Apparatus and method for speech rate modification

Abstract: In a speech rate modification, correlation functions between different segments of input speech signal are computed by a correlator (17), then amplitude of input signal is controlled by two multipliers (19, 20) which multiply the input speech signal by an increasing window function and by a decreasing window function, or vice versa, respectively, produced by a window function generator (18), and then output signals of the multipliers (19, 20) are added each other by an adder (21) at such relative delay within one unitary segment () as to make the larges value of the correlation function, and the input voice signal and the output of the adder (21) are selected by a multiplier (22), to be issued as rate-­ modified speech signal.

Description: BACKGROUND OF THE INVENTION 1. FIELD OF THE INVENTION The present invention relates to an apparatus for and a method of performing a speech rate modification in which only the time duration of a speech is changed without altering the fundamental frequency components of the speech signal. 2. DESCRIPTION OF THE PRIOR ART Heretofore, in order to perform a speed-up listening or a slow-down listening of speech signals recorded on audio tapes or the like, speech rate modification apparatus have been utilized. As the speech rate modification apparatus of prior art, there has been the U.S. Patent No. 3,786,195, to Schiffman et al., "Variable Delay Line Signal Processor for Sound Reproduction". This speech rate modification apparatus is comprised of a variable delay line, a ramp level and amplitude changer, a blanking circuit, a blanking pulse generator, and a ramp pulse-train generator. On the speech rate modification apparatus described above, its operation is elucidated below. The input signal is first written into the variable delay line. Next, the ramp pulse-train generator controls the ramp level and amplitude changer and the blanking pulse generator corresponding to a time-scale modification ratio. Then the level and amplitude changer performs the read-out operation of signals from the variable delay line with a speed which is different from that at the time of write-in operation depending on the time-axis modification ratio. That is, when the reproduction rate of a tape is increased, the read-out operation of the data from a memory is made slower than the write-in operation to the memory in order to restore raised tone (frequencies) to normal one; whereas when the reproduction rate of a tape is decreased, the read-out operation of the data from the memory is made faster than the write-in operation of the data to the memory in order to restore lowered tone to normal tone. Then, on discontinuous parts between respective speech blocks, the blanking circuit applies the muting action on the output of the variable delay line. In the conventional constitution as has been described above, however, when increasing the rate, degradations in the recognizability of consonants necessarily occur owing to the thinning use of data which is necessary for increasing the rate. And because of the above-mentioned muting, signal amplitude becomes discontinuous, causing the problem that only a speech voice having a poor naturalness can be obtained. Although there are other means using detection of pitch period, apart from the above-mentioned conventional speech rate modification apparatus, such pitch detection method cannot be applied in case of the background music or noise superimposed on the speech to be processed because the extraction of pitch is difficult in such case. Hence the above-mentioned method cannot be considered very suitable. In IEEE TRANSACTIONS ON ACOUSTICS, SPEECH AND SIGNAL PROCESSING, vol. ASSP-31, no. 1, part 2, February 1983, pages 258-272; R.V. COX et al.: "Real-time implementation of time domain harmonic scaling of speech for rate modification and coding", a pitch period p is derived from an input signal S(n), and the input signals S(n) are added by weighting with a triangular window Wc(n) or We(n) to obtain an output signal Sc(n) or Se(n), the speech signal is divided into windows with a predetermined window length Bc or Be of time-scale compression or time-scale extension, respectively. OBJECT AND SUMMARY OF THE INVENTION Purpose of the present invention is to offer a speech rate modification apparatus and method which are capable of issuing a speech voice having an ample naturalness with less data drop-offs. In order to achieve the above-mentioned purpose, a speech rate modification apparatus according to claim 1 and a speech rate modification method according to claim 7 is provided. According to the constitution described above, in consequence of controlling the signal amplitude by the multiplier, the discontinuities of signal amplitude or the drop-offs of data become less, and also in consequence of the addition calculation of signals by the correlator and the adder at a time point at which the correlation function takes a largest value, discontinuities in phase also become less. And furthermore, in consequence of the control of segments by which the input signal is directly issued through selection circuits, wide range of desired time-scale modification ratios are obtainable. BRIEF DESCRIPTION OF THE DRAWINGS FIG.1 is a block diagram of a speech rate modification apparatus in a first apparatus-embodiment of the present invention. FIG.2 is a flow chart representing a speech rate modification method in a first embodiment of the present invention. FIG.3 shows a schematic diagram of processing voice waveforms in accordance with the speech rate modification method in the first embodiment of the present invention. FIG.4 shows a schematic diagram of processing voice waveforms in accordance with the speech rate modification method in the first embodiment of the present invention. FIG.5 is a flow chart representing a speech rate modification method in a second embodiment of the present invention. FIG.6 shows a schematic diagram of processing voice waveforms in accordance with the speech rate modification method in the second embodiment of the present invention. FIG.7 shows a schematic diagram of processing voice waveforms in accordance with the speech rate modification method in the second embodiment of the present invention. FIG.8 is a flow chart representing a speech rate modification method in a third embodiment of the present invention. FIG.9 shows a schematic diagram of processing voice waveforms in accordance with the speech rate modification method in the third embodiment of the present invention. FIG.10 shows a schematic diagram of processing voice waveforms in accordance with the speech rate modification method in the third embodiment of the present invention. FIG.11 is a flow chart representing a speech rate modification method in a fourth embodiment of the present invention. FIG.12 shows a schematic diagram of processing voice waveforms in accordance with the speech rate modification method in the fourth embodiment of the present invention. FIG.13 is a block diagram of an improved embodiment of speech rate modification apparatus of the present invention. FIG.14 is a schematic diagram representing weighting functions to be applied to the correlation values in accordance with the speech rate modification apparatus in the second apparatus-embodiment of the present invention. FIG.15 is a schematic diagram representing weighting functions for the correlation values in accordance with the speech rate modification apparatus in the second apparatus-embodiment of the present invention. FIG.16 is a flow chart representing a speech rate modification method in a fifth embodiment of the present invention. FIG.17 shows a schematic diagram of processing voice waveforms in accordance with the speech rate modification method in the fifth embodiment of the present invention. FIG.18 shows a schematic diagram of processing voice waveforms in accordance with the speech rate modification method in the fifth embodiment of the present invention. FIG.19 is a flow chart representing a speech rate modification method in a sixth embodiment of the present invention. FIG.20 shows a schematic diagram of processing voice waveforms in accordance with the speech rate modification method in the sixth embodiment of the present invention. FIG.21 shows a schematic diagram of processing voice waveforms in accordance with the speech rate modification method in the sixth embodiment of the present invention. FIG.22 is a flow chart representing a speech rate modification method in a seventh embodiment of the present invention. FIG.23 shows a schematic diagram of processing voice waveforms in accordance with the speech rate modification method in the seventh embodiment of the present invention. FIG.24 shows a schematic diagram of processing voice waveforms in accordance with the speech rate modification method in the seventh embodiment of the present invention, It will be recognized that some or all of the Figures are schematic representations for purposes of illustration and do not necessarily depict the actual relative sizes or locations of the elements shown. DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is to offer a speech rate modification apparatus which is capable of giving a speech voice having an ample naturalness with less discontinuities in signal amplitude and phase and also with less data drop-offs and also which can be realized with a simple hardware. [FIRST APPARATUS-EMBODIMENT] In the following, elucidation is given on the first apparatus-embodiment of a speech rate modification of the present invention referring to FIG.1. FIG.1 is a block diagram of a speech rate modification apparatus in the present apparatus-embodiment. In FIG.1, numeral 11 is an A/D converter for converting input voice signal to digitized voice signal. A buffer 12 is for temporarily storing the digitized voice signal. A demultiplexer 14 switches to deliver the digitized voice signal to a first memory 15, to a second memory 16, and to a multiplexer 22, being controlled by a rate control circuit 13. A correlator 17 is for computing correlation function between outputs of the first memory 15 and the second memory 16. Output terminals of the correlator 17 are connected to the rate control circuit 13, to an adder 21 and to a window function generator 18. A first multiplier 19 and a second multiplier 20 are for multiplying output of the window function generator 18 on outputs of the first memory 15 and of the second memory 16, respectively. The output terminals of the multipliers 19 and 20 are connected to the adder 21 which adds outputs to each other being controlled by the output of the correlator 17. The multiplexer 22 is for combining outputs from the adder 21 and the demultiplexer 14 under control of the rate control circuit 13. Then a D/A converter 23 is for converting the combined digital signal to an analog output signal. On the speech rate modification apparatus constituted as has been described above, its operation is elucidated below. First, the input signal is converted into a digital signal by the A/D converter 11 and written into the buffer 12. Next, the rate control circuit 13 controls the demultiplexer 14 in accordance with a given time-scale modification ratio to supply the data in the buffer 12 to the first memory 15 and the second memory 16, and also to the multiplexer 22. Then, correlation functions between the contents of the first memory 15 and that of the second memory 16 are computed by the correlator 17, and the information of these correlation computation is supplied to the rate control circuit 13, the window function generator 18, and the adder 21. The window function generator 18 generates a first window function which gradually increases or gradually decreases, based on the information from the correlator 17 and on a given time-scale modification ratio, to supply it to the first multiplier 19. And the window function generator 18 also issues a second window function which is complementary to the above-mentioned first window function, to supply it to the second multiplier 20. Then the first multiplier 19 performs a multiplication calculation between the contents of the first memory 15 and the first window function issued from the window function generator 18; whereas the second multiplier 20 performs a multiplication calculation between the contents of the