Patent Publication Number: US-8111834-B2

Title: Vehicular active noise control system

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an active noise control system for reducing an in-compartment noise caused by a vibratory noise generated by a vibratory noise source on a vehicle with a canceling sound that is in opposite phase with the in-compartment noise. 
     2. Description of the Related Art 
     Heretofore, there has been known the technology of an active noise control apparatus for reducing an in-compartment noise at the position of a microphone placed in the passenger compartment of a vehicle, by detecting the in-compartment noise with the microphone and outputting, from a speaker placed in the passenger compartment, a canceling sound that is in opposite phase with the in-compartment noise based on the in-compartment noise and an engine rotation signal which is correlated to the vibratory noise of an engine on the vehicle (see Japanese Laid-Open Patent Publication No. 2006-084532 and Japanese Patent No. 3843082). The active noise control apparatus cancels out a noise (hereinafter also referred to as “engine noise” or “engine muffling sound”) in the passenger compartment which is caused by the vibratory noise of the engine, of the in-compartment noise. 
     The in-compartment noise also includes, in addition to the engine noise, a noise (hereinafter also referred to as “driveline noise”) in the passenger compartment that is caused by a vibratory noise of a rotating driveline component such as a propeller shaft, a drive shaft, or the like while the vehicle is running. According to Japanese Laid-Open Utility Model Publication No. 62-200034, it has been proposed to provide a torsional damper around a propeller shaft for dampening torsional vibrations of the propeller shaft thereby to reduce the noise generated by the differential. 
     The noise is generated by the differential because the propeller shaft which is relatively long and heavy is not well balanced upon rotation. The torsional damper disposed around the propeller shaft for reducing the noise makes the vehicle heavy as a whole and also makes the vehicle costly to manufacture. Alternatively, instead of the torsional damper, weights may be added to vibration-causing regions of the driveline, or production-induced variations of the components of the driveline may be strictly controlled, to reduce the driveline noise. These countermeasures, however, are still liable to make the vehicle heavy as a whole and also to make the vehicle costly to manufacture. 
     Attempts have been made to reduce the driveline noise with the active noise control apparatus described above. However, since the active noise control apparatus is based on the fact that the engine noise is generated in synchronism with the rotation of the output shaft of the engine, and generates the canceling sound using the frequency of the engine rotation signal depending on the rotational speed of the output shaft, the active noise control apparatus cannot directly be applied to reduce the driveline noise. 
     This is because the engine is occasionally disconnected from a transmission by a lockup control function of an automatic transmission vehicle or a clutch function of a manual transmission vehicle, making it difficult to calculate the rotational speed and rotation frequency of a driveline component such as a drive shaft, a propeller shaft, or the like at all times from the rotational speed of the output shaft of the engine. Therefore, even if the canceling sound is generated using the frequency of the engine rotation signal, it is difficult to reduce the noise in the passenger compartment (driveline noise) due to the vibratory noise of the driveline. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a vehicular active noise control system which is capable of reliably canceling out a driveline noise. 
     Another object of the present invention is to provide a vehicular active noise control system which is capable of making a vehicle that incorporates the vehicular active noise control system lower in weight and cost. 
     A vehicular active noise control system according to the present invention comprises a basic signal generator for generating a basic signal having a predetermined control frequency based on a frequency of a vibratory noise generated by a vibratory noise source of a vehicle, an adaptive filter for generating a control signal to cancel out an in-compartment noise produced in a passenger compartment of the vehicle by the vibratory noise, based on the basic signal, and a sound outputting device for outputting a canceling sound based on the control signal into the passenger compartment. The present invention further comprises an error signal detector for detecting a canceling error sound between the in-compartment noise and the canceling sound and outputting an error signal representing the detected canceling error sound, a reference signal generator for correcting the basic signal based on a corrective value representing transfer characteristics from the sound outputting device to the error signal detector corresponding to the control frequency, and outputting the corrected basic signal as a reference signal, and a filter coefficient updating unit for sequentially updating a filter coefficient of the adaptive filter to minimize the error signal, based on the error signal and the reference signal. 
     The vehicular active noise control system also includes a vehicle speed detector for detecting a vehicle speed of the vehicle and outputting a vehicle speed signal representing the detected vehicle speed, and a frequency calculating unit for calculating the control frequency which is a harmonic of a rotation frequency of a driveline rotary component of the vehicle which serves as the vibratory noise source, based on the vehicle speed signal, and outputting the calculated control frequency to the basic signal generator. The basic signal generator has a waveform data table for storing waveform data in one cyclic period, and generates the basic signal having the control frequency by successively reading the waveform data from the waveform data table at each sampling event. 
     The vehicular active noise control system also includes an engine rotational speed detector for detecting an engine rotational speed of an engine of the vehicle, and a frequency calculating unit for calculating the control frequency which is a harmonic of a rotation frequency of a driveline rotary component of the vehicle which serves as the vibratory noise source, based on the engine rotational speed, and outputting the calculated control frequency to the basic signal generator. The basic signal generator has a waveform data table for storing waveform data in one cyclic period, and generates the basic signal having the control frequency by successively reading the waveform data from the waveform data table at each sampling event. 
     With the above arrangements, the rotation frequency of the driveline rotary component is estimated from the engine rotational speed or the vehicle speed signal, the basic signal is generated which has the control frequency that is a harmonic of the rotation frequency, and the control signal is generated from the basic signal. Since the in-compartment noise produced in the passenger compartment due to the vibratory noise of the driveline rotary component is a driveline noise having a frequency that is a harmonic of the frequency of the vibratory noise, when the canceling sound based on the control signal is output from the sound outputting device into the passenger compartment, the driveline noise at the position of the error signal detector is reliably silenced. 
     As the driveline noise is silenced without the need for torsional dampers and weights, the vehicle as a whole can be reduced in weight and cost. 
     The driveline comprises an overall power transmitting mechanism from a clutch or a torque converter connected to the output shaft of the engine to tires of the vehicle. More specifically, the driveline includes a transmission, a propeller shaft, a differential, a drive shaft, and wheels, for example. The driveline rotary component refers to a component of the driveline which is rotatable when the vehicle is in operation, and includes the propeller shaft, the drive shaft, and tires, for example. 
     In the above system, the vehicle speed detector detects the rotational speed of a countershaft or the like of the vehicle, and outputs a pulse signal depending on the detected rotational speed as the vehicle speed signal to the frequency calculating unit. 
     Since the frequency calculating unit calculates the control frequency using the vehicle speed signal, the system can easily generate the control signal for canceling out the driveline noise. 
     The rotation frequency is estimated from the engine rotational speed as follows: 
