Noise-canceling apparatus
BACKGROUND OF THE INVENTION 
     This invention relates to a noise-canceling apparatus and, more particularly, to a noise-canceling apparatus capable of canceling noise at a prescribed position (observation point) in an automotive vehicle so that pleasant audio can be heard. 
     A known method of dealing with noise involves using a sound-absorbing material (this is a method of passive control). With a method that relies upon use of a sound-absorbing material, however, forming a silent area of little noise is troublesome and low-pitched sounds are not eliminated effectively. In particular, when noise within the passenger compartment of an automotive vehicle is prevented by passive control, the vehicle is increased in weight and the elimination of noise cannot be performed effectively. 
     For this reason, active-control methods in which a noise-canceling sound whose phase is the opposite of the noise is emitted from a speaker so as to reduce the noise have become the focus of attention and these methods are being put into practical use in factory and office interiors. Systems for reducing noise by active control have been proposed for the passenger compartments of automotive vehicles as well. 
     FIG. 9 is a block diagram of an apparatus for achieving the cancellation of sound. As shown in FIG. 9, an engine 11 which is a source of noise has its rotational speed R sensed by an rpm sensor 12. The output R of the sensor 12 is applied to a reference-signal generator 13, which generates a sinusoidal signal having a fixed amplitude and a frequency that conforms to the rotational speed R of the engine 11. The sinusoidal signal serves as a reference signal x n . When an engine is a source of noise, the noise generated by rotation of the engine has periodicity (this is periodic noise) and the frequency of the noise is dependent upon the engine rotational speed. In the case of a four-cylinder engine, for example, the frequency of periodic noise generated within the passenger compartment is 20 Hz when the rotational speed is 600 rpm (=10 rps) and 200 Hz when the rotational speed is 6000 rpm (=100 rps). These are secondary harmonics of the engine speed. Accordingly, the reference-signal generator 13 stores the sinusoidal data in a ROM and generates the reference signal x n  by reading out and delivering this data as necessary. The timing at which this data is read out and delivered is controlled in accordance with the engine rotational speed R so that the reference signal outputted will have a frequency conforming to the engine rotational speed R. 
     The reference signal x n  generated by the reference-signal generator 13 is applied to a noise-canceling controller 14 as an input. Also fed into the controller 14 is an error signal e n , which is a composite-sound signal that is a synthesis of noise S n  and a noise-canceling sound S c  at a noise-canceling position (an observation point, such as a point in the vicinity of the ears of the driver) within the passenger compartment. The noise-canceling controller 14 outputs a noise-canceling signal N c  by executing adaptive signal processing so as to minimize the error signal e n . The controller 14 includes an adaptive signal processor 14a, an adaptive filter 14b constructed as a digital filter, a DA converter 14c for converting the output of the adaptive filter 14b into the noise-canceling signal N c , which is an analog quantity, and a filter 14d for producing a filtered-X signal (a reference signal r n  for signal processing) by superimposing, on the reference signal x n , the propagation characteristic of a canceling-sound propagation system (secondary-sound propagation system) 18 extending from a speaker to the noise-canceling point. 
     A power amplifier 15 amplifies the noise-canceling signal N c  and applies the amplified signal to a canceling speaker 16, which emits the noise-canceling sound S c . An error microphone 17 is disposed at the noise-canceling point so as to detect the aforesaid composite-sound signal, which is a synthesis of the noise S n  and the noise-canceling sound S c , and output a composite-sound signal as the error signal e n . 
     The error signal e n  at the noise-canceling point and the filtered-X signal r n , which is produced by the filter 14d, enter the adaptive signal processor 14a, which decides the coefficients of the adaptive filter 14b by using these two signals to execute adaptive signal processing in such a manner that the noise at the noise-canceling point is canceled out. For example, the adaptive signal processor 14a decides the coefficients of the adaptive filter 14b in accordance with a well-known filtered-X LMS (least mean square) algorithm so as to minimize the error signal en that has entered from the error microphone 17. In accordance with the coefficients decided by the adaptive signal processor 14a, the adaptive filter 14b subjects the reference signal x n  to digital filtering processing so that the DA converter 14c will deliver the sound-canceling signal N c . It should be noted that the reference signal x n  must be a signal having a high correlation with respect to the noise S c  to be canceled; sounds having no correlation with the reference signal are not canceled out. 
     When the engine 11 rotates, its rotational speed R is sensed by the rpm sensor 12, the reference-signal generator 13 generates the reference signal x n  [see (a) in FIG. 10], whose frequency conforms to the engine rotational speed R, and the reference signal x n  enters the noise-canceling controller 14. At this time the periodic engine sound (periodic noise) generated by the engine 11 reaches the noise-canceling point upon propagating through space having a noise propagating system (a primary-noise propagating system) that exhibits a prescribed transfer function. Accordingly, the noise (engine sound) S n  at the noise-canceling point has a slightly lower level and a slight delay, as illustrated at (b) in FIG. 10. 
     Initially, the noise-canceling controller 14 produces the noise-canceling signal N c  so as to have a phase opposite that of the reference signal x n , as a result of which the canceling speaker 16 outputs the canceling sound S c  shown at (c) in FIG. 10, by way of example. However, since the level and phase of the noise S n  are displaced somewhat from the level and phase of the canceling sound S c , the noise is not canceled out by the canceling sound S c  and, hence, the error signal en is generated. The noise-canceling controller 14 decides the coefficients of the adaptive filter 14b by performing adaptive signal processing in such a manner that the error signal e n  is minimized. In an ideal case, the phase of the canceling sound S c  will be opposite that of the noise S n  and the levels thereof will be in agreement, as shown at (d) in FIG. 10, so that the noise is canceled out. 
