Abstract:
A band broadening apparatus includes a processor configured to analyze a fundamental frequency based on an input signal bandlimited to a first band, generate a signal that includes a second band different from the first band based on the input signal, control a frequency response of the second band based on the fundamental frequency, reflect the frequency response of the second band on the signal that includes the second band and generate a frequency-response-adjusted signal that includes the second band, and synthesize the input signal and the frequency-response-adjusted signal.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation application of International Application PCT/JP2010/055962, filed on Mar. 31, 2010 and designating the U.S., the entire contents of which are incorporated herein by reference. 
     
    
     FIELD 
       [0002]    The embodiments discussed herein are related to a band broadening apparatus and method. 
       BACKGROUND 
       [0003]    In a telephone call system such as a landline telephone system or a mobile telephone system, usually bandlimited audio signals are transmitted or received. For the purpose of enhancing the sound quality, a technique is known that extends the bandwidth of bandlimited audio signals. For example, a technique is known where the folding of a digital signal is bandlimited with a low pass filter that is switched between a low cutoff frequency for a voiced interval and a high cutoff frequency for an unvoiced interval, thereby broadening the bandwidth to a higher frequency within the unvoiced interval. Another example is where a waveform of a sound source is generated from a narrow band signal, a low frequency signal obtained through a low pass filter whose cutoff frequency is the lowest frequency of a narrow band, a period of the narrow band signal, and the amplitude of the narrow band signal; and an audio signal having a broadband width is obtained by the summation of a high frequency signal obtained through a high pass filter and a high frequency component signal of an unvoiced sound. Further another example is where a fundamental frequency of a narrow band signal is extracted; a linear predictive residual is obtained from the linear predictive analysis of the narrow band signal; the linear predictive residual is shifted toward the frequency axis by the amount of an integer multiple of the fundamental frequency; a band-extended signal is obtained by the linear predictive synthesis; and a broadband audio signal is obtained by adding the narrow band signal and the band-extended signal. 
         [0004]    For examples of the technologies above, refer to Japanese Laid-open Patent Publication Nos. 2002-82685, H9-258787, and H9-55778. 
         [0005]      FIGS. 1 and 2  are diagrams depicting one example of a spectrum of an audio signal (spectrum of broadband sound) where a high frequency component has been ideally estimated from a low frequency component of a bandlimited audio signal.  FIG. 1  depicts a spectrum of broadband sound when the fundamental frequency is high (345 Hz) and  FIG. 2  depicts a case of a low fundamental frequency (125 Hz). The average of the fundamental frequency of a male voice is about 100 Hz and of a female voice 200 Hz or more. The inventors of the present invention have found a characteristic of broadband sound in that when the fundamental frequency is high, the difference of volumes (difference of power) between a high frequency region and a low frequency region is small and when the fundamental frequency is low, the difference of volumes is large (see  FIGS. 1 and 2 ). 
         [0006]    However, the conventional techniques do not consider the characteristic depicted in  FIGS. 1 and 2 . According to the conventional techniques, the high frequency component is generated in a single way irrespective of fundamental frequency. This causes a problem in that when the high frequency component having as large volume as the low frequency component is generated under a low fundamental frequency, the volume of the high frequency component becomes too large compared to an ideal volume and the sound quality is degraded. When the high frequency component has a smaller volume than the low frequency component under a high fundamental frequency, the volume of the high frequency component becomes too small compared to an ideal volume and cannot obtain sufficient band broadening effect. In other words, high quality sound cannot be produced. 
       SUMMARY 
       [0007]    According to an aspect of an embodiment, a band broadening apparatus includes a processor configured to analyze a fundamental frequency based on an input signal bandlimited to a first band, generate a signal that includes a second band different from the first band based on the input signal, control a frequency response of the second band based on the fundamental frequency, reflect the frequency response of the second band on the signal that includes the second band and generate a frequency-response-adjusted signal that includes the second band, and synthesize the input signal and the frequency-response-adjusted signal. 
