Abstract:
A tone control device for an audio equipment comprises a first mixer amplifier, at least one second mixer, a third mixer and at least one band rejection filter, which are operatively engaged with each other such that a center frequency, Q value and gain thereof can be independently controlled.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a tone control device for an audio equipment. 
     One of the channels of a graphic equalizer, which is a tone control device, can be provided by using a circuit which satisfies the following transmission function T.sub.(s) : ##EQU1## where S is the Laplace operator, ω o  is center angular frequency, Q is sharpness, K 1  is attenuation factor at the angular frequency ω o  and K 2  is amplification factor at the angular frequency ω o . 
     By commonly dividing the numerator and the dominator of the right hand term of equation (1) by S 2  and then multiplying the results of the divisions with a certain common conditional parameters, respectively, the following equation (2) is obtained. ##EQU2## 
     The numerator and the dominator of the right hand term of the equation (2) are divided by a common constant ##EQU3## respectively. The result is as follows: ##EQU4## where B.sub.(s) is a transmissions function of a bandpass filter, which can be represented as follows: ##EQU5## Since T.sub.(s) =e o  /e i , where e i  and e o  are an input voltage and an output voltage of the channel, the followings are obtained: 
     
         x=e.sub.i -xB.sub.(s) K.sub.1 
    
     
         e.sub.o =x+xB.sub.(s) K.sub.2                              ( 5) 
    
     FIG. 1 shows a block diagram of a circuit which satisfies the equations (5) and FIG. 2 shows an example of concrete circuits embodying the block circuit in FIG. 1, in which a reference numeral 10 shows a band pass filter having transmission function B.sub.(s) and 13 and 14 are amplifiers. 
     FIG. 2 shows an example of the conventional tone control device which comprises a plurality (n) of parallel connected bandpass filters each constituted as shown in FIG. 2 to boost or cut the grains thereof at a corresponding number (n) of respective specific frequencies. In FIG. 2, outputs of the bandpass filter 10 1  to 10 n  are connected through variable resistors 12 1  to 12 n  to the amplifiers 13 and 14, respectively. In this conventional tone control device, there is a problem of degradation of signal quality. This problem becomes more severe when the Q values and the center angular frequencies are to be variable. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an improved tone control device by which the problem of the signal quality degradation is overcome. 
     Another object of the present invention is to provide an improved tone control device by which, in addition to the resolution of the signal quality problem, the circuit construction becomes simpler with a minimum number of circuit elements. 
     The above objects can be achieved by utilizing band rejection filter or filters rather than bandpass filters. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of a conventional tone control device having a single channel; 
     FIG. 2 is a circuit diagram of the device in FIG. 1; 
     FIG. 3 is a block diagram of a tone control device according to the present invention; 
     FIG. 4 is a schematic circuit diagram of the device in FIG. 3; 
     FIG. 5 is a circuit diagram of a band rejection filter of the device in FIG. 4; 
     FIG. 6 is a graph showing the frequency characteristics of the device in FIG. 4; 
     FIG. 7 is another embodiment of the present tone control device having a plurality of channels; 
     FIG. 8 is a graph showing the frequency characteristics of the device in FIG. 7, where three channels are involved; 
     FIG. 9 is an equivalent circuit of a portion of the band rejection filter in FIG. 5; and 
     FIG. 10 is another embodiment of the band rejection filter. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 3 shows, in block form, an embodiment of the present invention. The circuit in FIG. 3 satisfies the following equations which satisfy the equation (1): ##EQU6## where E.sub.(s) is a transmission function of a band rejection filter, which is as follows: ##EQU7## 
     FIG. 4 is an example of a concrete circuit embodying the block circuit in FIG. 3, which includes a first mixer amplifier 2, whose output is the functional parameter x, a second mixer amplifier 3, a third mixer amplifier 4 and a band rejection filter 5. The mixer amplifier 2 has an input for receiving an input signal e i  and another input connected to an output of the second mixer amplifier 3. The mixer amplifier 2 functions to mix the input signal e i  and the output of the second mixer 3 and to make the gain K 1  variable by changing the mixing ratio therebetween to provide a &#34;CUT&#34; function. 
     An output of the mixer amplifier 2 is connected to one input of the second mixer amplifier 3 and to an input of the band rejection filter 5 whose output is connected to the other input of the mixer amplifier 3. 
     The output of the mixer amplifier 3 is connected to one input of the third mixer amplifier 4 whose the other input is connected to the output of the filter 5. 
     The mixer amplifier 4 functions to combine the output of the mixer amplifier 3 and hence the amplified output of the mixer amplifier 2 and the output of the filter 5 to make the gain K 2  variable by changing the mixing ratio between the outputs of the mixer amplifier 3 and the filter 5 to provide a &#34;BOOST&#34; function. 
     FIG. 5 shows a circuit of the band rejection filter 5 in FIG. 4. In FIG. 5, the band rejection filter 5 is constituted with a pair of variable resistors R o , a capacitor C o  connected in parallel with the series variable resistors R o , a resistor R 1  &#34; connecting an input terminal of the parallel resistor-capacitor circuit to one input of a differential amplifier 6 whose other input is connected to an output terminal of the parallel resistor-capacitor circuit, a resistor R 2  connecting an output of the differential amplifier 6, a resistor 3 having one end connected to the output of the differential amplifier 6, a variable resistor R Q  having one end connected to the other end of the resistor R 3  and the other end grounded, and a capacitor C 1  having one end connected to a junction of the variable resistors and the other end connected to a slide contact of the variable resistor R Q , which constitutes, together with the latter variable resistors, a T type filter. 
     The center frequency of the T type filter can be independently varied by regulating the values of the variable resistors R o  simultaneously and Q value can also be independently varied by regulating the variable resistor R Q  which determines the positive feedback amount of the differential amplifier 6, as shown in FIG. 6. 
     According to the circuit in FIGS. 5 and 6, it becomes possible to vary the center frequency, the gain and the Q value independently of each other with a minimum number of the circuit elements. 
     For a plurality of n channels for making the frequency characteristics at a plurality of frequencies variable, it may be possible to cascade-connect the corresponding number of the circuits each shown in FIG. 4. It has been found that the cascade connection of the circuits in FIG. 4 is satisfactory for the purpose. In this case, however, another problem arises which is a degradation of the signal in view of the Klirr factor and noise. 
     According to another embodiment of the present invention, a plurality of n channels each shown in FIG. 4 are connected in parallel to minimize the effects of noise and Klirr factor. 
     In FIG. 7, the second embodiment in block diagram is shown, in which the mixer amplifier 2 in FIG. 4 is substituted by a mixer amplifier 20 which has 2n inputs and the mixer amplifier 4 in FIG. 4 is substituted by a mixer amplifier 40 which has 2n inputs, each being engaged with different one of n parallel channels and comprising the band rejection filters 5 i  and the mixer amplifier 3 i  connected as shown in FIG. 7, where i=1, 2 . . . , n. 
     The band rejection filters 5 i  have a transmission function E i (s) which is as follows: ##EQU8## 
     The gain of each channel is attenuated or &#34;CUT&#34; at a frequency f i  (=ωi/2π) when the output of the mixer amplifier 3 i  is amplified with amplification factor K 1  and combined in the mixer amplifier 20, and is increased (&#34;BOOST&#34;) at the frequency when the output of the mixer amplifier 3 i  is amplified with amplification (K 2  +1/i) and combined in the mixer amplifier 40. Furthermore, by changing the center frequency f i  and the value Q i  of the band rejection filter S i , respectively, a desired frequency characteristics can be obtained. 
     The transmission function T.sub.(s) =e o  /e i  of the whole circuit shown in FIG. 7 become as follows. ##EQU9## where K.sub.α is the mixing amplification factor with which the gain of the channel n is reduced and K.sub.β is the mixing amplification factor with which the gain of the channel n is increased. 
     FIG. 8 shows various frequency characteristics a to f of the circuit construction in FIG. 7 where n is 3, which the center frequencies f 1 , f 2  and f 3  of the band rejection filters 5 1 , and 5 2  and 5 3  are 500 Hz, 1 KHz and 2 KHz, respectively, and the Q value of each filter in commonly 3. 
     Table 1 shows the gains at the center frequencies of the respective filters 5 1  to 5 3 . 
     
