Abstract:
An audio signal processing apparatus includes a harmonic overtone adder and an equalizer. The harmonic overtone adder includes a high-pass filter for extracting from an audio signal higher than a first predetermined frequency, a filter for extracting a frequency component lower than half a second predetermined frequency, an harmonic overtone generator for generating a frequency-doubled harmonic overtone component from an output from the filter, and a first combining unit for combining the frequency component output from the high-pass filter and the harmonic overtone component output from the harmonic overtone generator. The equalizer includes a level detector for detecting a level of an overtone component contained in an output from the first combining unit, a gain controller for controlling dynamically the level of the harmonic overtone component contained in the output from the first combining unit, and a second combining unit for combining the output from the first combining unit with the harmonic overtone component.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
       [0001]    The present invention contains subject matter related to Japanese Patent Application JP 2006-283532 filed in the Japanese Patent Office on Oct. 18, 2006, the entire contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an audio reproducing apparatus. 
         [0004]    2. Description of the Related Art 
         [0005]    Small loudspeakers are employed in minicomponent stereo sets and flat-screen television receivers. In such applications, the enclosure of the loudspeaker (speaker box) is accordingly small. The resonance frequency f 0  of the loudspeaker is as high as or higher than 100 Hz. 
         [0006]    An audio signal having the resonance frequency f 0  or lower may be supplied to the loudspeaker. With the frequency being lowered, the fundamental component is increasingly lowered while distorted components (harmonic components) sharply increases in the sound output pressure thereof. 
         [0007]    Audio apparatuses having small loudspeakers cannot sufficiently reproduce low-frequency component lower than the resonance frequency f 0  of the loudspeaker. 
         [0008]    Two methods may be contemplated in the reproducing of the audio signal: 
         [0009]    (1) an equalizer is used to boost the low-frequency component, and 
         [0010]    (2) a harmonic overtone component of the low-frequency component is output to achieve a low-frequency sound effect. 
         [0011]    The method (1) provides the low-frequency sound effect by reinforcing the frequency component in the resonance frequency f 0  band of the loudspeaker. 
         [0012]    The method (2) takes advantage of the hearing of humans. More specifically, the sound of each musical instrument is composed of a fundamental component and harmonic components, and the ratio of the fundamental component to the harmonic components determines the tone of the sound. It has been psychoacoustically proved that if a sound without the fundamental component but with the harmonic components thereof is output humans hear as if the fundamental component is also output. The method (2) is based on such a human hearing property. 
         [0013]      FIG. 14  illustrates an audio apparatus. A loudspeaker  5  is used to improve the low frequency sound effect. An audio signal S 1  is supplied to a high-pass filter  2  at an input terminal  1 . As shown in  FIG. 15A , a middle to high-frequency component equal to or higher than the resonance frequency f 0  of the loudspeaker  5  is extracted and supplied to an adder  3 . The audio signal S 1  at the terminal  1  is supplied to a band-pass filter  7 . As shown in  FIG. 15B , a low-frequency component S 7  falling within a frequency band from f 0 /2 to f 0  is extracted and supplied to a pitch shifter  8 . 
         [0014]    The pitch shifter  8  doubles the frequency of the supplied low-frequency component S 7 . As shown in  FIG. 15C , a frequency multiplied component S 8  within a band from f 0  to  2   f   0 , namely, the low-frequency component S 8 , is output. 
         [0015]    The low-frequency component S 8  is supplied to the adder  3  to be added to the middle to high-frequency component S 2 . The adder  3  outputs an audio signal S 3  with the low-frequency component S 8  as the low-frequency component S 7  reinforced as shown in  FIG. 15D . The audio signal S 3  is output to the loudspeaker  5  via a power amplifier  4 . The loudspeaker  5  thus emits an acoustic sound having frequency characteristics of  FIG. 15D , namely, an acoustic sound with the low-frequency component S 8  as the reinforced low-frequency component S 7 . 
         [0016]    The sound of the low-frequency component S 7  is not output from the loudspeaker  5  and corresponds to the fundamental component. The corresponding low-frequency component S 8  is output from the loudspeaker  5 . A listener hears as if the low-frequency component S 7  is actually output. Even with the small loudspeaker  5 , the low frequency sound effect is thus provided. 