         [0008]    The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
         [0009]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0010]      FIG. 1  is a diagram depicting one example of an ideal spectrum of broadband sound when the fundamental frequency is high; 
           [0011]      FIG. 2  is a diagram depicting one example of an ideal spectrum of broadband sound when the fundamental frequency is low; 
           [0012]      FIG. 3  is a block diagram depicting a band broadening apparatus according to a first example; 
           [0013]      FIG. 4  is a flowchart of a band broadening method according to the first example; 
           [0014]      FIG. 5  is a block diagram depicting a cellular phone to which the band broadening apparatus according to a second example is applied; 
           [0015]      FIG. 6  is a block diagram depicting a hardware configuration of the band broadening apparatus according to the second example; 
           [0016]      FIG. 7  is a block diagram depicting a functional configuration of the band broadening apparatus according to the second example; 
           [0017]      FIG. 8  is a diagram depicting a high frequency component created by a high frequency component generating unit; 
           [0018]      FIG. 9  is a graph of an equation according to which a gradient α is obtained from a fundamental frequency f0; 
           [0019]      FIG. 10  is a graph depicting a frequency response controlled by a frequency response control unit; 
           [0020]      FIG. 11  is a diagram depicting an output spectrum synthesized by the spectrum synthesizing unit; 
           [0021]      FIG. 12  is a flowchart of a band broadening method according to the second example; 
           [0022]      FIG. 13  is a block diagram depicting a functional configuration of the band broadening apparatus according to a third example; 
           [0023]      FIG. 14  is a graph expressing an equation for obtaining f c  from f 0 ; 
           [0024]      FIG. 15  is a graph expressing an equation for obtaining G(f) from f c ; 
           [0025]      FIG. 16  is a flowchart of the band broadening method according to the third example; 
           [0026]      FIG. 17  is a block diagram depicting a functional configuration of the band broadening apparatus according to a fourth example; 
           [0027]      FIG. 18  is a graph expressing an equation for obtaining G L  from f 0 ; 
           [0028]      FIG. 19  is a graph expressing an equation for obtaining G(f) based on G L ; and 
           [0029]      FIG. 20  is a flowchart of the band broadening method according to the fourth example. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0030]    Preferred embodiments of a band broadening apparatus and method will be explained with reference to the accompanying drawings. The band broadening apparatus and method provides high quality sound by controlling the frequency response of a band such that the power difference between an input signal and a band-extended signal becomes smaller when the fundamental frequency is high than when the fundamental frequency is low and. Embodiments do not limit the invention in any way. 
         [0031]      FIG. 3  is a block diagram depicting a band broadening apparatus according to a first example. The band broadening apparatus includes a fundamental frequency analyzing unit  1 , an out-of-band component generating unit  2 , a frequency response control unit  3 , an out-of-band component adjusting unit  4  and a signal synthesizing unit  5 . Each unit is realized by a processor executing a band broadening program. The band broadening apparatus receives an input signal that is bandlimited to the first band. The fundamental frequency analyzing unit  1  analyzes the frequency of the fundamental frequency based on the input signal. The out-of-band component generating unit  2  generates a signal that includes the second band based on the input signal. The second band is a band outside of the first band and may be a higher frequency band or lower frequency band compared with the first band. 
         [0032]    The frequency response control unit  3  controls the frequency response of the second band such that the power difference between the input signal and the signal that includes the second band becomes smaller when the fundamental frequency is high than when the fundamental frequency is low. The out-of-band component adjusting unit  4  generates a signal that includes the second band with the frequency response adjusted by reflecting the frequency response of the second band controlled by the frequency response control unit  3  on the signal having the second band generated by the out-of-band component generating unit  2 . The signal synthesizing unit  5  synthesizes the input signal and the signal generated by the out-of-band component adjusting unit  4 . A signal generated by the signal synthesizing unit  5  is output as an output signal of the band broadening apparatus. The output signal is a broadband signal that includes the first band and the second band. 
         [0033]      FIG. 4  is a flowchart depicting a band broadening method according to the first example. As depicted in  FIG. 4 , when the band broadening process is started, the band broadening apparatus analyzes, by means of the fundamental frequency analyzing unit  1 , the frequency of the fundamental frequency based on the input signal (step S 1 ). The band broadening apparatus generates, by means of the out-of-band component generating unit  2 , a signal including the second band based on the input signal (step S 2 ). The order of steps S 1  and S 2  may be switched. 
