Patent Publication Number: US-6711538-B1

Title: Information processing apparatus and method, and recording medium

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an information processing apparatus and method, and to a recording medium therefor. More particularly, the present invention relates to an information processing apparatus and method capable of improving the accuracy of an excitation source in the band spreading of a speech signal, obtaining a wide-band signal having no gaps, and reducing the amount of computation thereof, and to a recording medium therefor. 
     2. Description of the Related Art 
     Speech signal transmission technology is becoming prevalent. Speech signal transmission technology is applied to portable telephones, wired telephones, voice recorders, etc. Conventionally, a narrow-band signal of 300 Hz to 3400 Hz is used for transmitting and receiving this speech signal. However, since the frequency band is narrow, there is a problem in that the sound quality is poor. Therefore, in order to overcome this problem, a technique has been developed in which a narrow-band signal is used at the transmission side or in a transmission line, and the receiving side performs a band-spreading process on the received narrow-band signal so that the signal is converted into a wide-band signal. 
     FIG. 1 is a block diagram showing the construction of a conventional band-spreading apparatus for converting a narrow-band speech signal into a wide-band speech signal. 
     An α band-widening section  1  causes a prediction coefficient α N  representing a narrow-band spectrum envelope of a narrow-band speech signal snd N  to represent a wider band, and outputs it as a prediction coefficient α W  representing a wide-band spectrum envelope to a wide-band LPC (Linear Predictive Code) combining section  4 . The details of this method of determining the prediction coefficient α W  from the prediction coefficient α N  is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 11-126098. 
     An adder  2  adds together an adaptive signal (signal containing pitch components) exc PN  and a noise signal exc NN  corresponding to the narrow-band speech signal snd N , and outputs the sum, as an excitation source exc N  for a narrow-band speech signal, to an exc band-widening section  3 . The adaptive signal exc PN  and the noise signal exc NN  correspond to an output from an adaptive code book and an output from a noise code book, respectively, when a coding apparatus employing a CELP (Code Excited Linear Prediction) method is used for each of them. 
     The exc band-widening section  3  performs band-widening on the excitation source exc N  for the input narrow-band speech signal, converts it into an excitation source exc W  for wide-band speech signal, and outputs it to the wide-band LPC combining section  4 . Specifically, based on the characteristics that the excitation source is almost white noise, aliasing is generated by inserting a zero value between adjacent samples, and the excitation source exc W  for a wide-band speech signal is generated. The details of this method of determining the excitation source exc W  for a wide-band speech signal from the excitation source exc N  for a narrow-band speech signal are also disclosed in, for example, Japanese Unexamined Patent Application Publication No. 11-126098 described above. 
     The wide-band LPC combining section  4  filter-synthesizes the excitation source exc W  input from the exc band-widening section  3  by using the prediction coefficient α W  input from the α band-widening section  1  as a filtering coefficient, converts it into a first wide-band speech signal, and outputs it to a band suppression section  5 . 
     The band suppression section  5  suppresses only the frequency band contained in the narrow-band speech signal within the input first wide-band speech signal, generates a second wide-band speech signal, and outputs it to an adder  7 . That is, since distortion is contained in the first wide-band speech signal, the frequency band of the narrow-band speech signal is replaced with a narrow-band speech signal input from an oversampling apparatus  6 . As a result, distortion of an amount corresponding to the frequency band contained in the original narrow-band speech signal is reduced. 
     The oversampling apparatus  6  oversamples the input narrow-band speech signal snd N  at the sampling frequency of the wide-band speech signal, causes the sampling frequency to coincide with the sampling frequency of the wide-band speech signal, and outputs it to the adder  7 . 
     The adder  7  adds together the second wide-band speech signal input from the band suppression section  5  and the signal input from the oversampling apparatus  6 , thereby generating a final wide-band speech signal snd W , and outputting this signal. 
     Not all of the prediction coefficient α N , the adaptive signal exc PN , the noise signal exc NN , and the narrow-band speech signal snd N  are independent. The prediction coefficient α N  can be determined by performing linear prediction analysis on the narrow-band speech signal snd N , and the adaptive signal exc PN  and the noise signal exc NN  can be determined by performing pitch analysis thereon. The noise signal exc NN  is a long-term predictive residual, and the sum of the adaptive signal exc PN  and the noise signal exc NN  becomes a linear predictive residual. Furthermore, the narrow-band speech signal snd N  can be determined by performing filter synthesis on the basis of the prediction coefficient α N , and the sum of the adaptive signal exc PN  and the noise signal exc NN . In addition, the prediction coefficient α N , the adaptive signal exc PN , and the noise signal exc NN  can also be determined by preprocessing the narrow-band speech signal snd N  and can also be determined on the basis of a quantized signal. 
     Next, a description is given of the operation when a conventional band-spreading apparatus converts the input narrow-band speech signal snd N  into a wide-band speech signal snd W . 
     The a band-widening section  1  causes the prediction coefficient α N  of the input narrow-band speech signal to represent a wider band, and outputs it as a prediction coefficient α W  of the wide-band speech signal to the wide-band LPC combining section  4 . 
     The adder  2  adds together the input adaptive signal exc PN  and the noise signal exc NN , and outputs an excitation source exc N  for the narrow-band speech signal to the exc band-widening section  3 . The exc band-widening section  3  performs band-widening on the excitation source exc N  for the input narrow-band speech signal, and outputs it as an excitation source exc W  for the wide-band speech signal to the wide-band LPC combining section  4 . 
     The wide-band LPC combining section  4  performs a filtering process on the excitation source exc W  for the wide-band speech signal on the basis of the prediction coefficient α W  of the input wide-band speech signal, generates a first wide-band speech signal, and outputs it to the band suppression section  5 . The band suppression section  5  suppresses the frequency band contained in the narrow-band speech signal within the input first wide-band speech signal, generates a second wide-band speech signal, and outputs it to the adder  7 . 
     The oversampling apparatus  6  oversamples the input narrow-band speech signal snd N  at the sampling frequency of the wide-band speech signal, and outputs it to the adder  7 . 
