Sound synthesizer

A sound synthesizer comprising:

• a sound source signal source for generating a sound source signal;
• a LSP parameter source for generating LSP parameters {.omega..sub.i } and {.theta..sub.i } expressed in angular frequencies respectively allowing the roots of polynomials P(Z) and Q(Z) assuming zero and defined by the following expressions:
• P(Z)=A.sub.p (Z)-Z.sup.p+1 A.sub.p (Z.sup.-1)
• Q(Z)=A.sub.p (Z)+Z.sup.p+ A.sub.p (Z.sup.-1)
• A.sub.p (Z)=1+.alpha..sub.1 Z+.alpha..sub.2 Z.sup.2 +. . . +.alpha..sub.p Z.sup.p
• where {.alpha..sub.i } are predictor coefficients determined from each predetermined number of samples of soundware signal;
• parameter transforming means for transforming the LSP parameters to control parameters of a type different from the LSP parameters;
• and a sound synthesis filter means having a transfer function H(Z)=.sigma./A.sub.p (Z), said sound synthesis filter means being supplied with the sound source signal from said sound source signal source while the characteristics thereof are controlled by the transformed control parameters.

A sound synthesizer according to claim 8 or 15 wherein said parameter transforming means is means for transforming said LSP parameters .omega..sub.i, .theta..sub.i to predictor coefficients .alpha., and wherein said sound synthesis filter means comprises:

• first adder means one input of which is supplied with said sound source signal;
• a cascade connection of a plurality of unit time delay means, the input of said cascade connection being connected to the output of said first adder means;
• a plurality of multiplier means each for multiplying the output of one of said unit time delay means and corresponding one of said predictor coefficients .alpha..sub.1 ;
• and second adder means for summing up all the outputs from said multiplier means and supplying the sum to another input of said first adder means.

A method of synthesizing sound comprising the steps of:

• extracting, from an original sound signal, sound source signal parameters representing a sound source signal for energizing a synthesis filter means having a transfer function H(Z) defined by ##EQU10## extracting features parameters {.omega..sub.i } and {.theta..sub.i } representing the feature of the original sound signal;
• transforming the feature parameters {.omega..sub.i } and {.theta..sub.i } to control parameters {a.sub.i } and {b.sub.i } in accordance with a.sub.i =-2 cos .omega..sub.i and b.sub.i =-2 cos .theta..sub.i ;
• generating a sound source signal in accordance with the sound source signal parameters;
• supplying the sound source signal to said synthesis filter means to energize it;
• and
• controlling the characteristics of said synthesis filter means by the control parameters {a.sub.i } and {b.sub.i } thereby producing a synthesized sound signal from said synthesis filter means;
• the step of extracting feature parameters {.omega..sub.i } and {.theta..sub.i } comprising the steps of:
• sampling the original sound signal at a predetermined period .DELTA.T;
• obtaining auto correlation coefficients from the samples in a predetermined time interval;
• calculating predictor coefficients {.alpha..sub.i } (i=1, 2, . . . p) from the auto correlation coefficients to determine the function A.sub.p (Z);
• calculating the feature parameters {.omega..sub.i } and {.theta..sub.i } which render polynomials P(Z) and Q(Z) to zero, the polynomials P(Z) and Q(Z) being defined by
• A.sub.p (Z)=1/2{P(Z)+Q(Z)}
• P(Z)=A.sub.p (Z)-Z.sup.p+1 A.sub.p (Z.sup.-1)
• Q(Z)=A.sub.p (Z)+Z.sup.p+1 A.sub.p (Z.sup.-1)
• where Z=e.sup.-j.omega., .omega. is a normalized angular frequency 2.pi.f.DELTA.T and p is the degree of analysis, and said synthesis filter means comprises two feedback paths respectively having transfer functions P(Z)-1 and Q(Z)-1 including cascade operating second-order filter means represented by (1+a.sub.i Z+Z.sup.2) and (1+b.sub.i Z+Z.sup.2), respectively.