Abstract:
The invention provides a method for controlling a level of a voice signal to reduce the noise which is superimposed on a voice signal without marring the content of the voice signal. To achieve the invention, the tracking signal is formed corresponding to a received RF signal strength. The tracking signal is reduced to together with the decrease of the received RF signal strength if the received RF signal strength decreases. The tracking signal increases at a constant rate gradually if the received RF signal strength increases. A level of a voice signal is controlled by a function corresponding to the tracking signal.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims the priority benefit of Japanese Patent Application No. 10-191752, filed Jul. 7, 1998, the entire subject matter of which is incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to a method for controlling a level of a voice signal, which is demodulated by a frequency modulation receiver, and a level-controlling device. 
     2. Description of the Related Art 
     The received RF(Radio Frequency) signal strength from a base station in a frequency modulation (FM) mode cellular phone fades intensively and dramatically in response to a change in the distance between the base station and the cellular phone or diffraction or reflection of radio wave by buildings. Noise added on a demodulated voice signal, is also fades intensively and dramatically in response to changes in the the received RF signal strength. As a result, the voice signal from the communicating party has a poor sound quality. Conventionally, a level-controlling circuit for a the voice signal in FM mode cellular phone controls the level of the voice signal in response to the received RF signal strength which is detected by a received RF signal strength detecting circuit. Concretely, The method utilized is that the level of the level of demodulate voice signal is attenuated by the received RF signal strength. For example, in a case that the received RF signal strength is weak, and high level of noise is superimposed on the voice signal, the voice signal and the noise are decayed. 
     However, in the conventional level-controlling circuit mentioned above, the level of the demodulated voice signal changes frequently in response to changes in the received RF signal strength. Therefore, the voice of the communicating party breaks up for short periods, or for long periods. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a method for controlling a level of a voice signal to reduce noise superimposed on a voice signal, without marring a content of the voice signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be more particularly described with reference to the accompanying drawings, in which: 
         FIG. 1  is a flow chart a method for controlling a level of a voice signal, according to a first embodiment of the invention; 
         FIG. 2  is a block diagram of a level-controlling circuit for carrying out the first embodiment of the invention; 
         FIG. 3  is a graph showing one example of the relationship between time and received RF signal strength which is received by the receiver such as a cellular phone; 
         FIG. 4  is a graph showing a characteristic property of a tracking signal in the first embodiment the invention; 
         FIG. 5  is a flow chart for carrying out the second embodiment of the invention; 
         FIG. 6  is a graph showing a characteristic property of a tracking signal in the second embodiment of the invention; 
         FIG. 7  is a block diagram of a level-controlling circuit for carrying out the third embodiment the invention; 
         FIG. 8  is a graph showing a characteristic property of tracking circuits  680  and  682  in the third embodiment; 
         FIG. 9  is a graph showing a characteristic property of a tracking signal which is an output of a circuit  64  in the third embodiment; 
         FIG. 10  is a block diagram of a level-controlling circuit for carrying out the fourth embodiment of the invention; 
         FIG. 11  is a graph showing the first example of a frequency characteristic of a filter  96  which is shown in  FIG. 10 ; and 
         FIG. 12  is a graph showing the second example of a frequency characteristic of a filter  96  which is shown in FIG.  10 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to  FIG. 2 , a control circuit  10  has a detecting circuit  12  which detects received RF signal strength, a tracking circuit  14  and multiplier  16 . The detecting circuit  12  outputs a detection signal  100  showing the received RF signal strength by detecting the received RF signtal strength. The detection signal  100  is input to the tracking circuit  14 . It is possible to use other detecting circuits, such as a signal-to-noise (S/N) detecting circuit to detect an S/N ratio of the received RF signal or a noise detecting circuit to detect a noise which is added on a RF signal, instead of the detecting circuit  12 . 
     The tracking circuit  14  outputs a tracking signal TR in response to the detection signal  100 . The tracking circuit has a memory  141 , an arithmetic circuit  142  and a comparator  143 . An output from the comparator  143  is stored in the memory  141 . A predetermined arithmetic explained later, is performed by the arithmetic circuit  142  with the data which is stored in the memory  141 . The comparator  143  compares an output from the arithmetic circuit  142  to the detection signal  100 , and outputs the tracking signal TR indicative of which of the signal  100  and the arithmetic circuit output signal, is smaller. 
     The tracking signal TR is stored in the memory  141 , and also is output to one of input nodes of the multiplier  16 . An input terminal  18  is connected to the other input node of the multiplier  16 . A voice signal  104  which is demodulated by the FM receiver is input to the input terminal  18 . The level of the voice signal  104  is multiplied by the level of the tracking signal TR in the multiplier  16 , then the multiplier  16  outputs the result to an output terminal  10  as a voice signal  106 . It is possible to use an alternative circuit which transforms a level of the voice signal  106  to a value proportional to the tracking signal TR, in stead of the multiplier  16 . 
     Each step shown in the flow chart in  FIG. 1  is explained below with reference to the control circuit  10  shown in FIG.  2 . 
     The received RF signal strength of the FM receiver which is loaded in a cellular phone or a car phone changes intensively a wide level range. Referring to  FIG. 3 , the received RF signal strength R of the received signal is measured along the vertical axis, and time is measured along the horizontal axis. The received RF signal strength R which is detected by the detecting circuit  12  is input to the tracking circuit  14  as the detection signal  100 . Then, the tracking signal TR is formed every cycle T 0  in response to the received RF signal strength R. The tracking signal is input to the multiplier  16 . The level of the voice signal  104  from the input terminal  18  is multiplied by the level of the tracking signal TR in the multiplier  16 ; then the multiplier  16  outputs the result to the output terminal  20 . The voice signal  106  is transformed in response to the tracking signal TR. 
     The initial value TR 0  of the tracking signal TR is preset in the memory  141 . A constant δ which has a very small value such as 0.01 is preset in the arithmetic circuit  142  (S  101  in FIG.  1 ). The initial value TR 0  is greater than the maximum value of the received RF signal strength R. 
     The tracking signal TR (t) is found every cycle T 0  by the following Equation (1): 
       TR ( 0 )= TR   0 
 