         [0034]    The band broadening apparatus controls, by means of the frequency response control unit  3 , the frequency response of the second band such that the power difference between the input signal and the signal including the second band becomes smaller when the fundamental frequency is high than when the fundamental frequency is low (step  3 ). The band broadening apparatus generates, by means of the out-of-band component adjusting unit  4 , a signal including the second band with the frequency response adjusted by reflecting the frequency response of the second band on the signal having the second band (step  4 ). The band broadening apparatus synthesizes, by means of the signal synthesizing unit  5 , the input signal and the signal including the second band with the frequency response adjusted (step S 5 ), and terminates the process. 
         [0035]    According to the first example, when the fundamental frequency of the input signal is high, the power difference (volume difference) between the input signal and the band-extended signal including the second band becomes smaller and thus an approximately ideal broadband sound spectrum as depicted in  FIG. 1  is obtained. Further, when the fundamental frequency of the input signal is low, the power difference (volume difference) between the input signal and the band-extended signal including the second band becomes larger and thus an approximately ideal broadband sound spectrum depicted in  FIG. 2  is obtained. In other words, the control of the frequency response of the second band according to the fundamental frequency of the input signal enables the provision of the high quality sound. 
         [0036]    The second example explains the application of the band broadening apparatus into a cellular phone. The application of the band broadening apparatus is not limited to a cellular phone but the band broadening apparatus is applicable to an apparatus for the a voice communication such as a telephone in the landline telephone system. In the second example, a high frequency region is generated from a bandlimited input signal, and the high frequency region and the input signal are synthesized to extend the band. The band of the input signal corresponds to the first band and the band of the high frequency component corresponds to the second band. 
         [0037]      FIG. 5  is a block diagram depicting a cellular phone to which the band broadening apparatus is applied. The cellular phone includes a decoder  11 , a band broadening apparatus  12 , a digital-analog converter  13 , an amplifier  14 , and a speaker  15 .  FIG. 5  depicts elements that broaden the band of a received sound signal and play the sound, and omits elements that convert sound into transmission data and do not relate to the sound processing such as communication, display, and operation. 
         [0038]    The decoder  11  demodulates and decodes a received signal, and outputs a signal having, for example, the bandwidth of 8 kHz. The band broadening apparatus  12  extends the bandwidth of an output signal from the decoder  11  and outputs a signal with the bandwidth of, for example, 16 kHz. The digital-analog converter  13  converts an output signal from the band broadening apparatus  12  to an analog signal. The amplifier  14  amplifies an output signal from the digital-analog converter  13 . The speaker  15  converts an output signal from the digital-analog converter  13  to sound and outputs the sound. 
         [0039]      FIG. 6  is a block diagram depicting a hardware configuration of the band broadening apparatus according to the second example. The band broadening apparatus  12  includes a central processing unit (CPU)  21 , a random access memory (RAM)  22 , and a read-only memory  23 , respectively connected by a bus  24 . 
         [0040]    The ROM  23  stores therein a band broadening program that causes the CPU  21  to perform a band broadening method that will be explained later. The RAM  22  is used as a work area of the CPU  21 . The RAM  22  stores data, output signals from the decoder  11 . The CPU  21  loads into the RAM  22 , the band broadening process program read from the ROM  23  and implements the band broadening process. 
         [0041]      FIG. 7  is a block diagram depicting a functional configuration of the band broadening apparatus according to the second example. The band broadening apparatus  12  includes a fast Fourier transformation (FFT) unit  31 , a power spectrum calculating unit  32 , and a high frequency component generating unit (out-of-band component generating unit)  33 . The fast Fourier transformation unit  31  performs a fast Fourier transformation process (for example, 256 points) for an input signal x(n) and works out an input spectrum X(f) where n is a sample number and f is a frequency number. 