     If the driveline rotary component comprises the propeller shaft, then the frequency calculating unit should preferably calculate the rotation frequency of the propeller shaft by multiplying a frequency depending on the engine rotational speed by a transmission gear ratio, a final gear ratio, a bevel gear ratio, and a transfer gear ratio. 
     In this manner, the frequency calculating unit can easily calculate the rotation frequency of the propeller shaft from the engine rotational speed. 
     The transmission gear ratio represents a gear ratio between a gear mounted on a main shaft of the transmission and a gear mounted on a countershaft. The final gear ratio represents a gear ratio between another gear mounted on the countershaft and a gear mounted on the drive shaft. The bevel gear ratio represents a gear ratio between a bevel gear mounted on the drive shaft and a bevel gear on the side of the propeller shaft which is held in mesh with the first-mentioned bevel gear within the differential. The transfer gear ratio represents a gear ratio between another gear mounted on a shaft which supports the bevel gear on the side of the propeller shaft and a gear mounted on the propeller shaft. 
     If the driveline rotary component comprises the drive shaft or the tires, then the frequency calculating unit should preferably calculate the rotation frequency of the drive shaft or the tires by multiplying a frequency depending on the engine rotational speed by the transmission gear ratio or the final gear ratio. 
     In this manner, the frequency calculating unit can easily calculate the rotation frequency of the drive shaft or the tires from the engine rotational speed. 
     The vehicular active noise control system should preferably further comprise a connected state output unit for outputting a disconnection signal indicating that the engine and the transmission of the vehicle are disconnected from each other, to the frequency calculating unit, and the frequency calculating unit should preferably stop calculating the rotation frequency when the disconnection signal is input thereto. 
     Therefore, when the disconnection signal is input to the frequency calculating unit while the frequency calculating unit is calculating the rotation frequency based on the engine rotational speed, the frequency calculating unit can quickly stop calculating the rotation frequency based on the engine rotational speed. 
     Further, the rotation frequency is estimated from the vehicle speed signal as follows: 
     If the driveline rotary component comprises the propeller shaft, then the frequency calculating unit calculates the rotation frequency of the propeller shaft by multiplying the frequency of the vehicle speed signal by a predetermined conversion value for conversion between the rotational speed of the countershaft and the vehicle speed signal, the final gear ratio, the bevel gear ratio, and the transfer gear ratio. 
     If the driveline rotary component comprises the drive shaft or the tires, then the frequency calculating unit calculates the rotation frequency of the drive shaft or the tires by multiplying the frequency of the vehicle speed signal by a predetermined conversion value for conversion between the rotational speed of the countershaft and the vehicle speed signal, and the final gear ratio. 
     In this manner, the rotation frequency of the propeller shaft, the drive shaft, or the tires can easily be calculated from the vehicle speed signal. 
     The vehicular active noise control system should preferably further comprise engine rotational speed detector for detecting the engine rotational speed of an engine of the vehicle, and a connected state output unit for outputting a disconnection signal indicating that the engine and the transmission are disconnected from each other, to the frequency calculating unit, and the frequency calculating unit should preferably calculate the rotation frequency based on the vehicle speed signal or the engine rotational speed when the disconnection signal is not input thereto, and calculate the rotation frequency based on the vehicle speed signal when the disconnection signal is input thereto. 
     Consequently, the frequency calculating unit continuously calculates the rotation frequency even when the engine and the transmission are disconnected from each other while the frequency calculating unit is calculating the rotation frequency. When the disconnection signal is input to the frequency calculating unit while the frequency calculating unit is calculating the rotation frequency based on the engine rotational speed, the frequency calculating unit quickly changes to the calculation of the rotation frequency based on the vehicle speed signal. 
     If the control frequency is a frequency which is represented by a real multiple of the rotation frequency, then the system reliably silences the driveline noise even if the driveline noise has a frequency which is of a given degree with respect to the vibratory noise. 
     Preferably, the control signal comprises a first control signal for canceling out a driveline noise produced in the passenger compartment by the vibratory noise generated by the driveline rotary component, and the vehicular active noise control system further comprises an active noise control apparatus for generating a second control signal to cancel out an engine noise produced in the passenger compartment by an engine vibratory noise generated by an engine of the vehicle which serves as the vibratory noise source, based on the engine vibratory noise, and a signal combining unit for combining the first control signal and the second control signal into a combined signal, and outputting the combined signal to the sound outputting device. 
     With the above arrangement, the in-compartment noise (the engine noise and the driveline noise) at the position of the error signal detector can well be silenced. 
     The vehicular active noise control system should preferably further comprise a comparing and adjusting unit for comparing a control frequency of the first control signal and a control frequency of the second control signal with each other, and stopping outputting one of the first and second control signals to the signal combining unit or changing an output level of one of the first and second control signals if the control frequencies of the first and second control signals are the same as or close to each other. 
     If the control frequencies are the same as each other, then the in-compartment noise is silenced using one of the control signals. If the control frequencies are close to each other, then a canceling sound based on one of the control signals which has a relatively large output level is output to cancel a noise which has the same frequency as the control frequency of the control signal having the relatively large output level, and a canceling sound based on the other control signal is output to reduce a noise which has a frequency close to the control frequency of the control signal having the relatively large output level. The comparing and adjusting unit makes it possible for the system to silence the in-compartment noise efficiently. 
     The above and other objects, features, and advantages of the present invention will become more apparent from the following description when taken in conjunction with the accompanying drawings in which preferred embodiments of the present invention are shown by way of illustrative example. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a side elevational view, partly in block form, of a vehicle incorporating a vehicular active noise control system according to a first embodiment of the present invention; 
         FIG. 2  is a schematic plan view showing a driveline of the vehicle shown in  FIG. 1 ; 
         FIG. 3  is a functional block diagram of the vehicular active noise control system shown in  FIG. 1 ; 
         FIGS. 4A and 4B  are diagrams showing specific data stored in a waveform data table shown in  FIG. 3 ; 
         FIGS. 5A through 5C  are diagrams showing the manner in which the data are read from the waveform data table shown in  FIG. 3 ; 
         FIG. 6  is a functional block diagram of the vehicular active noise control system shown in  FIG. 3 , with a signal transfer characteristics measuring device disposed in an electronic controller; 
         FIG. 7  is a side elevational view, partly in block form, of a vehicle incorporating a vehicular active noise control system according to a second embodiment of the present invention; 
         FIG. 8  is a schematic plan view showing a driveline of the vehicle shown in  FIG. 7 ; 
         FIG. 9  is a functional block diagram of the vehicular active noise control system shown in  FIG. 7 ; 
         FIG. 10  is a side elevational view, partly in block form, of a vehicle incorporating a vehicular active noise control system according to a third embodiment of the present invention; 
         FIG. 11  is a schematic plan view showing a driveline of the vehicle shown in  FIG. 10 ; 