 TR ( t )=min ( R ( t ), (1+δ)× TR ( t−T   0 ))  (1)
 
where R(t) is the received RF signal strength R at the time t, and TR(t) is the tracking signal TR at the time t.
 
     That is, the level of the tracking signal TR which is found by Equation (1) and which is stored in the memory  131  is multiplied by (1+δ) in arithmetic circuit  142 . The comparator  143  compares (1+δ)×TR, which is the output from the arithmetic circuit  142 , to the current received RF signal strength R then outputs which is smaller of these as the tracking signal TR to the multiplier  16  (S  102  in FIG.  1 ). The output from the comparator  143  is also stored in the memory  141 ; then it is used for forming a tracking signal TR after cycle T 0 . The level of the voice signal  104  is multiplied by the level of the tracking signal TR in the multiplier  16 , then the multiplier  16  outputs the result to an output terminal  10  as a voice signal  106  (S  103  in FIG.  1 ). 
     Referring to  FIG. 4 , the received RF signal strength R (dBm) is shown in solid line, and the level of the tracking signal TR (dB) is shown in broken line. The change of the tracking signal is explained below. 
     If the received RF signal strength R increases, the received RF signal strength R(t) is greater than the received RF signal strength R (t−T 0 ). As the constant δ is very small, the received RF signal strength R(t) is greater than (1+δ)×TR(t−T 0 ) (=(1+δ)×R(t−T 0 )). From Equation (1), (1+δ)×TR(t−T 0 ) (=(1+δ)×R(t−T 0 )) is selected as the tracking signal TR(t). 
     After the cycle T 0  is passed since the time t, the tracking signal TR (t+T 0 ) can be found by Equation (1). As a result of the calculation, the tracking signal TR (t+T 0 ) is (1+δ)×TR(t) (=(1+δ)×R(t)). Accordingly, if the received RF signal strength increases, the tracking signal TR which is found by Equation (1) increases at the rate δ gradually constantly. In  FIG. 4 , the tracking signals TR, which are increased gradually in certain periods with the constant rate δ, are shown as lines A 1  and A 3 . 
     If the received RF signal strength decreases, the received RF signal strength R(t) is smaller than the received RF signal strength R (t−T 0 ). As the constant δ is very small, the received RF signal strength R(t) is smaller than (1+δ)×TR(t−T 0 ) (=(1+δ)×R(t−T 0 )). From Equation (1), R(t) is selected as the tracking signal TR(t). 
     After the cycle T 0  is passed since the time t, the tracking signal TR (t+T 0 ) can be found by Equation (1). As a result, R(t+T 0 ) is selected as the tracking signal TR (t+T 0 ). Accordingly, if the received RF signal strength decreases, the tracking signal TR which is found by Equation (1) is equal to the received RF signal strength R. In  FIG. 4 , the tracking signal TR decreasing together with the decrease of the received RF signal strength R in a certain period, is shown as a line A 2   
     If the received RF signal strength R reaches a minimum value at the time t−T 0 , and then increases, the received RF signal strength R(t) is greater than the received RF signal strength R(t−T 0 ) and the tracking signal TR (t−T 0 ) is equal to the received RF signal strength R(t−T 0 ). That is, as the received RF signal strength R(t) is greater than (1+δ)×TR(t−T 0 ) (=(1+δ)×R(t−T 0 )), the tracking signal TR(t) is equal to (1+δ)×TR(t−T 0 ). After that, as mentioned above, the tracking signal TR increases at the rate δ gradually and constantly. Referring to  FIG. 4 , the received RF signal strength R reaches a minimum value at the time t 2 , and then increases. The tracking signal TR which increases gradually after the time t 2  in a certain period is shown as a line A 3 . 
     If the received RF signal strength R is changes to decreasing from increasing, the tracking signal TR increases at the rate δ gradually and constantly while the received RF signal strength R increases. If the received RF signal strength reaches a maximum value and then begins to decreasing, the tracking signal TR still increases at the rate δ constantly as long as the received RF signal strength R(t) remains greater than (1+δ)×TR(t−T 0 ). When the received RF signal strength R(t) becomes smaller than (1+δ)×TR(t−T 0 ), the tracking signal TR(t) becomes equal to the received RF signal strength R(t) in accordance with Equation (1). After that, the tracking signal TR decreases together with the decrease of the received RF signal strength R. In  FIG. 4 , the received RF signal strength is reached to the maximum value at time (t 1 ), and then it decreases. The tracking signal TR decreases a little behind the decrease of the received RF signal strength R. 
     In the first embodiment mentioned above, as the track signal TR is reduced when the received RF signal strength R of the received signal is reduced, the noise superimposed on the voice signal  106  is reduced, because the level of the voice signal  104  is multiplied by the level of the tracking signal TR. Further, as the tracking signal is reduced quickly together with a rapid decrease of the received RF signal strength R, the noise superimposed on the voice signal  104  is also reduced quickly. Furthermore, since the tracking signal TR is gradually increased when the received RF signal strength increases, the level of the voice signal  106  smoothly increases without breaking it up. It is possible to use Equation (2) set forth below to determine the tracking signal TR. In this case, the output from the memory and the constant δ are added in the arithmetic circuit  142 , and the result of the addition is then output from the arithmetic circuit  142 .
 
