Patent Publication Number: US-8112283-B2

Title: In-vehicle audio apparatus

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to in-vehicle audio apparatuses, and in particular, relates to a method for correcting audio signals in response to the level of noise that occurs in a vehicle that is being driven. 
     2. Description of the Related Art 
     In a case where a passenger listens to music through a compact disk (CD), a digital versatile disk (DVD), a radio broadcast, or the like in a vehicle cabin, when the passenger is exposed to background noise, such as an engine noise or a road noise, the passenger cannot hear music in a satisfactory condition. In view of this problem, a system exists in which the noise level is measured with a microphone provided in the vehicle cabin and an optimum correction (for example, correction of the volume) is performed in response to the noise level. Specifically, in this system, a microphone is used to collect sound in a vehicle cabin, and audio signals in external sound signals received by the microphone are cancelled to extract noise. 
     An apparatus that automatically adjusts the volume in response to noise is disclosed in Japanese Unexamined Patent Application Publication No. 10-335960. This apparatus includes a noise-signal extracting unit that extracts only noise signals from sound signals in a vehicle cabin detected by a microphone and a volume-adjusting unit that adjusts the volume of an audio apparatus in response to the noise signals detected by the noise-signal extracting unit, and this apparatus enables a passenger to hear audio signals from an audio signal source at an appropriate volume in consideration of the noise level in the vehicle cabin. 
     However, in the known in-vehicle audio apparatus, although played-back music can be distinguished from noise, a speech sound of a person in a vehicle cabin cannot be distinguished. Thus, correction is performed on noise that includes a speech sound. The frequency spectrum and volume of a speech sound of a person can suddenly and markedly change. Thus, when audio signals are corrected on the basis of the level of noise that includes a speech sound, a change in the amount of correction is inevitably large, resulting in an unnatural sound. 
     SUMMARY OF THE INVENTION 
     In view of the aforementioned problem, it is an object of the present invention to provide an audio apparatus that can remove a speech sound from noise and correct an audio signal. 
     An audio apparatus according the present invention has a function of correcting an audio signal in response to a noise level. The audio apparatus includes a correction unit that corrects an audio signal on the basis of a weighting factor, an output unit that produces a played-back audio sound on the basis of the corrected audio signal, a microphone for receiving an external sound that includes the played-back audio sound and noise, a noise-extracting unit that extracts a noise signal from an external sound signal detected by the microphone, the noise-extracting unit including a speech-removing unit that removes a speech signal from the noise signal on the basis of noise spectrum data that is prepared in advance, and a weighting factor calculation unit that calculates the weighting factor on the basis of the extracted noise signal and supplies the calculated weighting factor to the correction unit. 
     Moreover, a method for playing back an audio signal in an in-vehicle audio apparatus includes receiving an external sound that includes a played-back audio sound and noise in a vehicle interior space through a microphone, extracting a noise signal from an external sound signal detected by the microphone and removing a speech signal from the extracted noise signal with reference to noise spectrum data, calculating a weighting factor corresponding to the noise signal from which the speech signal is removed, correcting the audio signal on the basis of the calculated weighting factor, and producing the played-back audio sound on the basis of the corrected audio signal. 
     In the audio apparatus according to the present invention, a speech sound is removed from noise on the basis of noise spectrum data, and an audio signal is corrected on the basis of the noise from which the speech sound is removed. Thus, a played-back audio sound having a sound quality suitable for an actual noise level can be obtained by suppressing the influence of a speech sound in which a sudden change in the frequency or volume occurs. 
