Abstract:
An audio output apparatus includes a masking band determining unit configured to determine a first frequency band in which masking due to environmental sounds is likely to occur in audio signal output sounds; a band-component extracting unit configured to extract a signal component from an input audio signal in the first frequency band determined by the masking band determining unit; a pitch shift unit configured to perform pitch shifting of the signal component in the first frequency band extracted by the band-component extracting unit and generate a pitch shift signal containing a signal component of at least a doubled frequency; and a signal output unit configured to supply an audio signal containing the pitch shift signal acquired by the pitch shift unit to a connected speaker.

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
       [0001]    The present invention contains subject matter related to Japanese Patent Application JP 2008-044822 filed in the Japanese Patent Office on Feb. 26, 2008, 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 output apparatus and an audio output method particularly suitable for use in an environment with high levels of exogenous noise. 
         [0004]    2. Description of the Related Art 
         [0005]    In environments with high levels of exogenous noise, such as inside an automobile, it is often difficult to music and so on. 
         [0006]      FIG. 10  illustrates an example in which the noise level inside a traveling automobile is measured. As shown in the drawing, noise having a high level in a low frequency band is generated in a traveling automobile. 
         [0007]    When listening to music and so on with an in-vehicle audio system, in particular, low frequencies of the music are masked by such noise. 
       SUMMARY OF THE INVENTION 
       [0008]    When listening to music and so on with an in-vehicle audio system while driving when the level of exogenous noise is high, the user may increase the volume of the music (volume level) to a level similar to the noise, boost the low frequencies by the mechanic function of an equalizer, or carrying out small signal level boosting by compression. 
         [0009]    However, since the noise level inside the vehicle increases due to the driving speed, the music signal should be boosted to a level higher than that of the noise in order to prevent masking. 
         [0010]    Therefore, the volume level may increase to a level unexpected by the passengers, and thus, it is difficult to ensure a comfortable listening environment. 
         [0011]    Accordingly, it is desirable to provide an audio output apparatus and an audio output method that enables music and so on to be enjoyed comfortably even in an environment with high levels of exogenous noise. 
         [0012]    An audio output apparatus according to an embodiment of the present invention includes a masking band determining unit configured to determine a first frequency band in which masking due to environmental sounds is likely to occur in audio signal output sounds; a band-component extracting unit configured to extract a signal component from an input audio signal in the first frequency band determined by the masking band determining unit; a pitch shift unit configured to perform pitch shifting of the signal component in the first frequency band extracted by the band-component extracting unit and generate a pitch shift signal containing a signal component of at least a doubled frequency; and a signal output unit configured to supply an audio signal containing the pitch shift signal acquired by the pitch shift unit to a connected speaker. 
         [0013]    The band-component extracting unit may separate the signal component of the first frequency band and a signal component of a second frequency band and supply the signal component of the first frequency band to the pitch shift unit, and the signal output unit may supply an audio signal acquired by combining the signal component of the second frequency band and the pitch shift signal to a speaker. 
         [0014]    The band-component extracting unit may extract the signal component of the first frequency band from an input audio signal and supply the extracted signal component to the pitch shift unit, and the signal output unit may supply an audio signal acquired by combining the input audio signal and the pitch shift signal to a speaker. 
         [0015]    The masking band determining unit may carry out frequency analysis of environmental noise collected by a microphone and carry out determination of the first frequency band on the basis of an environmental noise level of each frequency band. 
         [0016]    The pitch shift unit may generate the pitch shift signal containing a signal component of at least a doubled frequency of the frequency of the signal component of the first frequency band and another harmonic component. 
         [0017]    An audio output apparatus according to another embodiment of the present invention includes a masking determining unit configured to determine whether or not masking due to environmental sounds occurs to audio signal output sounds; a band-component extracting unit configured to extract a signal component of a specific frequency band from an input audio signal when the masking determining unit determines that masking occurs; a pitch shift unit configured to perform pitch shifting of a signal component in a first frequency band extracted by the band-component extracting unit and generates a pitch shift signal containing a signal component of at least a doubled frequency; and a signal output unit configured to supply an audio signal containing the pitch signal acquired by the pitch shift unit to a connected speaker. 