     In order simplify the description, the foregoing example deals with one noise source, one source (the speaker) for generating the canceling sound, and one noise-canceling point (the observation point). In actuality, however, there is more than one noise source and more than point (observation point) at which noise is desired to be canceled. In such case, more than one speaker is necessary since noise at a plurality of points cannot be canceled with only one speaker. FIG. 11 is a block diagram of a conventional noise-canceling apparatus for a case in which there are K-number of noise sources, M-number of speakers and L-number of observation points. 
     Numeral 21 denotes a noise-canceling controller (which corresponds to the noise-canceling controller 14 in FIG. 9) that operates so as to cancel out noise at each of a number of observation points. Numeral 22 denotes a primary-sound hypothetical propagation system (noise propagation system), which expresses systems along which noise is propagated from each noise source (not shown) to each observation point. Numeral 23 represents a secondary-sound propagation system (noise-canceling sound propagation system), which expresses systems along which canceling sound is propagated from each speaker to each observation point. The system 23 includes the characteristics of the speakers (not shown). Numeral 24 designates a signal synthesizer, which implements the function of a microphone at each observation point. The signal synthesizer 24 includes adders 24 1  ˜24 1  &#39; corresponding to a microphone at a first observation point, adders 24 2  ˜24 2  &#39; corresponding to a microphone at a second observation point, . . . , and adders 24 L  ˜24 L  &#39; corresponding to a microphone at an L-th observation point. Further, d d1n  ˜d dLn  represent external noise that is not the object of cancellation at each of the observation points. 
     The noise-canceling controller 21 includes a multiple-input/multiple-output adaptive filter (hereinafter referred to simply as an adaptive filter) 21a for inputting noise-canceling signals y a1n  ˜y aMn  to the speakers upon being provided with inputs of reference signals x a1n  ˜x aKn  (outputted by a reference-signal generator, not shown) conforming to the noise components generated by the noise sources, a filtered-X signal producing filter 21b, which is fabricated using the elements (propagation elements) of a transfer-function matrix of the secondary-sound propagation system 23, this filter being provided with inputs of the reference signals x a1n  ˜x aKn  conforming to the noise generated by the noise sources, and an adaptive signal processor 21c, which is provided with inputs of error signals e 1n  ˜e Ln  prevailing at the observation points and filtered-X signals r 111n  ˜r LMKn  outputted by the filter 21b, for deciding the coefficients of the adaptive filter 21a by executing adaptive signal processing using these input signals so as to cancel out the noise at each observation point. 
     FIGS. 12A and 12B are diagrams for describing the primary-sound hypothetical propagation system 22. The noise generated by K-number of noise sources N G1  ˜NG K  reaches microphones (MIC 1  ˜MIC L ), which are provided at the respective observation points, upon propagating through the primary-sound propagation system 22 having prescribed frequency and phase characteristics. Accordingly, if we let H ji  represent the transfer characteristic of a propagation system in which noise from an i-th noise source NG i  reaches a j-th microphone MIC j , the primary-noise hypothetical propagation system 22 will be expressed as shown in FIG. 12B and the transfer-function matrix (H) thereof will be as follows: ##EQU1## 
     Each element H ij  of the transfer-function matrix (H) is implemented by a FIR-type digital filter shown in FIG. 13. More specifically, each element is realized by a digital filter comprising delay elements DL for successively delaying the input signal by one sampling period, multipliers ML for multiplying the outputs of the delay elements by coefficients h 0 , h 1 , h 2 , . . . , and adders AD for adding the outputs of the multipliers. 
     FIGS. 14A, 14B are views for describing the secondary-noise propagation system 23. As shown in FIG. 14A, noise-canceling sounds generated by speakers SP 1  ˜SP M  arrive at the microphones MIC 1  ˜MIC L , which are provided at the respective observation points, upon propagating through the secondary propagation system 23 having prescribed frequency and phase characteristics. Accordingly, if we let C ji  represent the transfer characteristic of a secondary-noise propagation system in which a canceling sound based upon an i-th noise-canceling signal y ain  reaches the j-th microphone MIC j , the secondary-noise propagation system 23 will have the form of the model shown in FIG. 14B and the transfer-function matrix (C) thereof will be as follows: ##EQU2## 
     Each element of the transfer-function matrix (C) is implemented by a FIR-type digital filter shown in FIG. 13, just as in the case of the primary-sound hypothetical propagation system 22. More specifically, each element is realized by a digital filter comprising delay elements DL for successively delaying the input signal by one sampling period, multipliers ML for multiplying the outputs of the delay elements by coefficients c 0 , c 1 , c 2 , . . . , and adders AD for adding the outputs of the multipliers. 
     FIG. 15 is a block diagram showing the filtered-X signal-producing filter 21b fabricated using each element C ij  of the transfer-function matrix (C) of the secondary-sound propagation system 23. 
     The adaptive signal processor 21c updates the coefficients of the adaptive filter 21a by executing adaptive signal processing based upon the reference signals x a1n  ˜x aKn  and the signals e 1n  ˜e Ln  that are a composite of the noise and canceling sounds at each of the observation points, and the adaptive filter 21a, to which the reference signals x a1n  -x aKn  are applied as inputs, generates the noise-canceling signals y a1n  ˜y aMn  and applies these signals to the speakers to cancel out the sound at each observation point. 