         [0017]    It is generally said that humans suffer from no unpleasant hearing impression on the low-frequency component S 8  lower than 200 Hz even if the frequency multiplied component S 5  is generated by multiplying the low-frequency component S 7 . 
         [0018]    Japanese Unexamined Patent Application Publication No. 8-213862 discloses one such technique. 
       SUMMARY OF THE INVENTION 
       [0019]    The method (1) of boosting the low-frequency component is not appropriate for the small loudspeaker that is originally unable to reproduce the low-frequency component. If an equalizer with a boost amount thereof fixed is used, signals are clipped, leading to noise and distortion. Supplying a deep bass component to the small loudspeaker is not preferable. 
         [0020]    The method (2) of adding the harmonic overtone is effective with the small loudspeaker since the low-frequency component is not supplied to the loudspeaker  5 . The low-frequency component S 8  is similar to a harmonic component of the band-pass filter  7 . If the low-frequency component S 8  is increased in amount to achieve the low frequency sound effect, the degree of distortion is also increased. If the low-frequency component S 8  is reduced in amount to lower the degree of distortion, the low frequency sound effect becomes insufficient. There is a trade-off between the low frequency sound effect and the degree of distortion. 
         [0021]    It is thus desirable to overcome such a problem. 
         [0022]    An audio signal processing apparatus includes a harmonic overtone adder and an equalizer. The harmonic overtone adder includes a high-pass filter for extracting from an audio signal a frequency component equal to or higher than a first predetermined frequency, a filter for extracting from the audio signal a frequency component equal to or lower than half a second predetermined frequency, an harmonic overtone generator for generating a frequency-doubled harmonic overtone component from an output from the filter, and a first combining unit for combining the frequency component output from the high-pass filter and the harmonic overtone component output from the harmonic overtone generator. The equalizer includes a level detector for detecting a level of an overtone component contained in an output from the first combining unit, a gain controller for controlling dynamically the level of the harmonic overtone component contained in the output from the first combining unit based on a detection output from the level detector, and a second combining unit for combining the output from the first combining unit with the harmonic overtone component output from the gain controller. 
         [0023]    In accordance with embodiments of the present invention, if the low-frequency component of the audio signal is lower than the first predetermined frequency, the harmonic overtone of the low-frequency component is output. The harmonic overtone provides the low frequency sound effect. Since the level of the harmonic overtone is dynamically varied, a crisp low frequency sound effect is obtained while the degree of distortion in the output sound is reduced. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0024]      FIG. 1  is a block diagram of a harmonic overtone adder in accordance with one embodiment of the present invention; 
           [0025]      FIGS. 2A-2F  illustrate frequency characteristics of the harmonic overtone adder in accordance with one embodiment of the present invention; 
           [0026]      FIG. 3  is a waveform diagram of the harmonic overtone adder; 
           [0027]      FIG. 4  is a control characteristic chart of the harmonic overtone adder; 
           [0028]      FIG. 5  is a block diagram of another harmonic overtone adder in accordance with one embodiment of the present invention; 
           [0029]      FIG. 6  is a block diagram illustrating yet another harmonic overtone adder in accordance with one embodiment of the present invention; 
           [0030]      FIGS. 7A-7C  illustrates frequency characteristics of the harmonic overtone adder of  FIG. 6 ; 
           [0031]      FIGS. 8A and 8B  illustrate frequency characteristics of the harmonic overtone adder of  FIG. 6 ; 
           [0032]      FIG. 9  illustrates still another harmonic overtone adder in accordance with one embodiment of the present invention; 
           [0033]      FIGS. 10A and 10B  illustrate frequency characteristics of the harmonic overtone adder of  FIG. 9 ; 
           [0034]      FIGS. 11A-11C  are waveform diagrams of the harmonic overtone adder; 
           [0035]      FIG. 12  illustrates a pitch shifter in the harmonic overtone adder; 
           [0036]      FIGS. 13A and 13B  are waveform diagrams of the patch shifter; 
           [0037]      FIG. 14  illustrates a harmonic overtone adder as a known art; and 
           [0038]      FIGS. 15A-15D  illustrate frequency characteristics of the known harmonic overtone adder. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0039]      FIG. 1  illustrates a harmonic overtone adder  10  in accordance with one embodiment of the present invention. A small loudspeaker  33  provides an improved low frequency sound effect. Let f 0  represent a resonance frequency of the loudspeaker  33 . The resonance frequency f 0  is 100 Hz or lower. Let f 1  represent a frequency upper limit below which a signal obtained by frequency multiplying a fundamental frequency signal causes no unpleasant hearing impression. The frequency upper limit f 1  is a frequency obtained by frequency multiplying a fundamental frequency of a signal. The frequency upper limit f 1  is about 200 Hz. Here, f 0 =f 1 /2 (or f 0 ≦f 1 /2). In the case of a two-channel stereophonic system or multi-channel stereophonic system, each channel has the structure of  FIG. 1 . 