     The adder  7  adds together the second wide-band speech signal input from the band suppression section  5  and the oversampled signal input from the oversampling apparatus  6 , generates a final wide-band speech signal snd W , and outputs it. 
     The band suppression section  5  may be a high-pass filter which, instead of strictly suppressing only the frequency band of the narrow-band speech signal, for example, suppresses only a low-frequency band, and also, the band suppression section  5  may multiply a gain factor or may perform a filtering process. 
     However, in the above-described method, originally, since the excitation source formed of the linear sum of an adaptive signal and a noise signal is band-widened by inserting zero values, there is a problem in that its accuracy is not high. 
     Also, for example, in a case where the sampling frequency is limited to 8 kHz, the sampling frequency of the wide-band signal is limited to 16 kHz, and the frequency of the narrow-band excitation source is limited to 300 to 3400 Hz, in the above-described method, the frequency band of the wide-band excitation source to be obtained becomes 300 to 3400 Hz and 4600 to 7700 Hz, and the intermediate frequency band of 3400 Hz to 4600 Hz which is between them is not generated (a gap occurs). For this reason, in this wide-band excitation source, even if wide-band LPC combining is performed, the intermediate frequency band of 3400 Hz to 4600 Hz is not generated, and there is a problem in that the wide-band speech signal becomes unnatural. 
     SUMMARY OF THE INVENTION 
     The present invention has been achieved in view of such circumstances. The present invention aims to improve the accuracy of an excitation source in band spreading of a speech signal and to obtain a wide-band signal having no gaps. 
     To achieve the above-mentioned object, according to a first aspect of the present invention, there is provided an information processing apparatus comprising first generation means for generating a second adaptive signal from a first adaptive signal of a narrow-band signal; second generation means for generating a second noise signal from a first noise signal of the narrow-band signal; and third generation means for generating an excitation source for a wide-band signal by combining the second adaptive signal generated by the first generation means and the second noise signal generated by the second generation means. 
     The first adaptive signal and the second adaptive signal may contain pitch components. 
     The first generation means may generate the second adaptive signal by performing band-widening on the first adaptive signal. 
     The first generation means may generate the second adaptive signal by interpolating the first adaptive signal. 
     The first generation means may generate the second adaptive signal by interpolating the first adaptive signal and by suppressing one or plural sample data before and after the sample data of the first adaptive signal which reaches a peak value. 
     The first generation means may generate the second adaptive signal by interpolating the first adaptive signal and by suppressing sample data of the first adaptive signal having a value equal to or greater than a predetermined value or by suppressing sample data whose absolute value is equal to or greater than a predetermined value. 
     The second generation means may generate the second noise signal by performing band-widening on the first noise signal. 
     The second generation means may generate the second noise signal by adding to the first noise signal a noise signal having components which are not contained in the first noise signal. 
     The second generation means may generate the second noise signal by adding to the second noise signal formed by band-widening the first noise a noise signal having components of a frequency band which is not contained therein. 
     According to a second aspect of the present invention, there is provided an information processing method comprising a first generation step of generating a second adaptive signal from a first adaptive signal of a narrow-band signal; a second generation step of generating a second noise signal from a first noise signal of the narrow-band signal; and a third generation step of generating an excitation source for a wide-band signal by combining the second adaptive signal generated in the first generation step and the second noise signal generated in the second generation step. 
     According to a third aspect of the present invention, there is provided a program of a recording medium, comprising a first generation step of generating a second adaptive signal from a first adaptive signal of a narrow-band signal; a second generation step of generating a second noise signal from a first noise signal of the narrow-band signal; and a third generation step of generating an excitation source for a wide-band signal by combining the second adaptive signal generated in a process of the first generation step and the second noise signal generated in a process of the second generation step. 
     According to a fourth aspect of the present invention, there is provided an information processing apparatus comprising first generation means for generating a second noise signal from a first noise signal of a narrow-band signal; and second generation means for directly generating an excitation source for a wide-band signal, from the second noise signal generated by the first generation means. 
     The first generation means may generate the second noise signal by adding to the first noise signal a noise signal having components which are not contained in the first noise signal. 
     The first generation means may generate the second noise signal by adding to the second noise signal formed by band-widening the first noise signal a noise signal having components of a frequency band which is not contained therein. 
     According to a fifth aspect of the present invention, there is provided an information processing method comprising a first generation step of generating a second noise signal from a first noise signal of a narrow-band signal; and a second generation step of directly generating an excitation source for a wide-band signal, from the second noise signal generated in a process of the first generation step. 
     According to a sixth aspect of the present invention, there is provided a program of a recording medium, comprising a first generation step of generating a second noise signal from a first noise signal of a narrow-band signal; and a second generation step of directly generating an excitation source for a wide-band signal, from the second noise signal generated in a process of the first generation step. 
     According to a seventh aspect of the present invention, there is provided an information processing apparatus comprising first extraction means for extracting a short-term predictive residual signal on the basis of the analysis result of a narrow-band signal; second extraction means for extracting a first adaptive signal and a first noise signal by performing long-term prediction on the basis of the short-term predictive residual signal extracted by the first extraction means; first generation means for generating a second adaptive signal from the first adaptive signal extracted by the second extraction means; second generation means for generating a second noise signal from the first noise signal extracted by the second extraction means; and third generation means for generating an excitation source for a wide-band signal by combining the second adaptive signal generated by the first generation means and the second noise signal generated by the second generation means. 
     The first adaptive signal and the second adaptive signal may contain pitch components. 
     The first generation means may generate the second adaptive signal by performing band-widening on the first adaptive signal. 
     The first generation means may generate the second adaptive signal by interpolating the first adaptive signal. 
     The first generation means may generate the second adaptive signal by interpolating the first adaptive signal and by suppressing one or plural sample data before or after sample data of the first adaptive signal which reaches a peak value. 