         [0018]    An audio output method according to an embodiment of the present invention includes the steps of determining a first frequency band in which masking due to environmental sounds is likely to occur in audio signal output sounds; extracting a signal component in the first frequency band from an audio signal; performing pitch shifting of the signal component in the extracted first frequency band and generating a pitch shift signal containing a signal component of at least a doubled frequency; and supplying an audio signal containing the pitch shift signal to a connected speaker. 
         [0019]    In the embodiments of the present invention, a signal component in a frequency band in an audio signal that is masked by noise is pitch shifted. 
         [0020]    For example, when listening to music in a vehicle, in particular, the low frequency band tends to be masked by noise, such as the engine noise and road noise caused during driving. Therefore, clear reproduction under a noise environment is possible by pitch shifting the frequency components of the audio signal of the masked music to a frequency band that is less likely to be masked depending on the noise level and the frequency band. 
         [0021]    By pitch shifting the audio signal, the musical pitch changes. However, embodiments of the present invention employ the missing fundamental illusion. 
         [0022]    The missing fundamental illusion is a phenomenon in which, for sounds including a harmonic series of the sounds in the fundamental frequency, human beings sense the sounds of the fundamental frequency even when the sounds of the fundamental frequency are not included. Thus, according to the embodiments of the present invention, pitch shifting of signal components of a masked frequency band is performed and the signal components having at least a doubled frequency are set as a pitch shift signal. In other words, the masked band components are moved to a frequency band less likely to be masked, and the user can sense the fundamental frequency components by the output sounds of the pitch shift signal components. 
         [0023]    According to embodiments of the present invention, by performing pitch shifting of signal components of a masked frequency band, setting the signal components having at least a doubled frequency as a pitch shift signal, and outputting an audio signal containing the pitch shift signal components to a speaker, sounds in a frequency band that is not heard due to a masking effect can be sensed by users through harmonic components that are less likely to be masked, and the sounds of the masked frequency band can be sensed by the missing fundamental illusion. 
         [0024]    In this way, the effect of masking due to noise can be reduced, and, even under high-noise conditions, music and so on can be enjoyed without increasing the output volume of the music and so on or by boosting the frequency band being masked. 
         [0025]    In-vehicle audio apparatuses according to first, second, and third embodiments of the present invention will be described below. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]      FIG. 1  is a block diagram of an audio apparatus according to a first embodiment of the present invention. 
           [0027]      FIG. 2  is a block diagram of a band dividing unit according to an embodiment. 
           [0028]      FIG. 3  is a block diagram of a pitch shift unit according to an embodiment. 
           [0029]      FIG. 4  is a flow chart illustrating the processing carried out by a spectrum analysis/control unit according to an embodiment. 
           [0030]      FIG. 5  is a schematic view of an operation image according to an embodiment. 
           [0031]      FIG. 6  is a block diagram of a pitch shift unit according to an embodiment. 
           [0032]      FIG. 7  is a block diagram of an audio apparatus according to a second embodiment of the present invention. 
           [0033]      FIG. 8  is a block diagram of a bandpass tunable filter unit according to an embodiment. 
           [0034]      FIG. 9  is a block diagram of an audio apparatus according to a third embodiment of the present invention. 
           [0035]      FIG. 10  illustrates the noise measurement result of a vehicle interior. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     First Embodiment  
       [0036]      FIG. 1  illustrates an in-vehicle audio apparatus  1  according to the first embodiment of the present invention. 
         [0037]    The audio apparatus  1  includes a microphone  2 , a microphone amplifier  3 , a spectrum analysis/control unit  4 , an audio reproduction unit  5 , a band dividing unit  6 , a pitch shift unit  7 , a combining unit  8 , a D/A converter  9 , a power amplifier  10 , and a speaker  15 . 
         [0038]    The microphone  2  is used to collect noise sensible inside the vehicle, i.e., road noise, and is installed in an appropriate location inside an automobile. 
         [0039]    Noise audio signals acquired by the microphone  2  are supplied to the spectrum analysis/control unit  4  via the microphone amplifier  3 . 
         [0040]    The spectrum analysis/control unit  4  performs spectrum analysis of the input noise audio signal and detects the level of each frequency band. As described below, the spectrum analysis/control unit  4  also controls the operation of the band dividing unit  6  and the pitch shift unit  7  in accordance with the detected result. 
