P v ,x)=U v + -u v Herein, p is the volume density of the air, c is the sound speed, A τ is the area of the trachea at the glottis, and A v is the area of the vocal tract at the glottis. Equations (1) and (3) indicate that the air stream through the glottis is equal to the air stream through vocal tract and trachea. Equations (2) and (4) indicate the relation between pressure and volume velocity. The time indices have been omitted for clarity. The quantities u τ + and u v " are known. Therefore, the above equations allow to calculate the other air speeds u v + and u v + in front of and behind the glottis, respectively. In turn, these speeds allow to calculate the control of the vocal tract and the eventual speech signal. In particular, the set consists of four non-linear equations with four unknowns. The solution may ensue by substituting equations (2) and (4) into (1) and (3), respectively. The expression for u G is relatively straightforward, and quite often a quadratic function. In the arrangement of Figure 6, block 32 represents the hydrodynamic sub-model according to the invention as governed by the glottal parameters inputted at 40. Block 32 according to the invention interacts with block 34, that contains the mechanical-, geometry-, and impact sub-models that have been considered earlier. Block 30, like block 36, represents a tube model of the human vocal tract as governed by its cross-sectional parameters inputted at left (42). The ultimate output quantity is the air pressure p[n] at the lips.

Claims: CLAIMS: 1. A method for modelling speech synthesis, based on a human vocal chord model that comprises two interlinked masses, which have positions determining glottis model parameters, characterized in that said human vocal chord model comprises, in addition to a first sub-model describing said dynamically interlinked masses acted upon by external forces, a second dynamic partial model describing a contour of the glottis in terms of its geometry parameters and including its dynamics, in dependence of position and velocity parameters produced by said first sub-model, and outputting a first part of said external forces in addition to transmitting said geometry parameters, and also a third hydrodynamic sub-model, describing hydrodynamics of an air flow through the vocal chords in dependence of said transmitted geometry parameters, and being acted upon by air pressure parameters, for outputting a second part of said external forces and an actual volume velocity. 2. A method as claimed in Claim 1, wherein said second dynamic partial model is decomposed into an instantaneous geometry sub-model describing an external geometry of said glottis, in dependence of behaviour of said first sub-model, and an impact sub-model fed by said external geometry describing a colliding behaviour of said vocal chords, for outputting the first part of said external forces and for transmitting said geometry parameters. 3. An apparatus for modelling speech synthesis, having modelling means for a human vocal chord model that comprises two interlinked masses, which have positions determining glottis model parameters, characterized in that said modelling means comprise, in addition to first sub-modelling means describing said dynamically interlinked masses acted upon by external forces, second partial modelling means for a dynamic partial model describing a contour of the glottis in terms of its geometry parameters and including its dynamics, in dependence of position and velocity parameters received from said first sub-modelling means, and having output means for outputting a first part of said external forces in addition to transmitting said geometry parameters, and also third hydrodynamic sub-modelling means for describing hydrodynamics of an air flow through the vocal chords in dependence of said transmitted parameters, and being acted upon by air pressure parameters, and having further output means for outputting a second part of said external forces and an actual volume velocity. 4. An apparatus as claimed in Claim 3, wherein said second dynamic partial modelling means are decomposed into instantaneous geometry sub-modelling means describing an external geometry of said glottis, in dependence of behaviour of said first sub¬ model, and impact sub-modelling means fed by said external geometry describing a colliding behaviour of the vocal chords, for outputting the first part of said external forces and for transmitting said geometry parameters. 5. A speech synthesizer appararatus as claimed in Claims 3 or 4, comprising an excitatory signal source feeding a trachea tube model, a vocal chord model having its hydrodynamic sub-model arranged downstream of the trachea tube-model, and a vocal tract tube-model fed by said hydrodynamic sub-model for outputting a lip pressure signal.

Inventors: BOGAERT IGOR JEAN MARIN, VELDHUIS RAYMOND NICOLAAS JOHA, BOGAERT, IGOR, JEAN, MARIN, VELDHUIS, RAYMOND, NICOLAAS, JOHAN
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1996
Main classification: G10L9
IPC codes: G10L11, G10L13