     The first generation means may generate the second adaptive signal by interpolating the first adaptive signal and by suppressing sample data of the first adaptive signal having a value equal to or greater than a predetermined value or by suppressing sample data whose absolute value is equal to or greater than a predetermined value. 
     The second generation means may generate the second noise signal by performing band-widening on the first noise signal. 
     The second generation means may generate the second noise signal by adding to the first noise signal a noise signal having components which are not contained in the first noise signal. 
     The second generation means may generate the second noise signal by adding to a noise signal formed by band-widening the first noise signal a noise signal having components of a frequency band, which are not contained therein. 
     According to an eighth aspect of the present invention, there is provided an information processing method comprising a first extraction step of extracting a short-term predictive residual signal on the basis of the analysis result of a narrow-band signal; a second extraction step of extracting a first adaptive signal and a first noise signal by performing long-term prediction on the basis of the short-term predictive residual signal extracted in a process of the first extraction step; a first generation step of generating a second adaptive signal from the first adaptive signal extracted in a process of the second extraction step; a second generation step of generating a second noise signal from the first noise signal extracted in a process of the second extraction step; and a third generation step of generating an excitation source for a wide-band signal by combining the second adaptive signal generated in a process of the first generation step and the second noise signal generated in a process of the second generation step. 
     According to a ninth aspect of the present invention, there is provided a program of a recording medium, comprising a first extraction step of extracting a short-term predictive residual signal on the basis of the analysis result of a narrow-band signal; a second extraction step of extracting a first adaptive signal and a first noise signal by performing long-term prediction on the basis of the short-term predictive residual signal extracted in a process of the first extraction step; a first generation step of generating a second adaptive signal from the first adaptive signal extracted in a process of the second extraction step; a second generation step of generating a second noise signal from the first noise signal extracted in a process of the second extraction step; and a third generation step of generating an excitation source for a wide-band signal by combining the second adaptive signal generated in a process of the first generation step and the second noise signal generated in a process of the second generation step. 
     According to a tenth aspect of the present invention, there is provided an information processing apparatus comprising first extraction means for extracting a short-term predictive residual signal on the basis of the analysis result of a narrow-band signal; second extraction means for extracting a first noise signal by performing long-term prediction on the basis of the short-term predictive residual signal extracted by the first extraction means; first generation means for generating a second noise signal from the first noise signal extracted by the second extraction means; and second generation means for directly generating an excitation source for a wide-band signal from the second noise signal generated by the first generation means. 
     The first generation means may generate the second noise signal by adding to the first noise signal a noise signal having components of a frequency band which is not contained in the first noise signal. 
     The first generation means may generate the second noise signal by adding to a noise signal of the wide-band signal formed by band-widening the first noise signal a noise signal having components of a frequency band which is not contained therein. 
     According to an eleventh aspect of the present invention, there is provided an information processing method comprising a first extraction step of extracting a short-term predictive residual signal on the basis of the analysis result of a narrow-band signal; a second extraction step of extracting a first noise signal by performing long-term prediction on the basis of the short-term predictive residual signal extracted in a process of the first extraction step; a first generation step of generating a second noise signal from the first noise signal extracted in a process of the second extraction step; and a second generation step of directly generating an excitation source for a wide-band signal on the basis of the second noise signal generated in a process of the first generation step. 
     According to a twelfth aspect of the present invention, there is provided a program of a recording medium, comprising a first extraction step of extracting a short-term predictive residual signal on the basis of the analysis result of a narrow-band signal; a second extraction step of extracting a first noise signal by performing long-term prediction on the basis of the short-term predictive residual signal extracted in a process of the first extraction step; a first generation step of generating a second noise signal from the first noise signal extracted in a process of the second extraction step; and a second generation step of directly generating an excitation source for a wide-band signal on the basis of the second noise signal generated in a process of the first generation step. 
     In the information processing apparatus, the information processing method, and the recording medium in accordance with the present invention, a second adaptive signal is generated from a first adaptive signal of a narrow-band signal, a second noise signal is generated from a first noise signal of the narrow-band signal, the generated second adaptive signal and the generated second noise signal are combined, and an excitation source for a wide-band signal is generated. 
     In the information processing apparatus, the information processing method, and the recording medium in accordance with the present invention, a second noise signal is generated from a first noise signal of a narrow-band signal, and an excitation source for a wide-band signal is generated directly from the generated second noise signal. 
     In the information processing apparatus, the information processing method, and the recording medium in accordance with the present invention, a short-term predictive residual signal is extracted from the analysis result of a narrow-band signal, long-term prediction is performed on the basis of the extracted short-term predictive residual signal, the first adaptive signal and the first noise signal are extracted, a second adaptive signal is generated from the extracted first adaptive signal, a second noise signal is generated from the extracted first noise signal, the generated second adaptive signal and the generated second noise signal are combined, and an excitation source for a wide-band signal is generated. 
     In the information processing apparatus, the information processing method, and the recording medium in accordance with the present invention, a short-term predictive residual signal is extracted from the analysis result of a narrow-band signal, long-term prediction is performed on the basis of the extracted short-term predictive residual signal, a first noise signal is extracted, a second noise signal is generated from the extracted first noise signal, and an excitation source for a wide-band signal is produced directly from the generated second noise signal. 
    
    
     The above and further objects, aspects and novel features of the invention will become more fully apparent from the following detailed description when read in conjunction with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing the construction of a conventional band-spreading apparatus. 
     FIG. 2 is a block diagram showing the construction of a band-spreading apparatus to which the present invention is applied. 
     FIG. 3 is a flowchart illustrating the operation of the band-spreading apparatus of FIG.  2 . 
     FIG. 4 is a block diagram showing the construction of a band-spreading apparatus to which the present invention is applied. 
     FIG. 5 is a block diagram showing the construction of a pitch band-widening section of FIG.  4 . 
     FIG. 6 is a block diagram showing the construction of the pitch band-widening section of FIG.  4 . 
     FIG. 7 is a flowchart illustrating the operation of the band-spreading apparatus of FIG.  4 . 