                       TABLE1______________________________________500 Hz        1KHz          2KHz______________________________________a   12dB.   BOOST     12dB. BOOST   12dB. BOOSTb   12dB.   BOOST     12dB. CUT     12dB. BOOSTc   6dB.    BOOST     6dB.  BOOST   6dB.  BOOSTd   2dB.    CUT       2dB.  BOOST   2dB.  CUTe   6dB.    CUT       6dB.  CUT     6dB.  CUTf   12dB.   CUT       12dB. CUT     12dB. CUT______________________________________ 
    
     According to this embodiment, there is no mutual interference between adjacent channels and the effects of noise and Klirr factor of the whole circuit are much improved thereby, with the advantages of the independent regulation of the center frequencies, the Q values and the gains with the minimum number of circuit element. 
     It may be advisable to provide an impedance transforming buffer amplifier having an input connected to the slide contact of the resistor R Q  and an output connected to one end of the capacitor C 1  having the other end connected to the junction of the variable resistors R o  so that the impedance as seen by capacitor C 1  is zero in order to balance the &#34;T&#34; type bridge circuit. When the provision of such buffer amplifier is undesirable while the effect thereof is still required, the buffer amplifier may be omitted by modifying the filter circuit itself. 
     According to another embodiment of the present invention the T type circuit which is shown in FIG. 5 and is composed of the series resistors R o , capacitor C o  connected in parallel to the series resistors and the capacitor C 1  connected to the junction of the resistors R o  is substituted by an equivalent T type circuit shown in FIG. 9. 
     In FIG. 9, the T type circuit comprises a pair of series connected capacitors C o , a resistor R o  connected in parallel to the series capacitors and a resistor R 1  &#39; connected to the junction of the capacitor C o . According to the present invention in FIG. 10 the resistor R 1  &#39; is eliminated and the function to be performed by the resistor R 1  &#39; is assigned to the feedback resistors R 1  and R 3 . 
     FIG. 10 shows the band rejection filter in FIG. 5 modified according to the usage of the equivalent circuit of the T type circuit in FIG. 9 and the elimination of the feedback determining resistor. 
     The transmission function T.sub.(s) of the circuit in FIG. 10 is as follows: ##EQU10## In order to operate this filter as a band rejection filter, it should be ##EQU11## Therefore 
     
         (1/R.sub.1)+(1/R.sub.3)=(1/R.sub.o) 
    
     
         R.sub.4 /R.sub.2 =2 
    
     By determining the values of R 1 , R 2 , R 3  and R o  such that they satisfy the above relations, the resistor R 1  &#39; to be connected to the junction of the capacitors C o  can be eliminated.