 TR ( 0 )= TR   0 
 
 TR ( t )=min ( R ( t ), ( TR +δ))  (2)
 
     A second embodiment of the invention is explained below. Although the operations of an arithmetic circuit and a comparator of the second embodiment differ from the arithmetic circuit  142  and the comparator  143  of the first embodiment, the components of a level-controlling circuit carrying out the second embodiment of the invention are the same as those of the level-controlling circuit  10  for carrying out the first embodiment. Therefore, the same reference numbers are used for the explanation of the second embodiment. 
     An initial value TR 0  of a tracking signal TR is preset in a memory  141 . The arithmetic circuit  142  has a counter CNT for counting a hang-over-period C. The tracking signal TR is kept at a predetermined value for the hang-over-period C to illuminate a jarring sound caused by small level-changes, and 5-8 msec is set to the hang-over-period C. An initial value of the counter CNT is set to zero. The initial value TR 0  is set to a value which is greater than the maximum value of a received RF signal strength R. Referring to  FIG. 5 , the tracking signal TR can be found at the end of every cycle T 0  by performing steps S 500  through S 510 . 
     In step S 500 , the arithmetic circuit  142  judges whether the counter CNT indicates zero or not. If the counter CNT indicates zero, a constant δ in the arithmetic circuit  142  is set to a very small value such as 0.01 by the arithmetic circuit  142  (Step S 502 ). If the counter CNT does not indicate zero, a constant δ is set to zero by decrementing the counter CNT(Step S 504 ). This means that one cycle (T 0 ) in the hang-over-period C has passed. 
     In step S 506 , the tracking signal TR(t) can be found by Equation (1). The level of the tracking signal TR(t−T 0 ) stored in the memory  141  is multiplied by (1+δ) in the arithmetic circuit  142 . In step S 508 , the comparator  143  compares (1+δ)×TR(t−T 0 ) which is the output from the arithmetic circuit  142  with the received RF signal strength R(t), the tracking circuit  14  outputs the smaller of these as the tracking signal TR. The output from the comparator  143  is stored in the memory  141 , and is then used for forming a tracking signal TR after the cycle T 0 . If the received RF signal strength R(t) is selected by the comparator  143 , the counter CNT is reset to the hang-over-period C(Step S 510 ). 
     Referring to  FIG. 6 , the received RF signal strength R(dBm) is shown in solid line, and the tracking signal TR (dB) is shown in broken line. The change of the tracking signal is explained below. 
     At the initial status, step S 502  follows S 500  because the counter CNT indicates zero. The constant δ is set to a predetermined value. In step S 506 , (1+δ)×TR(t−T 0 ) is compared with R(t). After that, as the following relationship is carried out at the initial status, R(t) is selected.
 