     The noise-canceling signals y a1n  ˜y aMn  outputted by the adaptive filter 21a do not reach the observation points as is. Rather, they reach the observation points upon being influenced by the frequency and phase characteristics of the secondary-sound propagation system 23. As a consequence, the adaptive signal processor 21c performs highly sophisticated noise-canceling control not by using the reference signals x a1n  ˜x aKn  as is but by employing a filtered-X LMS (multiple-error filtered X LMS, referred to as an &#34;MEFX LMS&#34;) algorithm, which uses signals obtained by impressing the characteristics of the secondary-sound propagation system 23 on the reference signals. In other words, on the basis of the filtered-X LMS algorithm, the adaptive signal processor 21c updates the coefficients of the adaptive filter 21a using signals r 111n  ˜r LMKn , which are result of filtering the reference signals x a1n  ˜x aKn  by the filter 21b, and the composite-sound signals (error signals) e 1n  ˜e Ln  at the observation points. 
     In FIG. 15, C ij  represents a FIR-type digital filter for realizing each element C ij  (see FIG. 14) of the transfer-function matrix (C) in the secondary-sound propagation system 23. The filter 21b is adapted so as to output the filtered-X signals r 111n  ˜r LMKn  upon impressing all of the propagation elements upon each of the reference signals x a1n  ˜x aKn  (i.e., passing each reference signals through filters corresponding to all of the propagation elements). More specifically, the propagation elements C 11  ˜C L1  from the first speaker to all of the observation points are made to act upon the reference signal x a1n  to produce the filtered-X signals r 111n  ˜r L11n , the propagation elements C 12  ˜C L2  from the second speaker to all of the observation points are made to act upon the reference signal x a1n  to produce the filtered-X signals r 121n  ˜r L21n , . . . , and the propagation elements C 1M  ˜C LM  from the M-th speaker to all of the observation points are made to act upon the reference signal x a1n  to produce the filtered-X signals r 1M1n  ˜r LM1n . All of the propagation elements are made to act upon each of the reference signals x a2n , x a3n , . . . x.sub. aKn in a similar manner. This may be expressed as follows: 
     
         R.sub.11 =(r.sub.111n, r.sub.211n, . . . r.sub.L11n) 
    
     
         R.sub.21 =(r.sub.121n, r.sub.221n, . . . r.sub.L21n) . . . R.sub.M1 =(r.sub.1M1n, r.sub.2M1n, . . . r.sub.LM1n) . . . R.sub.MK =(r.sub.1MKn, r.sub.2MKn, . . . r.sub.LMKn) 
    
     FIG. 16 is a block diagram showing the multiple-input/multiple-output adaptive filter 21a, which has a structure similar to that of the primary-sound hypothetical propagation system 22 or secondary-sound propagation system 23. FIR-type digital filters are shown at A 11n  ˜A MKn . By way of example, each of these filters may be realized by delay elements DL 1 , DL 2  . . . for successively delaying the input signal by one sampling period, multipliers ML 1 , ML 2 , ML 3  . . . for multiplying each delay-element output by coefficients a 0 , a 1 , a 2  . . . , and adders AD 1 , AD 2  . . . for adding the multiplier outputs. The number of delay stages is limited to two. 
     The noise-canceling signal y a1n  inputted to the first speaker is obtained by inputting the reference signals x a1n  ˜x aKn  to the digital filters A 11n  ˜A 1Kn  and then adding, the noise-canceling signal y a2n  inputted to the second speaker is obtained by inputting the reference signals x a1n  ˜x aKn  to the digital filters A 21n  ˜A 2Kn  and then adding, . . . , and the noise-canceling signal y aMn  inputted to the M-th speaker is obtained by inputting the reference signals x a1n  ˜x aKn  to the digital filters A M1n  ˜A MKn  and then adding. 
     When each of the FIR-type digital filters A 11n  ˜A MKn  in the adaptive filter 21a is composed of three coefficients (two delay stages), the adaptive signal processor 21c decides the values of the coefficients by executing adaptive signal processing for each of the three coefficients of the FIR-type digital filters A 11n  ˜A MKn . That is, the adaptive signal processor decides coefficients a 0 , a 1 , a 2  by performing the following operation with regard to these coefficients a 0 , a 1 , a 2  of one FIR-type digital filter A ijn  : ##EQU3## 
     In Equation (1), (n) signifies the value at the present sampling time, (n-1) the value one sampling earlier, (n-1) the value two samplings earlier, and (n+1) the value from the present time to the next sampling time. Accordingly, R ij  (n-2) signifies the output of the filter 21b that conforms to the reference signal two samplings earlier, R ij  (n-1) signifies the output of the filter that conforms to the reference signal one sampling earlier, and R ij  (n) signifies the output of the filter that conforms to the reference signal at the present time. Further, μ represents a constant (step-size parameter) of less than 1, and e n  represents the signal (error signal) that is the composite of the noise and canceling sound at each of the L-number of observation points. 
     In accordance with this noise-canceling apparatus, the adaptive signal processor 21c decides the coefficients of the FIR-type digital filters A 11n  ˜A MKn , which constitute the adaptive filter 21a, by executing adaptive signal processing based upon the filtered-X signals r 111n  ˜r LMKn , which are outputted by the filter 21b, and the composite-sound signals (error signals) e 1n  ˜e Ln  that are a composite of the noise and canceling sounds at each of the observation points. The adaptive filter 21a, to which the reference signals x a1n  ˜x aKn  are applied, generates the noise-canceling signals y a1n  ˜y aMn  and applies these signals to the speakers SP 1  ˜SP M  (FIG. 14). Each speaker generates a canceling sound to cancel out the noise at each observation point. 