         FIG. 12  is a functional block diagram of the vehicular active noise control system shown in  FIG. 10 ; 
         FIG. 13  is a side elevational view, partly in block form, of a vehicle incorporating a vehicular active noise control system according to a fourth embodiment of the present invention; 
         FIGS. 14A through 14C  are diagrams showing characteristic curves illustrative of a noise silencing control process carried out by the vehicular active noise control system shown in  FIG. 13 ; and 
         FIG. 15  is a functional block diagram of a vehicular active noise control system according to a fifth embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Like or corresponding parts are denoted by like or corresponding reference characters throughout views. 
       FIGS. 1 and 2  show in block form a vehicular active noise control system (hereinafter referred to as “system”)  10 A according to a first embodiment of the present invention, which is incorporated in a vehicle  12 . In  FIG. 2 , the vehicle  12  is shown as a 4WD (AWD) vehicle. 
     The system  10 A comprises a microphone  22  disposed on a roof lining of the vehicle  12  near the head rest  18  of a front seat  16  in a passenger compartment  14 , or specifically near the position of an ear of a passenger, not shown, seated on the front seat  16 , a microphone  28  disposed near the head rest  26  of a rear seat  24 , a speaker  30  mounted on a door near the front seat  16 , a speaker  32  disposed behind the rear seat  24 , and an electronic controller  34 . 
     The vehicle  12  has an engine  36  that is controlled by an engine control ECU  38 . The engine control ECU  38  is supplied with an engine rotation signal from an engine rotation sensor (engine rotational speed detector)  400 . The engine rotation signal is made up of engine rotation pulses that are output from the engine rotation sensor  400  in synchronism with the rotation of the output shaft of the engine  36 , and is correlated to a noise generated by the engine  36  (e.g., an engine sound and a periodic noise caused by vibratory forces produced upon rotation of the output shaft of the engine  36 ) and a vibratory noise representative of vibrations etc. of the engine  36 . 
     The engine control ECU  38  is also supplied with a gear position signal from a shift lever operation detector  404 . The gear position signal represents a transmission gear ratio of a transmission  45  depending on the operation by the passenger of a shift lever  402  if the vehicle  12  is a manual transmission vehicle. The engine control ECU  38  is also supplied with a clutch connection signal (disengagement signal) from a clutch connection detector (connected state output unit)  408 . The clutch signal represents a disengagement of a clutch  42  to disconnect the transmission  45  from the engine  36  when the passenger presses a clutch pedal  406 . The transmission gear ratio represents a gear ratio between a transmission gear  46  mounted on a main shaft  44  and a transmission gear  50  mounted on a countershaft  48  and held in mesh with the transmission gear  46  in the transmission  45  as shown in  FIG. 2 . 
     In the description which follows, it is assumed that the vehicle  12  is a manual transmission vehicle. However, if the vehicle  12  is an automatic transmission vehicle, then the clutch  42  is replaced with a torque converter, and when the transmission  45  is disconnected from the engine  36  by the torque converter, an automatic transmission (AT) controller (connected state detector)  410  (shown by the broken lines in  FIGS. 1 and 2 ) for controlling the torque converter and the transmission  45  generates a clutch connection signal (disengagement signal) indicating that the transmission  45  is disconnected from the engine  36 . The AT controller  410  also generates a gear position signal representative of the transmission gear ratio of the transmission  45 . 
     As shown in  FIG. 2 , the vehicle  12  has a driveline comprising a power transmitting mechanism from the clutch  42  connected to the output shaft of the engine  36  to tires  60 ,  62 ,  82 ,  84 . More specifically, the driveline includes the clutch  42 , the main shaft  44 , the countershaft  48 , the transmission gears  46 ,  50 , and a final gear  52  of the transmission  45 , a final gear  56 , bevel gears  64 ,  66 , transfer gears  70 ,  72 , and shafts  68 ,  74  of a front differential  54 , a drive shaft  58 , a propeller shaft  76 , a rear differential  78 , a drive shaft  80 , wheels  37 ,  39 ,  41 ,  43 , and the tires  60 ,  62 ,  82 ,  84 . 
     When the vehicle  12  is in operation, the driveline produces a vibratory noise upon rotation of driveline rotary components including the propeller shaft  76 , the drive shaft  58 , the tires  60 ,  62 ,  82 ,  84 , etc., and a driveline noise having harmonics of the frequency of the vibratory noise is generated in the passenger compartment  14  (see  FIG. 1 ) due to the vibratory noise. The components per se of the driveline are well known in the art, and will not be described in detail below. 
     A vehicle speed sensor (vehicle speed detector)  40  is disposed near the countershaft  48 . The vehicle speed sensor  40  supplies a vehicle speed signal (vehicle speed pulses) representing the vehicle speed of the vehicle  12  depending on the rotational speed of the countershaft  48 , to the electronic controller  34 . At this time, the vehicle speed sensor  40  converts countershaft pulses depending on the rotational speed of the countershaft  48  into the vehicle speed pulses using a predetermined statutory conversion value α for displaying a vehicle speed on a vehicle speedometer, not shown, and outputs the vehicle speed pulses to the electronic controller  34 . 
     The conversion value α is 0.8529, for example, indicating that the vehicle speed sensor  40  generates one vehicle speed pulse when the countershaft  48  makes a 0.8529 revolution. The conversion value α may be 1, so that the vehicle speed sensor  40  generates one vehicle speed pulse when the countershaft  48  makes one revolution. In the description which follows, the conversion value α is set to 0.8529. 
     Based on the vehicle speed signal, the electronic controller  34  generates control signals Sc 1 , Sc 2  for canceling an in-compartment noise including the driveline noise, and outputs the control signals Sc 1 , Sc 2  as canceling sounds to the speakers (sound outputting devices)  30 ,  32 , which output canceling sounds based on the control signals Sc 1 , Sc 2  into the passenger compartment  14 . The microphones (error signal detectors)  22 ,  28  detect canceling error sounds between the in-compartment noises and the canceling sounds, and output error signals e 1 , e 2  representing the detected canceling error sounds to the electronic controller  34 . 
       FIG. 3  is a functional block diagram of the electronic controller  34 . For an easier understanding of the present invention, it is assumed with respect to the electronic controller  34  shown in  FIG. 3  that the in-compartment noise including the driveline noise at the position of the microphone  22  in the passenger compartment  14  is reduced using the microphone  22  and the speaker  30  near the front seat  16 . The same assumption applies to all electronic controllers according to other embodiments of the present invention. 
     The electronic controller  34  is implemented by a microcomputer and has a control circuit  104  for generating a control signal Scp based on the vehicle speed signal, a D/A converter (hereinafter also referred to as “DAC”)  112 , and an A/D converter (hereinafter also referred to as “ADC”)  114 . 
     The control circuit  104  comprises a frequency detecting circuit (frequency calculating unit)  150 , a basic signal generator  316 , a reference signal generator  324 , adaptive filters  156 ,  158 , an adder  160 , and filter coefficient updating units  168 ,  176 . 
     The frequency detecting circuit  150  estimates the frequency (rotation frequency) fp of the propeller shaft  76  from the frequency fc of the vehicle speed pulses applied thereto. 
     A process of estimating the frequency fp from the frequency fc in the frequency detecting circuit  150  will be described below. 
     It is assumed that the gear ratio (final gear ratio) between the number Fr of teeth of the final gear  52  (see  FIG. 2 ) and the number Fn of teeth of the final gear  56  is represented by Fr/Fn, the gear ratio (bevel gear ratio) between the number Br of teeth of the bevel gear  64  and the number Bn of teeth of the bevel gear  66  by Br/Bn, and the gear ratio (transfer gear ratio) between the number Tr of teeth of the transfer gear  70  and the number Tn of teeth of the transfer gear  72  by Tr/Tn. The frequency detecting circuit  150  calculates (estimates) the frequency fp from the frequency fc according to the following equation (1): 
     