     FIG. 8 is a flowchart illustrating the operation of the pitch band-widening section of FIG.  5 . 
     FIG. 9 is a flowchart illustrating the operation of the pitch band-widening section of FIG.  6 . 
     FIG. 10 is a block diagram showing the construction of a band-spreading apparatus to which the present invention is applied. 
     FIG. 11 is a flowchart illustrating the operation of the band-spreading apparatus of FIG.  10 . 
     FIG. 12 is a block diagram showing the construction of a band-spreading apparatus to which the present invention is applied. 
     FIG. 13 is a flowchart illustrating the operation of the band-spreading apparatus of FIG.  12 . 
     FIG. 14 is a diagram illustrating media. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 2 is a block diagram showing the construction of an embodiment of a band-spreading apparatus to which the present invention is applied. In the description of the drawings of FIG.  2  and subsequent figures, portions corresponding to those of a conventional case or portions corresponding to those of FIG.  2  and subsequent figures are given the same reference numerals, and the descriptions thereof are omitted where appropriate. Also, the symbols of signals are the same as those of the conventional case. 
     In the band-spreading apparatus of FIG. 2, in place of an adder  2  and an exc band-widening section  3  of FIG. 2, an interpolation section  11 , a zero-filling section  12 , a noise addition section  13 , and an adder  14  are provided newly. 
     The band-spreading apparatus of FIG. 2 causes an adaptive signal exc PN  and a noise signal exc NN  of an input narrow-band speech signal to represent a wider band individually, after which the band-spreading apparatus adds together these signals in order to generate an excitation source exc W  for a wide-band speech signal. Exactly speaking, even if a process for band-widening is performed on the adaptive signal exc PN  of the narrow-band speech signal, there are cases in which the band is not widened. In the following, it is assumed that the adaptive signal exc PN  of the narrow-band speech signal, on which a process for band-widening is performed, is handled as a band-widened signal. 
     The interpolation section  11  increases the sampling frequency of the adaptive signal exc PN  of the input narrow-band speech signal, performs linear interpolation thereon, generates an adaptive signal exc PW  of the wide-band speech signal, and outputs it to the adder  14 . The interpolation method may be a method other than linear interpolation. For example, zero-order holding or spline interpolation may be used, and a backward linear filtering process of a zero-filling process (to be described later), a non-linear process, etc., may be used. 
     When the sampling frequency of the band-widened speech signal is n times as high as the sampling frequency of the noise signal exc NN  of the input narrow-band speech signal, the zero-filling section  12  inserts (n−1) zero values between adjacent sampling values, performs band-widening thereon at the sampling frequency, generates a noise signal of the first wide-band speech signal, and outputs it to a noise addition section  13 . That is, this insertion of the zero value causes aliasing components to be generated in the noise signal exc NN  of the narrow-band speech signal. Thereupon, since the frequency characteristics of the narrow-band speech signal are almost flat, aliasing becomes also almost flat, and the signal which is output can be used as a noise signal exc NW  of the wide-band speech signal. 
     The noise addition section  13  adds a noise signal of the frequency band which is a gap within the noise signal of the input first wide-band speech signal, generates a noise signal exc NW  of the final wide-band speech signal, and outputs it to the adder  14 . That is, in the zero-filling section  12 , when the noise signal exc NN  of the narrow-band speech signal from 0 Hz to a Nyquist frequency is not flat, the aliasing component is not flat. For example, in a case where the sampling frequency is limited to 8 kHz, the sampling frequency of the wide-band signal is limited to 16 kHz, and the noise signal of the narrow-band speech signal is limited to 300 Hz to 3400 Hz, when a zero value is inserted every other sample, the frequency band of the noise signal of the wide-band speech signal becomes from 300 Hz to 3400 Hz and 4600 Hz to 7700 Hz, and the frequency band of the noise signal of the frequency band of 3400 Hz to 4600 Hz becomes a gap. For this reason, the noise addition section  13  adds a noise signal of the wide-band speech signal of the frequency band of 3400 Hz to 4600 Hz, which is a gap. 
     The adder  14  adds together the adaptive signal exc PW  of the wide-band speech signal input from the interpolation section  11  and the noise signal exc NW  of the wide-band speech signal input from the noise addition section  13 , and outputs it as the excitation source exc W  for the wide-band speech signal to the wide-band LPC combining section  4 . 
     Next, referring to the flowchart in FIG. 3, a description is given of the operation when the band-spreading apparatus of FIG. 2 converts an input narrow-band speech signal snd N  to a wide-band speech signal snd W . 
     A prediction coefficient α N  of the narrow-band speech signal is input to the a band-widening section  1 , the adaptive signal exc PN  and the noise signal exc NN  of the narrow-band speech signal are input to the interpolation section  11  and the zero-filling section  12 , respectively, and the narrow-band speech signal snd N  is input to the oversampling apparatus  6 , thereby starting processing. 
     In step S 1 , the α band-widening section  1  causes the prediction coefficient α N  of the input narrow-band speech signal to represent a wider band, generates a prediction coefficient α W  of the wide-band speech signal, and outputs it to the wide-band LPC combining section  4 . Furthermore, the oversampling apparatus  6  oversamples the input narrow-band speech signal snd N  at the sampling frequency of the wide-band speech signal, and stores it. 
     In step S 2 , the interpolation section  11  performs linear interpolation on the adaptive signal exc PN  of the input narrow-band speech signal, causes the sampling frequency to coincide with the sampling frequency of the wide-band speech signal, generates an adaptive signal exc PW  of the wide-band speech signal, and outputs it to the adder  14 . When the sampling frequency of the wide-band speech signal is n times as high as the sampling frequency of the noise signal exc NN  of the input narrow-band speech signal, the zero-filling section  12  inserts (n−1) zero values between adjacent samples of the input narrow-band speech signal, performs band-widening thereon, generates a noise signal of the wide-band speech signal, and outputs it to the noise addition section  13 . The noise addition section  13  adds a noise signal of a frequency band, which is a gap of the noise signal of the input wide-band speech signal, to the noise signal of the input wide-band speech signal, generates a noise signal exc NW  of a final wide-band speech signal, and outputs it to the adder  14 . 