     FIG. 17 is a block diagram illustrating the details of the conventional noise-canceling apparatus for a case in which there are one noise source (K=1), two speakers (M=2) and two observation points, i.e., two microphones (L=2). Numeral 21a denotes the adaptive filter, which is composed of two FIR-type digital filters A 11n , A 21n , numeral 21b denotes the filtered-X signal producing filter, which is obtained by using digital filters to construct each of the propagation elements C 11 , C 21 , C 12 , C 22  of the transfer-function matrix of the secondary propagation system, numerals 21c-1, 21c-2 denote adaptive signal processors (MEFX LMS) for deciding the coefficients of each of the digital filters in the adaptive filter 21a, SP 1 , SP 2  represent speakers, and MC 1 , MC 2  designate microphones disposed at the observation points. 
     FIG. 18 is a block diagram illustrating the details of the conventional noise-canceling apparatus for a case in which there are one noise source (K=1), four speakers (M=4) and four observation points, i.e., four microphones (L=4). Numeral 21a denotes the adaptive filter, which is composed of four FIR-type digital filters A 11n , A 21n , A 12n , A 22n , numeral 21b denotes the filtered-X signal producing filter, which is obtained by using digital filters to construct each of the propagation elements C 11 , C 21 , C 31 , C 41  . . . , C 44  of the transfer-function matrix of the secondary propagation system, numerals 21c-1 through 21c-4 denote adaptive signal processors (MEFX LMS), SP 1  ˜SP 4  represent speakers, and MC 1  ˜MC 4  designate microphones disposed at the observation points. 
     The frequency characteristics, inclusive of the speaker characteristics, of the secondary propagation system from the speakers to each observation point are not flat but vary as a function of frequency. FIG. 19 is a characteristic diagram showing the characteristics of speaker frequency. A frequency characteristic up to a noise frequency of 200 Hz, which corresponds to an engine rotational speed of 6000 rpm (=100 rps), varies approximately linearly in conformity with frequency. The frequency characteristic of the secondary-sound propagation system 23, which is the result of adding the frequency characteristic within the passenger compartment to this speaker characteristic, varies in conformity with frequency. 
     If the frequency of the noise to be canceled is constant, the coefficient convergence characteristic of the adaptive filter that relies upon adaptive signal processing is improved so that the coefficient values of the adaptive filter quickly converge to their optimum values. As a result, a satisfactory noise-canceling effect is capable of being achieved. 
     However, the frequency of the noise to be canceled fluctuates from one moment to the next. For example, the engine frequency fluctuates from one moment to the next and in dependence upon vehicle velocity, and the frequency of the engine sound also varies. When the frequency of noise fluctuates, gain varies in accordance with the frequency characteristic of the secondary-sound propagation system 23, and the coefficient convergence characteristic of the adaptive filter that relies upon adaptive signal processing deteriorates (i.e., there is a decline in the follow-up capability). The result is that the noise-canceling effect cannot be manifested satisfactorily. More specifically, in the adaptive signal processor, processing for deciding adaptive filter coefficients that conform to the present frequency characteristic (gain) of the secondary-sound propagation system is executed. However, when the frequency characteristic (gain) fluctuates at the next point in time, the coefficients that have been decided do not take on appropriate values that conform to the frequency characteristic at this next point in time and the coefficients of the adaptive filter do not converge quickly. This causes a decline in the follow-up capability. 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a noise-canceling apparatus in which, even if the frequency of noise fluctuates so that there is a variation in the gain of the secondary-sound propagation system, the noise is canceled by applying compensation in such a manner that the gain is rendered constant. 
     Another object of the present invention is to provide a noise-canceling apparatus in which the effects of noise cancellation can be enhanced even if the frequency of noise fluctuates from one moment to the next. 
     A further object of the present invention is to provide a noise-canceling apparatus in which follow-up performance is improved so that the effects of noise cancellation can be enhanced. 
     According to the present invention, the foregoing objects are attained by providing a noise-canceling apparatus in which a frequency-characteristic correcting unit is provided on the input side of an adaptive filter and has a frequency characteristic that is approximately symmetrical with respect to the frequency characteristic of a canceling-sound propagation system about a 0 dB line. Adaptive signal processing is executed using a signal obtained by inputting a reference signal to the frequency-characteristic correcting unit as a true reference signal. More specifically, in accordance with the noise-canceling apparatus of the present invention, the overall frequency characteristic of the frequency-characteristic correcting unit and canceling-sound propagation system can be made substantially flat to improve the coefficient convergence of the adaptive filter that relies upon adaptive signal processing. This makes it possible to achieve a satisfactory noise-canceling effect. 
     Further, the foregoing objects are attained by providing a noise-canceling apparatus in which a frequency-characteristic correcting unit is provided either on the input side of a canceling-noise generating source or in a feedback section for feeding back a composite-sound signal (error signal) to a noise-canceling controller. The overall frequency characteristic of the frequency-characteristic correcting unit and canceling-sound propagation system is made substantially flat. In accordance with the noise-canceling apparatus of the invention, the overall frequency characteristic of the frequency-characteristic correcting unit and canceling-sound propagation system is made substantially flat to improve the coefficient convergence of the adaptive filter that relies upon adaptive signal processing. This makes it possible to achieve a satisfactory noise-canceling effect. 