(1+δ)× TR ( t−T   0 )&gt; R ( t )
 
In step S 508 , as the tracking signal TR is equal to the received RF signal strength R, step S 510  is performed after step S 508 . That is, the hang-over-period C is set in the counter.
 
     After the cycle T 0  has passed, the counter CNT is in the hang-over-period C, not zero in step S 500 . Therefore, the next step following step S 500  is step  504 . The counter CNT is decremented, and the constant δ is set to zero. In the next step S 506 , (1+δ)×TR(t) is equal to R(t). The different operation will be performed in the consequent steps by the variation of the received RF signal strength R. 
     If the received RF signal strength R increases, the received RF signal strength R(t+T 0 ) is greater than the received RF signal strength R (t). Therefore, the received RF signal strength R (t+T 0 ) is greater than the tracking signal TR (t) (=R(t)). Accordingly, the tracking signal TR (t) is selected as the tracking signal TR (t+T 0 ) from Equation (1) (Step S 506 ). Next, the tracking signal TR (t+T 0 ) is compared with the received RF signal strength R (t+T 0 ) in step S 508 . As they are not equal, step S  510  is not performed. As mentioned in step S 504 , the counter CNT is kept in the decremented status. After that, the operations of step S 500  and steps S 504  through S 510  are repeatedly performed every cycle T 0  being passed. The counter CNT decreases by one. During this period, the tracking signal TR is not changed. Referring to  FIG. 6 , the tracking signal TR which is not changed in the certain period is shown as a line B 1 . 
     After that, supposing the counter CNT becomes zero at the time (t 3 ) by increasing the received RF signal strength R, step S 502  is performed at the time (t 3 ) and The constant δ is set to the predetermined value. In step S 506 , the tracking signal TR (t 3 ) is found by Equation (1). The received RF signal strength R (t 3 ) is greater than the tracking signal TR (t 3 −T 0 ) at the time (t 3 −T 0 ). Accordingly, the tracking signal TR (t 3 ) becomes (1+δ)×TR(t 3 −T 0 ). Next, the tracking signal TR (t 3 ) is compared with the received RF signal strength R (t 3 ) in step S 508 . As they are not equal, step S  510  is not performed. The counter CNT is kept in zero. After that, the operations of steps S 500 , S 502 , S 506  and S 508  are repeatedly performed every cycle T 0  being passed. During this period, the tracking signal increases with at rate δ constantly. Referring to  FIG. 6 , the tracking signal TR which increases at constant rate δ gradually is shown as a line B 4 . 
     If the received RF signal strength decreases, the received RF signal strength R(t+T 0 ) is smaller than the received RF signal strength R (t) and the tracking signal TR(t) is equal to the received RF signal strength R(t). Therefore, the tracking signal TR (t+T 0 ) is equal to the received RF signal strength R (t+T 0 ) from Equation (1). Next, the tracking signal TR (t+T 0 ) is compared with the received RF signal strength R (t+T 0 ) in step S 508 . As they are equal, step S  510  is performed, and then the counter is set in the hang-over-period C. After that, the operations of steps S 500  and S 504  and steps S 506  through S 510  are repeatedly performed every cycle T 0  being passed. The tracking signal TR decreases together with the decrease of the received RF signal strength R. Referring to  FIG. 6 , the tracking signal TR which decreases together with the received RF signal strength R is shown as a line B 2 . 