     In step S 3 , the adder  14  adds together the adaptive signal exc PW  and the noise signal exc NW  of the input wide-band speech signal, generates an excitation source exc W  for the wide-band speech signal, and outputs it to the wide-band LPC combining section  4 . 
     In step S 4 , the wide-band LPC combining section  4  performs a filtering process on the excitation source exc W  of the input band signal by using the prediction coefficient α W  of the input wide-band speech signal as a filtering coefficient, generates a first wide-band speech signal, and outputs it to the band suppression section  5 . 
     In step S 5 , the band suppression section  5  suppresses the components of the frequency band contained in the narrow-band speech signal within the frequency band of the input first wide-band speech signal, generates a second wide-band speech signal, and outputs it to the adder  7 . Furthermore, the oversampling apparatus  6  outputs the stored, oversampled narrow-band signal to the adder  7 . 
     In step S 6 , the adder  7  adds together the input second wide-band speech signal and the oversampled narrow-band speech signal, and outputs a final wide-band speech signal snd W , terminating the processing. 
     Next, referring to FIGS. 4 to  6 , a description is given of an example in which a band-widening technique differing from a band-widening technique for the adaptive signal exc PN  and the noise signal exc NN  of the narrow-band speech signal of FIG. 2 is used. 
     In the band-spreading apparatus shown in FIG. 4, in place of the interpolation section  11 , the zero-filling section  12 , and the noise addition section  13  in FIG. 2, a pitch band-widening section  21 , a noise addition section  22 , and a zero-filling section  23  are provided newly, and the remaining construction is the same as that in FIG.  2 . 
     The pitch band-widening section  21  performs band-widening on the pitch components of the adaptive signal exc PN  of the narrow-band speech signal, generates an adaptive signal exc PW  of the wide-band speech signal, and outputs it to the adder  14 . Examples of the construction of the pitch band-widening section  21  are shown in FIGS. 5 and 6. 
     An interpolation section  31  of the pitch band-widening section  21  of FIG. 5 performs an interpolation process on the adaptive signal exc PN  of the input narrow-band speech signal, causes the sampling frequency to coincide with that of the wide-band speech signal, and outputs the signal to a peak sharpening section  32 . 
     The peak sharpening section  32  detects a peak value exceeding a predetermined threshold value, of the interpolated adaptive signal exc PW  of the wide-band speech signal, forms the peak value to a more sharpened waveform by suppressing the sample values before and after the detected peak value, and outputs it to the adder  14  at a subsequent stage. As a result, higher-frequency components occur in the adaptive signal exc PW  of the band-widened speech signal. 
     This predetermined threshold value may be fixed or variable depending on a signal. Also, the amount of suppression of the sample value before and after a peak value may be at a fixed ratio or at a ratio which varies depending on a signal. Alternatively, all the sample values before and after the peak value may be suppressed to a zero value so as to obtain a pulse waveform. In addition, the number of sample values before and after the peak value, which should be suppressed, may be one or plural. 
     A gain adjustment section  41  of the pitch band-widening section  21  of FIG. 6 increases the gain of the adaptive signal exc PN  of the input narrow-band speech signal by a predetermined multiplying factor, and outputs it to an interpolation section  42 . 
     In a manner similar to the interpolation section  31  of FIG. 5, the interpolation section  42  performs an interpolation process on the adaptive signal exc PN  of the input narrow-band speech signal, causes the sampling frequency to coincide with that of the wide-band speech signal, and outputs it to a clipping section  43 . 
     The clipping section  43  detects a sample value exceeding a predetermined threshold value, clips a waveform by replacing the detected sample value with that predetermined threshold value, and outputs it to the adder  14  at a subsequent stage. Alternatively, the waveform may be clipped by a method in which the amount exceeding the threshold value may be suppressed at a predetermined ratio, and is added to the threshold value. As a result, harmonic components occur in the adaptive signal exc PW  of the band-widened speech signal. 
     Whereas the noise addition section  13  of FIG. 2 adds a noise signal of a wide-band speech signal having a frequency band which is a gap to a band-widened noise signal, the noise addition section  22  of FIG. 4 generates a noise signal of a flat narrow-band speech signal by adding to the noise signal exc NN  of the narrow-band speech signal a noise signal of a narrow-band speech signal of a frequency band which becomes a gap after being band-widened. 
     Whereas the zero-filling section  12  of FIG. 2 inserts a zero value between adjacent samples of a noise signal exc NN  of a narrow-band speech signal which is not formed flat, the zero-filling section  23  of FIG. 4 inserts a zero value to a noise signal of a narrow-band speech signal which is formed flat. 
     Next, referring to the flowchart in FIG. 7, a description is given of the operation when the band-spreading apparatus of FIG. 4 converts an input narrow-band speech signal snd N  into a wide-band speech signal snd W . 
     A prediction coefficient α N  of the narrow-band speech signal is input to the a band-widening section  1 , an adaptive signal exc PN  and a noise signal exc NN  of the narrow-band speech signal are input to the pitch band-widening section  21  and the noise addition section  22 , respectively, and a narrow-band speech signal snd N  is input to the oversampling apparatus  6 , thereby starting processing. 
     In step S 11 , the α band-widening section  1  causes the prediction coefficient α N  of the input narrow-band speech signal to represent a wider band, generates a prediction coefficient α W  for the wide-band speech signal, and outputs it to the wide-band LPC combining section  4 . Furthermore, the oversampling apparatus  6  oversamples the input narrow-band speech signal snd N  at the sampling frequency of the wide-band speech signal, and stores it. 