second memory 16 and the second window function issued also from the window function generator 18. The adder 21 performs an addition calculation between these windowed outputs from the first multiplier 19 and from the second multiplier 20 after displacing their mutual position making a relative delay so that the computed correlation function takes a largest value within a time-length of unitary segment, based on the information from the correlator 17. And the adder 21 supplies the sum output to the multiplexer 22. Then, the multiplexer 22 selects the output of the adder 21 and the output of the demultiplexer 14 and supplies the selected result to the D/A converter 23, which converts the resultant digital signal to an analog signal. As has been described above, according to the present embodiment, by using the first multiplier 19 and the second multiplier 20, the contents of the first memory 15 and the contents of the second memory 16 are multiplied respectively by paired window functions. These paired window functions are complementary to each other, one being a gradually increasing window function and the other being a gradually decreasing window function, both generated from the window function generator 18. Then, those windowed outputs from respective multipliers are added to each other by the adder 21, thus making a digitized speech voice having an ample naturalness with less discontinuities in the signal amplitude and also with relatively small data drop-offs. The correlator 17 computes a correlation function between the contents of the first memory 15 and the contents of the second memory 16. The adder 21 performs an addition calculation between the outputs from the first multiplier 19 and from the second multiplier 20 after displacing their mutual position to make delay so that the computed correlation function takes a largest value within a time-length of unitary segment. Thus, a high quality speech voice signal with less discontinuities in the signal phase can be obtained. Moreover, the length of segments in which the input signal is directly Issued is controlled by the action of the rate control circuit 13, the demultiplexer 14 and the multiplexer 22. Thereby, time-scale modification ratio can easily be changed. And at the same time. according to the above-mentioned controlling, it becomes possible to rapidly absorb such deviations in the time-scale modification ratio that might be caused by the addition calculation performed by displacing the mutual position of those windowed signals to make the correlation function take a largest value within a time-length of unitary segment. [1st METHOD-EMBODIMENT] In the following, elucidation is given on the first embodiment of the speech rate modification method of the present invention referring to the accompanying drawings, FIG.2 through FIG.4. The present invention is to offer a method of speech rate modification which is capable of giving a speech voice having an ample naturalness with less discontinuities in signal amplitude and phase and also with less data drop-offs for a range of the time-scale modification ratio of α ≧ 1.0. Hereupon, the time-scale modification ratio α is defined as Time-Scale modification Ratio: α = Reproduction Time Duration after Time-Scale modification Reproduction Time Duration at Normal Rate FIG.2 is a flow chart representing a speech rate modification method in the present embodiment. Its operation is elucidated below. First, an input pointer is reset (step 202). Then, a signal X A having a time-length as long as T time-units starting from a time point designated by this input pointer is inputted from the demultiplexer 14 to the first memory 15 (step 203). Then, T is added to the input pointer to update it (step 204). Next, a signal X B having thus the same time-length as long as T time-units starting from a time point designated by this updated input pointer is inputted from the demultiplexer 14 to the second memory 16 (step 205). Then a correlation function between X A and X B is computed (step 206). Based on this correlation function thus obtained, X A is multiplied by a window of a gradually increasing function (step 207). Also based on this correlation function obtained, X B is multiplied by a window of a gradually decreasing function (step 208). Then based also on the correlation function obtained, these windowed X A and X B are displaced to each other by a time units T c (as shown also in FIG.3) so that the correlation function between X A and X B takes a largest value within a time-length of unitary segment and they are added, issuing the added result (step 209). Next, a signal X C which has a time-length of T/(α-1) time-units from a time point designated by the updated input pointer, is inputted from the demultiplexer 14 and directly issued to the multiplexer 22 (step 210). Then T/(α-1) is added to the input pointer to update it (step 211). Then, step returns to the step 203. FIG.3 schematically illustrates actual exemplary cases, wherein the horizontal direction corresponds to the time lapse and the vertical heights corresponds to the amplitude level of voice signal. FIG.3(a) schematically shows a succession of segments, designated by 1, 2, 3, original voice signal on which speech rate modification process is to be carried out. In FIGs.3, (b) and (c) schematically represent embodiments that the time-scale modification ratios α are 2.0 and 3.0, respectively. In Fig.3(c), f stands for the fore part of a segment, while h stands for the hind part thereof. In FIGs.3, (d) and (e) schematically illustrate examples of individual detailed process of the addition calculation. FIG.3(d) illustrates a case of addition calculation designated by D in FIG.3(b) and FIG.3(c), wherein the addition calculation is done under a condition that the correlation function takes a largest value when X B is displaced to the positive side by T c time-units with respect to X A , resulting in extension of arise time sections outside the leading and rear edges of their overlapping time interval. FIG.3(e) illustrates another case of addition calculation designated by E in FIG.3(b) and in FIG.3(c), wherein the addition calculation for the same condition is done when X B is displaced to the negative side by T c time-units with respect to X A . In the exemplary cases shown in FIGs.3(b) and (c), there are time intervals designated by D which correspond to the time interval D of FIG.3(d). In these time intervals, time sections extending outside the overlapping time interval may overlap also to adjacent time intervals and hence it is necessary to perform the amplitude adjustments also in those adjacent time intervals. Hereinafter, also in FIGs.4, 6, 7, 9, 10, 12, 17, 18, 20, 21, 23, and 24, the same convention as has been employed in FIG.3 is applied. As has been described above, according to the present embodiment, signals X A and X B are multiplied respectively by window functions which are complementary to each other, one being a gradually increasing window function and the other being a gradually decreasing window function. And a signal obtained by adding these windowed signals is inserted at a time point corresponding to the beginning of the input signal part X B , and this process is repeated. Thus, a speech voice having an ample naturalness with less discontinuities in signal amplitude and also with less data drop-offs can be issued for a range of the time-scale modification ratio of α ≧ 1.0. And by computing a correlation function between X A and X B , and adding windowed X A and X B by displacing their mutual position so that the computed correlation function takes a largest value within a time-length of unitary segment, a high quality speech voice with less discontinuities in the signal phase is obtainable. Moreover, by changing the length of X C , it becomes possible to easily change the time-scale modification ratio. FIG.4 schematically illustrates modified exemplary cases obtained by modifying the above-mentioned embodiment. FIG.4(a) schematically shows a succession of segments 1, 2, 3 each having a time-length of T time-units of an original voice signal on which the speech rate modification process is to be carried out. FIG.4(b) and FIG.4(c) schematically represent embodiments that the time-scale modification ratios α are 2.0 and 3.0, respectively, and FIG.4(d) and FIG.4(e) schematically illustrate examples of detailed individual process of the addition calculation. FIG.4(d) illustrates a case of addition calculation designated by D in FIG.4(b) and FIG.4(c), wherein the addition calculation is done under a condition that the correlation function takes a largest value when X B is displaced to the positive side by T c time-units with respect to X A and time sections extending outside the leading and rear edges of the overlapping time interval are discarded. FIG.4(e) illustrates another case of addition calculation, designated by E in FIG.4(b) and FIG.4(c), wherein the addition calculation for the same condition is done when X B is displaced to the negative side by T c time-units with respect to X A . In these exemplary cases shown in FIGs.4(b) and (c), too, there are time intervals designated by D which correspond to the time interval D of FIG.4(d). In these time intervals, time sections extending outside the overlapping time interval are discarded as shown in FIG.4(d). This modified method can be realized by changing the window function. This modified method enables realizing a simplification of process described above without suffering a degradation in the recognizability of the speech voice. [2nd METHOD-EMBODIMENT] In the following, elucidation is given on the second embodiment of the speech rate modification method of the present invention referring to FIGs.5 through 7. The present embodiment is to offer a method of speech rate modification which is capable of giving a speech voice having an ample naturalness with less discontinuities in signal amplitude and phase and also with less data drop-offs for a range of the time-scale modification ratio of 0.5 ≦ α ≦ 1.0. FIG.5 shows a flow chart representing a speech rate modification method in the present embodiment, and the same hardware as shown in FIG.1 is used. Its operation is elucidated below. First, an input pointer is reset (step 502). Then, a signal X A having a time-length as long as T time-units starting from a time point designated by this input pointer is inputted (step 503). Then, T is added to the input pointer to update it (step 504). Next, a signal X B having thus the same time-length as long as T time-units starting from a time point designated by this updated input pointer is inputted (step 505). And T is added to the input pointer to update it (step 506). Then a correlation function between X A and X B is computed (step 507). Based on this correlation function thus obtained, X A is multiplied by a window of a gradually decreasing function (step 508). Also based on this correlation function obtained, X B is multiplied by a window of a gradually increasing function(step 509). Then based also on the correlation obtained, these windowed X A and X B are added to each other after they are mutually displaced at a time point at which the correlation function takes a largest value within a time-length of unitary segment and the added result is issued (step 510). Next, a signal X C having a time-length of (2α-1)T/(α-1) time-units starting from a time point designated by the updated input pointer is inputted and directly issued (step 511). Then (2α-1)T/(α-1) is added to the input pointer to update it (step 512). Then, step returns to the step 503. FIG.6 schematically represents actual exemplary cases, wherein FIG.6(a) schematically shows a succession of segments each having a time-length of T time-units of original voice signals on which speech rate modification process is to be carried out, FIG.6(b) and FIG.6(c) schematically represent embodiments that the time-scale modification ratios α are 2/3 and 0.5, respectively. And FIG.6(d) and FIG.6(e) schematically illustrate examples of individual detailed process of the addition calculation with mutual; FIG.6(d) illustrates a case of addition calculation designated by D in FIG.6(b) and FIG.6(c), wherein the addition calculation under the condition that the correlation function takes a largest value when X B is displaced to the positive side by T c time-units with