       
         
           
             
               
                 
                   fp 
                   = 
                   
                     fc 
                     × 
                     α 
                     × 
                     
                       ( 
                       
                         Fr 
                         ⁢ 
                         
                           / 
                         
                         ⁢ 
                         Fn 
                       
                       ) 
                     
                     × 
                     
                       ( 
                       
                         Br 
                         ⁢ 
                         
                           / 
                         
                         ⁢ 
                         Bn 
                       
                       ) 
                     
                     × 
                     
                       ( 
                       
                         Tr 
                         ⁢ 
                         
                           / 
                         
                         ⁢ 
                         Tn 
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
     For example, if fc=58.8 [Hz], (Fr/Fn)×(Br/Bn)×(Tr/Tn)=0.629764, then fp=37 [Hz]. 
     According to the above estimating process, since the gear ratio (transmission gear ratio) Hr/Hn between the number Hr of teeth of the transmission gear  46  and the number Hn of teeth of the transmission gear  50  is not included in the equation (1), the frequency detecting circuit  150  can calculate the frequency fp from the frequency fc using the vehicle speed signal regardless of the connected state between the engine  36  and the transmission  45 , i.e., regardless of whether the engine  36  and the transmission  45  are connected or not. 
     The frequency detecting circuit  150  then calculates a control frequency fp′ which is a harmonic (e.g., a first harmonic represented by a real multiple) of the frequency fp, from the frequency fp of the propeller shaft  76  estimated according to the equation (1), and outputs the calculated control frequency fp′ to the basic signal generator  316 . 
     The frequency detecting circuit  150  also generates a timing signal (sampling pulses) having a sampling period of the microcomputer (the control circuit  104 ), and the microcomputer performs a processing operation according to an LMS algorithm, to be described later, based on the timing signal generated by the frequency detecting circuit  150 . 
     The basic signal generator  316  comprises an address shifter  312 , a waveform data table  314  as a memory, a cosine wave generating circuit  320 , and a sine wave generating circuit  322 . Based on waveform data in one cyclic period stored in the waveform data table  314 , the basic signal generator  316  generates basic signals (a basic cosine wave signal xp 1  and a basic sine wave signal xp 2 ) having the control frequency fp′, and outputs the generated basic signals to the adaptive filters  156 ,  158  and the reference signal generator  324 . 
     As shown in  FIGS. 4A and 4B , the waveform data table  314  stores instantaneous value data as waveform data at respective addresses, the instantaneous value data representing a predetermined number (N) of instantaneous values into which the waveform of a sine wave in one cyclic period is divided at equal intervals along a time axis {the phase axis in FIG.  4 B}. The addresses (i) are indicated by integers (i=0, 1, 2, . . . , N−1) ranging from 0 to (the predetermined number−1). An amplitude value A shown in  FIGS. 4A and 4B  are represented by 1 or any desired positive real number. Therefore, the waveform data at the address i is calculated as A·sin(360°×i/N). Stated otherwise, one cycle of sine waveform is divided into N sampled values at sampling points spaced over time, and data generated by quantizing the instantaneous values of the sine wave at the respective sampling points are stored as waveform data at respective addresses, which are represented by the respective sampling points, in the waveform data table  314  (see  FIG. 3 ). 
     Addresses based on the control frequency fp′ from the frequency detecting circuit  150  are specified for the sine wave generating circuit  322  to access the waveform data table  314 , and addresses produced when the address shifter  312  shifts the above addresses based on the control frequency fp′ by a ¼ period are specified for the cosine wave generating circuit  320  to access the waveform data table  314 . 
       FIGS. 5A through 5C  schematically illustrate the manner in which the basic signal generator  316  (see  FIG. 3 ) generates the basic signals (the basic cosine wave signal xp 1  and the basic sine wave signal xp 2 ). A process of generating the basic cosine wave signal xp 1  with the cosine wave generating circuit  320  and a process of generating the basic sine wave signal xp 2  with the sine wave generating circuit  322  will be described in specific detail below with reference to  FIGS. 3 through 5C . 
     In  FIGS. 5A through 5C , n refers to an integer of 0 or greater, and represents a count of sampling pulses (timing signal count).  FIG. 5A  schematically shows the relationship between the addresses and the waveform data of the waveform data table  314  (see  FIG. 3 ).  FIG. 5B  schematically shows the generation of the basic sine wave signal xp 2 , and  FIG. 5C  shows the generation of the basic cosine wave signal xp 1 . 
     The frequency detecting circuit  150  outputs a timing signal at a fixed sampling period according to a fixed sampling process. The predetermined number (N) is assumed to be 3600. The addresses are i=0, 1, 2, . . . , N−1=0, 1, 2, . . . , 3599, and the shift for the ¼ period is represented by N/4=900. For the sake of brevity, the sampling interval (time) is set to t=1/N= 1/3600 [s]. 
     Since the sampling interval is 1/3600 [s] (1/N [s]), each time a sampling pulse is input from the frequency detecting circuit  150 , a read address i(n) for the waveform data table  314  is specified at an address interval “is” based on the control frequency fp′ according to the following equation (2): 
     
       
         
           
             
               
                 
                   
                     
                       
                         
                           Address 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           interval 
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           
                             : 
                           
                           ⁢ 
                           
                               
                           
                           ⁢ 
                           is 
                         
                         = 
                           
                         ⁢ 
                         
                           
                             N 
                             ⁢ 
                             
                                 
                             
                             × 
                             
                               fp 
                               ′ 
                             
                             × 
                             
                                 
                             
                             ⁢ 
                             t 
                           
                           = 
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           3600 
                           ⁢ 
                           
                               
                           
                           × 
                           
                             fp 
                             ′ 
                           
                           × 
                           
                             ( 
                             
                               1 
                               / 
                               3600 
                             
                             ) 
                           
                         
                       
                     
                   
                   
                     
                       
                         = 
                           
                         ⁢ 
                         
                           fp 
                           ′ 
                         
                       
                     