     In step S 12 , the pitch band-widening section  21  performs band widening on an adaptive signal exc PN  of the input narrow-band speech signal, generates an adaptive signal exc PW  of the wide-band speech signal, and outputs it to the adder  14 . The detailed operations of the pitch band-widening section  21  will be described later with reference to the flowcharts in FIGS. 8 and 9. Also, the noise addition section  22  adds to the noise signal exc NN  of the input narrow-band speech signal a noise signal of a narrow-band speech signal having components of a frequency band which is a gap after being band-widened, generates a noise signal of a flat narrow-band speech signal, and outputs it to the zero-filling section  23 . When the sampling frequency of the wide-band speech signal is n times as high as the sampling frequency of the noise signal exc NN  of the input flat narrow-band speech signal, the zero-filling section  23  inserts (n−1) zero values between adjacent samples of the noise signal exc NN  of the input narrow-band speech signal, performs band widening thereon, generates a noise signal exc NW  of the wide-band speech signal, and outputs it to the adder  14 . 
     In step S 13 , the adder  14  adds together the adaptive signal exc PW  of the input wide-band speech signal and the noise signal exc NW  of the input wide-band speech signal, generates an excitation source exc W  for the wide-band speech signal, and outputs it to the wide-band LPC combining section  4 . 
     In step S 14 , the wide-band LPC combining section  4  performs a filtering process on the excitation source exc W  of the input band signal by using the prediction coefficient α W  of the input wide-band speech signal as a filtering coefficient, generates a first wide-band speech signal, and outputs it to the band suppression section  5 . 
     In step S 15 , the band suppression section  5  suppresses the components of the frequency band contained in the narrow-band speech signal within the frequency band of the input first wide-band speech signal, generates a second wide-band speech signal, and outputs it to the adder  7 . Furthermore, the oversampling apparatus  6  outputs the stored, oversampled narrow-band signal to the adder  7 . 
     In step S 16 , the adder  7  adds together the input second wide-band speech signal and the oversampled narrow-band speech signal, and outputs a final wide-band speech signal snd W , terminating the processing. 
     Next, referring to the flowchart in FIG. 8, a description is given of the operation when the pitch band-widening section  21  of FIG. 4 is constructed as shown in FIG.  5 . 
     When the adaptive signal exc PN  of the narrow-band speech signal is input, the pitch band-widening section  21  starts processing. In step S 21 , the interpolation section  31  of the pitch band-widening section  21  performs an interpolation process, and when the sampling frequency of the adaptive signal exc PN  of the narrow-band speech signal differs from the sampling frequency of the wide-band speech signal, the sampling frequency is made to coincide with the sampling frequency of the wide-band speech signal, and the signal is output to the peak sharpening section  32 . 
     In step S 22 , the peak sharpening section  32  detects a peak value exceeding a predetermined threshold value within the input signal, suppresses the sample values before and after the peak value, generates an adaptive signal exc PW  of the wide-band speech signal, and outputs it to the adder  14 , terminating the processing. 
     Next, referring to the flowchart in FIG. 9, a description is given of the operation when the pitch band-widening section  21  of FIG. 4 is constructed as shown in FIG.  6 . 
     When the adaptive signal exc PN  of the narrow-band speech signal is input, the pitch band-widening section  21  starts processing. In step S 31 , a gain adjustment section  41  increases the gain of the adaptive signal exc PN  of the input narrow-band speech signal by a predetermined multiplying factor, and outputs it to an interpolation section  42 . 
     In step S 32 , the interpolation section  42  performs an interpolation process on the adaptive signal exc PN  of the input narrow-band speech signal, causes the sampling frequency to coincide with that of the wide-band speech signal, and outputs it to the clipping section  43 . 
     In step S 33 , the clipping section  43  detects a sample value exceeding a predetermined threshold value from the input signal, clips the waveform by replacing the detected sample value with that predetermined threshold value, and outputs it to the adder  14  at a subsequent stage, terminating the processing. 
     Next, referring to FIG. 10, a description is given of an example of a band-spreading apparatus in which an input signal is only a narrow-band speech signal snd N . In the band-spreading apparatus of FIG. 10, an LPC analysis section  51  and a pitch analysis section  52  are provided newly. An adaptive signal exc PN  output from the pitch analysis section  52  is supplied to the interpolation section  11 , and a noise signal exc NN  is supplied to the noise addition section  22 . The output of the interpolation section  11  is supplied to the adder  14 , and the output of the noise addition section  22  is supplied to the adder  14  via the zero-filling section  23 . The remaining construction of the apparatus is the same as that of the band-spreading apparatus of FIG. 2 or  4 , and the operations are also the same. 
     The LPC analysis section  51  performs short-term prediction analysis on the input narrow-band speech signal snd N  by linear prediction analysis, outputs the prediction coefficient α N  to the a band-widening section  1 , and outputs the predictive residual exc N  to the pitch analysis section  52 . This short-term prediction is not limited to linear prediction analysis, and may be PARCOR (Partial Auto-Correction Coefficient) analysis, etc. 
     The pitch analysis section  52  performs long-term prediction analysis on the input predictive residual exc N . That is, the pitch analysis section  52  calculates the difference from a past signal which is away by an amount corresponding to a pitch lag of the input predictive residual exc N , and selects a pitch lag such that the power of the residual becomes small. Alternatively, an ABS (Analysis by Synthesis) method, which is well known in CELP, etc., is used. Then, the residual signal is assumed to be the adaptive signal exc PN  of the narrow-band speech signal, the long-term predictive residual signal is assumed to be the noise signal exc NN  of the narrow-band speech signal, and these signals are output to the interpolation section  11  and the noise addition section  22 , respectively. 
     Next, referring to the flowchart in FIG. 11, a description is given of the operation of the band-spreading apparatus of FIG. 10 when a narrow-band speech signal snd N  is input thereto. 
     When the narrow-band speech signal snd N  is input, the processing is started. In step S 41 , the LPC analysis section  51  performs prediction analysis on the input narrow-band speech signal snd N , outputs the prediction coefficient α N  to the α band-widening section  1 , and outputs the predictive residual to the pitch analysis section  52 . Furthermore, the oversampling apparatus  6  oversamples the input narrow-band speech signal snd N  at the sampling frequency of the wide-band speech signal, and stores it. 