respect to X A . FIG.6(e) illustrates another case of addition calculation, designated by E in FIG.6(b) and FIG.6(c), wherein the addition calculation is done for the same condition is done when X B is displaced to the negative side by T c time-units with respect to X A . In the exemplary cases shown in FIG.6(b) and FIG.6(c), there are time intervals designated by E which correspond to the time interval E of FIG.6(e). In these time intervals, time sections extending outside the overlapping time interval may overlap also to adjacent time intervals and hence it is necessary to perform the amplitude adjustments also in those adjacent time intervals. As has been described above, according to the present embodiment, signals X A and X B are multiplied respectively by window functions which are complementary to each other, one being a gradually decreasing window function and the other being a gradually increasing window function. And a signal obtained by adding these windowed signals is issued and then the signal X C is issued, and this process is repeated. Thus, a speech voice having an ample naturalness with less discontinuities in signal amplitude and also with less data drop-offs can be issued for a range of the time-scale modification ratio of 0.5 ≦ α ≦ 1.0. And by computing a correlation function between X A and X B , and adding windowed X A and X B by displacing their mutual position so that the computed correlation function takes a largest value within a time-length of unitary segment, a high quality speech voice with less discontinuities In its signal phase can be obtained. Moreover, by changing the length of X C , it becomes possible to easily change the time-scale modification ratio. FIG.7 schematically illustrates modified exemplary cases obtained by modifying the above-mentioned embodiment, wherein FIG.7(a) schematically shows a succession of segments each having a time-length of T time-units of an original voice signal on which the speech rate modification process is to be carried out, FIG.7(b) and FIG.7(c) schematically represent embodiments that the time-scale modification ratios α are 2/3 and 0.5, respectively. And, FIG.7(d) and FIG.7(e) schematically illustrate examples of detailed individual process of the addition calculation. FIG.7(d) illustrates a case of addition calculation designated by D in FIG.7(b) and FIG.7(c), wherein the addition calculation is done under a condition that the correlation function takes a largest value when X B is displaced to the positive side by T c time-units with respect to X A . FIG.7(e) illustrates another case of addition calculation designated by E in FIG.7(b) and FIG.7(c), wherein the addition calculation for the same condition is done when X B is displaced to the negative side by T c time-units with respect to X A and time sections extending outside the leading and rear edges of the overlapping time Interval are discarded. In these exemplary cases shown In FIG.7(b) and FIG.7(c), too, there are time intervals designated by E which correspond to the time interval E of FIG.7(e). In these time intervals, time sections extending outside the overlapping time interval are discarded as shown in FIG.7(e). This modified method can be realized by changing the window function. This modified method enables realizing a simplification of process described above without suffering a degradation in the recognizability of the speech voice. [3rd METHOD-EMBODIMENT] In the following, elucidation is given on the third embodiment of the speech rate modification method of the present invention referring to drawings of FIG.8 through FIG.10. The present embodiment is to offer a method of speech rate modification which is capable of giving a speech voice having an ample naturalness with less discontinuities in signal amplitude and phase for a range of the time-scale modification ratio of α ≦ 0.5. FIG.8 shows a flow chart representing a speech rate modification method in the present embodiment, and the same hardware as shown in Fig. 1 is used. Its operation is elucidated below. First, an input pointer is reset (step 802). Then, a signal X A having a time-length as long as T time-units starting from a time point designated by this input pointer is inputted (step 803). Then, (1-α)T/α is added to the input pointer to update it (step 804). Next, a signal X B having the same time-length as long as T time-units starting from a time point designated by this updated input pointer is inputted (step 805). And T is added to the input pointer to update (step 806). Then a correlation function between X A and X B is computed (step 807). Based on this correlation function thus obtained, X A is multiplied by a window of a gradually decreasing function (step 808). Also based on this correlation function obtained, X B is multiplied by a window of a gradually increasing function (step 809). Then based also on the correlation function obtained, these windowed X A and X B are added to each other after they are displaced at a point at which the correlation function between X A and X B takes a largest value within a time-length of unitary segment and the added result is issued (step 810). Then the step returns to the step 803. FIG.9 schematically represents actual exemplary cases, wherein FIG.9(a) schematically shows a succession of segments each having a time-length of T time-units of original voice signals on which speech rate modification process is to be carried out, FIGs.9(b) and (c) schematically represent embodiments that the time-scale modification ratios α are 1/3 and 1/4, respectively, and FIGs.9(d) and (e) schematically illustrate examples of individual detailed process of the addition calculation with mutual; FIG.9(d) illustrates a case of addition calculation designated by D in FIG.9(b) and FIG.9(c), wherein the addition calculation under the condition that the correlation function takes a largest value when X B is displaced to the positive side by T c time-units with respect to X A . FIG.9(e) illustrates another case of addition calculation designated by E in FIG.9(b) and FIG.9(c), wherein the addition calculation is done for the same condition when X B is displaced to the negative side by T c time-units with respect to X A . In the exemplary cases shown in FIGs.9(b) and (c), there are time intervals designated by E which correspond to the time interval E of FIG.9(e). In these time intervals, time sections extending outside the overlapping time interval may overlap also to adjacent time intervals and hence it is necessary to perform the amplitude adjustments also in those adjacent time intervals. As has been described above, according to the present embodiment, signals X A and X B are multiplied respectively by window functions which are complementary to each other, one being a gradually increasing window function and the other being a gradually decreasing window function. And a signal obtained by adding these windowed signals is issued. And this process is repeated. Thus, a speech voice having an ample naturalness with less discontinuities in signal amplitude can be issued for a range of the time-scale modification ratio of α ≦ 0.5. And by computing a correlation function between X A and X B , and adding windowed X A and X B by displacing their mutual position so that the computed correlation function takes a largest value within a time-length of unitary segment, a high quality speech voice with less discontinuities in the signal phase can be issued. Moreover, by changing the time interval between X A and X B , it becomes possible to easily change the time-scale modification ratio. FIG.10 schematically illustrates modified exemplary cases obtained by modifying the above-mentioned embodiment, wherein FIG.10(a) schematically shows a succession of segments each having a time-length of T time-units of an original voice signal on which the speech rate modification process is to be carried out, FIGS.10(b) and (c) schematically represent embodiments that the the time-scale modification ratios α are 1/3 and 1/4, respectively, and FIGS.10(d) and (e) schematically illustrate examples of detailed individual process of the addition calculation. FIG.10(d) illustrates a case of addition calculation designated by D in FIG.10(b) and FIG.10(c), wherein the addition calculation is done under a condition that the correlation function takes a largest value when X B is displaced to the positive side by T c time-units with respect to X A . FIG.10(e) illustrates another case of addition calculation designated by E in FIG.10(b) and FIG.10(c). wherein the addition calculation for the same condition is done when X B is displaced to the negative side by T c time-units with respect to X A , and time sections extending outside the leading and rear edges of the overlapping time Interval are discarded. In these exemplary cases shown in FIGS.10(b) and (c), too, there are time intervals designated by E which correspond to the time interval E of FIG.10(e). In these time intervals, time sections extending outside the overlapping time interval are discarded as shown in FIG.10(e). This modified method can be realized by changing the window function. This modified method enables realizing a simplification of process described above without suffering a degradation in the recognizability of the speech voice. [4th METHOD-EMBODIMENT] In the following, elucidation is given on the fourth embodiment of the speech rate modification method of the present invention referring to FIGs.11 and 12. The present embodiment is to offer a method of speech rate modification which is capable of giving a speech voice having an ample naturalness with less discontinuities in signal amplitude and phase and also with less data drop-offs also for a range of the time-scale modification ratio of α ≦ 0.5. FIG.11 shows a flow chart representing a speech rate modification method in the present method-embodiment, and the same hardware as shown in FIG.1 is used. Its operation is elucidated below. First, an input pointer is reset (step 1102). Next, an output pointer is reset (step 1103). Then, a signal X having a time-length as long as T/(1-α) time-units starting from a time point designated by this input pointer is inputted (step 1104). Then, T/(1-α) is added to the input pointer to update it (step 1105). Next, a correlation function between X and the output of one segment before is computed by having a time point of the output pointer as its reference (step 1106). Based on this correlation function thus obtained, X is multiplied by a window of a gradually increasing function at its leading-half part and a gradually decreasing function at its rear-half part (step 1107). Then based also on the correlation function obtained, this windowed X is added to the output signal so that the correlation function takes a largest value within a time-length of unitary segment and the added result is issued (1108). Then αT/(1-α) is added to the output pointer to update it (step 1109). Next, step returns to the step 1104. FIG.12 schematically represents actual exemplary cases, wherein the time-scale modification ratios α are 1/3 and 1/4. As has been described above, according to the present embodiment, X is multiplied by a window function which increases gradually at its leading-half part and a gradually decreasing function at its rear-half part on X. Then this windowed X is added on the output signal and issued. And this process is repeated. Thus, a speech voice having an ample naturalness with less discontinuities in signal amplitude and also with less data drop-offs can be issued for a range of the time-scale modification ratio of α ≦ 0.5. And by computing a correlation function between X and one segment before, and adding them by displacing their mutual position so that their correlation function takes a largest value within a time-length of unitary segment, a high quality speech voice with less discontinuities in the signal phase can be issued. Moreover, by changing the amount of shifting between the input pointer and the output pointer, it becomes possible to easily change the time-scale modification ratio. The present invention is to offer a speech rate modification apparatus which is capable of giving a speech voice having an ample naturalness with less discontinuities in signal amplitude and phase and also with less data drop-offs and also which can be realized with a simple hardware. [SECOND