     Other features and advantages of the present invention will be apparent from the following description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram showing a first embodiment of the present invention; 
     FIG. 2 is a characteristic diagram for describing the frequency characteristic of a frequency-characteristic correcting unit; 
     FIG. 3 is an explanatory view for a case in which the frequency-characteristic correcting unit is constituted by an IIR-type digital filter; 
     FIG. 4A is an explanatory view of a noise-canceling effect according to a prior-art apparatus, and FIG. 4B is an explanatory view of a noise-canceling effect according to a first embodiment of the invention; 
     FIG. 5 is a block diagram showing a second embodiment of the present invention; 
     FIG. 6 is a characteristic diagram for describing the frequency characteristic of a frequency-characteristic correcting unit; 
     FIG. 7 is an explanatory view for a case in which the frequency-characteristic correcting unit is constituted by an equalizer; 
     FIG. 8 is a block diagram showing a third embodiment of the present invention; 
     FIG. 9 is a block diagram showing a noise-canceling apparatus according to the prior art; 
     FIG. 10 is a diagram of waveforms for describing a noise-canceling operation; 
     FIG. 11 is a block diagram showing a prior-art noise-canceling apparatus for a case in which there are a plurality of noise sources, speakers and observation points; 
     FIG. 12A is an explanatory view of a primary-sound hypothetical propagation system, and FIG. 12B shows an example in which a primary-sound hypothetical propagation system is realized; 
     FIG. 13 is a block diagram showing a digital filter for realizing each element of a transfer-function matrix; 
     FIG. 14A is an explanatory view of a secondary-sound propagation system, and FIG. 14B shows an example in which a secondary-sound propagation system is realized; 
     FIG. 15 is a block diagram showing a filter for producing a filtered-X signal; 
     FIG. 16 is a block diagram of an adaptive filter; 
     FIG. 17 is a block diagram showing a prior-art noise-canceling apparatus for a case having one noise source, two speakers and two observation points; 
     FIG. 18 is a block diagram showing a prior-art noise-canceling apparatus for a case having one noise source, four speakers and four observation points; and 
     FIG. 19 is a characteristic diagram showing the frequency characteristic of a speaker. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     (a) First embodiment of the invention 
     Overall configuration 
     FIG. 1 is a block diagram showing a noise-canceling apparatus according to a first embodiment of the present invention. Functional blocks identical with those of the prior-art apparatus shown in FIG. 9 are designated by like reference characters. 
     As shown in FIG. 1, the engine 11 which is the source of noise has its rotational speed R sensed by the rpm sensor 12. The output R of the sensor 12 is applied to the reference-signal generator 13, which generates the sinusoidal signal having a fixed amplitude and a frequency that conforms to the rotational speed R of the engine 11. The sinusoidal signal serves as the reference signal x n . When the engine is a source of noise, the noise generated by rotation of the engine has periodicity (periodic noise) and the frequency of the noise is dependent upon the engine rotational speed. Accordingly, the reference-signal generator 13 stores the sinusoidal data in a ROM and generates the reference signal x n  by reading out and delivering this data as necessary. 
     The reference signal x n  generated by the reference-signal generator 13 is applied to the noise-canceling controller 14 as an input. Also fed into the controller 14 is the error signal en, which is a composite-sound signal that is a synthesis of the noise S n  and the noise-canceling sound S c  at the noise-canceling position (the observation point, such as a point in the vicinity of the ears of the driver) within the passenger compartment. The noise-canceling controller 14 outputs a noise-canceling signal N c  by executing adaptive signal processing so as to minimize the error signal e n . The power amplifier 15 amplifies the noise-canceling signal N c  and applies the amplified signal to the canceling speaker (canceling-sound generating source) 16, which emits the noise-canceling sound S c . The error microphone 17 is disposed at the noise-canceling point (observation point) so as to detect the aforesaid composite-sound signal, which is a synthesis of the noise S n  and the noise-canceling sound S c , and output the composite-sound signal as the error signal e n . Numeral 18 denotes the canceling-sound propagation system (secondary-sound propagation system) in which the canceling sound is propagated from the speaker to the noise-canceling point. 
     In order to simplify the description, FIG. 1 illustrates an arrangement having one noise source, one speaker and one error microphone. However, the present invention is not limited to this arrangement but can be applied also to an arrangement in which a plurality of noise sources, a plurality of speakers and a plurality of microphones are provided. 
     Noise-canceling controller 
     The noise-canceling controller 14 includes the adaptive signal processor 14a, the adaptive filter 14b constructed as a digital filter, the DA converter 14c for converting the output of the adaptive filter 14b into the analog noise-canceling signal N c , the filter 14d for producing the filtered-X signal used in adaptive signal processing, and a frequency-characteristic correcting unit 14e. 
     The frequency-characteristic correcting unit 14e has a frequency characteristic that is approximately symmetrical with respect to the frequency characteristic of the secondary-sound propagation system (which includes the speaker) 18 about a 0 dB line. The reference signal x n  is applied to the correcting unit 14e as an input signal. FIG. 2 is a characteristic diagram showing the frequency characteristic of the frequency-characteristic correcting unit 14e. The dashed line indicates the frequency characteristic of the secondary-sound propagation system 18, and the solid line indicates the frequency characteristic of the frequency-characteristic correcting unit 14e. 