     In step S 42 , the α band-widening section  1  causes the prediction coefficient α N  of the input narrow-band speech signal to represent a wider band, generates a prediction coefficient α W  of the wide-band speech signal, and outputs it to the wide-band LPC combining section  4 . 
     In step S 43 , the interpolation section  11  performs linear interpolation on an adaptive signal exc PN  of the input narrow-band speech signal, causes the sampling frequency to coincide with the sampling frequency of the wide-band speech signal, generates an adaptive signal exc PW  of the wide-band speech signal, and outputs it to the adder  14 . Also, the noise addition section  22  adds to the noise signal exc NN  of the input narrow-band speech signal a noise signal of the narrow-band speech signal having components of a frequency band which is a gap after being band-widened, generates a noise signal of a flat narrow-band speech signal, and outputs it to the zero-filling section  23 . Then, when the sampling frequency of the wide-band speech signal is n times as high as the sampling frequency of the noise signal exc NN  of the input flat narrow-band speech signal, the zero-filling section  23  inserts (n−1) zero values between adjacent samples of the noise signal exc NN  of the input narrow-band speech signal, performs band widening thereon, generates a noise signal exc NW  of the wide-band speech signal, and outputs it to the adder  14 . 
     In step S 44 , the adder  14  adds together the adaptive signal exc PW  of the input wide-band speech signal and the noise signal exc NW  for the wide-band speech signal, generates an excitation source exc W  for the wide-band speech signal, and outputs it to the wide-band LPC combining section  4 . 
     In step S 45 , the wide-band LPC combining section  4  performs a filtering process on the excitation source exc W  of the input band signal by using the prediction coefficient α W  of the input wide-band speech signal as a filtering coefficient, generates a first wide-band speech signal, and outputs it to the band suppression section  5 . 
     In step S 46 , the band suppression section  5  suppresses the components of the frequency band contained in the narrow-band speech signal within the frequency band of the input first wide-band speech signal, generates a second wide-band speech signal, and outputs it to the adder  7 . Furthermore, the oversampling apparatus  6  outputs the stored, oversampled narrow-band signal to the adder  7 . 
     In step S 47 , the adder  7  adds together the input second wide-band speech signal and the oversampled narrow-band speech signal, and outputs a final wide-band speech signal snd W , terminating the processing. 
     Next, referring to FIG. 12, a description is given of an example of a band-spreading apparatus which does not require the adaptive signal exc PN  of the narrow-band speech signal as an input signal. 
     In the band-spreading apparatus of FIGS. 2 and 4, as an input signal, a wide-band speech signal snd N  is generated based on the prediction coefficient α N  of the narrow-band speech signal, the adaptive signal exc PN  and the noise signal exc NN  of the narrow-band speech signal, and the narrow-band speech signal snd N . 
     Generally speaking, the pitch components of a speech signal have characteristics such that the higher the frequency, the lower the intensity. Therefore, also for the excitation source for performing wide-band LPC combining, it is preferable that the higher the frequency, the lower the intensity in a similar manner. However, in order to uniquely determine the degree of this decrease in the intensity of the pitch components, there is a difficulty, such as computations becoming complex. Therefore, it is assumed that the pitch components are contained only in the frequency band of the input narrow-band speech signal and are not present in the band other than that. 
     At this time, the band suppression section  5  suppresses the frequency band of the original narrow-band speech signal within the input first wide-band speech signal, and outputs the signal as a second wide-band speech signal to the adder  7 . In this case, since pitch components are not contained in the original narrow-band speech signal, the pitch components are also not contained in this second wide-band speech signal. 
     In addition, the fact that pitch components are not contained in the second wide-band speech signal means that the excitation source for the wide-band LPC combining need not contain pitch components. That is, the excitation source for the wide-band speech signal needs only the noise signal. 
     Accordingly, FIG. 12 shows a band-spreading apparatus from which a section for processing the adaptive signal exc PN  of the narrow-band speech signal is omitted. In this apparatus, the interpolation section  11  and the adder  14  of FIG. 2 are omitted, and the noise signal exc NN  of the wide-band speech signal, which is output from the noise addition section  13 , is directly supplied to the wide-band LPC combining section  4  (supplied without adding to the adaptive signal exc PN ). 
     Next, referring to the flowchart in FIG. 13, a description is given of the operation when the band-spreading apparatus of FIG. 12 converts an input narrow-band speech signal snd N  into a wide-band speech signal snd W . 
     The processing is started when a prediction coefficient α N  of the narrow-band speech signal is input to the α band-widening section  1 , a noise signal exc NN  of the narrow-band speech signal is input to the zero-filling section  12 , and a narrow-band speech signal snd N  is input to the oversampling apparatus  6 . 
     In step S 51 , the α band-widening section  1  causes the prediction coefficient α N  of the input narrow-band speech signal to represent a wider band, generates a prediction coefficient α W  of the wide-band speech signal, and outputs it to the wide-band LPC combining section  4 . Furthermore, the oversampling apparatus  6  oversamples the input narrow-band speech signal snd N  at the sampling frequency of the wide-band speech signal, and stores it. 
     In step S 52 , when the sampling frequency of the wide-band speech signal is n times as high as the sampling frequency of the noise signal exc NN  of the input narrow-band speech signal, the zero-filling section  12  inserts (n−1) zero values between adjacent samples of the noise signal exc NN  of the input narrow-band speech signal, performs band widening thereon, generates a noise signal of the wide-band speech signal, and outputs it to the noise addition section  13 . The noise addition section  13  adds a noise signal having components of a frequency band, which is a gap of the noise signal of the input wide-band speech signal, to the noise signal of the input wide-band speech signal, generates a noise signal exc NW  of a final wide-band speech signal, and outputs it as the excitation source exc W  for the wide-band speech signal to the wide-band LPC combining section  4 . 
     In step S 53 , the wide-band LPC combining section  4  performs a filtering process on the excitation source exc W  of the input band signal by using the prediction coefficient α W  of the input wide-band speech signal as a filtering coefficient, generates a first wide-band speech signal, and outputs it to the band suppression section  5 . 