         [0042]    The power spectrum calculating unit  32  works out a power spectrum S(f) from the input spectrum X(f) according to Equation (1) below. The high frequency component generating unit  33  shifts, according to Equation (2), the input spectrum X(f) over the frequency numbers 64 to 127 toward the high frequency region of the frequency number 128 and the subsequent frequency numbers, and generates a high frequency spectrum X h (f).  FIG. 8  is a diagram depicting a high frequency component created by the high frequency component generating unit. As depicted in FIG,  8 , the high frequency component generating unit  33  only shifts an input signal (expressed by a two-dot line) toward a high frequency region. At present, the attenuation profile of a high frequency component (expressed by a solid line) is not adjusted. 
         [0000]        S ( f )=10 log 10 (| X ( f )| 2 )   (1)
 
         [0000]        X   h ( f+ 64)= X ( f )  f= 64 to 127   (2)
 
         [0043]    The band broadening apparatus  12  further includes a fundamental frequency analyzing unit  34 , a frequency response control unit  35 , and a high frequency component adjusting unit (out-of-band component adjusting unit)  36 . The fundamental frequency analyzing unit  34  works out the fundamental frequency f 0  from the autocorrelation of the power spectrum S(f) according to, for example, Equation (3) below. 
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         [0044]    The frequency response control unit  35  works out a gradient α of the attenuation profile in the high frequency region based on the fundamental frequency f 0  according to, for example, an equation expressed by a graph in  FIG. 9 .  FIG. 9  is a graph of an equation according to which the gradient α is obtained from the fundamental frequency f 0 . In  FIG. 9 , the frequency number 4 corresponds to 125 Hz, generally the fundamental frequency (about 150 Hz) of men. The frequency number 8 corresponds to 250 Hz, generally the fundamental frequency (about 300 Hz) of women. The fundamental frequency f 0  varies in and near the range between 125 Hz and 250 Hz. 
         [0045]    In  FIG. 9 , when the fundamental frequency f 0  is in the range below the frequency number 4, the gradient α is at a constant value of −12 dB/kHz. When the fundamental frequency f 0  is in the range between the frequency number 4 and 8, the gradient α increases at a constant rate and comes to 0 dB/kHz. When the fundamental frequency f 0  is in the range above the frequency number 8, the gradient α is at a constant value of 0 dB/kHz. The specific numerical values on the horizontal and vertical axes in  FIG. 9  are mere examples. The frequency response control unit  35  works out the attenuation profile G(f) in the high frequency region from the gradient α of the attenuation profile in the high frequency region according to Equation (4) below. When 0 is substituted into f in Equation (4), the attenuation profile G(f) at the frequency number 128 becomes 0 dB. This means that an amount of the amplification at the boundary between the band of the input signal and the band of the high frequency component is 0 dB. 
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         [0046]      FIG. 10  is a graph depicting a frequency response controlled by the frequency response control unit. In  FIG. 10 , the amplification in the band of the input signal is 0 dB. The amplification is 0 dB at the boundary between the band of the input signal and the band of the high frequency component and is less than 0 dB in the higher frequency region. In the high frequency region, the attenuation becomes larger at the rate a as the frequency becomes higher. In the example of  FIG. 10 , the attenuation profile of the high frequency region is expressed by a function proportional to the frequency. 
         [0047]    When the gradient α becomes smaller as the fundamental frequency f 0  becomes higher as explained in  FIG. 9 , the line over the high frequency region in  FIG. 10  becomes shallower. On the other hand, when the gradient α becomes larger as the fundamental frequency f 0  becomes lower, the line over the high frequency region in  FIG. 10  becomes steeper. Therefore, in the high frequency region, the attenuation under a low fundamental frequency is larger than that under a high fundamental frequency. Numerical values on the vertical axis in  FIG. 10  are mere examples. 
         [0048]    The high frequency component adjusting unit  36  multiplies the high frequency spectrum X h (f) by the attenuation profile G(f) according to Equation (5) and generates the high frequency spectrum X h ′(f) with the frequency response adjusted. 
         [0000]        X   h ′( f )= X   h ( f )· G ( f )   (5)
 
         [0049]    The band broadening apparatus  12  further includes a spectrum synthesizing unit (signal synthesizing unit)  37  and an inverse FFT unit  38 . The spectrum synthesizing unit  37  synthesizes the input spectrum output from the FFT unit  31  and the frequency-response-adjusted high frequency spectrum X h ′(f) output from the high frequency component adjusting unit  36 , and generates an output spectrum Y(f). The output spectrum Y(f) equals to the input spectrum X(f) over the range between the frequency number 0 and 127 and equals to the frequency-response-adjusted high frequency spectrum X h ′(f) over the range between the frequency number 128 and 255 as expressed by Equation (6) below. 