         [0041]    The audio reproduction unit  5  is, for example, an optical disk reproduction unit, a hard disk drive (HDD), a memory card drive, or a magnetic tape player. In other words, the audio reproduction unit  5  is a section that reproduces an audio signal SA 1 , such as music content, on a recording medium, such as an optical disk, a hard disk, a memory card, or a magnetic tape. 
         [0042]    The audio signal SA 1  output from the audio reproduction unit  5  a digital audio signal. However, the audio signal SA 1  may otherwise be an analog audio signal. 
         [0043]    In this embodiment, the audio reproduction unit  5  is the audio source of the audio signal SA 1 . However, this is merely an example, and so long as the audio source is a section that outputs the audio signal SA 1 , it may not necessarily be a reproduction unit of recording medium. The audio reproduction unit  5  may instead be an audio output system, such as a radio tuner, a television tuber, or a video reproduction unit. 
         [0044]    To simplify the description, only one circuit system (band dividing unit  6 , pitch shift unit  7 , combining unit  8 , D/A converter  9 , power amplifier  10 , and speaker  15 ) corresponding to an audio signal SA is described. However, for a stereo system, two of these systems are provided. When a multi channel system is employed, a similar configuration is provided for each channel. Alternatively, the configuration shown in  FIG. 1  may be provided for some of the channels in the multi channel system. 
         [0045]    The band dividing unit  6  performs band division on the audio signal SA 1  from the audio reproduction unit  5  and outputs band-division audio signals SA 2  and SA 3 . One of the divided bands is supplied to the pitch shift unit  7  as the audio signal SA 3  of a frequency band subjected to pitch shift processing. 
         [0046]    As shown in  FIG. 2 , the band dividing unit  6  includes switches SW 1  and SW 2 , a bandpass tunable low-pass filter (LPF)  30 , a bandpass tunable high-pass filter (HPF)  31 . 
         [0047]    The switches SW 1  and SW 2  are turned on or off by a control signal C 1  form the spectrum analysis/control unit  4 . In this case, only one of the switches SW 1  and SW 2  is turned on. 
         [0048]    The cutoff frequencies of the bandpass tunable LPF  30  and the bandpass tunable HPF  31  are controlled in an interlocking manner by a control signal C 2  from the spectrum analysis/control unit  4 . 
         [0049]    As shown in  FIG. 2 , when the switch SW 1  is turned on and the switch SW 2  is turned off, the audio signal SA 1  is supplied to the bandpass tunable LPF  30  and the bandpass tunable HPF  31 . When the cutoff frequency of the bandpass tunable LPF  30  and the bandpass tunable HPF  31  is controlled to 100 Hz by the control signal C 2 , signal components of a frequency band of 100 Hz or lower are extracted at the bandpass tunable LPF  30 , and these signal components are output to the pitch shift unit  7  as the audio signal SA 3  of a frequency band subjected to pitch shift processing. At the bandpass tunable HPF  31 , signal components of a frequency band of 100 Hz or higher pass. These signal components are output as the audio signal SA 2  and are supplied to the combining unit  8 . 
         [0050]    Alternatively, when the switch SW 1  is turned off and the switch SW 2  is turned on, the audio signal SA 1  is output as the audio signal SA 2  without being divided. In such a case, an audio signal SA 2  for the pitch shift unit  7  is not output. 
         [0051]    When such a configuration is employed, the band dividing unit  6  outputs the audio signal SA 2  and SA 3 , as shown in  FIG. 1 . 
         [0052]    The audio signal SA 3  output from the band dividing unit  6  is input to the pitch shift unit  7 . The pitch shift unit  7  performs pitch shift of the audio signal SA 3  and outputs a pitch shift signal SA 3 ′ including signal components of at least a doubled frequency. 
         [0053]    An example configuration of the pitch shift unit  7  is illustrated in  FIG. 3 . For example, the pitch shift unit  7  includes a memory  20 , a memory controller  21 , and a multiplier  22 . The memory  20  is, for example, a dynamic random access memory (DRAM), a synchronous dynamic random access memory (SDRAM), which is a type of DRAM, or a static random access memory (SRAM). 