     FIG. 3 shows an example in which the frequency-characteristic correcting unit 14e is constituted by an IIR-type digital filter. The correcting unit 14e includes delay elements DLi (i=1, 2, . . . , N-1) for successively delaying the input signal by one sampling period, a coefficient unit CE for storing coefficients a 0 , a 1 , a 2  . . . , multipliers MLi (i=0, 1, 2, . . . , N-1) for multiplying delay-element outputs x n , x n-1 , x n-2  . . . by the coefficients a 0 , a 1 , a 2  . . . , respectively, delay elements DLi&#39; (i=1, 2, . . . , N-1) for successively delaying the output signal by one sampling period, a coefficient unit CE&#39; for storing coefficients b 0 , b 1 , b 2  . . . , multipliers MLi&#39; (i=0, 1, 2, . . . , N-1) for multiplying delay-element outputs y n , y n-1 , y n-2  . . . by the coefficients b 0 , b 1 , b 2  . . . , respectively, and an adder ADD for adding the outputs of all of the multipliers and producing a signal y n  indicative of the sum. Thus, the frequency-characteristic correcting unit 14e outputs a reference signal x n  &#39; (=y n ) by performing an operation in accordance with the following equation: 
     
         x.sub.n &#39;=Σa.sub.i ·x.sub.n-i -Σb.sub.j ·y.sub.n-j (i =0, 1, 2, . . . , N-1; j=1, 2, . . . M). 
    
     By adopting appropriate values for the coefficients a i , b j , it is possible to set a frequency characteristic that is approximately symmetrical with respect to the frequency characteristic of the secondary-sound propagation system 18 about a 0 dB line. 
     The filter 14d for producing a filtered-X signal is constructed based upon the transfer function of the secondary-sound propagation system. The input signal thereto is the reference signal x n  &#39; outputted by the frequency-characteristic correcting unit 14e. The error signal e n  at the noise-canceling point and the filtered-X signal r n , which is produced by the filter 14d, enter the adaptive signal processor 14a, which decides the coefficients of the adaptive filter 14b by using these two signals to execute adaptive signal processing in accordance with Equation (1) in such a manner that the noise at the noise-canceling point is canceled out. More specifically, the adaptive signal processor 14a decides the coefficients of the adaptive filter 14b in accordance with the well-known filtered-X LMS algorithm so as to minimize the error signal e n  that has entered from the error microphone 17. In accordance with the coefficients decided by the adaptive signal processor 14a, the adaptive filter 14b subjects the reference signal x n  &#39; to digital filtering processing so that the noise-canceling signal N c  will be produced. 
     Overall operation 
     When the engine 11 rotates, the rotational speed R thereof is sensed by the rpm sensor 12 and the reference-signal generator 13 generates the reference signal x n  that conforms to the engine rotational speed R. This signal enters the noise-canceling controller 14. At this time the periodic engine sound (periodic noise) generated by the engine 11 reaches the noise-canceling point upon propagating through space having a noise propagating system (primary-noise propagating system) that exhibits a prescribed transfer function. This sound is the noise S n . 
     The error microphone 17 detects the composite sound that is the combination of the noise S n  and canceling sound S c  at the noise-canceling point and applies the resultant sound signal (the error signal) e n  to the adaptive signal processor 14a. 
     In concurrence with the foregoing operation, the frequency-characteristic correcting unit 14e impresses a frequency characteristic, which is the reverse of that of the secondary-sound propagation system 18, upon the reference signal x n  and applies the resulting signal x n  &#39; to the adaptive filter 14b and filtered-X signal producing filter 14d. The filter 14d superimposes the transfer function of the secondary-sound propagation system 18 upon the reference signal x n  &#39; outputted by the frequency-characteristic correcting unit 14e, thereby generating the filtered-X signal r n  used in adaptive signal processing. This signal is fed into the adaptive signal processor 14a. 
     The adaptive signal processor 14a decides the coefficients of the adaptive filter 14b by performing adaptive signal processing in accordance with Equation (1) using the composite-sound signal (error signal) e n  and the filtered-X signal r n , which is outputted by the filter 14d. 
     On the basis of the coefficients decided by the adaptive signal processor 14a, the adaptive filter 14b produces the noise-canceling signal y n  by applying digital filtering processing to the reference signal x n  &#39; that enters from the frequency-characteristic correcting unit 14e. The DA converter 14c subjects the adaptive filter output to a DA conversion to generate the analog noise-canceling signal N c , which enters the speaker 16 via the power amplifier 15. As a result, the speaker outputs a noise-canceling sound that arrives at the noise-canceling point via the secondary-sound propagation system 18 to cancel out the noise S n . The foregoing operation is repeated to cancel out the noise in a rapid manner. 
     In the foregoing, the frequency characteristic of the frequency-characteristic correcting unit 14e is symmetrical to the frequency characteristic of the secondary-sound propagation system about the 0 dB level. The overall frequency characteristic therefore is flat. Accordingly, the second term μR ij  e n  on the right side of Equation (1) may be written as follows if we let C represent the characteristic of the secondary-sound propagation system and C&#39; the characteristic of the frequency-characteristic correcting unit 14e: ##EQU4## Consequently, the adaptive signal processor 14a is capable of executing adaptive signal processing just as if the secondary-sound propagation system possessed a frequency characteristic having a constant gain. The result is that the coefficient convergence characteristic of the adaptive algorithm can be advanced to improve follow-up with respect to any fluctuation in noise, thereby making it possible to manifest a satisfactory noise-canceling effect. 