         [0000]        Y ( f )= X ( f )  f= 0 to 127 
         [0000]        Y ( f )= X   h ′( f )  f= 128 to 255   (6)
 
         [0050]      FIG. 11  is a diagram depicting an output spectrum synthesized by the spectrum synthesizing unit. The spectrum in the high frequency region is not a mere translation of the spectrum in the band of the input signal toward the high frequency region but is a spectrum more attenuated than the input signal according to the fundamental frequency f 0 . The inverse FFT unit  38  performs the inverse FFT process for the output spectrum Y(f) (for example, 512 points) and works out an output signal y(n). Each unit in the functional configuration of the band broadening apparatus  12  is realized by the CPU  21  loading a band broadening program in the RAM  22  and executing the band broadening process. 
         [0051]      FIG. 12  is a flowchart of the band broadening method according to the second example. As depicted in  FIG. 12 , when the band broadening process is started, the band broadening apparatus  12  conducts the FFT process for an input signal x(n) by means of the FFT unit  31  and transforms the input signal x(n) into an input spectrum X(f) (step S 11 ). The band broadening apparatus  12  works out a power spectrum S(f) from the input spectrum X(f) based on Equation (1) by means of the power spectrum calculating unit  32  (step S 12 ). The band broadening apparatus  12  generates a high frequency spectrum X h (f) from the input spectrum X(f) based on Equation (2) by means of the high frequency component generating unit  33  (step S 13 ). 
         [0052]    The band broadening apparatus  12  analyzes the fundamental frequency f 0  based on the autocorrelation of the power spectrum S(f) according to, for example, Equation (3) by means of the fundamental frequency analyzing unit  34  (step S 14 ). The band broadening apparatus  12  calculates, by means of the frequency response control unit  35 , a gradient α of the attenuation profile in the high frequency region corresponding to the fundamental frequency f 0  according to, for example, an equation expressed by a graph in  FIG. 9  (step S 15 ). The band broadening apparatus  12  conducts the calculation of Equation (4) by means of the frequency response control unit  35  and calculates the attenuation profile G(f) in the high frequency region from the gradient α of the attenuation profile in the high frequency region (step S 16 ). 
         [0053]    The band broadening apparatus  12  multiplies, by means of the high frequency component adjusting unit  36 , the high frequency spectrum X h (f) by the attenuation profile G(f) according to Equation (5) and generates the frequency-response-adjusted high frequency spectrum X h ′(f) (step S 17 ). Step S 13  may be conducted anytime after step S 11  and before step S 17 . 
         [0054]    The band broadening apparatus  12  synthesizes, by means of the spectrum synthesizing unit  37 , the input spectrum X(f) (spectrum in low frequency spectrum) and the frequency-response-adjusted high frequency spectrum X h ′(f) and generates the output spectrum Y(f) (step S 18 ). The band broadening apparatus  12  performs the inverse FFT process for the output spectrum Y(f) by means of the inverse FFT unit  38 , and transforms the output spectrum Y(f) into the output signal y(n) (step S 19 ) and ends the whole band broadening process. 
         [0055]    According to the second example, when the fundamental frequency of an input signal is high, the power difference (volume difference) between the input signal and the high frequency component signal becomes small and thus an approximately ideal broadband sound spectrum as depicted in  FIG. 1  is obtained. Further, when the fundamental frequency of the input signal is low, the power difference (volume difference) between the input signal and the high frequency component signal becomes larger and thus an approximately ideal broadband sound spectrum depicted in  FIG. 2  is obtained. Accordingly, the high quality sound can be provided. 
         [0056]    The third example explains the application of the band broadening apparatus into an audio conferencing apparatus. The application of the band broadening apparatus is not limited to an audio conferencing apparatus but the band broadening apparatus is applicable to an apparatus for the audio communication such as a telephone in the landline telephone system and a cellular phone. In the third example, a high frequency region is generated from a bandlimited input signal, and the high frequency region and the input signal are synthesized to extend the band. The band of the input signal corresponds to the first band and the band of the high frequency component corresponds to the second band. 