         [0054]    The memory controller  21  is provided with a clock signal CK 1  having a frequency fs as a writing clock signal. The clock signal CK 1  is doubled at the multiplier  22  to generate a clock signal CK 2  having a frequency (2fs), and this clock signal CK 2  is supplied to the memory controller  21  as a reading clock signal. 
         [0055]    The memory controller  21  writes the input audio signal SA 3  in the memory  20  according to the clock signal CK 1 . The memory controller  21  reads out the audio signal SA 3  written in the memory  20  two consecutive times at each predetermined unit according to the doubled clock signal CK 2 . By outputting the readout signal consecutively, the audio signal SA 3  can be output as a pitch shift signal SA 3 ′ having a doubled frequency. In other words, a pitch shift signal SA 3 ′ in which the fundamental pitch components included as the audio signal SA 3  are second harmonic overtones is output. 
         [0056]    The memory controller  21  performs such a pitch shifting operation on the basis of a control signal C 3 . 
         [0057]    As shown in  FIG. 1 , the pitch shift signal SA 3 ′ output from the pitch shift unit  7  and the audio signal SA 2  from the band dividing unit  6  are supplied to the combining unit  8 . 
         [0058]    The combining unit  8  additively combines the pitch shift signal SA 3 ′ and the audio signal SA 2  to generate an audio signal SA 4  to be supplied to the speaker  15 . 
         [0059]    The audio signal SA 4  is amplified at the power amplifier  10  after being converted into an analog audio signal at the D/A converter  9  and is output from the speaker  15  as sound, i.e., reproduced sound, such as music. 
         [0060]    With reference to the configuration shown in  FIG. 1 , the correspondence between the features of the claims and the specific elements disclosed in an embodiment of the present invention is as follows: 
         [0061]    masking band determining unit: spectrum analysis/control unit  4   
         [0062]    band-component extracting unit: band dividing unit  6   
         [0063]    pitch shift unit: pitch shift unit  7   
         [0064]    signal output unit: combining unit  8   
         [0065]    The operation of the audio apparatus  1  will be described. 
         [0066]    As shown in  FIG. 10 , the level of vehicle interior noise generated during driving is high in at low frequency and low at high frequency. Therefore, music signal components at low frequency tend to be masked by the driving noise. In this embodiment, to prevent such masking, the vehicle interior noise is collected by the microphone  2 , and the low frequency bands are appropriately shifted to frequencies less likely to be masked. 
         [0067]    The processing for this operation performed by the spectrum analysis/control unit  4  is illustrated in  FIG. 4 .  FIG. 5  is a schematic view of an image of the operation process corresponding to the process shown in  FIG. 4 . 
         [0068]    The process illustrated in  FIG. 4  is carried out repeatedly by the spectrum analysis/control unit  4  while music and so on from the audio reproduction unit  5  is reproduced by the audio apparatus  1 . 
         [0069]    In Step F 101 , noise is input to the spectrum analysis/control unit  4 . In other words, a noise audio signal is input to the spectrum analysis/control unit  4  via the microphone  2  and the microphone amplifier  3 . 
         [0070]    In Step F 101 , the spectrum analysis/control unit  4  performs spectrum analysis of the input noise audio signal in predetermined units. In Step F 103 , as a result of the spectrum analysis, the level of each frequency band is detected and a frequency band in which masking of the reproduced music is more likely to occur is determined. 
         [0071]    For example, the probability of masking may be determined by comparing the noise level in each frequency band to a predetermined threshold level th. 
         [0072]      FIGS. 5A and 5B  illustrate examples of the results of noise spectrum analysis. In  FIG. 5A , the noise level is below the threshold level th even in the low frequency band, and thus, it is determined that masking will not occur. This, for example, corresponds to a case in which the vehicle is not driving, and thus the noise level is low. 
         [0073]    On the other hand,  FIG. 5B  illustrates a case in which road noise is great due to an increase in driving speed. The noise exceeds the threshold level th in the low frequency band. When a noise level exceeding the threshold level th is detected, it is determined that masking of the speaker output sound is likely to occur. 
         [0074]    As a result of the determination, if the noise level is low, as shown in  FIG. 5A , and masking is less likely to occur, the spectrum analysis/control unit  4  proceeds from Step F 104  to Step F 107  and carries out pitch shift non execution control. 