     The audio apparatus according to the present invention can be used in a navigation apparatus or a navigation system that includes an in-vehicle audio unit, in-vehicle electronic equipment that receives a radio broadcast or a television broadcast, or the like. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing the structure of an in-vehicle audio apparatus according to a first embodiment of the present invention; 
         FIG. 2  is a graph showing the noise spectrum of a vehicle that is being driven; 
         FIG. 3A  shows threshold values that are generated for individual frequency bands having a predetermined bandwidth on the basis of the noise spectrum; 
         FIG. 3B  shows threshold values that are generated for individual frequency bands on the basis of the noise spectrum so that a gain decreases by a predetermined value; 
         FIG. 4  is a graph showing a speech spectrum; 
         FIG. 5  is a flowchart showing the operation of the audio apparatus according to the first embodiment; and 
         FIG. 6  is a flowchart showing the operation of an audio apparatus according to a second embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS AND THE PRESENTLY PREFERRED EMBODIMENTS 
     Audio apparatuses according to embodiments of the present invention will now be described in detail with reference to the drawings. The audio apparatuses according to the embodiments of the present invention are preferably embodied in in-vehicle audio apparatuses or in-vehicle audio systems. 
     Embodiments 
     In the present invention, noise of a vehicle that is being driven is distinguished from a speech sound of a person on the basis of the difference between spectra of the noise and the speech sound so that correction is not performed on the speech sound.  FIG. 1  is a block diagram showing the structure of an in-vehicle audio apparatus according to a first embodiment of the present invention. The in-vehicle audio apparatus  10  includes an audio playback unit  12  that generates audio signals corresponding to data stored in, for example, a CD or a DVD, or audio signals from a radio broadcast or a television broadcast, an acoustic weighting filter  14  that receives the audio signals from the audio playback unit  12  as input and corrects the audio signals for the frequency characteristics, the volume, or the like on the basis of an acoustic weighting factor C, an amplifier  16  that amplifies the corrected audio signals, and a speaker  18  that produces the amplified audio signals in a vehicle interior space as an audible sound. 
     The in-vehicle audio apparatus  10  further includes a microphone  20  for receiving an external sound in a vehicle interior space, a noise-extracting unit  30  that receives external sound signals M detected by the microphone  20  as input and extracts noise signals from the external sound signals M, and an acoustic weighting factor calculation unit  40  that calculates the acoustic weighting factor C on the basis of the noise signals extracted by the noise-extracting unit  30  and provides the calculated acoustic weighting factor C to the acoustic weighting filter  14 . 
     In an acoustic space of a vehicle, sound of conversation among passengers and a driving noise, such as an engine noise and a road noise, which occurs in a vehicle that is being driven, are mixed with a played-back audio sound produced from the speaker  18 . These mixed sounds are received by the microphone  20  as an external sound. Thus, the external sound signals M detected by the microphone  20  include played-back audio signals Sp and noise signals N (M=Sp+N). The noise signals N include speech signals Ns and other driving noise signals Np (N=Ns+Np). 
     The noise-extracting unit  30  according to this embodiment includes an audio-canceling unit  32 , a speech determining/removing unit  34 , and a noise-band-level generating/storing unit  36 . The audio-canceling unit  32  receives audio signals S from the acoustic weighting filter  14  and the external sound signals M (M=Sp+N) from the microphone  20  as inputs and cancels the played-back audio signals Sp on the basis of the difference between the audio signals S and the external sound signals M. At this time, the audio-canceling unit  32  corrects the audio signals S with transfer characteristics H of audio signals in an acoustic space of a vehicle so that Sp=H·S. Thus, the played-back audio signals Sp are removed from the external sound signals M and the noise signals N (N=Ns+Np) are extracted. 
     The speech determining/removing unit  34  receives the noise signals N from the audio-canceling unit  32 , reads threshold values Th from the noise-band-level generating/storing unit  36 , and compares the noise signals N with the threshold values Th to determine whether the noise signals N include the speech signals Ns. The speech determining/removing unit  34  removes the speech signals Ns from the noise signals N upon determining that the noise signals N include the speech signals Ns. In this way, the driving noise signals Np are extracted and supplied to the acoustic weighting factor calculation unit  40 . 
     The noise-band-level generating/storing unit  36  generates the threshold values Th on the basis of the noise spectrum data that is prepared in advance and stores the threshold values Th in a memory or the like. For example, the noise spectrum of a vehicle as shown in  FIG. 2  can be used as the noise spectrum data. This noise spectrum represents typical frequency characteristics of noise that occurs in a vehicle that is being driven. In  FIG. 2 , the abscissa indicates a frequency (represented on a logarithmic scale), and the ordinate indicates a gain (represented in units of decibels). It can be seen that a driving noise of a vehicle is large at low frequencies (bass) and gradually decreases as the frequency becomes higher in this noise frequency spectrum. 