         [0057]    Units of the audio conferencing apparatus that extend a band of a received audio signal and play sound are similar to the configuration depicted in  FIG. 5  and thus a redundant explanation will be omitted. 
         [0058]    The hardware configuration of a band broadening apparatus according to the third example is similar to the configuration depicted in  FIG. 6  and thus a redundant explanation will be omitted. 
         [0059]      FIG. 13  is a block diagram depicting a functional configuration of the band broadening apparatus according to the third example. Elements identical to that of the second example are given identical reference numerals as in the second example and the explanation thereof will be omitted. As depicted in  FIG. 13 , the band broadening apparatus  12  includes a high frequency component generating unit  41  serving as the FFT unit  31  and an out-of-band component generating unit. As for the FFT unit  31 , see the second example. The high frequency component generating unit  41  folds back the input spectrum X(f) over the frequency number 31 to 127 toward the high frequency region and generates a high frequency spectrum X h (f) corresponding to the frequency number 128 and the subsequent frequency numbers. At this point, the attenuation profile of the high frequency component is not adjusted. 
         [0000]        X   h ( f+ 128)= X (127 −f )  f= 0 to 96   (7)
 
         [0060]    The band broadening apparatus  12  includes a fundamental frequency analyzing unit  42 , a fundamental frequency smoothing unit  43 , a frequency response control unit  44 , the high frequency component adjusting unit  36 , the spectrum synthesizing unit  37 , and the inverse FFT unit  38 . The fundamental frequency analyzing unit  42  works out the fundamental period t 0  from the autocorrelation of the input signal x(n) according to Equation (8) below. The fundamental frequency analyzing unit  42  works out the fundamental frequency f 0  from the fundamental period t 0  according to Equation (9) below. 
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         [0061]    The fundamental frequency smoothing unit  43  works out a cut-off frequency f c  of the high frequency region from the fundamental frequency f 0  based on, for example, the graph depicted in  FIG. 14 .  FIG. 14  is a graph expressing an equation for obtaining f c  from f 0 . In  FIG. 14 , specific numerical values, frequency numbers 4 and 8, and frequencies 125 Hz and 250 Hz, are one example as explained in the second example. 
         [0062]    According to  FIG. 14 , when the fundamental frequency f 0  is less than the frequency number 4, f c  is at a constant value of 5000 Hz. As the fundamental frequency f 0  moves from the frequency numbers 4 and 8, f c  goes to 7000 Hz at a constant gradient. When the fundamental frequency f 0  is more than the frequency number 8, f c  is at a constant value of 7000 Hz. Specific values on the vertical and horizontal axes in  FIG. 14  have been given as an example. 
         [0063]    The frequency response control unit  44  works out the attenuation profile G(f) of the high frequency region from the cut-off frequency f c  according to, for example, the graph depicted in  FIG. 15 .  FIG. 15  is a graph expressing an equation for obtaining G(f) from f c . 
         [0064]    According to  FIG. 15 , when the fundamental frequency f 0  is less than f c −16, G(f) is constant value taking 0 dB. When the fundamental frequency f 0  moves from f c −16 to f c +16, G(f) goes to −30 dB at a constant gradient. When the fundamental frequency f 0  is more than f c +16, G(f) is constant taking −30 dB. Specific values on the vertical and horizontal axes in  FIG. 15  have been given as an example. The cut-off frequency f 0  of the high frequency region fluctuates in a range between 5000 Hz and 7000 Hz in  FIG. 14 . 
         [0065]    As for the high frequency component adjusting unit  36 , the spectrum synthesizing unit  37 , and the inverse FFT unit  38 , see the second example. Each functional element of the band broadening apparatus  12  is realized by the CPU  21  loading the band broadening program to the RAM  22  and executing the band broadening process. 