         [0075]    In other words, in such a case, the control signal C 1  turns on the switch SW 1  of the band dividing unit  6 , which is shown in  FIG. 2 , turns on the switch SW 2 , and does not allow the pitch shift unit  7  to carry out a pitch shifting operation by the control signal C 3 . 
         [0076]    Therefore, in this case, the audio signal SA 1  from the audio reproduction unit  5  is directly supplied to the combining unit  8  as the audio signal SA 2 , which is not divided by the band dividing unit  6 . The pitch shift signal SA 3 ′ is not input to the combining unit  8 . 
         [0077]    The combining unit  8  directly outputs the audio signal SA 2  (=SA 1 ) as the audio signal SA 4  (i.e., SA 4 =SA 1 ) for speaker output. Therefore, in such a case, the audio signal SA 1  from the audio reproduction unit  5  is directly output from the speaker. 
         [0078]    As shown in  FIG. 5B , when the road noise level is high and masking is likely to occur, the spectrum analysis/control unit  4  proceeds from Step F 104  to F 105 . In Step F 105 , the frequency band to which pitch shifting is to be carried out is determined. 
         [0079]    For example, in  FIG. 5B , if a noise level exceeding the threshold level th is observed in a frequency band below a frequency fx, the frequency band below the frequency fx is selected as the frequency band to which pitch shifting is carried out. 
         [0080]    Then, pitch shift execution control is carried out in Step F 106  to the selected frequency band. 
         [0081]    In other words, in such a case, the control signal C 1  turns on the switch SW 1  of the band dividing unit  6 , which is shown in  FIG. 2 , and turns off the switch SW 2 . The control signal C 2  sets a cutoff frequency to fx. Moreover, the control signal C 3  instructs the pitch shift unit  7  to execute the pitch shifting operation. 
         [0082]    An image of an audio signal in such a case is illustrated in  FIGS. 5C ,  5 D,  5 E, and  5 F. 
         [0083]    The audio signal SA 1  from the audio reproduction unit  5  is illustrated in  FIG. 5C  along a frequency axis. 
         [0084]    In such a case, signal components of the frequency band above a frequency fx shown in  FIG. 5D  are output as the audio signal SA 2  from the band dividing unit  6 , and the signal components of a frequency band below the frequency fx shown in  FIG. 5E  are supplied to the pitch shift unit  7  as the audio signal SA 3 . 
         [0085]    The pitch shift unit  7  carries out pitch shifting processing on the audio signal SA 3  and outputs the pitch shift signal SA 3 ′ including signal components shown in  FIG. 5F . 
         [0086]    At the combining unit  8 , the audio signal SA 2  of  FIG. 5D  and the pitch shift signal SA 3 ′ of  FIG. 5F  are additively combined, and the result is output to the speaker  15  as the audio signal SA 4  (SA 4 =SA 2 +SA 3 ′). 
         [0087]    For example, if the frequency fx is 100 Hz, signal components in a frequency band equal to and below 100 Hz is pitch shifted to a doubled frequency. The pitch shifted components are added to the signal components in a frequency band equal to and above 100 Hz and are output to the speaker  15 . 
         [0088]    By carrying out the processing according to this embodiment, the masking effect due to noise can be reduced. Accordingly, even under a high-noise conditions, such as inside a driving vehicle, music and so on can be enjoyed without increasing the output volume of the music and so on reproduced by the audio reproduction unit  5  or by boosting the frequency band being masked. 
         [0089]    In other words, by pitch shifting signal components in the audio signal SA 1  in the frequency band masked by noise, the signal components in the frequency band, i.e., the signal components that are not heard by listeners due to masking, are shifted to a frequency band that is less likely to be masked. Thus, the audio output after carrying out pitch shifting can be heard by the users. 
         [0090]    When the audio signal components, for example, signal components of 100 Hz, are pitch shifted to 200 Hz, masking may be prevented, but the pitch of the signal components may change. However, due to the missing fundamental illusion, the user will sense the music and so on normally. 