         [0040]    An audio signal S 11  to be reproduced is supplied to a harmonic overtone adder  10 . A harmonic overtone component providing a low-frequency sound effect is thus added to the audio signal S 11 . The audio signal S 1  is supplied to a high-pass filter  12  via an input terminal  11 . As shown by a solid line in  FIG. 2A , a low-frequency component S 12  equal to or higher than the resonance frequency f 0  of the loudspeaker  33 , namely 100 Hz, in this case is extracted and then supplied to an adder  13 . The audio signal S 11  is supplied from the input terminal  11  to a low-pass filter  14 . As shown by a broken line in  FIG. 2A , a low-frequency component S 14  equal to or lower than the resonance frequency f 0  of the loudspeaker  33 , namely 100 Hz, in this case is extracted by the low-pass filter  14 . The low-frequency component S 14  is then supplied to an attenuator  15 . The attenuator  15  adjusted the low-frequency component S 14  to a predetermined level and the resulting level adjusted component is supplied to the adder  13 . 
         [0041]    The audio signal S 11  is supplied from the input terminal  11  to a band-pass filter  16 . As shown by a solid line in  FIG. 2B , a low-frequency component S 16  falling within a frequency band f 0 /2 to f 1 /2, namely, 50 Hz to 100 Hz, is extracted. The low-frequency component S 16  is then supplied to a pitch shifter  17 . 
         [0042]    One example of the pitch shifter  17  will be described later. The pitch shifter  17  doubles the frequency of the supplied low-frequency component S 16 . As shown by a broken line in  FIG. 2B , a frequency doubled, harmonic overtone component S 17  is extracted. Since the low-frequency component S 16  falls within the frequency band f 0 /2-f 1 /2, the frequency band of the harmonic overtone component S 17  becomes f 0 -f 1 . The frequency-doubled harmonic overtone component S 17  is adjusted to a predetermined level by an attenuator  18  and the level-adjusted harmonic overtone component S 17  is then supplied to the adder  13 . 
         [0043]    As shown in  FIG. 2C , the adder  13  outputs an audio signal S 13 . The audio signal S 13  is obtained by adding the low-frequency component S 14  and the frequency-doubled harmonic overtone component S 17  at predetermined ratios to the middle to high-frequency component S 12 . 
         [0044]    The audio signal S 13  is supplied to a gain-controlling type equalizer  20 . The equalizer  20  performs an equalization operation accounting for low frequency sound effect and distortion effect. The audio signal S 13  is supplied to an adder  21  and a band-pass filter  22 . As shown in  FIG. 2D , a low-frequency component S 22  falling within a frequency band f 0 -f 1 , namely, 100 Hz-200 Hz, is extracted. The low-frequency component S 22  is supplied to the adder  21  via a gain controller  23  to be discussed later. 
         [0045]    As shown in  FIG. 2D , the low-frequency component S 22  contains a low-frequency component S 121  of the middle to high-frequency component S 12  ( FIG. 2A ) and the harmonic overtone component S 17 . A signal component S 23  as an output signal of the gain controller  23  is obtained by level controlling the low-frequency component S 22 , and contains the low-frequency component S 121  and the pitch shifter  17 . 