     FIG. 4 is useful in describing the noise-canceling effect of the present invention. FIG. 4A is an explanatory view of the noise-canceling effect obtained with the prior-art apparatus, in which the frequency-characteristic correcting unit 14e is not included, and FIG. 4B is an explanatory view of the noise-canceling effect according to the apparatus of the present invention having the frequency-characteristic correcting unit 14e. In FIGS. 4A and 4B, engine rotational speed in rpm (frequency of noise in Hz) is plotted along the horizontal axis, and noise level (dB SpL ) is plotted along the vertical axis. Further, NS represents noise sound-pressure level at an observation point in a case where noise is not canceled, and NSC represents noise sound-pressure level at an observation point in a case where noise is canceled. Noise-canceling effects indicated by the hatching in each of FIGS. 4A and 4B are obtained. A comparison of FIGS. 4A and 4B reveals that the noise-canceling effect provided by the noise-canceling apparatus of the present invention is superior to that provided by the conventional apparatus. It should be noted that NG in FIGS. 4A and 4B indicates an augmented area in which noise is amplified. 
     The foregoing relates to a case in which the frequency-characteristic correcting unit is digitally constructed. However, the correcting unit can be constructed in analog fashion using a graphic equalizer or the like. 
     (b) Second embodiment of the invention 
     Overall configuration 
     FIG. 5 is a block diagram showing a noise-canceling apparatus according to a second embodiment of the present invention. Functional blocks identical with those of the first embodiment shown in FIG. 1 are designated by like reference characters. 
     As shown in FIG. 5, the engine 11 which is the source of noise has its rotational speed R sensed by the rpm sensor 12. The output R of the sensor 12 is applied to the reference-signal generator 13, which generates the sinusoidal signal having a fixed amplitude and a frequency that conforms to the rotational speed R of the engine 11. The sinusoidal signal serves as the reference signal x n . The reference signal x n  generated by the reference-signal generator 13 is applied to the noise-canceling controller 14 as an input. Also fed into the controller 14 is the error signal e n , which is a composite-sound signal that is a synthesis of the noise S n  and the noise-canceling sound S c  at the noise-canceling position within the passenger compartment. The noise-canceling controller 14 outputs a noise-canceling signal N c  &#39; by executing adaptive signal processing so as to minimize the error signal e n . The power amplifier 15 amplifies the noise-canceling signal N c  &#39; and applies the amplified signal to the canceling speaker (canceling-sound generating source) 16, which emits the noise-canceling sound S c . The error microphone 17 is disposed at the noise-canceling point (observation point) so as to detect the aforesaid composite-sound signal, which is a synthesis of the noise S n  and the noise-canceling sound S c , and output the composite-sound signal as the error signal e n . The canceling-sound propagation system (secondary-sound propagation system) 18 is that in which the canceling sound is propagated from the speaker to the noise-canceling point. 
     Noise-canceling controller 
     The noise-canceling controller 14 includes the adaptive signal processor 14a, the adaptive filter 14b constructed as a digital filter, the DA converter 14c for converting the output y n  of the adaptive filter 14b into the analog noise-canceling signal N c , the filter 14d for producing the filtered-X signal used in adaptive signal processing, and a frequency-characteristic correcting unit 14f. The frequency-characteristic correcting unit 14f has a frequency characteristic that is set in such a manner that the overall frequency characteristic in combination with the frequency characteristic of the canceling-sound propagation system 18 is substantially flat. FIG. 6 is a diagram for describing the characteristic correction performed by the frequency-characteristic correcting unit 14f. The solid line indicates the frequency characteristic of the secondary-sound propagation system 18, and the dashed line indicates the ideal overall frequency characteristic that results after the insertion of the frequency-characteristic correcting unit 14f. 
     FIG. 7 is a diagram useful in describing a case in which the frequency-characteristic correcting unit 14f is constituted by a graphic equalizer. Here the frequency characteristics in three bands F 1 , F 2 , F 3  are controlled independently. As shown in FIG. 7, the correcting unit includes a characteristic controller 14f-1 for controlling the characteristic of band F 1 , a characteristic controller 14f-2 for controlling the characteristic of band F 2 , a characteristic controller 14f-3 for controlling the characteristic of band F 3 , a bridge amplifier 14f-4, an output circuit 14f-5, and variable resistors VR 1  ˜VR 3  for setting the gain or attenuation quantities of each of the bands F 1  ˜F 3 , respectively. The noise-canceling signal N c  outputted by the DA converter 14c enters the + terminal of the bridge amplifier 14f-4 and one end of each of the variable resistors VR 1  ˜VR 3  of the respective characteristic controllers 14f-1˜14f-3. The other ends of the variable resistors VR 1  ˜VR 3  are tied together and connected to the - terminal of the bridge amplifier 14f-4. By virtue of this arrangement, the frequency characteristics of each of the bands F 1  ˜F 3  are controlled based upon the set values of the variable resistors VR 1  ˜VR 3 , as a result of which a prescribed overall frequency characteristic is obtained. Though a case in which the frequency characteristics of only three bands are controlled has been described for the sake of simplifying the explanation, it goes without saying that the frequency-characteristic correcting unit can be constructed in similar fashion for controlling the frequencies of four or more bands. 
     The filtered-X signal producing filter 14d is constructed using an overall transfer function from the frequency-characteristic correcting unit 14e to the noise-canceling point. Since the frequency characteristic is flat, the filtered-X signal producing filter 14d can be constructed solely of delay elements having a fixed gain. 
     The error signal e n  at the noise-canceling point and the filtered-X signal r n , which is produced by the filter 14d, enter the adaptive signal processor 14a, which decides the coefficients of the adaptive filter 14b by using these two signals to execute adaptive signal processing in accordance with Equation (1) in such a manner that the noise at the noise-canceling point is canceled out. More specifically, the adaptive signal processor 14a decides the coefficients of the adaptive filter 14b in accordance with the filtered-X LMS algorithm so as to minimize the error signal e n  that has entered from the error microphone 17. In accordance with the coefficients decided by the adaptive signal processor 14a, the adaptive filter 14b subjects the reference signal x n  to digital filtering processing so that the noise-canceling signal y n  will be produced. 