         [0091]    According to the related art, the missing fundamental illusion is a phenomenon in which, for sounds including a harmonic series of the sounds in the fundamental frequency, human beings sense the sounds of the fundamental frequency even when the sounds of the fundamental frequency are not included. Even when components of the fundamental frequency (for example, 100 Hz) are not included, human beings sense the fundamental frequency (100 Hz) if the second harmonic overtone (200 Hz) is included. Due to this phenomenon, even when pitch shifting is performed as described in this embodiment, the image of the original music and so on is not lost. Therefore, the effect of masking due to noise can be reduced, and music and so on can be enjoyed. In particular, the low frequency band that is masked can be clearly heard. In this way, an increase in the speaker output volume is unnecessary. 
         [0092]    In this embodiment, the pitch shift unit  7  carries out pitch shift to a doubled frequency. 
         [0093]    In order to sense the fundamental frequency under the missing fundamental illusion, at least a second harmonic overtone of the fundamental frequency should be present, and it is preferable that a harmonic series including, for example, a third harmonic overtone and a fourth harmonic over tone be present. As described above, when the audio signal is music, the audio signal contains a harmonic series. In other words, since, not only the pitch shift signal SA 3 ′, which is the second harmonic overtone, is output, but also the audio signal SA 2  is mixed and output, the final speaker output includes, in addition to the high level second harmonic overtone, components of a harmonic series is included. Therefore, the user can sense the fundamental frequency. 
         [0094]    The pitch shift signal SA 3 ′ may not only contain the second harmonic overtone but also other components of the harmonic series. 
         [0095]    For example, the pitch shift unit  7  may be constructed in such a manner illustrated in  FIG. 6 . The pitch shift unit  7  includes, in addition to the memory  20 , the memory controller  21 , and the multiplier  22  shown in  FIG. 3 , a memory  23 , a memory controller  24 , a multiplier  25 , and an adder  26 . 
         [0096]    The clock signal CK 1  having the frequency fs is supplied to the memory controller  24  as a writing clock signal, and the clock signal CK 2 , which is acquired by multiplying the clock signal CK 1  by four to a frequency (4fs) at the multiplier  25 , is supplied as a reading clock signal. 
         [0097]    The memory controller  21  writes the input audio signal SA 3  on the memory  20  according to the clock signal CK 1  and reads out, at every predetermined unit, the audio signal SA 3  written on the memory  20  two consecutive times according to the clock signal CK 2  having a doubled frequency. In this way, a signal acquired by pitch shifting the audio signal SA 3  to a doubled frequency is output. 
         [0098]    The memory controller  24  writes the audio signal SA 3  on the memory  23  according to the clock signal CK 1  and reads out, at every predetermined unit, the audio signal SA 3  written on the memory  23  four consecutive times according to the clock signal CK 3 . In this way, a signal acquired by pitch shifting the audio signal SA 3  to quadrupled frequency is output. 
         [0099]    The adder  26  adds the signal pitch shifted to a double frequency and the signal pitch shifted to a quadrupled frequency and outputs the added signals as the pitch shift signal SA 3 ′. 
         [0100]    In this way, not only a second harmonic overtone but also other harmonic series components may be actively added to the pitch shift signal SA 3 ′. 
         [0101]    Third, fifth, and/or sixth harmonic overtones may be included in the pitch shift signal SA 3 ′. 
         [0102]    In this embodiment, the microphone  2  is configured to collects noise and not to collect sound, such as the reproduced music, based on the audio signal SA 1 . 
         [0103]    It is desirable to configure the microphone  2  such that the sounds output from the speaker  15  are less likely to be collected by selecting an appropriate installation site and orientation of the microphone  2  in the vehicle. 
         [0104]    Alternatively, since road noise is mainly in a low frequency band, for example, 200 Hz or lower, the low frequency components of 200 Hz or lower of the audio signal collected at the microphone  2  may be supplied to the spectrum analysis/control unit  4 . 
         [0105]    Moreover, the audio signal SA 1  from the audio reproduction unit  5  is phase-reversed and supplied to the spectrum analysis/control unit  4  as a reversed phase signal. By adding the reversed phase signal to the audio signal collected at the microphone  2  and cancel out the components of the audio signal SA 1 , the road noise components may be analyzed at the spectrum analysis/control unit  4 . 