         [0046]    The low-frequency component S 22  is supplied from the band-pass filter  22  to a level detector  24 . As represented by a solid wave line in  FIG. 3 , one cycle of the low-frequency component S 22 , i.e., a duration Tx from a reversal from negative to positive to a next reversal from negative to positive is determined as one period. A peak level V 22  (absolute value), detected within one period Tx, is referred to as a detected signal S 24  (absolute value). The detected signal S 24  is supplied to the gain controller  23  as a gain control signal. 
         [0047]      FIG. 4  illustrates control characteristics of the gain controller  23 . The abscissa represents the input level of the low-frequency component S 22  supplied to the gain controller  23 , namely, the peak level V 22  of the band-pass filter  22  within one period Tx from which the signal S 24  is detected. The ordinate represents an output level V 23  of a low-frequency component S 23  output from the gain controller  23 . A broken line B represents characteristics with gain being 1 regardless of the input level V 22  (peak level) for reference only in  FIG. 4 . 
         [0048]    Control characteristics of the gain controller  23  is represented by line A. Let VLM represent a predetermined upper limit and VTH represent a predetermined threshold level (VLM&gt;VTH). V 23 =VMAX holds if V 22 ≧VLM. V 23  is in linear proportion to level V 22  with relationship V 22 &lt;VLM held. Gain is greater than 1 if relationship VTH&lt;V 22 &lt;VLM holds. Gain is 1 if relationship V 22 =VTH holds. Gain is smaller than 1 if relationship V 22 &lt;VTH holds. 
         [0049]    The gain controller  23  level controls the low-frequency component S 22  on a per period Tx in accordance with the detected signal S 24  and the control characteristics A. The peak level V 22  cannot be known until one period Tx is completed. For simplicity, it is assumed the peak level V 22  within the one period Tx can be detected at the start of the period Tx. For detection and control, the low-frequency component S 22  to be level controlled is pre-delayed to be synchronized with the corresponding detected signal S 24 . 
         [0050]    The adder  21  adds the low-frequency component S 23  level controlled by the gain controller  23  to the audio signal S 13  from the adder  13 . As shown in  FIG. 2E , the adder  13  outputs an audio signal S 21  composed of the frequency components S 14 , S 121 , S 17  and S 12 . The audio signal S 21  is then supplied to a low-frequency cutoff filter  31 . As shown in  FIG. 2F , the low-frequency cutoff filter  31  removes a deep bass component detrimental to the small loudspeaker  33 , i.e., the low-frequency cutoff filter  31  outputs an audio signal S 31 . Although almost no standard low frequency sound is output from the loudspeaker  33 , a low-frequency component becoming a distorted component (harmonic component) could be output. The low-frequency cutoff filter  31  removes such a low-frequency component. The low-frequency cutoff filter  31  is then supplied to the loudspeaker  33  via a power amplifier  32 . 
         [0051]    With the above-described arrangement, the audio signal S 31  of  FIG. 2F  is output with the audio signal S 11  input to the input terminal  11 . The audio signal S 31  is then supplied to the loudspeaker  33 , and the resulting sound is output from the loudspeaker  33 . As shown in  FIG. 2F , the audio signal S 31  supplied to the loudspeaker  33  contains the harmonic overtone component S 17  twice as high in frequency as the low-frequency component S 16  ( FIG. 2B ). 
         [0052]    Although almost no sound of the fundamental component lower than the resonance frequency f 0  is output from the loudspeaker  33 , the harmonic overtone component S 17  twice as high as the low-frequency component S 16  is output. A listener may hear the sound as if the sound lower than the resonance frequency f 0  is output. Even with the small loudspeaker  33 , the low frequency sound effect is provided. 
         [0053]    The gain controller  23  level controls the low-frequency component S 22  containing the harmonic overtone component S 17  to the low-frequency component S 23 . Since the low-frequency component S 23  has control characteristics as shown in  FIG. 4 , the output low-frequency component S 23  has the waveform (level) as represented by a broken line in  FIG. 3 . If the peak level V 22  of the low-frequency component S 22  within the one period Tx is lower than the threshold level VTH, the output level V 23  of the low-frequency component S 23  becomes smaller than the original magnitude as represented by an arrow C in  FIG. 4 . If the peak level V 22  of the low-frequency component S 22  within the one period Tx is higher than the threshold level VTH, the output level V 23  of the low-frequency component S 23  becomes larger than the original magnitude as represented by an arrow D in  FIG. 4 . 