     The noise-band-level generating/storing unit  36  generates the threshold values Th for individual predetermined frequency bands on the basis of the noise spectrum shown in  FIG. 2 . For example, as shown in  FIG. 3A , average gains Th 1 , Th 2 , Th 3 , and Th 4  of the noise spectrum shown in  FIG. 2  are calculated for corresponding frequency bands  1  to  4  having ranges of 10 to 100 Hz, 100 Hz to 1 kHz, 1 kHz to 1 GHz, and greater than or equal to 1 GHz, respectively, and these average gains are set as the threshold values Th. 
     Alternatively, average gains of the noise spectrum shown in  FIG. 2  may be calculated for bands  1  to n so that the average values decrease by 10 dB, as shown in  FIG. 3B , and these average gains may be set as threshold values Th 1  to Thn for the corresponding bands  1  to n. 
     On the other hand, the voice of a person has the speech spectrum shown in  FIG. 4 . In  FIG. 4 , the abscissa indicates a frequency (represented in units of kilohertz), and the ordinate indicates a long-time effective value per hertz (represented in units of decibels). A solid line indicates the spectrum at a distance of 33.5 cm, and a dashed line indicates the spectrum at a distance of 3 cm. It can be seen from the drawing that the long-time effective value per hertz reaches a peak at middle frequencies and is small at low and high frequencies in the speech spectrum. 
     The noise signals N extracted by the audio-canceling unit  32  include the speech signals Ns and the driving noise signals Np. That is to say, the noise signals N are based on a spectrum that is obtained by superimposing the speech spectrum shown in  FIG. 4  on the spectrum of driving noise shown in  FIG. 2 . The speech determining/removing unit  34  compares the superimposed noise signals N with the threshold values Th for the individual frequency bands. When the noise signals N exceed a threshold value Th for each frequency band, the speech determining/removing unit  34  determines that the noise signals N include the speech signals Ns, and replaces the noise signals with the threshold value Th for each frequency band. In this way, the speech signals Ns are removed from the noise signals N. 
     The acoustic weighting factor calculation unit  40  calculates an appropriate acoustic weighting factor C on the basis of the driving noise signals Np extracted by the noise-extracting unit  30  and provides the calculated acoustic weighting factor C to the acoustic weighting filter  14 . 
     The operation of the speech determining/removing unit  34  in the noise-extracting unit  30  will now be described with reference to a flowchart of  FIG. 5 . It is assumed that a played-back audio sound is produced from the speaker  18  and an external sound in the vehicle interior space is received by the microphone  20  when a vehicle is being driven. When the speech determining/removing unit  34  receives the noise signals N from the audio-canceling unit  32  in step S 101 , the speech determining/removing unit  34  determines for individual frequency bands whether the noise signals N include the speech signals Ns and performs other processes. In this case, the band level is sequentially increased from band  1  to n in order. 
     In step S 102 , the speech determining/removing unit  34  selects the lowest band level, i.e., band  1 , as the first band level at which the speech determining/removing unit  34  determines whether the noise signals N include the speech signals Ns. In step S 103 , the speech determining/removing unit  34  compares the noise signals N in band  1  with a threshold value Th corresponding to band  1 . When the noise signals N exceed the threshold value Th, it is determined that the noise signals N include the speech signals Ns in step S 104 , and the noise signals N are replaced with the threshold value Th in step S 105 . On the other hand, when the noise signals N are equal to or less than the threshold value Th, it is determined that the noise signals N do not include the speech signals Ns in step S 106 , and the noise signals N are set as the driving noise signals Np (Np=N) in step S 107 . Then, noise is determined and extracted in band  1  in step S 108 . 