         [0106]    Masking is determined at the spectrum analysis/control unit  4  by comparing the noise level at each frequency band with a predetermined threshold level th. The threshold level th may be the same level for each frequency band, or different threshold levels th may be set for each frequency band. 
         [0107]    The threshold level th for masking determination may be variable according to the volume of the audio signal output from the speaker  15 . 
         [0108]    The audio signal SA 1  from the audio reproduction unit  5  may be supplied to the spectrum analysis/control unit  4 , and the level of the audio signal SA 1  may be detected for each frequency band in a similar manner as for noise. Then, the noise level of each frequency level and the audio signal level may be compared to detect whether masking occurs and in which frequency band masking occurs. 
         [0109]    In  FIG. 1 , the audio signal SA 1  is a digital audio signal. However, the audio signal SA 1  may be an analog audio signal, and the band dividing unit  6 , the pitch shift unit  7 , the combining unit  8 , and so on may carry out processing for analog audio signals. 
         [0110]    Although not repeated, the above-described aspects, i.e., a pitch shift signal SA 3 ′ containing many harmonic components, detection method of noise by the microphone  2 , masking determination method, and convertibility of digital processing and analog processing of an audio signal, can be employed in second and third embodiments described below. 
       Second Embodiment  
       [0111]    The configuration of an audio apparatus  1  according to a second embodiment is illustrated in  FIG. 7 . The components that are same as those in  FIG. 1  will be indicated by the same reference numerals, and descriptions thereof will not be repeated. 
         [0112]    In such a case, an audio signal SA 1  from an audio reproduction unit  5  is directly supplied to a combining unit  8  and is supplied to a bandpass tunable filter unit  11 . 
         [0113]    The bandpass tunable filter unit  11  includes, for example, a switch SW 1  and a bandpass tunable LPF  30 , as shown in  FIG. 8 . The switch SW 1  is turn on or off by a control signal C 1  from a spectrum analysis/control unit  4 . The cutoff frequency of the bandpass tunable LPF  30  is variably set by a control signal C 2  from the spectrum analysis/control unit  4 . 
         [0114]    The output from the bandpass tunable LPF  30  is supplied to a pitch shift unit  7  as an audio signal SA 3  of a frequency band subjected to a pitch shifting. 
         [0115]    At the pitch shift unit  7 , a pitch shift signal SA 3 ′ acquired by pitch shifting the audio signal SA 3  at to at least a second harmonic overtone is generated and output to the combining unit  8 . 
         [0116]    With reference to the configuration shown in  FIG. 8 , frequency-band-component extracting unit of the claims corresponds to the bandpass tunable filter unit  11 . 
         [0117]    Also in the second embodiment, the spectrum analysis/control unit  4  carried out the processing illustrated in  FIG. 4 . 
         [0118]    When the process in  FIG. 4  proceeds to Step F 107  when it is determined that masking does not occur, the spectrum analysis/control unit  4  carries out pitch shifting non-execution control in which the switch SW 1  of the bandpass tunable filter unit  11  is turned off and the pitch shift unit  7  is prohibited from carrying out pitch shifting operation by a control signal C 3 . 
         [0119]    Therefore, in such a case, the audio signal SA 1  from the audio reproduction unit  5  is directly output from the combining unit  8  as a speaker output audio signal SA 4  (SA 4 =SA 1 ). 
         [0120]    When the process of the spectrum analysis/control unit  4  proceeds to Step F 105  when the road noise level is high and it is determined that masking is likely to occur, the frequency band to be subjected to pitch shifting is determined on the basis of the result of spectrum analysis, and pitch shifting execution control is carried out in Step F 106 . 
         [0121]    In other words, in such a case, the control signal C 1  turns on the switch SW 1  of the bandpass tunable filter unit  11 , shown in  FIG. 2 , and the control signal C 2  instructs the cutoff frequency of the bandpass tunable LPF  30 . Then, the control signal C 3  instructs the pitch shift unit  7  to execute pitch shifting operation. 
         [0122]    The audio signal SA 3  of the low frequency band extracted by the bandpass tunable LPF  30  is supplied to the pitch shift unit  7 . The pitch shift unit  7  generates a pitch shift signal SA 3 ′ from the audio signal SA 3  and outputs the pitch shift signal SA 3 ′ to the combining unit  8 . 