         [0054]    As represented by the broken line in  FIG. 3 , the output level V 23  of the low-frequency component S 23  output from the gain controller  23  becomes more smaller throughout one period Tx during which the peak level V 22  is smaller than the threshold level VTH, and becomes more larger throughout one period Tx during which the peak level V 22  is larger than the threshold level VTH. Since the harmonic overtone component S 17  contained in the low-frequency component S 23  dynamically changes in level in the same way, a crisp low frequency sound effect is reached while distortion effect is restricted. 
         [0055]    Since part of the low-frequency component S 14  contained in the audio signal S 11  still remains as shown in  FIGS. 2A and 2F , the low frequency sound effect becomes natural. A deep bass component detrimental to the small loudspeaker  33 , out of the low-frequency component S 14 , is removed by the low-frequency cutoff filter  31 , and outputting a large amount of distorted component (harmonic component) is avoided. 
         [0056]    In the above discussion, the low-frequency component S 22  output from the band-pass filter  22  is supplied to the detector  24 . As represented by a broken line in  FIG. 1 , the audio signal S 13  output from the adder  13  may be supplied to the detector  24  to obtain the detected signal S 24 . 
         [0057]      FIG. 5  illustrates a harmonic overtone adder  10  in accordance with a second embodiment of the present invention. In the harmonic overtone adder  10 , the band-pass filter  16  is removed, and the low-frequency component S 14  output from the low-pass filter  14  is supplied to the pitch shifter  17 . The band-pass filter  22  is replaced with a low-pass filter  25  having the upper limit frequency f 1  as a cutoff frequency. The rest of the harmonic overtone adder of  FIG. 5  is identical to the harmonic overtone adder of  FIG. 1 . 
         [0058]    The harmonic overtone component S 17  of the low-frequency component S 14  is output as a sound. The listener thus hears the sound as if the sound below the resonance frequency f 0  is also output. Even the small loudspeaker  33  provides the low frequency sound effect. 
         [0059]    The gain controller  23 , having the control characteristics of  FIG. 4 , level controls the low-frequency component S 22  containing the harmonic overtone component S 17  to the low-frequency component S 23 . The harmonic overtone component S 17  contained in the low frequency component S 23  output from the gain controller  23  dynamically changes the level thereof. A crisp low frequency sound effect results while distortion effect is controlled. 
         [0060]    As represented by a broken line in  FIG. 5 , the audio signal S 13  output from the adder  13  is supplied to the detector  24 , and the detector  24  outputs the detected signal S 24 . 
         [0061]      FIG. 6  illustrates a first modification of the harmonic overtone adder  10 . As shown in  FIG. 6 , part of the harmonic overtone adder  10  is identical to the harmonic overtone adder  10  of  FIG. 1 . As shown in  FIG. 7A , a high-pass filter  12  and the low-pass filter  14  respectively extract the middle to high frequency component S 12  and the low-frequency component S 14  from the audio signal S 11 , and then supply these components to the adder  13 . 
         [0062]    As shown in  FIG. 7B , the band-pass filter  16  extracts the low-frequency component S 16  falling within a range of f 0 /2 to f 1 /2, and the pitch shifter  17  doubles the frequency of the low-frequency component S 16  to the harmonic overtone component S 17 . The harmonic overtone component S 17  is supplied to the adder  13  via the attenuator  18 . 
         [0063]    The audio signal S 11  is supplied to a band-pass filter  46  via the input terminal  11 . As shown in  FIG. 7B , a low-frequency component S 46  falling within a frequency range f 0 /4-f 1 /4 is thus extracted. The low-frequency component S 46  is frequency multiplied by a pitch shifter  47  into a quadrupled frequency, harmonic overtone component S 47 . The harmonic overtone component S 47  is supplied to the adder  13  via an attenuator  48 . 