APPARATUS-EMBODIMENT] In the following, elucidation is given on the second or improved apparatus-embodiment of a speech rate modification of the present invention referring to FIGs.13 through 15. The apparatus is improved to achieve an intended accurate time scale of the rate-modified speech, and is applicable to the foregoing 1st through 4th method embodiments. FIG.13 is a block diagram of the improved speech rate modification apparatus in the present embodiment. In FIG.13, numeral 11 is an A/D converter for converting input voice signal to digitized voice signal. A buffer 12 is for temporarily storing the digitized voice signal. A demultiplexer 14 switches to deliver the digitized voice signal to a first memory 15, to a second memory 16, and to a multiplexer 22, being controlled by a rate control circuit 13. A correlator 17 is for computing correlation function between outputs of the first memory 15 and the second memory 16. Output terminals of the correlator 17 are connected to a third multiplier 26, which multiplies the output of a weighting function generator 25 on the output of the correlator 17. The weighting function generator 25 generates weighting functions depending upon the output of a time-scale modification ratio detector 24, which detects the difference between the number of data supplied to the demultiplexer 14 and the number of data issued from the multiplexer 22 under the control of the rate control circuit 13. The output of the third multiplier 26 is supplied to the rate control circuit 13, the window function generator 18, and an adder 21. A first multiplier 19 and a second multiplier 20 are for multiplying output of the window function generator 18 on outputs of the first memory 15 and of the second memory 16, respectively. The output terminals of the multipliers 19 and 20 are connected to the adder 21 which adds outputs to each other being controlled by the output of the third multiplier 26. The multiplexer 22 is for combining outputs from the adder 21 and the demultiplexer 14 under control of the rate control circuit 13. Then a D/A converter 23 is for converting the combined digital signal to an analog output signal. On the speech rate modification apparatus constituted as has been described above, its operation is elucidated below. First, the input signal is converted into a digital signal by the A/D converter 11 and written into the buffer 12. Next, the rate control circuit 13 controls the demultiplexer 14 in accordance with a given time-scale modification ratio to supply the data in the buffer 12 to the first memory 15 and the second memory 16, and also to the multiplexer 22. And the time-scale modification ratio detector 24 detects a time-scale modification ratio presently being processed by judging from the number of data supplied to the demultiplexer 14 and the number of data issued from the multiplexer 22. And monitoring the deviation from the target time-scale modification ratio which is set in the rate control circuit 13, information thus obtained is issued to the weighting function generator 25. Next, the weighting function generator 25 corrects the weighting function to be issued in a manner that the time-scale modification ratio of speech voice data presently being processed does not deviate largely corresponding to an amount of the deviation with respect to the target time-scale modification ratio obtained from the time-scale modification ratio detector 24. Then, a correlation function between the contents of the first memory 15 and that of the second memory 16 is computed by the correlator 17. The third multiplier 26 performs a multiplication calculation between the output of the correlator 17 and the output of the weighting function generator 25. Then the information thus obtained is supplied to the rate control circuit 13, the window function generator 18, and the adder 21. And the window function generator 18 supplies a window function to the first multiplier 19 and the second multiplier 20 based on the information from the third multiplier 26. Then the first multiplier 19 performs a multiplication calculation between the contents of the first memory 15 and the first window function issued from the window function generator 18, whereas the second multiplier 20 performs a multiplication calculation between the contents of the second memory 16 and the second window function issued also from the window function generator 18. The adder 21 performs an addition calculation between the output of the first multiplier 19 and the output of the second multiplier 20 after displacing their mutual position so that the weighted correlation function takes a largest value within a time-length of unitary segment based on the information from the third multiplier 26 and supplies its output to the multiplexer 22. Then the multiplexer 22 selects the output of the adder 21 and the output of the multiplexer 14 and supplies the selected result to the D/A converter 23, which converts the resultant digital signal to an analog signal. FIG.14 and FIG.15 show examples of weighting functions issued from the weighting function generator 25. In these figures, each abscissa represents mutual delay between two segments whereon the correlation function is computed. Fig.14 shows a weighting function by which the largest value of the correlation function is searched only at a side wherein the deviation is made less. FIG.14(a) shows a case that the deviation from the target time-scale modification ratio increases when the largest value of the correlation function is present on the negative side. FIG.14(b) shows a case that the presently processed time-scale modification ratio does not deviate from the target time-scale modification ratio. And, FIG.14(c) shows a case that the deviation from the target time-scale modification ratio increases when the largest value of the correlation function is present at the positive side. FIG.15 shows a weighting function which searches, in case that the presently processed time-scale modification ratio deviates from the target time-scale modification ratio, the largest value of the correlation function by putting a weight on the side on which the deviation is made less. FIG.15(a) shows a case that the deviation from the target time-scale modification ratio increases when the largest value of the correlation function is present on the negative side. FIG.15(b) shows a case that the presently processed time-scale modification ratio does not deviate from the target time-scale modification ratio. And, FIG.15(c) shows a case that the deviation from the target time-scale modification ratio increases when the largest value of the correlation function is present on the positive side. As has been described above, according to the present embodiment, similarly to the first apparatus embodiment of FIG.1, by using the first multiplier 19 and the second multiplier 20, the contents of the first memory 15 and the contents of the second memory 16 are multiplied respectively by a window function generated from the window function generator 18. Then those windowed outputs from respective multipliers are added to each other by the adder 21. Thus, a speech voice having an ample naturalness with less discontinuities in the signal amplitude and also with less data drop-offs can be obtained. And the correlator 17 computes a correlation function between the contents of the first memory 15 and the contents of the second memory 16. The adder 21 performs an addition calculation between the outputs from the first multiplier 19 and from the second multiplier 20 after displacing their mutual position so that the correlation function between the output of the first multiplier 19 and the output of the second multiplier 20 takes a largest value within a time-length of unitary segment. Thus, thereby the discontinuities in the phase of the signal is reduced. When the addition calculations are performed succesively at those parts at which the correlation function takes a largest value within a time-length of unitary segment, the time-scale modification ratio actually obtained may deviates from the target time-scale modification ratio. Then, according to the configuration of FIG.13, the time-scale modification ratio actually being processed is detected by the time-scale modification ratio detector 24, and thereby the deviation from the target value is monitored. Responding to the deviation, the weighting function generator 25 changes the weighting function and issues it. Thus, the deviation from the target time-scale modification ratio can easily be reduced and and also a time position at which the correlation function takes a largest value within a time-length of unitary segment can be found. Thereby a high quality processed speech voice with less time scale fluctuations can be obtained with a desired time-scale modification ratio. [5th METHOD-EMBODIMENT] In the following, elucidation is given on the fifth embodiment of the speech rate modification method of the present invention referring to FIGs.16 through 18. The present embodiment is to offer a method of speech rate modification which is capable of giving a speech voice having an ample naturalness with less discontinuities in signal amplitude and phase and also with less data drop-offs for a range of the time-scale modification ratio of α ≧ 1.0. FIG.16 shows a flow chart representing a speech rate modification method in the present embodiment. Its operation is elucidated below. First, an A-pointer is set to be 0 (step 1602), while a B-pointer is set to be T (step 1603). Then, a signal X A having a time-length as long as T time-units starting from a time point designated by the A-pointer is inputted (step 1604). And, a signal X B having a time interval as long as T time-units starting from a time point designated by the B-pointer is inputted (step 1605). Then, the B-pointer is updated by inputting a number obtained by adding T on the contents of the A-pointer (step 1606). Then a correlation function between X A and X B is computed (step 1607). A time point T c (which corresponds to a time point displaced by T c from the time point when two segments completely overlap.) at which the correlation function takes its largest value within a time-length of one unitary segment is searched (step 1608). Based on this correlation function thus obtained, X A is multiplied by a window of a gradually increasing function (step 1609). Also based on this correlation function obtained, X B is multiplied by a window of a gradually decreasing function (step 1610). Then based also on the correlation function obtained, these windowed X A and X B are added to each other after they are mutually displaced at a time point at which the correlation function takes a largest value within one unitary segment (step 1611). Next, in case that T-T c is less than αT/(α-1), added signal is all issued (step 1613), further a signal X C of a time-length as long as T/(α-1)+T c time-units starting from a time point designated by the B-pointer is directly issued (step 1615). On the other hand, in case that αT/(α-1) is less than T-T the added signal is issued only for a time-length of αT/(α-1) time-units (step 1614). Next, T/(α-1)+T c is added to the B-pointer to update it (step 1616). And T/(α-1) is added to the A-pointer to update it (step 1617). Then, step returns to the step 1604. FIG.17 schematically represents actual exemplary cases, wherein FIG.17(a) schematically shows a succession of segments each having a time-length of T time-units of original voice signals on which speech rate modification process is to be carried out, FIG.17(b) and FIG.17(c) schematically represent embodiments that the time-scale modification ratios α are 2.0 and 3.0, respectively, and FIG.17(d) and FIG.17(e) schematically illustrate examples of individual detailed process of the mutual addition calculation. FIG.17(d) illustrates a case of addition calculation designated by D in FIG.17(b) and FIG.17(c), wherein the addition calculation under the condition that the correlation function takes a largest value when X B is displaced to the positive side by T c time-units with respect to X A , whereas FIG.17(e) illustrates another case of addition calculation designated by E in FIG.17(b) and FIG.17(c), wherein the addition calculation is done for the same condition when X B is displaced to the negative side by T c time-units with respect to X A . In the exemplary