                   
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
     Therefore, the address i(n) at a certain timing is given according to the following equation (3):
 
 i ( n )= i ( n− 1)+ is=i ( n− 1)+ fp′   (3)
 
     If i(n)&gt;3599 (=N−1), the address i(n) at a certain timing is given according to the following equation (4):
 
 i ( n )= i ( n− 1)+ fp′− 3600  (4)
 
     The sine wave generating circuit  322  (see  FIG. 3 ) generates a basic sine wave signal xp 2 (n) by reading waveform data from the waveform data table  314  at the address interval “is” corresponding to the control frequency fp′ each time a sampling pulse is generated by the frequency detecting circuit  150 . For example, if the control frequency fp′ is 40 [Hz], then when the control process has started, the sine wave generating circuit  322  generates a basic sine wave signal xp 2 (n) of 40 [Hz] by reading waveform data from the addresses i(n)=0, 40, 80, 120, . . . , 3560, 0, . . . in response to each sampling pulse from the frequency detecting circuit  150 , i.e., at each interval of 1/3600 [s]. 
     The address shifter  312  (see  FIG. 3 ) produces addresses by shifting (adding) the read addresses i(n) for the basic sine wave signal xp 2 (n) by a ¼ period based on sin(θ+π/2)=cos θ, according to the equation (5) shown below, and specifies the produced addresses as read addresses i′(n) for the cosine wave generating circuit  320  to access the waveform data table  314 .
 
 i ′( n )= i ( n )+ N/ 4 =i ( n )+900  (5)
 
     If i′(n)&gt;3599 (=N−1), the addresses i′(n) are given according to the following equation (6):
 
 i ′( n )= i ( n )+900−3600  (6)
 
     Therefore, the cosine wave generating circuit  320  generates a basic cosine wave signal xp 1 (n) by reading waveform data from the waveform data table  314  at the address interval “is” corresponding to the control frequency fp′ each time a sampling pulse is generated by the frequency detecting circuit  150 , based on the addresses i′(n) produced by shifting the read addresses i(n) for the reference sine wave signal xp 2 (n) by the ¼ period. 
     For example, if the control frequency fp′ is 40 [Hz], then when the control process has started, the cosine wave generating circuit  320  generates a basic cosine wave signal xp 1 (n) of 40 [Hz] by reading waveform data from the addresses i′(n)=900, 940, 980, 1020, . . . , 860, 900, . . . in response to each sampling pulse from the frequency detecting circuit  150 , i.e., at each interval of 1/3600 [s]. 
     According to the fixed sampling process, as described above, the basic signals {the basic cosine wave signal xp 1 (n) and the basic sine wave signal xp 2 (n)} are generated by changing the read address interval for the waveform data depending on the control frequency fp′. 
     If the frequency detecting circuit  150  outputs a timing signal at a sampling period in synchronism with the rotational speed of the propeller shaft  76  (see  FIG. 2 ), i.e., the rotational speed based on vehicle speed pulses (variable sampling process), then the basic signals {the basic cosine wave signal xp 1 (n) and the basic sine wave signal xp 2 (n)} can be generated by changing the value of the predetermined number (N) and the sampling period in synchronism with the rotational speed of the propeller shaft  76 , according to the process of generating a basic signal based on the synchronous sampling process as disclosed in Japanese Laid-Open Patent Publication No. 2006-084532 (variable sampling process) and also the above fixed sampling process. 
     The basic cosine wave signal xp 1  and the basic sine wave signal xp 2  thus generated are basic signals having a harmonic frequency of the frequency fp of the propeller shaft  76 . The control frequency fp′ which is a harmonic frequency corresponds to the frequency of the driveline noise that is generated in the passenger compartment  14  due to the vibratory noise of the propeller shaft  76 . 
     The adaptive filter  156  corrects the basic cosine wave signal xp 1  with a filter coefficient Wp 1 , and outputs a corrected basic cosine wave signal xp 1 ·Wp 1  to the adder  160 . The adaptive filter  158  corrects the basic sine wave signal xp 2  with a filter coefficient Wp 2 , and outputs a corrected basic sine wave signal xp 2 ·Wp 2  to the adder  160 . The adder  160  adds the signal xp 1 ·Wp 1  from the adaptive filter  156  and the signal xp 2 ·Wp 2  from the adaptive filter  158  into a control signal Scp for canceling out the driveline noise in the passenger compartment  14  which is caused due to the vibratory noise produced upon rotation of the propeller shaft  76  (see  FIG. 2 ). 
     The control signal Scp is converted from a digital signal into an analog signal by the DAC  112 . The analog control signal Scp (Sc 1 ) is supplied to the speaker  30 , which outputs a canceling sound based on the control signal Scp into the passenger compartment  14 . The microphone  22  detects a canceling error sound between the in-compartment noise including the driveline noise at the position of the microphone  22  and the canceling sound, and outputs an error signal e 1  based on the detected canceling error sound. The error signal e 1  is converted from an analog signal into a digital signal by the ADC  114 . The digital error signal e 1  is output to the filter coefficient updating units  168 ,  176 . 
     The reference signal generator  324  comprises correctors  326 ,  328  each having a corrective value C representative of signal transfer characteristics C 11  from the speaker  30  (see  FIGS. 1 and 3 ) to the microphone  22 . The correctors  326 ,  328  correct the respective basic signals xp 1 , xp 2  with the corrective value Ĉ, thereby generating respective reference signals rp 1 , rp 2 , and output the reference signals rp 1 , rp 2  to the filter coefficient updating units  168 ,  176 . 