         [0064]    As shown in  FIG. 7C , the adder  13  adds to the middle to high-frequency component S 12 , the low-frequency component S 14 , the frequency-doubled, harmonic overtone component S 17  and the frequency-quadrupled, harmonic overtone component S 47  at predetermined ratios through the attenuators  15 ,  18  and  48 , thereby outputting the resulting audio signal S 13 . 
         [0065]    The subsequent process is identical to the process discussed with reference to  FIG. 1 . The audio signal S 13  is supplied to the loudspeaker  33  via the equalizer  20 , the low-frequency cutoff filter  31  and the power amplifier  32 , though the discussion thereof is omitted herein. 
         [0066]    With this arrangement, the loudspeaker  33  emits almost no sound of the fundamental component below the resonance frequency f 0 , but emits the sound of the harmonic overtone component S 17  and the harmonic overtone component S 47 . The listener thus hears the sound as if the sound lower than the resonance frequency f 0  is output. Even with the small loudspeaker  33 , the low frequency sound effect is achieved. 
         [0067]    If the low-frequency component S 16  (S 46 ) is 35 Hz as shown in  FIG. 8A , the harmonic overtone component S 17  (represented by an arrow-headed broken line) obtained by doubling the frequency of the low-frequency component S 16  has a frequency of 70 Hz, and the loudspeaker  33  is still unable to reproduce the harmonic overtone component S 17  as represented by frequency characteristics F 33  of the loudspeaker  33 . 
         [0068]    In the harmonic overtone adder  10  of  FIG. 6 , the low-frequency component S 16  having a frequency of 35 Hz, namely, the low-frequency component S 46  is supplied to the pitch shifter  47  via the band-pass filter  46 . The pitch shifter  47  frequency quadruples the low-frequency component S 16  to the frequency-quadrupled, harmonic overtone component S 47  (arrow-headed solid line) having a frequency of 140 Hz. The harmonic overtone component S 47  is supplied to the adder  13 . With the low-frequency component S 16  having a frequency of 35 Hz, the frequency quadrupled, harmonic overtone component S 47  results in a low frequency sound corresponding to the harmonic overtone component S 17 . 
         [0069]    As shown in  FIG. 8B , the low-frequency component S 46  (S 16 ) might have a frequency of 60 Hz. If the low-frequency component S 46  is quadrupled in frequency to the harmonic overtone component S 47  (as represented by an arrow-headed broken line), the resulting frequency is 240 Hz. The resulting frequency of 240 Hz is above the frequency upper limit f 1  (≅200 Hz) in the addition of the harmonic overtone. If the resulting harmonic overtone component S 47  is supplied to the loudspeaker  33 , an output sound results in an unpleasant hearing impression in human ears. 
         [0070]    The low-frequency component S 46  might have a frequency of 60 Hz in the harmonic overtone adder  10  of  FIG. 6 . The band-pass filter  46 , namely, the low-frequency component S 16  is supplied to the pitch shifter  17  via the band-pass filter  16 . The pitch shifter  17  frequency doubles the low-frequency component S 16  to the frequency-doubled, harmonic overtone component S 17  (represented by an arrow-headed solid line) having a frequency of 120H. The resulting harmonic overtone component S 17  is supplied to the adder  13 . Even when the low-frequency component S 46  has a frequency of 60 Hz, the frequency-doubled, harmonic overtone component S 17  results in a low frequency sound corresponding to the low-frequency component S 46 . 
         [0071]      FIG. 9  illustrates a second modification of the harmonic overtone adder  10 . As the harmonic overtone adder  10  of  FIG. 6 , the harmonic overtone adder  10  of  FIG. 9  adds the frequency-quadrupled, harmonic overtone component S 47  if the fundamental frequency component is low. The harmonic overtone components S 17  and S 47  respectively output from the pitch shifters  17  and  47  are supplied to an adder  19  via the attenuators  18  and  48 . As shown in  FIG. 10A , a harmonic overtone component S 19  containing the harmonic overtone components S 17  and S 47  is extracted and then supplied to a low-pass filter  49 . 
         [0072]    The low-pass filter  49  has frequency characteristics F 49  of  FIG. 10B . The low-pass filter  49  has the frequency upper limit f 1  as the cutoff frequency thereof at which the input signal is almost cut off. 