     Subsequently, it is determined in step S 109  whether noise is determined and extracted in all of the bands, and steps S 103  to S 108  are repeated for band  2  and the succeeding bands. When noise has been determined and extracted in all of the bands, the speech determining/removing unit  34  provides the driving noise signals Np to the acoustic weighting factor calculation unit  40  in step S 110 . 
     In the in-vehicle audio apparatus according to this embodiment, a system for correcting audio signals can be achieved which is insensitive to a speech sound in a loud environment. 
     A second embodiment according to the present invention will now be described. In the aforementioned embodiment, the noise signals N are compared with a threshold value Th in a corresponding frequency band to determine whether the noise signals N include the speech signals Ns. On the other hand, in the second embodiment, the noise signals N in one frequency band are compared with those in another frequency band that is one level lower than the one frequency band. The spectrum of the speech sound has a peak in a middle frequency range, as described above. Thus, when the speech spectrum shown in  FIG. 4  is superimposed on the noise spectrum shown in  FIG. 2 , a decrease in a gain of the noise signals N is moderate from a low frequency range to a middle frequency range. The speech spectrum has the peak in a frequency range of about 300 Hz to 1 kHz, and a decrease in a gain of the noise signals N is moderate in this frequency range. 
     For example, in a case where the frequency range of 300 Hz to 1 kHz is divided into bands b 1  to bn having a predetermined bandwidth, it is determined whether the difference between noise signals N 2  in band b 2  and noise signals N 1  in band b 1 , which is one level lower than band b 2 , exceeds a threshold value. When the difference is less than the threshold value, a decrease in a gain of the noise is moderate. Thus, it is determined that the noise includes the speech sound, and the noise signals N 2  are replaced with the noise signals N 1 . When the difference exceeds the threshold value, it is determined that the noise does not include the speech sound, and the noise signals N 2  are set as the driving noise signals Np. 
       FIG. 6  is a flowchart showing the operation of the second embodiment. When the speech determining/removing unit  34  receives the noise signals N from the audio-canceling unit  32  in step S 201 , the speech determining/removing unit  34  selects a low band level, i.e., band  2 , as the first band level at which the speech determining/removing unit  34  determines whether the noise signals N include the speech signals Ns in step S 202 . In step S 203 , the speech determining/removing unit  34  compares the difference between noise signals N 2  in band  2  and noise signals N 1  in band  1  with a threshold value. The noise signals N 1  and N 2  are averages in bands  1  and  2 , respectively. The threshold value is an adjustment factor which can be adjusted for individual vehicles. 
     When the difference between the noise signals N 2  and the noise signals N 1  is less than the threshold value, it is determined that the noise signals N 2  include the speech signals Ns in step S 204 , and the noise signals N 2  are replaced with the noise signals N 1  in step S 205 . On the other hand, when the difference between the noise signals N 2  and the noise signals N 1  is equal to or more than the threshold value, it is determined that the noise signals N 2  do not include the speech signals Ns in step S 206 , and the noise signals N 2  are set as the driving noise signals Np (Np=N 2 ) in step S 207 . Then, noise is determined and extracted in band  2  in step S 208 . 
     Subsequently, it is determined in step S 209  whether noise is determined and extracted in all of the bands, and steps S 203  to S 208  are repeated for band  3  and the succeeding bands. When it is determined that noise has been determined and extracted in all of the bands, the speech determining/removing unit  34  provides the driving noise signals Np to the acoustic weighting factor calculation unit  40  in step S 210 . 
     In the process described above, the middle frequency range of the noise signals can be removed. Thus, the speech signals can be removed from the noise signals so that correction is not performed on the speech signals. 
     While there has been illustrated and described what is at present contemplated to be preferred embodiments of the present invention, it will be understood by those skilled in the art that various changes and modifications may be made, and equivalents may be substituted for elements thereof without departing from the true scope of the invention. In addition, many modifications may be made to adapt a particular situation to the teachings of the invention without departing from the central scope thereof. Therefore, it is intended that this invention not be limited to the particular embodiments disclosed, but that the invention will include all embodiments falling within the scope of the appended claims.