     The signal transfer characteristics are actually measured as follows. As shown in  FIG. 6 , a signal transfer characteristics measuring device  500  which comprises a Fourier transforming device is connected between the input terminal of the DAC  112  and the output terminal of the ADC  114 . The signal transfer characteristics measuring device  500  measures signal transfer characteristics based on a test signal that is input from the adder  160  of the control circuit  104  to the DAC  112  and a signal output from ADC  114  to the filter coefficient updating units  168 ,  176  of the control circuit  104 . In  FIGS. 3 and 6 , the signal transfer characteristics measured by the signal transfer characteristics measuring device  500  are set as the corrective value Ĉ in the correctors  326 ,  328  of the reference signal generator  324 . Therefore, depending on how the signal transfer characteristics measuring device  500  measures signal transfer characteristics, the corrective value Ĉ may represent the signal transfer characteristics from the speaker  30  to the microphone  22  or the signal transfer characteristics from the output terminal of the adder  160  to the input terminals of the input terminals of the filter coefficient updating units  168 ,  176 , including the signal transfer characteristics from the speaker  30  to the microphone  22 , measured as described above. 
     The filter coefficient updating units  168 ,  176  (see  FIGS. 3 and 6 ), which comprise least mean square (LMS) algorithm operators, perform an adaptive arithmetic process for adaptively calculating the filter coefficients Wp 1 , Wp 2  based on the reference signals rp 1 , rp 2  and the error signal e 1 , i.e., an arithmetic process for calculating the filter coefficients Wp 1 , Wp 2  according to the least mean square method in order to minimize the error signal e 1 , and successively update the filter coefficients Wp 1 , Wp 2  based on the calculated results in response to each sampling pulse. 
     As described above, the system  10 A according to the first embodiment estimates the (rotation) frequency fp of the propeller shaft  76  as a driveline rotary component from the frequency fc of vehicle speed pulses, generates the basic signals (the basic cosine wave signal xp 1  and the basic sine wave signal xp 2 ) having the control frequency fp′ which is a harmonic of the frequency fp, and generates the control signal Scp (Sc 1 ) from the basic signals. Since the noise generated in the passenger compartment  14  due to the vibratory noise produced upon rotation of the propeller shaft  76  is a driveline noise having a harmonic frequency of the frequency of the vibratory noise, when the speaker  30  outputs a canceling sound based on the control signal Scp into the passenger compartment  14 , the driveline noise at the position of the microphone  22  can reliably be canceled out. 
     Since the driveline noise is silenced without the need for torsional dampers and weights, the vehicle  12  as a whole can be reduced in weight and cost. 
     The frequency detecting circuit  150  calculates the frequency fp and the control frequency fp′ using the frequency fc of vehicle speed pulses. Consequently, the system  10 A can easily generate the control signal Scp for canceling out the driveline noise. 
     As the frequency detecting circuit  150  calculates the frequency fp of the propeller shaft  76  from the frequency fc of vehicle speed pulses according to the equation (1), the frequency detecting circuit  150  can easily calculate the frequency fp of the propeller shaft  76  from the vehicle speed pulses. 
     Since the control frequency fp′ is a real-multiple harmonic frequency of the frequency fp, the system  10 A can reliably silence a driveline noise which may have been generated in the passenger compartment  14  at a frequency of a given degree with respect to the vibratory noise. 
     A system  10 B according to a second embodiment of the present invention will be described below with reference to  FIGS. 7 through 9 . Those parts of the system  10 B which are identical to the system  10 A according to the first embodiment (see  FIGS. 1 through 6 ) are denoted by identical reference characters, and will not be described in detail below. 
     In the system  10 B, the electronic controller  34  is not supplied with the vehicle speed signal, but with the engine rotation signal, the gear position signal, and the clutch connection signal from the engine control ECU  38 . Based on the engine rotation signal, the gear position signal, and the clutch connection signal, the electronic controller  34  generates control signals Sc 1 , Sc 2 . The electronic controller  34  has a switch  300  and a switch controller  302 . 
     In  FIGS. 7 and 8 , if the vehicle  12  is an automatic transmission vehicle, then the AT controller  410  supplies the electronic controller  34  with the gear position signal and the clutch connection signal. However, it is assumed that the vehicle  12  is a manual transmission vehicle in the second embodiment and other subsequent embodiments. 
     When the switch controller  302  is supplied with the clutch connection signal from the engine control ECU  38 , the switch controller  302  outputs a disconnection signal Ss indicating that the clutch  42  has disconnected the transmission  45  from the engine  36 , to the control circuit  104  and the switch  300 . When the disconnection signal Ss is not input to the switch  300 , the switch  300  is turned on, supplying the engine rotation signal to the control circuit  104 . When the disconnection signal Ss is input to the switch  300 , the switch  300  is turned off, stopping supplying the engine rotation signal to the control circuit  104 . 
     When the disconnection signal Ss is not input to the frequency detecting circuit  150 , the frequency detecting circuit  150  estimates the frequency (rotation frequency) fp of the propeller shaft  76  (see  FIG. 8 ) from the frequency fe of the engine rotation signal (engine rotation pulses) supplied from the switch  300 . 
     A process of estimating the frequency fp from the frequency fe in the frequency detecting circuit  150  will be described below. 
     The frequency detecting circuit  150  calculates (estimates) the frequency fp from the frequency fe according to the following equation (7): 
     