     In the second embodiment mentioned above, as the track signal TR is reduced when the received RF signal strength R is reduced, the noise superimposed on the voice signal  106  is reduced because the level of the voice signal  104  is multiplied by the level of the tracking signal TR. Further, as the tracking signal is reduced quickly together with the rapid decrease of the received RF signal strength R, the noise superimposed on the voice signal  104  is reduced quickly. Furthermore, as the tracking signal TR increases gradually after the hang-over-period C has passed when the received RF signal strength is change to increase, the voice signal  106  is smoothly increased without breaking it. It is possible to use Equation (2) instead of Equation (1) in step S 506  to get the same benefits. 
     The feature of the third embodiment is to add a step for forming a tracking signal which interpolates the differences between the first minimum value of the received RF signal strength and the next minimum value during the hang-over-period C, to the second embodiment. 
     Referring to  FIG. 7 , a level-controlling circuit  60  of the third embodiments has a detecting circuit  12 , a multiplier  16 , input terminal  18 , a delay circuit  66  which is connected between the input terminal  18  and the multiplier  16 , and a circuit  64  which is connected between the detecting circuit  12  and the multiplier  16 . The detecting circuit  12  and the multiplier  16  in  FIG. 2  are used for the third embodiment. The voice signal  104  from the input terminal  18  is delayed for the hang-over-period C by the delay circuit  66  to output. 
     The circuit  64  has a tracking circuit  680 , a delay circuit  681 , a tracking circuit  682 , a storing circuit  683  and a switch  684 . One of two outputs from the delay circuit  681  and form the storing circuit are selected by the switch  684 , and then the selected output is input to the multiplier  16 . If a control signal  609  from the tracking circuit  682  is zero, the output from the delay circuit  681  is input to the multiplier  16  as the tracking signal TR. If a control signal  609  from the tracking circuit  682  is one, the output from the storing circuit  683  is input to the multiplier  16  as the tracking signal TR. 
     Referring to  FIG. 8 , the tracking signal TR 1  which is shown in broken line is formed by the tracking circuit  680  when the detection signal  100  which shows the received RF signal strength R shown in solid line is input to the tracking circuit  680 . The tracking signal TR 1  is delayed for the hang-over-period C by the delay circuit  681 , and then it is input to the switch  684 . The tracking circuit  680  is the same as the tracking circuit mentioned in the second embodiment. 
     The tracking signal TR 2  shown in a single dash line, which interpolates the differences between minimum values, is formed when the detection signal  100  which shows the received RF signal strength R shown in solid line is input to the tracking circuit  682 . The tracking circuit  682  has a supplemental counter for counting the passing time from the time when the minimum value is detected. The predetermined pulse, which is synchronized with the cycle T 0  of the tracking circuit  680 , is counted by the supplement counter. The minimum value of the detection signal  100  is watched by the tracking circuit  682 . The tracking circuit is restarted every detection of the minimum value of the detection signal  100  by resetting the supplemental counter. 
     Referring to  FIG. 8 , the operation of the tracking circuit  682  is explained below. 
     When the tracking circuit  682  detects that the received RF signal strength R (t 2 ) is the minimum value, the tracking signal TR 2  (t 1 ) . . . TR 2  (t 2 ) is calculated by Equation (3) mentioned below using the minimum value R (t) which is stored in the internal memory and a value N 2  (=t 2 −t 1 ) of the supplemental counter. The tracking signal TR 2  (t 1 ) . . . TR 2  (t 2 ) is stored in the storing circuit  683 . After the calculation is completed, the minimum value R (t 1 ) which is stored in the memory of the tracking circuit  682  is replaced to the minimum value R (t 2 ). These operations are competed before the next minimum value is detected.
 