     In step S 54 , the band suppression section  5  suppresses the components of the frequency band contained in the narrow-band speech signal within the frequency band of the input first wide-band speech signal, generates a second wide-band speech signal, and outputs it to the adder  7 . Furthermore, the oversampling apparatus  6  outputs the stored, oversampled narrow-band signal to the adder  7 . 
     In step S 55 , the adder  7  adds together the input second wide-band speech signal and the oversampled narrow-band speech signal, and outputs a final wide-band speech signal snd W , terminating the processing. 
     The LPC analysis section  51  and the pitch analysis section  52  of FIG. 10 may also be provided in the band-spreading apparatus of FIG. 4 or  12 . Furthermore, in the examples shown in FIGS. 2,  4 , and  10 , the construction may be formed in such a way that the section for processing the adaptive signal exc PN  of the narrow-band speech signal is omitted, as shown in the example of FIG.  12 . 
     In the foregoing description, since the processing means for an adaptive signal and a noise signal are independent from each other, each process described in each embodiment may be interchanged as desired so as to be combined. 
     As a method of performing band widening by increasing the sampling frequency of a noise signal, zero-filling has been taken as an example. However, other methods may be used, for example, a process for performing full-wave rectification or half-wave rectification may be used. In addition, in the foregoing description, an example in which a speech signal is used has been described. However, other signals may be used, for example, a video signal may be used, and furthermore, applications to a process other than frequency conversion are also possible. 
     As has thus been described, it is possible to improve the accuracy of an excitation source for a wide-band speech signal and to improve the sound quality of a speech signal of a wide-band speech signal. Also, in a case where pitch components are contained in only the frequency band of an input narrow-band speech signal and are not present in bands other than that, it is possible to simplify the construction of an apparatus and computation processing for converting the narrow-band speech signal into a wide-band speech signal. 
     Although the above-described series of processing can be performed by hardware, it can also be performed by software. When a series of processing is performed by software, the programs making up the software are installed from a recording medium into a computer which is built into dedicated hardware or into, for example, a general-purpose computer which is capable of performing various functions by installing various programs. 
     FIG. 14 shows the construction of an embodiment of a personal computer. A CPU  101  of the personal computer controls the overall operations of the personal computer. Also, when an instruction is input by a user from an input section  106  formed of a keyboard, a mouse, etc., via a bus  104  and an input-output interface  105 , the CPU  101  executes a program stored in a ROM (Read Only Memory)  102  in response to the instruction. Alternatively, the CPU  101  loads into a RAM (Random Access Memory)  103  a program which is read from a magnetic disk  131 , an optical disk  132 , a magneto-optical disk  133 , or a semiconductor memory  134 , which is connected to a drive  110 , and which is installed into a storage section  108 , and executes it. Furthermore, the CPU  101  performs communications with the outside by controlling a communication section  109  so that data is exchanged. 
     This recording medium, as shown in FIG. 14, is constructed by not only package media formed of the magnetic disk  131  (including a floppy disk), the optical disk  132  (including a CD-ROM (Compact Disk-Read Only Memory), and a DVD (Digital Versatile Disc)), the magneto-optical disk  133  (including an MD (Mini-Disk)), or the semiconductor memory  134 , in which programs are recorded, which is distributed separately from the computer so as to distribute programs to a user, but also by the ROM  102  in which programs are recorded, a hard disk contained in the storage section  108 , etc., which are distributed to a user in a state in which these are installed in advance into the computer. 
     In this specification, steps which describe a program recorded in a recording medium, of course, include processes which are performed in a time-series manner along a written sequence and include processes which area performed in parallel or individually although these are not necessarily processed in a time-series manner. 
     According to the information processing apparatus, the information processing method, and the recording medium of the present invention, a second adaptive signal is generated from a first adaptive signal of a narrow-band speech signal, a second noise signal is generated from a first noise signal of the narrow-band speech signal, the generated second adaptive signal and the generated second noise signal are combined, and an excitation source for a wide-band speech signal is generated. Thus, it is possible to eliminate gaps of the excitation source for the wide-band speech signal and to improve the sound quality of a speech signal of the wide-band speech signal. 
     According to the information processing apparatus, the information processing method, and the recording medium of the present invention, a second noise signal is generated from a first noise signal of a narrow-band speech signal, and an excitation source for a wide-band speech signal is generated directly from the generated second noise signal. Thus, it is possible to simplify the construction of an apparatus and computation processing for converting a narrow-band speech signal into a wide-band speech signal. 
     According to the information processing apparatus, the information processing method, and the recording medium of the present invention, a short-term prediction residual signal is extracted from the analysis result of a narrow-band signal, long-term prediction is performed on the basis of the extracted short-term prediction residual signal, a first adaptive signal and a first noise signal are extracted, a second adaptive signal is generated from the extracted first adaptive signal, a second noise signal is generated from the extracted first noise signal, the generated second adaptive signal and the generated second noise signal are combined, and an excitation source for a wide-band speech signal is generated. Thus, it is possible to eliminate gaps of the excitation source for the wide-band speech signal and to improve the sound quality of a speech signal of the wide-band speech signal. 
     According to the information processing apparatus, the information processing method, and the recording medium of the present invention, a short-term prediction residual signal is extracted from the analysis result of a narrow-band signal, long-term prediction is performed on the basis of the extracted short-term prediction residual signal, a first noise signal is extracted, a second noise signal is generated from the extracted first noise signal, and an excitation source for a wide-band speech signal is generated directly from the generated second noise signal is generated from the extracted first noise signal. Thus, it is possible to simplify the construction of an apparatus and computation processing for converting a narrow-band speech signal into a wide-band speech signal. 
     Many different embodiments of the present invention may be constructed without departing from the spirit and scope of the present invention. It should be understood that the present invention is not limited to the specific embodiments described in this specification. To the contrary, the present invention is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the invention as hereafter claimed. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications, equivalent structures and functions.