       
         
           
             
               
                 
                   fp 
                   = 
                   
                     fe 
                     × 
                     
                       ( 
                       
                         Hr 
                         ⁢ 
                         
                           / 
                         
                         ⁢ 
                         Hn 
                       
                       ) 
                     
                     × 
                     
                       ( 
                       
                         Fr 
                         ⁢ 
                         
                           / 
                         
                         ⁢ 
                         Fn 
                       
                       ) 
                     
                     × 
                     
                       ( 
                       
                         Br 
                         ⁢ 
                         
                           / 
                         
                         ⁢ 
                         Bn 
                       
                       ) 
                     
                     × 
                     
                       ( 
                       
                         Tr 
                         ⁢ 
                         
                           / 
                         
                         ⁢ 
                         Tn 
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
     For example, if the transfer gear ratio Hr/Hn indicated by the gear position signal input to the frequency detecting circuit  150  is a 5th-speed gear ratio, (Hr/Hn)×(Fr/Fn)×(Br/Bn)×(Tr/Tn)=1.5357, and the engine rotational speed is 3000 [rpm], then since fe=50 [Hz] (=3000 [rpm]/60 [s]), fp=76.8 [Hz]. 
     The process of estimating the frequency fp of the propeller shaft  76  according to the equation (7) is applicable when the engine  36  and the transmission  45  are connected to each other by the clutch  42 . In other words, when the frequency detecting circuit  150  is supplied with the disconnection signal Ss, the frequency detecting circuit  150  stops estimating the frequency fp of the propeller shaft  76 . 
     As described above, when the engine  36  and the transmission  45  are connected to each other by the clutch  42 , the system  10 B estimates the (rotation) frequency fp of the propeller shaft  76  as a driveline rotary component from the frequency fe of engine rotation pulses, and generates the basic signals (the basic cosine wave signal xp 1  and the basic sine wave signal xp 2 ) which have the control frequency fp′ that is a harmonic of the frequency fp. Therefore, as with the system  10 A according to the first embodiment, the system  10 B is capable of well silencing the driveline noise at the position of the microphone  22 , and allows the vehicle  12  as a whole to be reduced in weight and cost. 
     The frequency detecting circuit  150  calculates the frequency fp and the control frequency fp′ using the frequency fe of engine rotation pulses. Consequently, the system  10 B also can easily generate the control signal Scp for canceling out the driveline noise. 
     As the frequency detecting circuit  150  calculates the frequency fp of the propeller shaft  76  from the frequency fe of engine rotation pulses according to the equation (7), the frequency detecting circuit  150  can easily calculate the frequency fp of the propeller shaft  76  from the engine rotation pulses. 
     A system  10 C according to a third embodiment of the present invention will be described below with reference to  FIGS. 10 through 12 . 
     In the system  10 C, the electronic controller  34  is supplied with the vehicle speed signal from the vehicle speed sensor  40 , and is also supplied with the engine rotation signal, the gear position signal, and the clutch connection signal from the engine control ECU  38 . Based on the vehicle speed signal, the engine rotation signal, the gear position signal, and the clutch connection signal, the electronic controller  34  generates control signals Sc 1 , Sc 2 . The electronic controller  34  has a switch  300  and a switch controller  302 . The switch  300  is a selector switch which supplies the engine rotation signal to the control circuit  104  when the disconnection signal Ss is not input to the switch  300 , and supplies the vehicle speed signal to the control circuit  104  when the disconnection signal Ss is input to the switch  300 . 
     When the disconnection signal Ss is not input to the frequency detecting circuit  150 , the frequency detecting circuit  150  estimates the frequency fp of the propeller shaft  76  (see  FIG. 11 ) from the frequency fe of the engine rotation signal according to the equation (7). When the disconnection signal Ss is input to the frequency detecting circuit  150 , the frequency detecting circuit  150  estimates the frequency fp of the propeller shaft  76  from the frequency fc of vehicle speed pulses according to the equation (1). 
     As described above, in the system  10 B according to the third embodiment, when the switch controller  302  outputs the disconnection signal Ss to the switch  300  and the frequency detecting circuit  150 , the switch  300  changes its connections to supply vehicle speed pulses, rather than engine rotation pulses, to the frequency detecting circuit  150 . Based on the input disconnection signal Ss, the frequency detecting circuit  150  quickly changes from the calculation of the frequency fp based on the engine rotation pulses to the calculation of the frequency fp based on the vehicle speed pulses. Therefore, the frequency detecting circuit  150  continuously calculates the frequency fp. Since the frequency detecting circuit  150  can output the control frequency fp′ based on the frequency fp to the basic signal generator  316  even when the engine  36  and the transmission  45  are disconnected from each other by the clutch  42 , the control circuit  104  can continuously silence the driveline noise at the position of the microphone  22 . 
     In the third embodiment, the frequency detecting circuit  150  changes from the calculation of the frequency fp based on the engine rotation pulses to the calculation of the frequency fp based on the vehicle speed pulses, based on the disconnection signal Ss input thereto. However, regardless of whether the disconnection signal Ss is input or not, the switch  300  may supply vehicle speed pulses to the frequency detecting circuit  150  to enable the frequency detecting circuit  150  to calculate the frequency fp based on the vehicle speed pulses. 
     A system  10 D according to a fourth embodiment of the present invention will be described below with reference to  FIGS. 13 through 14C . 
     The system  10 D is different from the system  10 C (see  FIGS. 10 through 12 ) as to the following features. The vibratory noise source comprises the propeller shaft  76  (see  FIG. 11 ), the engine  36 , the drive shaft  58 , or the tires  60 ,  62 . The system  10 D includes, in addition to the control circuit  104  for reducing the driveline noise at the position of the microphone  22  due to the vibratory noise produced upon rotation of the propeller shaft  76 , a control circuit  102  for reducing an engine noise (engine muffled sound) at the position of the microphone  22  due to the vibratory noise produced by the engine  36 , and a control circuit  106  for reducing a driveline noise at the position of the microphone  22  due to the vibratory noise produced upon rotation of the drive shaft  58  or the tires  60 ,  62 . The control circuits  102 ,  104 ,  106  generate respective control signals Sce, Scp, Sct, which are combined into a control signal Sc 1 . The speaker  30  outputs a canceling sound based on the control signal Sc 1  into the passenger compartment  14  to reduce the in-compartment noise including the engine noise and the driveline noises. 
     The control circuits  102 ,  104 ,  106  are substantially identical in structure to each other. Specifically, the control circuits  102 ,  104 ,  106  have respective frequency detecting circuits  120 ,  150 ,  180 , respective basic signal generators  316 ,  334 ,  364 , respective reference signal generators  324 ,  340 ,  370 , respective pairs of adaptive filters  126 ,  128 ,  156 ,  158 ,  186 ,  188 , and respective pairs of filter coefficient updating units  138 ,  146 ,  168 ,  176 ,  198 ,  206 . 
     In the control circuit  102  for reducing the engine noise, the frequency detecting circuit  120  generates a control frequency fe′ which is a harmonic (a real multiple) of the frequency fe of engine rotation pulses based on the engine rotation signal (engine rotation pulses). The basic signal generator  334  generates a basic cosine signal xe 1  and a basic sine signal xe 2  of the control frequency fe′, and the reference signal generator  340  generates reference signals re 1 , re 2  based on the basic cosine signal xe 1  and the basic sine signal xe 2 . 
     In the control circuit  106  for reducing a driveline noise due to the rotation of the drive shaft  58  or the tires  60 ,  62 , the frequency detecting circuit  180  estimates a frequency ft of the drive shaft  58  or the tires  60 ,  62  based on the frequency fe of engine rotation pulses or the frequency fc of vehicle speed pulses supplied from the switch  300 , and calculates a control frequency ft′ which is a harmonic (a real multiple) of the frequency ft. 
     Specifically, when the engine rotation pulses are input to the frequency detecting circuit  180 , the frequency detecting circuit  180  estimates the frequency ft of the drive shaft  58  or the tires  60 ,  62  from the frequency fe of engine rotation pulses according to the following equation (8):
 
 ft=fe ×( Hr/Hn )×( Fr/Fn )  (8)
 
     When the vehicle speed pulses are input to the frequency detecting circuit  180 , the frequency detecting circuit  180  estimates the frequency ft from the frequency fc of the vehicle speed pulses according to the following equation (9):
 
 ft=fc ×α×( Fr/Fn )  (9)
 