cases shown in FIG.17(b) and FIG.17(c), there are time intervals designated by D which correspond to the time interval D of FIG.17(d). In these time intervals, time sections extending outside the overlapping time interval may overlap also to adjacent time intervals and hence it is necessary to perform the amplitude adjustments also in those adjacent time intervals. As has been described above, according to the present embodiment, signals X A and X B are multiplied respectively by window functions which are complementary to each other, one being a gradually increasing window function and the other being a gradually decreasing window function. And a signal obtained by adding these windowed signals is issued, and a signal X C subsequent to X A is issued, and these process is repeated. Thus, a speech voice having an ample naturalness with less discontinuities in signal amplitude and also with less data drop-offs can be issued for a range of the time-scale modification ratio of α ≧ 1.0. And by computing a correlation function between X A and X B , and adding windowed X A and X B by displacing their mutual position so that the correlation function obtained takes a largest value within a time-length of one unitary segment, a high quality speech voice with less discontinuities in the signal phase can be issued. Moreover, by adjusting the segment length of X C in which the input signal is directly issued, it becomes possible to easily change the time-scale modification ratio. Also, according to the above-mentioned controlling, it becomes possible to rapidly absorb such deviations in the time-scale modification ratio that might be caused by the addition calculation performed by displacing the mutual position of those windowed signals to make the correlation function take a largest value within a time-length of one unitary segment. FIG.18 schematically illustrates modified exemplary cases obtained by modifying the above-mentioned embodiment, wherein FIG.18(a) schematically shows a succession of segments each having a time-length of T time-units of an original voice signal on which the speech rate modification process is to be carried out, FIG.18(b) and FIG.18(c) schematically represent embodiments that the the time-scale modification ratios α are 2.0 and 3.0, respectively, and FIGS.18(d) and (e) schematically illustrate examples of detailed individual process of the addition calculation. FIG.18(d) illustrates a case of addition calculation designated by D in FIG.18(b) and FIG.18(c), wherein the addition calculation is done under a condition that the correlation function takes a largest value when X B is displaced to the positive side by T c time-units with respect to X A and time sections extending outside the leading and rear edges of the overlapping time interval are discarded. FIG.18(e) illustrates another case of addition calculation designated by E in FIG.18(b) and FIG.18(c), wherein the addition calculation for the same condition is done when X B is displaced to the negative side by T c time-units with respect to X A . In these exemplary cases shown in FIG.18(b) and FIG.18(c), too, there are time intervals designated by D which correspond to the time interval D of FIG.18(d). In these time intervals, time sections extending outside the overlapping time interval are discarded as shown in FIG.18(d). This modified method can be realized by changing the window function. This modified method enables realizing a simplification of process described above without suffering a degradation in the recognizability of the speech voice. [6th METHOD-EMBODIMENT] In the following, elucidation is given on the sixth embodiment of the speech rate modification method of the present invention referring to FIGS.19 through 21. The present embodiment is to offer a method of speech rate modification which is capable of giving a speech voice having an ample naturalness with less discontinuities in signal amplitude and phase and also with less data drop-offs also for a range of the time-scale modification ratio of 0.5 ≦ α ≦ 1.0. FIG.19 shows a flow chart representing a speech rate modification method in the present embodiment, and the same hardware as shown in FIG.1 is used. Its operation is elucidated below. First, an A-pointer is set to be 0 (step 1902), while a B-pointer is set to be T (step 1903). Then, a signal X A having a time-length as long as T time-units starting from a time point designated by the A-pointer is inputted (step 1904). And, a signal X B having a time interval as long as T time-units starting from a time point designated by the B-pointer is inputted (step 1905). Then, the A-pointer is updated to be a number obtained by adding T on the contents of the B-pointer (step 1906). Then a correlation function between X A and X B is computed (step 1907). A time point T c at which the correlation function takes its largest value in a time-length of one unitary segment is searched (step 1908). Based on this correlation function thus obtained, X A is multiplied by a window of a gradually decreasing function (step 1909). Also based on this correlation function obtained, X B is multiplied by a window of a gradually increasing function is multiplied on X B (step 1910). Then based also on the correlation function obtained, these windowed X A and X B are added to each other after they are mutually displaced at a time point at which the correlation function takes a largest value within a time-length of one unitary segment (step 1911). Next, in case that T+T c is less than αT/(1-a), added signal is all issued (step 1913). Further a signal X C of a time interval as long as (2α-1)T/(1-α)-T c time-units starting from a time point designated by the A-pointer is directly issued (step 1915). On the other hand, in case that αT/(1-α) is less than T+T c , the added signal is issued only for a time-length of αT/(1-α) time-units (step 1914). Next, (2α-1)T/(1-α)-T c is added to the A-pointer to update it (step 1916). And T/(1-α) is added to the B-pointer to update it (step 1917). Then, step returns to the step 1904. FIG.20 schematically represents actual exemplary cases, wherein FIG.20(a) schematically shows a succession of segments each having a time-length of T time-units of original voice signals on which speech rate modification process is to be carried out, FIG.20(b) and FIG.20(c) schematically represent embodiments that the time-scale modification ratios α are 2/3 and 0.5, respectively, and FIG.20(d) and FIG.20(e) schematically illustrate examples of individual detailed process of the mutual addition calculation. FIG.20(d) illustrates a case of addition calculation, designated by D in FIG.20(b) and FIG.20(c), wherein the addition calculation under the condition that the correlation function takes a largest value when X B is displaced to the positive side by T c time-units with respect to X A . FIG.20(e) illustrates another case of addition calculation designated by E in FIG.20(b) and FIG.20(c), wherein the addition calculation is done for the same condition when X B is displaced to the negative side by T c time-units with respect to X A . In the exemplary cases shown in FIG.20(b) and FIG.20(c), there are time intervals designated by E which correspond to the time interval E of FIG.20(e). In these time intervals, time sections extending outside the overlapping time interval may overlap also to adjacent time intervals and hence it is necessary to perform the amplitude adjustments also in those adjacent time intervals. As has been described above, according to the present embodiment, signals X A and X B are multiplied respectively by window functions which are complementary to each other, one being a gradually increasing window function and the other being a gradually decreasing window function. And a signal obtained by adding these windowed signals is issued, and a signal X C subsequent to X B is issued, and these process is repeated. Thus, a speech voice having an ample naturalness with less discontinuities in signal amplitude and also with less data drop-offs can be issued for a range of the time-scale modification ratio of 0.5 ≦ α ≦ 1.0. And by computing a correlation function between X A and X B , and adding windowed X A and X B by displacing their mutual position so that the correlation function obtained takes a largest value within a time-length of one unitary segment, a high quality speech voice with less discontinuities in the signal phase can be issued. Moreover, by adjusting the segment length of X C in which the input signal is directly issued, it becomes possible to easily change the time-scale modification ratio. Also, according to the above-mentioned controlling, it becomes possible to rapidly absorb such deviations in the time-scale modification ratio that might be caused by the addition calculation performed by displacing the mutual position of those windowed signals to make the correlation function take a largest value within a time-length of one unitary segment. FIG.21 schematically illustrates modified exemplary cases obtained by modifying the above-mentioned embodiment, wherein FIG.21(a) schematically shows a succession of segments each having a time-length of T time-units of an original voice signal on which the speech rate modification process is to be carried out, FIG.21(b) and FIG.21(c) schematically represent embodiments that the time-scale modification ratios α are 2/3 and 0.5, respectively, and FIG.21(d) and FIG.21(e) schematically illustrate examples of detailed individual process of the addition calculation. FIG.21(d) illustrates a case of addition calculation designated by D in FIG.21(b) and FIG.21(c), wherein the addition calculation is done under a condition that the correlation function takes a largest value when X B is displaced to the positive side by T c time-units with respect to X A . FIG.21(e) illustrates another case of addition calculation, designated by E in FIG.21(b) and FIG.21(c), wherein the addition calculation for the same condition Is done when X B is displaced to the negative side by T c time-units with respect to X A and time sections extending outside the leading and rear edges of the overlapping time interval are discarded. In these exemplary cases shown in FIG.21(b) and FIG.21(c), too, there are time intervals designated by E which correspond to the time interval E of FIG.21(e). In these time intervals, time sections extending outside the overlapping time interval are discarded as shown in FIG.21(e). This modified method can be realized by changing the window function. This modified method enables realizing a simplification of process described above without suffering a degradation in the recognizability of the speech voice. [7th METHOD-EMBODIMENT] In the following, elucidation is given on the seventh embodiment of the speech rate modification method of the present invention referring to FIGs.22 through 24. The present embodiment is to offer a method of speech rate modification which is capable of giving a speech voice having an ample naturalness with less discontinuities in signal amplitude and phase for a range of the time-scale modification ratio of α ≦ 0.5. FIG.22 shows a flow chart representing a speech rate modification method in the present embodiment, and the same hardware as shown in FIG.1 is used. Its operation is elucidated below. First, an A-pointer is set to be 0 (step 2202), while a B-pointer is set to be (1-α)T/α (step 2203). Then, a signal X A having a time interval as long as T segments starting from a time point designated by the A-pointer is inputted (step 2204). And, a signal X B having a time interval as long as T segments starting from a time point designated by the B-pointer is inputted (step 2205). Then, the A-pointer is updated to be a number obtained by adding T on the contents of the B-pointer (step 2206). Then a correlation function between X A and X B is computed (step 2207). A time point T c at which the correlation function takes its largest value is searched (step 2208). Based on this correlation function thus obtained, X A is multiplied by a window of a gradually decreasing function (step 2209). Also based on this correlation function obtained, X B is multiplied by a window of a gradually increasing function. (step 2210). Then, based also on the correlation function obtained, these windowed X A and X B are added to each other after they are mutually displaced at a time point at which the