         [0066]      FIG. 16  is a flowchart of the band broadening method according to the third example. When the band broadening process is started, the band broadening apparatus  12  performs the FFT process for the input signal x(n) by means of the FFT unit  31  transforming the input signal x(n) to the input spectrum X(f) (step S 21 ). The band broadening apparatus  12  generates the high frequency spectrum X h (f) from the input spectrum X(f) by means of the high frequency component generating unit  41  according to Equation (7) (step S 22 ). 
         [0067]    The band broadening apparatus  12  performs the calculation of Equations (8) and (9) by means of the fundamental frequency analyzing unit  42  and analyzes the fundamental period t 0  and the fundamental frequency f 0  (step S 23 ). The band broadening apparatus  12  works out, by means of the fundamental frequency smoothing unit  43 , the cut-off frequency f c  of the high frequency region from the fundamental frequency f 0  based on the graph depicted in  FIG. 14  (step S 24 ). The band broadening apparatus  12  works out, by means of the frequency response control unit  44 , the attenuation profile G(f) of the high frequency region from the cut-off frequency f c  based on the graph depicted in  FIG. 15  (step S 25 ). 
         [0068]    The subsequent steps are identical to steps S 17  to S 19  of the second example (step S 26  to step S 28 ) and the whole process ends. Step S 22  may be performed anytime after step S 21  and before step S 26 . The third example presents a similar advantage as the second example. 
         [0069]    The fourth example explains the application of the band broadening apparatus into a cellular phone, generating a low frequency component from a bandlimited input signal and synthesizing the low frequency component and the input signal to extend a band. The application of the band broadening apparatus is not limited to a cellular phone but the band broadening apparatus is applicable to an apparatus for an audio communication. The band of the input signal corresponds to the first band and the band of the low frequency component corresponds to the second band. 
         [0070]    Units of the cellular phone that extend a band of a received audio signal and play sound are similar to the configuration depicted in  FIG. 5  and thus a redundant explanation will be omitted. In the fourth example, the band broadening apparatus  12  extends a band of the output signal from the decoder  11  and outputs a signal with an 8-kHz bandwidth. 
         [0071]    The hardware configuration of a band broadening apparatus according to the fourth example is similar to the configuration depicted in  FIG. 6  and thus a redundant explanation will be omitted. 
         [0072]      FIG. 17  is a block diagram depicting a functional configuration of the band broadening apparatus according to the fourth example. Elements identical to those of the second example are given identical reference numerals as in the second example and the explanation thereof will be omitted. The band broadening apparatus  12  includes the FFT unit  31 , the power spectrum calculating unit  32 , and the fundamental frequency analyzing unit  34 . See the second example for the detail of the FFT unit  31 , the power spectrum calculating unit  32 , and the fundamental frequency analyzing unit  34 . 
         [0073]    The band broadening apparatus  12  includes a low frequency component generating unit  51  and a frequency response control unit  52  that serve as an out-of-band component generating unit, and a low frequency component adjusting unit  53  that serves as a out-of-band component adjusting unit. The low frequency component generating unit  51  shifts toward the low frequency region the input spectrum X(f) ranging from the frequency number corresponding to the fundamental frequency f 0  to the frequency number corresponding to three times of f 0  and generates the low frequency spectrum X L (f) ranging from the frequency number 0 to the frequency number corresponding to twice of f 0 . At this point, the attenuation profile of the low frequency component is not adjusted. 
         [0000]        X   L ( f )= X ( f+f   0 )  f= 0 to 2· f   0    (10)
 
         [0074]    The frequency response control unit  52  works out a target amount of attenuation G L  in the low frequency region from the fundamental frequency f 0  based on a graph depicted in  FIG. 18 .  FIG. 18  is a graph expressing an equation for obtaining G L  from f 0 . The specific numerical values, frequency numbers 4 and 8 and frequencies 125 Hz and 250 Hz, are mere examples as explained in the second example. 
         [0075]    In  FIG. 18 , when the fundamental frequency f 0  is less than the frequency number 4, G L  is constant at 0 dB. When the fundamental frequency f 0  moves from the frequency number 4 to the frequency number 8, G L  goes to −12 dB. When the fundamental frequency f 0  is more than the frequency number 8, G L  is constant at −12 dB. The specific values on the vertical and horizontal axes in  FIG. 18  have been given as an example. 