         [0073]    The low-pass filter  49  outputs, out of the low-frequency component S 19 , a frequency-doubled and frequency-quadrupled harmonic overtone component S 49  (hatched area) which causes no unpleasant hearing impression. The harmonic overtone component S 49  is supplied to the adder  13 . 
         [0074]    The rest of the harmonic overtone adder of  FIG. 5  is identical to the harmonic overtone adder of  FIG. 1 . The audio signal S 13  is supplied to the loudspeaker  33  via the equalizer  20 , the low-frequency cutoff filter  31  and the power amplifier  32 , though such path is not shown. 
         [0075]    As the harmonic overtone adder  10  of  FIG. 6 , the pitch shifters  17  and  47  output the frequency-doubled harmonic overtone component S 17  and the frequency-quadrupled harmonic overtone component S 47  by doubling the frequency of the low-frequency component S 16  and by quadrupling the frequency of the low-frequency component S 46 , respectively. The listener may hear the sound as if the sound lower than the resonance frequency f 0  is emitted. Even with the small loudspeaker  33 , the low frequency sound effect is achieved. 
         [0076]    The low-pass filter  49  lowers more the level of the low-frequency component S 19  output from an adder  19  as frequency becomes closer to the frequency upper limit f 1 . Even if the low-frequency component S 19  contains a frequency component close to or even above the frequency upper limit f 1 , an unpleasant hearing impression is controlled. A low frequency sound effect is thus provided even within the frequency range f 0 &gt;f 1 /2 without any unpleasant hearing impression. 
         [0077]    The harmonic overtone component S 17  twice in frequency the low-frequency component S 16  and the pitch shifter  47  four times in frequency the low-frequency component S 46  are produced as shown in  FIGS. 11A-11C . As shown in  FIG. 11A , digital data DA for digital-to-analog converting one period of a sinusoidal signal SA is now stored on a memory. Each solid dot symbol “” represents a sampling point, and one sample is stored at one corresponding address. A duration TA represents one sample cycle of the sinusoidal signal SA, and a duration 1/fc is one sample period. 
         [0078]    If the digital data DA is read at a clock frequency fc identical to a write clock, one cycle of the sinusoidal signal SA can be read for the duration TA. 
         [0079]    The digital data DA may be read from one at every two addresses at the clock frequency fc identical to the write clock, and the reading operation is repeated twice as shown in  FIG. 11B . A sinusoidal signal SB of two cycles twice in frequency the sinusoidal signal SA is obtained for the duration TA. More specifically, during the duration TA, the harmonic overtone component SB twice in frequency the sinusoidal signal SA is obtained. 
         [0080]    The digital data DA may be read from one at every four addresses at the clock frequency fc identical to the write clock, and the reading operation is repeated four times as shown in  FIG. 11C . A sinusoidal signal SC of four cycles four times in frequency the sinusoidal signal SA is obtained for the duration TA. More specifically, during the duration TA, the harmonic overtone component SC four times in frequency the sinusoidal signal SA is obtained. 
         [0081]    The pitch shifters  17  and  47  have a structure as shown in  FIG. 12 .  FIG. 12  illustrates a memory  17 M having large number of addresses (large capacity) composed of a ring buffer. The low frequency component S 16  may have a waveform of  FIG. 13A , digital data D 16  may be obtained by analog-to-digital converting the waveform, and fc may represent a sampling frequency (clock frequency). 
         [0082]    Let tx represent the timing at which the polarity of the digital data D 16  (low-frequency component S 16 ) is reversed from negative to positive, and Tx represent a duration from one tx to the next tx, namely, one period of the low-frequency component S 16 . 
         [0083]    As shown in  FIG. 12 , the digital data D 16  is supplied to the memory  17 M via an input terminal  17 A. As shown in  FIG. 13A , the digital data D 16  is written on the addresses of the memory  17 M every one sample. The duration Tx of  FIGS. 13A and 13B  correspond to the duration TA of  FIG. 11A .  FIG. 13A  corresponds to  FIG. 11A . 