correlation function takes a largest value within a time-length of one unitary segment (step 2211). Next, in case that T c is negative, added signal is all issued (step 2213). Further a signal X C of a time interval as long as -T c time-units starting from a time point designated by the A-pointer is issued (step 2215). On the other hand, in case that T c is not negative, the added signal is issued only for a time interval of T time-units (step 2214). Next, -T c is added to the A-pointer to update it (step 2216). And T/α is added to the B-pointer (step 2217). Then the step returns to the step 2204. FIG.23 schematically represents actual exemplary cases, wherein FIG.23(a) schematically shows a succession of segments each having a time-length of T time-units of original voice signals on which speech rate modification process is to be carried out, FIG.23(b) and FIG.23(c) schematically represent embodiments that the time-scale modification ratios α are 1/3 and 1/4, respectively. FIG.23(d) and FIG.23(e) schematically illustrate examples of individual detailed process of the mutual addition calculation. FIG.23(d) illustrates a case of addition calculation designated by D in FIG.23(b) and FIG.23(c), wherein the addition calculation under the condition that the correlation function takes a largest value when X B is displaced to the positive side by T c time-units with respect to X A . FIG.23(e) illustrates another case of addition calculation, designated by E in FIG.23(b) and FIG.23(c), wherein the addition calculation is done for the same condition when X B is displaced to the negative side by T c time-units with respect to X A . In the exemplary cases shown in FIGs.23(b) and (c), there are time intervals designated by E which correspond to the time interval E of FIG.23(e). In these time intervals, time sections extending outside the overlapping time interval may overlap also to adjacent time intervals and hence it is necessary to perform the amplitude adjustments also in those adjacent time intervals. As has been described above, in accordance with the present embodiment, signals X A and X B are multiplied respectively by window functions which are complementary to each other, one being a gradually increasing window function and the other being a gradually decreasing window function. And a signal obtained by adding these windowed signals is issued, and a signal X C subsequent to X B is issued, and these process is repeated. Thus, a speech voice having an ample naturalness with less discontinuities in signal amplitude can be issued for a range of the time-scale modification ratio of α ≦ 0.5. And by computing a correlation function between these windowed X A and X B , and adding windowed X A and X B by displacing their mutual position so that the computed correlation function takes a largest value within a time-length of one unitary segment, a high quality speech voice with less discontinuities in the signal phase can be obtained. Moreover, by adjusting the position of the B-pointer with respect to the A-pointer, it becomes possible to easily change the time-scale modification ratio. Also, according to the above-mentioned controlling, it becomes possible to rapidly absorb such deviations in the time-scale modification ratio that might be caused by the addition calculation performed by displacing the mutual position of those windowed signals to make the correlation function take a largest value within a time-length of one unitary segment. FIG.24 schematically illustrates modified exemplary cases obtained by modifying the above-mentioned embodiment, wherein FIG.24(a) schematically shows a succession of segments each having a time-length of T time-units of an original voice signal on which the speech rate modification process is to be carried out, FIG.24(b) and FIG.24(c) schematically represent embodiments that the the time-scale modification ratios α are 1/3 and 1/4, respectively, and FIG.24(d) and FIG.24(e) schematically illustrate examples of detailed individual process of the addition calculation. FIG.24(d) illustrates a case of addition calculation designated by D in FIG.24(b) and FIG.24(c), wherein the addition calculation is done under a condition that the correlation function takes a largest value when X B is displaced to the positive side by T c time-units with respect to X A . FIG.24(e) illustrates another case of addition calculation, designated by E in FIG.24(b) and FIG.24(c), wherein the addition calculation for the same condition is done when X B is displaced to the negative side by T c time-units with respect to X A and time sections extending outside the leading and rear edges of the overlapping time interval are discarded. In these exemplary cases shown in FIG.24(b) and FIG.24(c), too, there are time intervals designated by E which correspond to the time interval E of FIG.24(e). In these time intervals, time sections extending outside the overlapping time interval are discarded as shown in FIG.24(e). This modified method can be realized by changing the window function. This modified method enables realizing a simplification of process described above without suffering a degradation in the recognizability of the speech voice.

Claims: A speech rate modification apparatus comprising: a window function generator (18) for outputting a pair of window functions, a pair of multipliers (19, 20) each for controlling amplitude of different segments of an input signal by the pair of window functions output from said window function generator (18), and an adder (21) for performing an addition calculation of output signals of said two multipliers (19, 20) at a relative delay, characterized in that a correlator (17) is provided for calculating a correlation function between said different segments of an input signal and outputting data of a time point at which the value of the correlation function is maximum, said window function generator is for outputting said pair of window functions on the basis of the output of said correlator, said relative delay is defined as the delay at which said correlation function takes a highest value, said adder is for receiving the output of said correlator (17), and a selection circuit (22) is provided for switching over said input signal and the output of said adder (21) on the basis of a time-scale modification ratio α (= output time duration/input time duration). A speech rate modification apparatus in accordance with claim 1, characterized in that a first memory (15) is provided for storing said input signal, a second memory (16) is provided for storing said input signal subsequent to the contents of said first memory (15), said correlator (17) computes said correlation function between contents of said first memory (15) and contents of said second memory (16) and outputs data of a time point at which the value of the correlation function is maximum, said window function generator (18) is generating and issuing two complementary window functions based on said output of said correlator (17), a first multiplier (19) of said pair of multipliers is for multiplying said contents of said first memory (15) by one output of said window function generator, and a second multiplier (20) of said pair of multipliers is for multiplying said contents of said second memory (16) by the other output of said window function generator (18). A speech rate modification apparatus according to claim 1, characterized in that a time-scale modification ratio detector (24) is provided for detecting the deviation of an actual time-scale modification ratio from a target time-scale modification ratio, a weighting function generator (25) is provided for generating a weighting function based on the output of said time-scale modification ratio detector (24), a third multiplier (26) is provided for multiplying the output of said correlator (17) by an output of said weighting function generator (25), said adder (21) is for performing an addition calculation of said signals at a time point at which a weighted correlation function takes a highest value on the basis of the output of said third multiplier (26). A speech rate modification apparatus in accordance with claim 3, characterized in that a first memory is provided for memorizing an input signal, a second memory is provided for memorizing said input signal subsequent to contents of said first memory, said correlator computes said correlation function between said contents of said first memory and said contents of said second memory, said target time-scale modification ratio is α (= output time duration/input time duration), said weighting function generator generates weighting functions based upon output of said time-scale modification ratio detector, a third multiplier is provided for multiplying the output of said correlator by the output of said weighting function generator, a maximum value detector is provided for deriving a time point at which the output of said third multiplier is maximum, a window function generator is provided for generating two complementary window functions on the basis of the output of said maximum value detector, a first multiplier is provided for multiplying the contents of said first memory by one output of said window function generator, a second multiplier is provided for multiplying said contents of said second memory by the other output of said window function generator, and there are provided an adder for performing an addition calculation of the output of said first multiplier and the output of said second multiplier at a time point at which said correlation function takes a highest value based on the output of said maximum value detector, and a selection circuit for switching between the input signal and the output of said adder on the basis of the time-scale modification ratio. A speech rate modification apparatus in accordance with claim 4, wherein: said weighting function generator issues said weighting function on the basis of said deviation between a target time-scale modification ratio α (= output time duration/input time duration) and an actually resulted time-scale modification ratio issued from said time-scale modification ratio detector, in a manner that: in case of the actually resulted time-scale modification ratio being longer than a target time-scale modification ratio α, the highest value of the correlation function is selected at a time point at which a time length of a time-part of the output of the adder wherein the weighted addition is performed, is made shorter with a higher probability than in case of said weighting function not being used, and in case of the actually resulted time-scale modification ratio being shorter than said target time-scale modification ratio α, the highest value of the correlation function is selected at a time point at which a time length of a time-part of the output of the adder wherein the weighted addition is performed, is made longer with a higher probability than in case of said weighting function not being used. A speech rate modification apparatus according to claim 1, characterized in that said selection circuit is switching over said input signal and the output of said adder on the basis of the value of the time-scale modification ration α (= output time duration/input time duration) and a time point T c at which the correlation function is maximum. Method for modifying a speech rate comprising the following steps: computing a correlation function between a first signal and a second signal subsequent to said first signal and deriving a time point at which the value of the correlation function is maximum, displacing said first signal and said second signal mutually at said time point at which the correlation function takes a highest value, determining two complementary window functions on the basis of said time point at which the value of the correlation function is maximum, multiplying said first signal and said second signal by said complementary window functions, adding said first signal multiplied by said first window function to said second signal multiplied by said second window function to issue an added result, issuing a third signal subsequent to said added output signal for a time interval decided on the basis of the time-scale modification ratio desired, and repeating all the above-mentiond steps. Method for modifying a speech rate for changing the speech reproduction time interval by 