     Overall operation 
     When the engine 11 rotates, the rotational speed R thereof is sensed by the rpm sensor 12 and the reference-signal generator 13 generates the reference signal x n  that conforms to the engine rotational speed R. This signal enters the noise-canceling controller 14. At this time the periodic engine sound (periodic noise) generated by the engine 11 reaches the noise-canceling point upon propagating through space having a noise propagating system (primary-noise propagating system) that exhibits a prescribed transfer function. 
     The error microphone 17 detects the composite sound that is the combination of the noise S n  and canceling sound S c  at the noise-canceling point and applies the resultant sound signal (the error signal) e n  to the adaptive signal processor 14a. 
     In concurrence with the foregoing operation, the filtered-X signal producing filter 14d receives the reference signal x n  as an input, generates the filtered-X signal r n  used in the filtered-X LMS algorithm processing and applies this signal to the adaptive signal processor 14a. 
     The adaptive signal processor 14a decides the coefficients of the adaptive filter 14b by performing adaptive signal processing in accordance with Equation (1) using the error signal e n  and the filtered-X signal r n , which is outputted by the filter 14d. 
     In accordance with the coefficients decided by the adaptive signal processor 14a, the adaptive filter 14b produces the noise-canceling signal y n  by applying digital filtering processing to the reference signal x n . The DA converter 14c subjects the adaptive filter output y n  to a DA conversion and inputs the resulting analog quantity to the frequency-characteristic correcting unit 14e. The latter impresses the preset frequency characteristic upon the noise-canceling signal inputted thereto and applies the resulting signal to the speaker 16 via the power amplifier 15. As a result, the speaker outputs a noise-canceling sound that arrives at the noise-canceling point via the secondary-sound propagation system 18 to cancel out the noise. The foregoing operation is repeated to cancel out the noise in a rapid manner. 
     In the foregoing, the overall frequency characteristic of the frequency-characteristic correcting unit 14e and secondary-sound propagation system 18 is substantially flat, and therefore the adaptive signal processor 14a need only perform noise-canceling control in a system having a fixed gain. In other words, the adaptive signal processor 14a need only perform noise-canceling control in which the gains of the filtered-X signal producing filters 
     
         (C.sub.ii, C.sub.2i, C.sub.3i, . . . , C.sub.Li) 
    
     in Equation (2) are fixed. The result is that the coefficient convergence characteristic of the adaptive algorithm can be advanced to improve follow-up with respect to any fluctuation in noise, thereby making it possible to manifest a satisfactory noise-canceling effect. 
     The second embodiment provides a noise-canceling effect similar to that of the first embodiment. That is, the noise sound-pressure level is as indicated at NSC in FIG. 4B in the second embodiment as well, and the noise-canceling effect obtained is as indicated by the hatched area. 
     (c) Third embodiment of the invention 
     Overall configuration 
     FIG. 8 is a block diagram showing a noise-canceling apparatus according to a third embodiment of the present invention. Functional blocks identical with those of the second embodiment are designated by like reference characters. 
     The third embodiment differs from the second embodiment in the location of the frequency-characteristic correcting unit 14f. In the second embodiment, the frequency-characteristic correcting unit 14f is provided on the input side of the speaker 16 (the output signal of the DA converter 14c). In the third embodiment, the frequency-characteristic correcting unit 14f is provided in the feedback path that feeds back the error signal e n  to the adaptive signal processor 14a. By adopting this arrangement, effects identical with those of the first and second embodiments are obtained. That is, since the overall frequency characteristic of the frequency-characteristic correcting unit 14f and secondary-sound propagation system 18 is flat, the second term μR ij  e n  on the right side of Equation (1) may be written as follows if we let C represent the characteristic of the secondary-sound propagation system and C&#39; the characteristic of the frequency-characteristic correcting unit 14f: ##EQU5## Consequently, the adaptive signal processor 14a is capable of executing adaptive signal processing just as if the secondary-sound propagation system possessed a frequency characteristic having a constant gain. The result is that the coefficient convergence characteristic of the adaptive algorithm can be advanced to improve follow-up with respect to any fluctuation in noise, thereby making it possible to manifest a satisfactory noise-canceling effect. 
     In the second and third embodiments, the frequency-characteristic correcting unit is described as being composed of a graphic equalizer. However, the correcting unit can be constructed using an IIR-type digital filter. 
     In accordance with the present invention as described above, a frequency-characteristic correcting unit is provided on the input side of an adaptive filter in a noise-canceling controller and the frequency characteristic of the correcting unit is set so as to be approximately symmetrical to that of the canceling-sound propagation system about a 0 dB line. As a result, the overall frequency characteristic of the frequency-characteristic correcting unit and canceling-sound propagation system becomes substantially flat and the coefficient convergence characteristic of the adaptive filter based upon adaptive signal processing is improved. This makes it possible to achieve a satisfactory noise-canceling effect. 
     Further, in accordance with the present invention, a frequency-characteristic correcting unit is provided either on the input side of a canceling-noise generating source or in a feedback section for feeding back an error signal to a noise-canceling controller. The overall frequency characteristic of the frequency-characteristic correcting unit and canceling-sound propagation system is made substantially flat (i.e., gain is made constant) and the coefficient convergence of the adaptive filter that relies upon adaptive signal processing is improved. This makes it possible to achieve a satisfactory noise-canceling effect. 
     As many apparently widely different embodiments of the present invention can be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.