         [0076]    The frequency response control unit  52  calculates the attenuation profile G(f) of the low frequency region based on the target amount G L  and the graph depicted in  FIG. 19 .  FIG. 19  is a graph expressing an equation for obtaining G(f) based on G L . In  FIG. 19 , when the frequency is less the fundamental frequency f 0 , G(f) is constant at G L . When the frequency moves from f 0  to twice of f 0 , G(f) goes to −60 dB, maximum G MAX , at a constant gradient. When the frequency is more than twice the fundamental frequency f 0 , G(f) is constant at maximum G MAX . Specific values on the horizontal axis in  FIG. 19  have been given as an example. 
         [0077]    The low frequency component adjusting unit  53  multiples, as taught by Equation (11) below, the low frequency spectrum X L (f) generated by the low frequency component generating unit  51  by the attenuation profile G(f) of the low frequency region controlled by the frequency response control unit  52  and generates the frequency-response-adjusted low frequency spectrum X L ′. 
         [0000]        X   L ′( f )= X   L ( f )· G ( f )   (11)
 
         [0078]    The band broadening apparatus  12  further includes a spectrum synthesizing unit  54  and an inverse FFT unit  55 . The spectrum synthesizing unit  54  synthesizes the input spectrum X(f) output from the FFT unit  31  and the frequency-response-adjusted low frequency spectrum X L ′(f) output from the low frequency component adjusting unit  53  and generates the output spectrum Y(f) according to Equation (12) below. 
         [0000]        Y ( f )= X ( f )+ X   L ′( f )  f= 0 to 127   (12)
 
         [0079]    The inverse FFT unit  55  performs the inverse FFT process (for example 256 points) for the output spectrum Y(f) and works out the output signal y(n). Each element in the functional configuration of the band broadening apparatus  12  is realized by the CPU  21  loading the band broadening program to the RAM  22  and executing the band broadening process. 
         [0080]      FIG. 20  is a flowchart of the band broadening method according to the fourth example. When the band broadening process is started, the band broadening apparatus  12  transforms the input signal x(n) into the input spectrum X(f) in a similar manner as step Sll of the second example (step S 31 ). The band broadening apparatus  12  transforms the input spectrum X(f) to the power spectrum S(f) in a similar manner as step S 12  of the second example (step S 32 ). The band broadening apparatus  12  analyzes the fundamental frequency f 0  based on the power spectrum S(f) in a similar manner as step S 14  of the second example (step S 33 ). 
         [0081]    The band broadening apparatus  12  generates the low frequency spectrum X L (f) from the input spectrum X(f) and the fundamental f 0  according to Equation (10) by means of the low frequency component generating unit  51  (step S 34 ). The band broadening apparatus  12  works out the target amount of attenuation G L  from the fundamental frequency f 0  based on the graph depicted in  FIG. 18  by means of the frequency response control unit  52  (step S 35 ). The band broadening apparatus  12  works out, by means of the frequency response control unit  52 , the attenuation profile G(f) of the low frequency region based on G L  according to the graph depicted in  FIG. 19  (step S 36 ). Step S 34  may be conducted anytime before step S 33  and before step S 37 . 
         [0082]    The band broadening apparatus  12  multiplies the low frequency spectrum X L (f) by the attenuation profile G(f) of the low frequency region according to Equation (11) by means of the low frequency component adjusting unit  53  and generates the frequency-response-adjusted low frequency spectrum X L ′(f) (step S 37 ). The band broadening apparatus  12  synthesizes, by means of the spectrum synthesizing unit  54 , the input spectrum X(f), the spectrum of the high frequency region and the frequency-response-adjusted low frequency spectrum X L ′(f) according to Equation (12) and generates the output spectrum Y(f) (step S 38 ). The band broadening apparatus  12  performs the inverse FFT process for the output spectrum Y(f) by means of the inverse FFT unit  55  and transforms the output spectrum Y(f) to the output signal y(n) (step S 39 ) and the whole process ends. According to the fourth embodiment, the extension of a band toward the low frequency region also presents the advantages similar to the second example. 
         [0083]    According to one aspect of the invention, high quality sound can be output. 
         [0084]    All examples and conditional language provided herein are intended for pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although one or more embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.