1.0 times or more in accordance with claim 7, characterized in that said first window function gradually increases, said second window function gradually decreases, said third signal is issued subsequent to said first signal of an original input signal for a time interval decided on the basis of the time-scale modification ratio. Method for modifying a speech rate for changing the speech reproduction time interval by 1.0 times or more in accordance with claim 8 comprising the following steps: deriving a correlation function in a range, being shorter than a time length T with respect to a positive direction in which said second signal is moved, to a direction with respect to said first signal, and a negative direction in which said second signal is moved, to the inverse direction of said direction with respect to said first signal from a reference time point at which the starting point of said first signal is in coincidence with the starting point of said second signal, in said first signal of the time length T and said second signal of the time length T, and deriving a time point T c at which the value of said correlation function becomes a maximum value, displacing said first signal with respect to said second signal at said time point at which the correlation function takes a highest value, multiplying said first signal by a window function whose amplitude, decided on the basis of the time point at which the value of the correlation function is maximum, gradually increases, multiplying said second signal by a window function whose amplitude, decided on the basis of the time point at which the value of the correlation function is maximum, gradually decreases, adding said first signal multiplied by said window function to said second signal multiplied by said window function and outputting them, issuing said third signal of a time-length of {T/(α-1)} time-units subsequent to the first signal decided on the basis of the time-scale modification ratio α (= output time duration/input time duration), taking a starting point of said first signal at the next process to be a point at which the starting point of said first signal is delayed by a time interval of {T/(α-1)} time-units, and repeating all the above-mentioned steps. Method for modifying a speech rate for changing the speech reproduction time interval of a range of from 0.5 times to 1.0 times in accordance with claim 7 comprising the following steps: computing a correlation function between a first signal and a second signal subsequent to the first one and deriving a time point at which the value of the correlation function is maximum, displacing said second signal with respect to said first signal at a time point at which the correlation function takes a highest value, multiplying said first signal by a first window function whose amplitude, decided on the basis of the time point at which the value of the correlation function is maximum, gradually decreases, multiplying said second signal by a second window function whose amplitude, decided on the basis of the time point at which the value of the correlation function is maximum, gradually increases, adding said first signal multiplied by said first window function to said second signal multiplied by said second window function to issue the added result, issuing a third signal subsequent to said second signal of an original input signal for a time interval decided on the basis of the time-scale modification ratio, repeating all the above-mentioned steps. Method for modifying a speech rate for changing the speech reproduction time interval of a range of from 0.5 times to 1.0 times in accordance with claim 10 comprising the following steps: deriving a correlation function in a range being shorter than a time length T with respect to a positive direction, in which said second signal is moved, to a direction with respect to said first signal and a negative direction, in which said second signal is moved, to the inverse direction of said direction with respect to said first signal from a reference time point at which the starting point of said first signal is in coincidence with the starting point of said second signal, in said first signal of the time length T and said second signal of the time length T, and deriving a time point T c at which the value of said correlation function becomes a maximum value, displacing said second signal with respect to said first signal at said time point at which the correlation function takes a highest value, multiplying said first signal by a window function whose amplitude, decided on the basis of the time point at which the value of the correlation function is maximum, gradually decreases, multiplying said second signal by a window function whose amplitude, decided on the basis of the time point at which the value of the correlation function is maximum, gradually increases, adding said first signal multiplied by said first window function to said second signal multiplied by said second window function to issue an added result, issuing the third signal of a time-length of {(2α-1)T/(1-α)} time-units subsequent to the second signal decided on the basis of the time-scale modification ratio, taking a starting point of said first signal at the next process to be a next point to a terminal point of said third signal, and repeating all the above-mentioned steps. Method for modifying a speech rate for changing the speech reproduction time interval by 0.5 or less in accordance with claim 7 comprising the following steps: setting a starting point of a second signal to a time point at which a first signal is delayed by such a time interval so as to make desired time-scale modification ratio α (= output time duration/input time duration), computing a correlation function between a first signal and a second signal and deriving a time point at which the value of the correlation function is maximum, displacing said second signal with respect to said first signal to a time point at which said correlation function takes a highest value, multiplying said first signal by a window function whose amplitude, decided on the basis of the time point at which the value of the correlation function is maximum, gradually decreases, multiplying said second signal by a window function whose amplitude, decided on the basis of the time point at which the value of the correlation function is maximum, gradually increases, adding said first signal multiplied by said first window function to said second signal multiplied by said second window function to issue an added result, taking a starting point of said first signal at the next process to be a point next to a terminal point of said second signal, and repeating all the above-mentioned steps. Method for modifying a speech rate for changing the speech reproduction time interval by 0.5 or less in accordance with claim 12 comprising the following steps: setting a starting point of a second signal to a time point at which the starting point of a first signal is delayed by a time interval of {(1-α)T/α)} time-units wherein T is a time-length of one unitary segment and α is a time-scale modification ratio, deriving a correlation function in a range being shorter than a time length T with respect to a positive direction, in which said second signal is moved, to a direction with respect to said first signal, and a negative direction, in which said second signal is moved, to the inverse direction of said direction with respect to said first signal from a reference time point at which the starting point of said first signal is in coincidence with the starting point of said second signal, in said first signal of the time length T and said second signal of the time length T, and deriving a time point T c at which the value of said correlation function becomes a maximum value, displacing said second signal with respect to said first signal to a time point T c at which the correlation function takes a highest value, multiplying said first signal by a window function whose amplitude, decided on the basis of the time point at which the value of the correlation function is maximum, gradually decreases, multiplying said second signal by a window function whose amplitude, decided on the basis of the time point at which the value of the correlation function is maximum, gradually increases, adding said first signal multiplied by said first window function to said second signal multiplied by said second window function to issue the added result, taking a starting point of said first signal at the next process to be a point at which the starting point of said second signal is delayed by a time interval of T time-units, and repeating all the above-mentioned steps. Method for modifying a speech rate for changing the speech reproduction time interval by 0.5 times or less in accordance with claim 7 comprising the following steps: displacing an input signal with respect to a preceding output signal on the basis of a time-scale modification ratio α (= output time duration/input time duration), computing a correlation function between said preceding output signal and said input signal and deriving a time point at which the value of the correlation function is maximum, displacing said input signal further to a time point at which the correlation function takes a highest value, multiplying said input signal by a window function whose amplitude, decided on the basis of the time point at which the value of the correlation function is maximum, gradually increases at its front-half part and gradually decreases at its rear-half part, adding said input signal multiplied by said window function to said output signal to issue the added result, and repeating all the above-mentioned steps. Method for modifying a speech rate for changing the speech reproduction time interval by 0.5 times or less in accordance with claim 14 comprising the following steps: displacing an input signal of a time length of {T/(1-α)} time-units to a point at which a starting point of a preceding output signal is delayed by a time interval of {αT/(1-α)} time-units, computing a correlation function between said preceding output signal and said input signal and deriving a time point at which the value of the correlation function is maximum, displacing said input signal to a time point at which said correlation function takes a highest value, multiplying said input signal by a window function whose amplitude, decided on the basis of the value of the time-scale modification ratio α and the time point at which the value of the correlation function is maximum, gradually increases at its front-half part and gradually decreases at its rear-half part, adding said input signal multiplied by the window function to said output signal, taking a starting point of said input signal at the next process to be a point at which the starting point of said input signal is delayed by a time interval of {T/(1-α)} time-units, and repeating all the above-mentioned steps. Method for modifying a speech rate according to claim 7, characterized in that a correlation function between a first signal and a second signal is computed and a time point at which the value of the correlation function is maximum, is derived and said third signal is issued subsequent to said added output signal for a time interval, decided on the basis of the time-scale modification ratio α, and a time point T c at which the value of the corrrelation function is maximum to produce a desired time-scale modification ratio. Method for modifying a speech rate for changing the speech reproduction time interval by 1.0 times or more in accordance with claim 16, characterized in that said third signal is issued subsequent to said first signal for a time-length which is determined on the basis of a time-scale modification ratio α and a time point T c at which said correlation function takes a highest value within a time-length of one unitary segment in a manner that a desired time-scale modification ratio α (= output time duration/input time duration) is realized, afterwards the starting time point of the first signal in the next process is set to be a time point at which a starting time point of said first signal is delayed by a time interval such that a desired time-scale modification ratio is produced, the starting time point of the second signal in the next process is set to be a subsequent time point of a terminal