     For example, if fc=58.8 [Hz] and Fr/Fn=0.1854, then ft=10.9 [Hz]. 
     The frequency detecting circuit  180  then calculates a control frequency ft′ (=10.9×3=32.7 [Hz]) which is a harmonic (e.g., of a third degree) of the frequency ft, and outputs the calculated control frequency ft′ to the basic signal generator  364 . 
     The basic signal generator  364  generates a basic cosine signal xt 1  and a basic sine signal xt 2  of the control frequency ft′, and the reference signal generator  370  generates reference signals rt 1 , rt 2  based on the basic cosine signal xt 1  and the basic sine signal xt 2 . 
     The operation of the basic signal generators  334 ,  364  to generate the basic cosine signals xe 1 , xt 1  and basic sine signals xe 2 , xt 2 , and the operation of the reference signal generators  340 ,  370  to generate the reference signals re 1 , re 2 , rt 1 , rt 2  are essentially the same as the operation of the basic signal generator  316  to generate the basic signals xp 1 , xp 2  and the operation of the reference signal generator  324  to generate the reference signals rp 1 , rp 2 , and will not be described in detail below. 
     The adaptive filters  126 ,  128 ,  186 ,  188  and the filter coefficient updating units  138 ,  146 ,  198 ,  206  operate in essentially the same manner as the adaptive filters  156 ,  158  and the filter coefficient updating units  168 ,  176 , and hence their operations will not be described in detail below. 
     The control signals Scp, Sct output from the control circuits  104 ,  106  are added by the adder  108  into a sum signal, which is output to the adder  110 . The adder  110  adds the control signal Sce from the control circuit  102  and the sum signal (Scp+Sct) from the adder  108  into a control signal, which is output through the DAC  112  to the speaker  30  as a control signal Sc 1 . 
       FIGS. 14A through 14C  show characteristic curves indicative of reductions achieved by the system  10 D in the in-compartment noise at the position of the microphone  22 .  FIG. 14A  shows characteristic curves indicative of a reduction in the driveline noise caused by the vibratory noise of the propeller shaft  76  (see  FIG. 11 ).  FIG. 14B  shows characteristic curves indicative of a reduction in the driveline noise caused by the vibratory noise of the drive shaft  58  or the tires  60 ,  62 .  FIG. 14C  shows characteristic curves indicative of a reduction in the engine noise. It can be seen from the characteristic curves shown in  FIGS. 14A through 14C  that the above noises are silenced when the control circuits  102 ,  104 ,  106  perform their silencing control processes (ANC turned on), but not when the control circuits  102 ,  104 ,  106  do not perform their silencing control processes (ANC turned off). 
     Specifically, the control circuit  104  (see  FIG. 13 ) generates the control signal Scp of the control frequency fp′ which is a harmonic based on the frequency fp of the propeller shaft  76  (see  FIG. 11 ), and the canceling sound based on the control signal Scp is output from the speaker  30  to reduce the driveline noise at the position of the microphone  22  due to the vibratory noise of the propeller shaft  76  (see  FIG. 14A ). The control circuit  106  generates the control signal Sct of the control frequency ft′ which is a harmonic based on the frequency ft of the drive shaft  58  or the tires  60 ,  62 , and the canceling sound based on the control signal Sct is output from the speaker  30  to reduce the driveline noise at the position of the microphone  22  due to the vibratory noise of the drive shaft  58  or the tires  60 ,  62  (see  FIG. 14B ). The control circuit  102  generates the control signal Sce of the control frequency fe′ which is a harmonic based on the frequency fe, and the canceling sound based on the control signal Sce is output from the speaker  30  to reduce the engine noise at the position of the microphone  22  (see  FIG. 14C ). 
     The system  10 D according to the fourth embodiment offers the same advantages as those of the system  10 C according to the third embodiment (see  FIGS. 10 through 12 ), and is also capable of silencing the driveline noise caused by the drive shaft  58  or the tires  60 ,  62  and also the engine noise. Therefore, the system  10 D is highly effective to cancel out the in-compartment noise. 
     A system  10 E according to a fifth embodiment will be described below with reference to  FIG. 15 . 
     The system  10 E is different from the system  10 D according to the fourth embodiment (see  FIG. 13 ) in that the electronic controller  34  additionally includes a comparing and adjusting unit  260  having a comparator  250  and variable-gain amplifiers  252 ,  254 ,  256  and connected to the output terminals of the control circuits  102 ,  104 ,  106 . 
     The comparator  250  compares the control frequency fe′ of the control signal Sce, the control frequency fp′ of the control signal Scp, and the control frequency ft′ of the control signal Sct, and adjusts the gains of the variable-gain amplifiers  252 ,  254 ,  256  if these control frequencies fe′, fp′, ft′ are the same as or close to each other. 
     Specifically, if the control frequencies fe′, fp′, ft′ are the same as each other (fe′=fp′=ft′), then the comparator  250  adjusts the gains of the variable-gain amplifiers  254 ,  256  to zero (0). Therefore, only the control signal Sce is supplied through the adder  110  and the DAC  112  to the speaker  30 , so that the noise in the passenger compartment  14  is silenced based on the control signal Sce. 
     If the control frequencies fe′, fp′, ft′ are close to each other, then the comparator  250  adjusts the gains of the variable-gain amplifiers  252 ,  254 ,  256  such that the gains of the variable-gain amplifiers  254 ,  256  are lower than the gain of the variable-gain amplifier  252 . Therefore, the control signal Sce and the control signals Scp, Sct which are lower in output level than the control signal Sce are supplied to the adder  110 , so that the noise in the passenger compartment  14  is silenced based on the control signals Sce, Scp, Sct. Specifically, the canceling sound based on the control signal Sce which has the relatively high output level silences the noise of the same frequency as the control frequency fe′ of the control signal Sce, and the canceling sound also reduces noises having frequencies close to the control frequency fe′ of the control signal Sce. The reduced noises are silenced by the canceling sounds based on the control signals Scp, Sct having the lower output levels. The in-compartment noise is reliably canceled out. 
     The system  10 E according to the fifth embodiment offers the advantages of the systems  10 C,  10 D according to the third and fourth embodiments (see  FIGS. 10 through 13 ), and is additionally capable of efficiently canceling out the in-compartment noise at the position of the microphone  22  because of the comparing and adjusting unit  260 . 
     In each of the above embodiments, the vehicle speed sensor  40  outputs a vehicle speed signal (vehicle speed pulses) representing the rotational speed of the countershaft  48 . However, another signal in synchronism with the vehicle speed, such as the rotational speed of the main shaft  44 , the rotational speeds of the drive shafts  58 ,  80 , or the rotational speed of the propeller shaft  76 , may directly be detected by the vehicle speed sensor  40 , and vehicle speed pulses depending on the detected rotational speed may be output from the vehicle speed sensor  40  to the electronic controller  34  for the control circuits  104 ,  106  to reduce the driveline noise. 
     In each of the above embodiments, the vehicle  12  has been described as a 4WD (AWD) vehicle. However, the present invention is also applicable to vehicles of other drive types, such as FF, FR, RR, MR types, as the electronic controller  34  may comprise an appropriate combinations of control circuits  102 ,  104 ,  106 . 
     In each of the above embodiments, when the control circuits  102 ,  104 ,  106  start or stop operating or the switch  300  changes its connections, if the values of the filter coefficients We 1 , We 2 , Wp 1 , Wp 2 , Wt 1 , Wt 2  are sequentially reduced or increased to smoothly attenuate or amplify the canceling sound output from the speaker  30  according to a fade-out or fade-in process, then uncomfortable vibratory noises are prevented from being produced at the time the control circuits  102 ,  104 ,  106  start or stop operating or the switch  300  changes its connections. 
     In each of the above embodiments, the reduction of the in-compartment noise at the position of the microphone  22  has been described. The in-compartment noise at the position of the microphone  28  can also be reduced by the control circuits  102 ,  104 ,  106 . 
     Although certain preferred embodiments of the present invention have been shown and described in detail, it should be understood that various changes and modifications may be made therein without departing from the scope of the appended claims.