 R=R   1 +( R   2 − R   1 )× n/N   2   (3)
 
( N =0, 1, 2 , . . . , N 2 ) 
 
The tracking circuit  682  judges whether the value N 2  of the supplemental counter is greater than the predetermined hang-over-period C or not. In this case, since the predetermined hang-over-period C is greater than the value N 2 , the control signal  690  from the tracking circuit  682  which is input to the switch  684  becomes one after the hang-over-period C has been passed from the time (t 2 ). Then, the tracking signal TR 2  (t 1 ) . . . TR 2  (t 2 ) which is stored in the storing circuit  683  is output from the switch  684  as the tracking signal TR, and it is input to the multiplier  16 . Referring to  FIG. 8 , the tracking signal TR 2  (t 1 ) . . . TR 2  (t 2 ) is shown in the single dash line. However, the tracking signal is delayed for the hang-over-period C, practically. The tracking signal TR 2  (t 2 ) . . . TR 2  (t 3 ), which interpolates the difference between the minimum value R (t 2 ) and the minimum value R (t 3 ), is also formed.
 
     In the case that the next minimum value is not detected after the hang-over-period C has been passed from the time when the minimum vale was detected, that is, the value of the supplemental counter becomes the hang-over-period C, the control signal  690  from the tracking circuit  682 , which is input to the switch  684 , is changed from one to zero. Then, the formation of the tracking signal TR 2  is halted. The output from the delay circuit  681  is input to the multiplier  16  as the tracking signal TR through the switch  684 . 
     A switching signal  695  is output to the tracking circuit  682  from the tracking circuit  680  when the tracking signal TR 1  is changed from the signal which increases at the rate δ constantly to the signal which decreases together with the decrease of the received RF signal strength R. When the first minimum value of the received RF signal strength R is detected after receiving the switching signal, the tracking circuit  682  stores the its minimum value in the memory, and resets the supplemental counter. The supplemental counter restarts to count, and the tracking signal which interpolates the difference the minimum values is formed after that. 
     Referring to  FIG. 9 , the tracking signal TR from the switch  684  is shown in the broken line. In this drawing, the received RF signal strength R is tilted to the right side with the hang-over-period C from the actual plots. 
     The voice signal  104  from the input terminal  18  is delayed for the hang-over-period C by the delay circuit  66 . A phase of the voice signal  606  which is the output from the delay circuit  66  is identical to a phase of the tracking signal TR. The level of the voice signal  606  is multiplied by the level of the tracking signal TR in the multiplier  16 , then the result of the multiplication is output to the output terminal  20  as the voice signal  608 . 
     According to the third embodiment, the tracking signal is smoothly changed because the minimum value is interpolated between the minimum values. Therefore, the voice signal is smoothly changed. 
     In the fourth embodiment, step for filtering the voice signal  104  is added instead of step S 103  which is explained in the FIG.  1 . Referring to  FIG. 10 , a level-controlling circuit  90  has a filter  96  instead of the multiplier  16  which is mentioned in FIG.  2 . The detecting circuit  12  and the tracking circuit  14  are the same as what are used in the first and second embodiments. The filter  96  can control a gain and a passing band. 
     Referring to  FIG. 11 , this frequency characteristic can be obtained by a linear infinite impulse response (IIR) low band passing filter. A transfer function H (z) is obtained by Equation (4)
 
 H ( z )=(1−δ)×(1 −δ×z   −1 )  (4)
 
(0≦δ≦1)
 
     The frequency characteristics of this filter is changed to the direction indicated by an arrow in consequence of the change of the coefficient σ from one to zero. The gain is fallen when the value of the coefficient σ is getting smaller. Specifically, the gain in the high band is dramatically fallen comparing to the gain in the low band. The noise of the high band constituent of the voice signal  104  can be reduced by controlling the coefficient σ following to the tracking signal TR. 
     Referring to  FIG. 11 , this frequency characteristic can be obtained by a quadratic IIR filter. A transfer function H (z) is obtained by Equation (5)
 
 H ( z )=σ 8 (1−1.764×σ −1   ×z   −1 +0.873×σ −2   ×z   −2 )/(1−1.764× z   −1 +0.837× z   −2 )  (5)
 
(0≦σ≦1)
 
     The frequency characteristic of this filter resembles a spectrum of the voice signal. Specifically, the reduction ratio around 500 Hz in which plenty of the information is included is small, and the reduction ratio in the other band is large. The frequency characteristic of this filter is changed to the direction indicated by an arrow in  FIG. 12  in consequence of the change of the coefficient σ from one to zero. The noise of the high band constituent of the voice signal  104  can be reduced by controlling the coefficient σ following to the tracking signal TR. 
     It is possible to use a finite impulse Response type filter as the filter  96  other than two filters mentioned above. It is also possible to use a combination of the IIR type filter and the FIR type filter to compose the filter  96 . As the degree and the coefficient of the filter mentioned above is one of the examples to carry out the invention, it is further possible to use the combination of other degree and coefficient. It is furthermore possible to use the filter  96  mentioned in the fourth embodiment as the replacement of the multiplier  16  in the level-controlling circuit  60  in the third embodiment described in FIG.  7 . 
     According to the invention, the level of the voice signal is controlled by the tracking signal which changes smoothly the deference between the minimum values of the received RF signal strength. Therefore, the level-controlling circuit of the invention has smooth noise reduction property. The level-controlling circuit is specifically used for the FM mode receiver such as FM mode cellular phone or a car FM receiver to obtain the benefit mentioned above. 
     While the invention has been described with reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications of the illustrated embodiments, as well as other embodiments of the invention, will be apparent to those skilled in the art on reference to this description. It is therefore contemplated that the appended claims will cover any such modifications or embodiments as fall within the true scope of the invention.