time point of said third signal, and all the above-mentioned steps are repeated. Method for modifiying a speech rate for changing the speech reproduction time interval by 1.0 times or more in accordance with claim 17 comprising the following steps: deriving a correlation function in a range being shorter than a time length T with respect to a positive direction, in which said second signal is moved, to a direction with respect to said first signal and a negative direction, in which said second signal is moved, to the inverse direction of said direction with respect to said first signal from a reference time point at which the starting point of said first signal is in coincidence with the starting point of said second signal, in said first signal of the time length T and said second signal of the time length T, and deriving a time point T c at which the value of said correlation function becomes a maximum value, displacing said first signal to a time position T c with respect to said second signal at which said correlation function takes a highest value, multiplying said first signal by a window function whose amplitude, decided on the basis of the time point T c at which the value of said correlation function is maximum, gradually increases, multiplying said second signal by a window function whose amplitude, decided on the basis of the time point T c at which the value of said correlation function is maximum, gradually decreases, adding said first signal multiplied by said first window function to said second signal multiplied by said second window function to issue an added result, issuing a third signal of a time interval of {T/(α-1)+T c } time-units subsequent to said first signal, setting a starting time of said first signal in the next process to such a time point that starting point of said first signal is delayed by a time interval of {T/(α-1)} time-units, setting said starting time of said second signal in the next process to such a time point that starting point of said first signal is delayed by a time interval of {αT/(α-1)+T c } time units, and repeating all the above-mentioned steps. Method for modifying a speech rate in accordance with claim 18, wherein: when said first signal multiplied by the first window function is added to said second signal multiplied by the second window function and an added result is issued, in case of the time interval of the added signal exceeding a time interval of {αT/(α-1)} time-units, said added signal is issued only for a time interval of {αT/(α-1)} time-units from the start of said added signal, and said third signal is not issued. Method for modifying a speech rate for changing the speech reproduction time interval of from 0.5 to 1.0 times in accordance with claim 16 comprising the following steps: computing a correlation function between a first signal and a second signal, and deriving a time point T c at which the value of the correlation function is maximum, displacing said second signal with respect to said first signal to a time point T c at which the correlation function takes a highest value, multiplying said first signal by a window function whose amplitude, decided on the basis of the time point T c at which the value of said correlation function is maximum, gradually decreases, multiplying said second signal by a window function whose amplitude, decided on the basis of the time point T c at which the value of said correlation function is maximum, gradually increases, adding said first signal multiplied by said first window function to said second signal multiplied by said second window function to issue an added result, issuing a third signal subsequent to said second signal for a time-length which is determined on the basis of time-scale modification ratio α and a time point T c at which said correlation function takes a highest value in a manner that a desired time-scale modification ratio α (= output time duration/input time duration) is realized, setting the starting time point of said first signal in the next process to be a subsequent time point of a terminal time point of said third signal, setting said starting time point of said second signal in the next process to be a time point at which a starting time point of said second signal is delayed by a time interval such that a desired time-scale modification ratio α is produced, and repeating all the above-mentioned steps. Method for modifying a speech rate for changing the speech reproduction time interval of from 0.5 to 1.0 times or more in accordance with claim 20 comprising the following steps: deriving a correlation function in a range being shorter than a time length T with respect to a positive direction, in which said second signal is moved, to a direction with respect to said first signal and a negative direction, in which said second signal is moved, to the inverse direction of said direction with respect to said first signal from a reference time point at which the starting point of said first signal is in coincidence with the starting point of said second signal, in said first signal of the time length T and said second signal of the time length T, and deriving a time point T c at which the value of said correlation function becomes a maximum value, displacing said second signal to a time position T c with respect to said first signal at which said correlation function takes a highest value within a time-length of T time-units, multiplying said first signal by a window function whose amplitude, decided on the basis of the time point T c at which the value of said correlation function is maximum, gradually decreases, multiplying said second signal by a window function whose amplitude, decided on the basis of the time point T c at which the value of said correlation function is maximum, gradually increases, adding said first signal multiplied by said first window function and said second signal multiplied by said second window function to each other to issue an added result, issuing a third signal of a time interval of {(2α-1)T/(1-α)-T c } time-units subsequent to said second signal, wherein α is time-scale modification ratio (output time duration/input time duration), setting said starting time of said first signal in the next process to such a time point that starting point of said second signal is delayed by a time interval of {αT/(1-α)-T c } time-units, setting said starting time of said second signal in the next process to be such a time point that said starting point of said second signal is delayed by a time interval of {T/(1-α)} time-units, and repeating all the above-mentioned steps. A speech rate modification method in accordance with claim 21, wherein: when said first signal multiplied by the first window function is added to said second signal multiplied by the second window function and said added result is issued, in case of the time-length of said added result exceeding a time interval of {αT/(1-α)} time-units, the added result is issued only for a time interval of {αT/(1-α)} time-units from the start of the added result, and the third signal is not issued. Method for modifying a speech rate for changing the speech reproduction time interval of 0.5 times or less in accordance with claim 16 comprising the following steps: setting initially the starting point of a second signal to such a time point that the starting point of a first signal is delayed by such a time interval as to produce a desired time-scale modification ratio α (= output time duration/input time duration), computing a correlation function between a first signal and a second signal, and deriving a time point T c at which the value of the correlation function is maximum, displacing said second signal with respect to said first signal at a time point T c at which said correlation function takes a highest value, multiplying said first signal by a window function whose amplitude, decided on the basis of the time point T c at which the value of said correlation function is maximum, gradually decreases, multiplying said second signal by a window function whose amplitude, decided on the basis of the time point T c at which the value of said correlation function is maximum, gradually increases, adding said first signal multiplied by said first window function to said second signal multiplied by said second window function to issue an added result, issuing said added signal as well as a third signal, which is subsequent to said second signal, for a time-length such that a desired time-scale modification ratio is made, setting a starting time of the first signal in the next process to be a next time point of the terminal time point of the issued signal, setting a starting time of the second signal in the next process to be such a time point that said starting point of said second signal is delayed by such a time interval as to produce a desired time-scale modification ratio, and repeating all the above-mentioned steps except for said initial setting. Method for modifying a speech rate for changing the speech reproduction time interval of 0.5 times or less in accordance with claim 23 comprising the following steps: setting initially the starting point of a second signal to such a time point that the starting point of a first signal is delayed by a time interval of {(1-α)T/α} time-units, deriving a correlation function in a range being shorter than a time length T with respect to a positive direction, in which said second signal is moved, to a direction with respect to said first signal and a negative direction, in which said second signal is moved, to the inverse direction of said direction with respect to said first signal from a reference time point at which the starting point of said first signal is in coincidence with the starting point of said second signal, in said first signal of the time length T and said second signal of the time length T, and deriving a time point T c at which the value of said correlation function becomes a maximum value, displacing said second signal to a time point T c at which the correlation function takes a highest value, multiplying said first signal by a window function whose amplitude, decided on the basis of the time point T c at which the value of said correlation function is maximum, gradually decreases, multiplying said second signal by a window function whose amplitude, decided on the basis of the time point T c at which the value of said correlation function is maximum, gradually increases, adding said first signal multiplied by said first window function to said second signal multiplied by said second window function to issue an added result, issuing, when T c is negative, a third signal of a time length of -T c subsequent to said second signal after issuing said added result, issuing, when T c is not negative, said added result for a time length of T time-units from said starting point of the added result, setting the starting time of said first signal in the next process at such a time point that the starting point of the second signal is delayed by a time interval of {T-T c } time-units, setting said starting point of said second signal in the next process at such a time point that the starting point of said second signal is delayed by a time interval of {T/α} time-units, and repeating all the above-mentioned steps except for said initial setting. A speech rate modification method in accordance with one of the claims 7 to 24, wherein: said first signal and said second signal are multiplied respectively by window functions which are complementary to each other, one being a gradually increasing window function and the other being a gradually decreasing window function to result a first windowed signal and a second windowed signal, and when said first windowed signal and said second windowed signal are mutually displaced so that a correlation function between said first signal and said second signal takes a highest value, and when they are afterwards added to one another, in the case where those parts gradually decreased are extending from the both edges of an overlapping part, the window functions are replaced by such a new pair of window functions which make the amplitudes of those parts extending from the both edges zero.

Inventors: MISAKI MASAYUKI, SUZUKI RYOJI, MISAKI, MASAYUKI, SUZUKI, RYOJI
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1990
Main classification: G10L3
IPC codes: G10L21