         [0123]    Therefore, in such a case, the combining unit  8  additively combines the audio signal SA 1  shown in  FIG. 5C  and the pitch shift signal SA 3 ′ shown in  FIG. 5F . The result is output to the speaker  15  as an audio signal SA 4 . 
         [0124]    The difference with the first embodiment is that the audio signal SA 4  (SA 4 =SA 1 +SA 3 ′) for speaker output is generated by adding the pitch shift signal SA 3 ′ to the audio signal SA 1  of all frequency bands including the frequency ban din which masking is likely to occur. 
         [0125]    The same advantages as the first embodiment can also be achieved by the second embodiment. 
       Third Embodiment  
       [0126]    A third embodiment will be described with reference to  FIG. 9 . The components that are the same as those in  FIG. 1  will be represented by the same reference numerals, and descriptions thereof will not be repeated. 
         [0127]    The configuration illustrated in  FIG. 9  is the same as that illustrated in  FIG. 1 , except that a low-band noise detection/control unit  14  is provided instead of the spectrum analysis/control unit  4 . The low-band noise detection/control unit  14  is a section that performs simple spectral analysis. The low-band noise detection/control unit  14  extracts only the low frequency band of the noise audio signal collected at a microphone  2  using an LPF having a cutoff frequency of a specific frequency fx and detects the noise level of the extracted frequency band. Then, the low-band noise detection/control unit  14  determines whether or not masking has occurred according to the detected noise level. 
         [0128]    An audio signal SA 1  from an audio reproduction unit  5  is supplied to a combining unit  8  and, when a switch  12  is turned on, is supplied to a pitch shift unit  7  via an LPF  13 . The LPF  13  has a fixed cutoff frequency of frequency fx. 
         [0129]    In such a case, the low-band noise detection/control unit  14  detects the noise level in a frequency band below the frequency fx. Then, according to the detected result, when the noise level in the low frequency band is low and it is determined that masking will not occur, the switch  12  is turned off by a control signal C 1 . Furthermore, a control signal C 3  prohibits the pitch shift unit  7  from carrying out pitch shifting. 
         [0130]    Therefore, in such a case, the audio signal SA 1  from the audio reproduction unit  5  is directly output from the combining unit  8  as an audio signal SA 4  (SA 4 =SA 1 ). 
         [0131]    Alternatively, when the road noise level is high and it is determined that the low-band noise level increases, causing masking to occur, the low-band noise detection/control unit  14  turns on the switch  12  by the control signal C 1  and instructs the pitch shift unit  7  to carry out pitch shifting by the control signal C 3 . 
         [0132]    In this way, the low-band audio signal SA 3  extracted by the LPF  13  is supplied to the pitch shift unit  7 . Then, the pitch shift unit  7  generates a pitch shift signal SA 3 ′ from the audio signal SA 3  and outputs the pitch shift signal SA 3 ′ to the combining unit  8 . 
         [0133]    Therefore, in such a case, the combining unit  8  additively combines the audio signal SA 1  and the pitch shift signal SA 3 ′ and outputs the result as an audio signal SA 4  to the speaker  15 . 
         [0134]    In other words, the third embodiment simplifies the configuration and processing by fixing the frequency band of the audio signal SA 3  supplied to the pitch shift unit  7 . 
         [0135]    For example, the frequency band in which the low-band noise detection/control unit  14  carries out level detection is fixed to 100 Hz and lower, and the cutoff frequency of the LPF  13  is fixed to 100 Hz. In this way, when masking occurs in the frequency band of 100 Hz and lower, the same advantages as those achieved in the first and second embodiments can be achieved by pitch shifting and adding the frequency band of the audio signal SA 1 . 
         [0136]    By fixing the frequency band to be pitch shifted, fine control corresponding to the actual noise level may not be carried out. However, it is suitable for achieving the advantages of the first and second embodiments by a simple configuration. 
         [0137]    The present invention is not limited to the first, second, and third embodiments described above, and various modifications and applications thereof may be made. 
         [0138]    The present invention is applied to an audio apparatus used in a vehicle. In addition, the present invention may be suitably applied to an audio system used under environments with noise, such as audio apparatuses used in an aircraft or a train and audio apparatuses installed in factories and shops. 
         [0139]    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.