         [0084]    At the same time as the digital data D 16  is written on the memory  17 M, the digital data D 16  is read from the memory  17 M. For simplicity of explanation, the duration Tx for the write operation equals the duration Tx for the read operation in  FIGS. 13A and 13B . 
         [0085]    The read operation from the pitch shifter  17  is performed in the same manner as described with reference to  FIG. 11B . More specifically, the digital data D 16  is read from one at every two addresses at the clock frequency fc equal to the one for the write operation. During the duration Tx, the read operation is repeated twice. Read digital data D 17  is digital-to-analog converted, and the harmonic overtone component S 17  twice in frequency the original low-frequency component S 16  is obtained. 
         [0086]    Similarly, the low-frequency component S 46  is analog-to-digital converted and written on the memory  17 M. The written data is then read as shown in  FIG. 11C . The digital data D 16  is read from one at every four addresses at the clock frequency fc equal to the one for the write operation. During the duration Tx, the read operation is repeated four times. Read digital data is digital-to-analog converted, and the harmonic overtone component S 47  four times in frequency the original low-frequency component S 46  is obtained. 
         [0087]    Even when the low-frequency component S 16  is lower in frequency than the resonance frequency f 0  of the loudspeaker  33 , the pitch shifter  17  converts the low-frequency component S 16  into the harmonic overtone component S 17  higher in frequency than the resonance frequency f 0  of the loudspeaker  33 . The harmonic overtone component S 17  is added to the middle to high-frequency component S 12  and the resulting component is supplied to the loudspeaker  33 . Even the small loudspeaker  33  can provide a low frequency sound effect. 
         [0088]    Since the harmonic overtone component S 17  contained in the low-frequency component S 23  dynamically changes in level as shown in  FIG. 3 , a crisp low frequency sound effect is provided while distortion effect is restricted. 
         [0089]    The low-frequency component S 16  is frequency-doubled or frequency-quadrupled so that the resulting harmonic overtone components fall within the frequency range from the resonance frequency f 0  to the frequency upper limit f 1 . As a result, no unpleasant hearing impression is caused. 
         [0090]    For example, since a harmonic overtone component with the frequency thereof three times the fundamental component has no octaval relationship with the fundamental component, an unpleasant hearing impression is given to the listener. The harmonic overtone components twice or four times in frequency the fundamental frequency have one octave or two octaves higher than the fundamental frequency and cause no unpleasant hearing impression to the listener. 
         [0091]    In the above embodiments, the level detector  24  detects the peak level V 22  within the one period Tx, and controls gain within the one period Tx. Alternatively, a mean level may be detected within the one period Tx, and gain is controlled within the one period Tx. Alternatively, the level of the harmonic overtone component supplied to the detector  24  is detected for each sample, in other words, the envelope of the harmonic overtone component is detected and used to control gain. Alternatively, gain may be controlled taking into consideration the characteristics of the loudspeaker  33  such as attack time and release time. 
         [0092]    The control characteristics of the gain controller  23  of  FIG. 4  may be inverted. More specifically, gain is set to be smaller than 1 with relationship V 22 &gt;VTH held, and gain is set to be 1 with relationship V 22 =VTH held. Gain may be set to be larger than 1 with relationship V 22 &lt;VTH held. In such a case, automatic gain control (AGC) also works and the low frequency sound effect is thus provided. 
         [0093]    The process of obtaining intermediate signals and the resulting audio signal S 31  from the audio signal S 11  may be a digital process performed by a digital signal processor or other dedicated hardware. In such a case, the buffer memory may be shared with the digital signal process, for example. 
         [0094]    In the above discussion, the pitch shifters  17  and  47  frequency multiplies the input digital data D 16  (S 16 ) every period of the digital data D 16 . Alternatively, the digital data D 16  may be doubled in frequency every predetermined duration of time. In such a case, an end point of one period and a start point of the next period may be concatenated in a smooth fashion. 
         [0095]    The low-frequency cutoff filter  31  may be arranged between the harmonic overtone adder  10  and the equalizer  20 . The resonance frequency f 0  is the resonance frequency of the loudspeaker  33 . In actual products, however, another frequency may be set as the frequency f 0  taking into consideration a frequency at which a low frequency sound effect is desired. 
         [0096]    It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof.