Patent Publication Number: US-9432765-B2

Title: Condenser microphone

Description:
TECHNICAL FIELD 
     The present invention relates to a condenser microphone that can adjust an input level to an impedance converter without deterioration of the signal-to-noise ratio of an output signal. 
     BACKGROUND ART 
     A condenser microphone includes a condenser microphone unit functioning as an electro-acoustic transducer having high impedance and thus needs an impedance converter including, for example, a field-effect transistor (hereinafter referred to as “FET”). An impedance converter needs an operation power source. The voltage of the operation power source limits the maximum output level of the condenser microphone. This causes distortion of an output signal when the acoustic pressure of sound waves inputted to the condenser microphone unit is so high as to exceed the maximum output level of the unit. 
     Such distortion of an output signal is prevented using an attenuator called a “pad” attenuating the input level to the impedance converter. The pad includes a capacitor connected in parallel to the condenser microphone unit and attenuates the input signal level of the impedance converter in response to the ratio of capacitance of the capacitor to that of the condenser microphone unit. This can prevent an excessive input to the impedance converter. 
     Meanwhile, the impedance converter generates inherent noise. The noise level is constant independent of an input signal level. This decreases the signal-to-noise ratio of a condenser microphone output when the pad is used to attenuate the input level to the impedance converter for prevention of distortion due to an excessive input. 
     A conventional condenser microphone is known that prevents a decrease in the signal-to-noise ratio by converting an unbalanced output of the impedance converter into a balanced output to reduce the distortion of an audio signal outputted from the impedance converter (for example, see Japanese Unexamined Patent Application Publication No. 2006-101302). 
     SUMMARY OF INVENTION 
     Technical Problem 
     A condenser microphone described in Japanese Unexamined Patent Application Publication No. 2006-101302 can offset the second-order distortion of the impedance converter to reduce output distortion. A pad is however necessary for inputted sound waves having excessive acoustic pressure above the maximum output level, and thus cannot avoid a decrease in the signal-to-noise ratio. 
     As illustrated in  FIG. 9 , another conventional condenser microphone is also known that can switch the function (active or inactive) of the pad in response to the amplitude of an input level. A condenser microphone  100  as illustrated in  FIG. 9  includes a first condenser microphone unit  21 , a second condenser microphone unit  22 , a first impedance converter  31 , a second impedance converter  32 , and a pad  40 . The pad  40  includes a first capacitor  41 , a second capacitor  42 , and two switches  43  and  43  activating or deactivating the capacitors  41  and  42 . 
     The condenser microphone unit  21  includes a fixed electrode  212  connected to the input terminal of the impedance converter  31 . The first capacitor  41  for the pad is connected in parallel to the condenser microphone unit  21  through the single switch  43 . The condenser microphone unit  22  includes a diaphragm  221  connected to the input terminal of the impedance converter  32 . The second capacitor  42  for the pad is connected in parallel to the condenser microphone unit  22  through the single switch  43 . The condenser microphone unit  21  has a diaphragm  211  grounded while the condenser microphone unit  22  has a fixed electrode  222  grounded. 
     The first and second capacitors  41  and  42  can be connected or disconnected by one of the switches  43  to turn on or off the pad including the capacitors  41  and  42 . If a low acoustic pressure level of sound waves is inputted to the condenser microphone  100 , the switch  43  is turned off to deactivate the pad  40 . If a high acoustic pressure level of sound waves is inputted, the switch  43  is turned on to activate the pad  40 . The pad  40  is appropriately activated or deactivated by a user operation of the switch  43 . 
     Using a pad  40  including multiple switches and capacitors having different capacitances, changes in capacitances of these capacitors by the switches can provide a gradual attenuation of the input signal level to the impedance converter. Unfortunately, this configuration cannot provide a continuously variable attenuation value. Since a signal is not amplified by the impedance converter, the signal-to-noise ratio cannot be maintained at a low level even if the input level to the impedance converter is low. 
     It is an object of the present invention to provide a condenser microphone that can function as a pad for excess acoustic pressure and continuously increase the signal level in response to a low input level without a variation in signal-to-noise ratio. 
     Solution to Problem 
     A condenser microphone according to the present invention includes a first condenser microphone unit and a second condenser microphone unit that generate output signals having phases opposite to each other; and a twin variable resistor connected to an output terminal of each of the first condenser microphone unit and the second condenser microphone unit. In the condenser microphone, the output signal of the first condenser microphone unit is combined with the output signal of the second condenser microphone unit in a first variable resistor included in the twin variable resistor and the composite signal is applied to an input of the first condenser microphone unit, and the output signal of the first condenser microphone unit is combined with the output signal of the second condenser microphone unit in a second variable resistor included in the twin variable resistor and the composite signal is applied to an input of the second condenser microphone unit. 
     Advantageous Effects of Invention 
     A condenser microphone according to the present invention can provide a continuously variable attenuation value without a variation in signal-to-noise ratio even when the input level to the impedance converter is attenuated. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a circuit diagram illustrating a condenser microphone according to an embodiment of the present invention. 
         FIG. 2  is a circuit diagram illustrating an operation when each wiper of a twin variable resistor is shifted toward one terminal of the variable resister in the embodiment. 
         FIG. 3  is a circuit diagram illustrating another operation when each wiper of the twin variable resistor is shifted toward one terminal of the variable resister in the embodiment. 
         FIG. 4  is a circuit diagram illustrating an operation when each wiper of the twin variable resistor is shifted toward the other terminal of the variable resister in the embodiment. 
         FIG. 5  is a circuit diagram illustrating another operation when each wiper of the twin variable resistor is shifted toward the other terminal of the variable resister in the embodiment. 
         FIG. 6  is a graph illustrating frequency response characteristics when each wiper of the twin variable resistor is positioned at the middle point in the embodiment. 
         FIG. 7  is a graph illustrating frequency response characteristics when each wiper of the twin variable resistor is positioned on the positive end in the embodiment. 
         FIG. 8  is a graph illustrating frequency response characteristics when each wiper of the twin variable resistor is positioned on the negative end in the embodiment. 
         FIG. 9  is an example circuit diagram illustrating a conventional condenser microphone. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     A condenser microphone according to an embodiment of the present invention will now be described with reference to the accompanying drawings. With reference to  FIG. 1 , a condenser microphone  10  includes a first condenser microphone unit  11 , a second condenser microphone unit  12 , a first impedance converter  13 , a second impedance converter  14 , and a twin variable resistor  15 . 
     The first condenser microphone unit  11  includes a diaphragm  111  and a fixed electrode  112 . The diaphragm  111  and the fixed electrode  112  are separated by a spacer (not illustrated) defining a predetermined gap and are accommodated in a microphone case (not illustrated). The second condenser microphone unit  12  includes a diaphragm  121  and a fixed electrode  122 . The diaphragm  121  and the fixed electrode  122  are separated by a spacer (not illustrated) defining a predetermined gap and are accommodated in the microphone case (not illustrated). 
     The first and second impedance converters  13  and  14  each include an FET as an active element for impedance conversion. In the first condenser microphone unit  11  functioning as a fixed electrode output, the fixed electrode  112  as an output terminal is connected to the gate terminal of the FET as an input terminal of the first impedance converter  13 . In the second condenser microphone unit  12  functioning as a diaphragm output, the diaphragm  121  as an output terminal is connected to the gate terminal of the FET as an input terminal of the second impedance converter  14 . 
     The output signal of the first condenser microphone unit  11  is outputted from the drain terminal of the FET as an output terminal of the first impedance converter  13 . The output signal of the second condenser microphone unit  12  is outputted from the drain terminal of the FET as an output terminal of the second impedance converter  14 . The first condenser microphone unit  11  functions as a fixed electrode output while the second condenser microphone unit  12  functions as a diaphragm output. In other words, the output signal of the first condenser microphone unit  11  has a phase opposite to that of the output signal of the second condenser microphone unit  12 . The condenser microphone  10  thus generates a balanced output. As illustrated in  FIG. 1 , the output terminal of the first impedance converter  13  functions as a hot terminal of the balanced output while the output terminal of the second impedance converter  14  functions as a cold terminal of the balanced output. 
     The variable resistor  15  is connected between the output terminals of the first and second impedance converters  13  and  14 . The twin variable resistor  15  includes a first variable resistor  151  and a second variable resistor  152 . The first variable resistor  151  includes a wiper  153  interlocked with a wiper  154  of the second variable resistor  152 . 
     The first variable resistor  151  is connected between the output terminal of the first impedance converter  13  (the output terminal for the first condenser microphone unit  11 ) and the output terminal of the second impedance converter  14  (the output terminal for the second condenser microphone unit  12 ). The second variable resistor  152  is connected between the output terminal of the second impedance converter  14  (the output terminal for the second condenser microphone unit  12 ) and the output terminal of the first impedance converter  13  (the output terminal for the first condenser microphone unit  11 ). 
     Hereinafter, a terminal of the first variable resistor  151  adjacent to the first impedance converter  13  is referred to as a “first terminal  21 ”, a terminal of the first variable resistor  151  adjacent to the second impedance converter  14  to as a “second terminal  22 ”, a terminal of the second variable resistor  152  adjacent to the second impedance converter  14  to as a “third terminal  23 ”, and a terminal of the second variable resistor  152  adjacent to the first impedance converter  13  to as a “fourth terminal  24 ”. Hereinafter, the first and third terminals  21  and  23  are designated with a “positive end” while the second and fourth terminals  22  and  24  are designated with a “negative end”. 
     The first terminal  21  of the first variable resistor  151  is connected to the fourth terminal  24  of the second variable resistor  152  while the second terminal  22  of the first variable resistor  151  is connected to the third terminal  23  of the second variable resistor  152 . 
     The “interlock” between the first and second wipers  153  and  154  will be described below. The shift of the first wiper  153  toward the first terminal  21  leads to the shift of the second wiper  154  toward the third terminal  23 . The shift of the first wiper  153  toward the second terminal  22  leads to the shift of the second wiper  154  toward the fourth terminal  24 . The shift of the second wiper  154  toward the third terminal  23  leads to the shift of the first wiper  153  toward the first terminal  21 . The shift of the second wiper  154  toward the fourth terminal  24  leads to the shift of the first wiper  153  toward the second terminal  22 . In this way, the first and second variable resistors  151  and  152  constitute a twin variable resistor including the first and second wipers  153  and  154  shifted in cooperation with each other. 
     The first wiper  153  is connected to the diaphragm  111  of the first condenser microphone unit  11 . The second wiper  154  is connected to the fixed electrode  122  of the second condenser microphone unit  12 . 
     The diaphragm  111  of the first condenser microphone unit  11  is supplied with a composite signal including output signals of the first and second condenser microphone units  11  and  12  through the first wiper  153 . The output signal of the first condenser microphone unit  11  has a phase opposite to that of the output signal of the second condenser microphone unit  12 . Resistance values on the positive end and the negative end of the first variable resistor  151  are determined by a position of the first wiper  153  and affect the output signal of the first and second condenser microphone units  11  and  12 . Output signal levels, which have phases opposite to each other, of the first and second condenser microphone units  11  and  12  are determined depending on the position of the first wiper  153 . The output signals having phases opposite to each other are combined. 
     Similarly, the fixed electrode  122  of the second condenser microphone unit  12  is supplied with a composite signal including output signals of the first and second condenser microphone units  11  and  12  through the second wiper  154 . As described above, the output signal of the second condenser microphone unit  12  has a phase opposite to that of the output signal of the first condenser microphone unit  11 . Resistance values, which affect the respective output signals, on the positive end and the negative end of the second variable resistor  152  are determined by the position of the second wiper  154 . Output signal levels, which have phases opposite to each other, of the first and second condenser microphone units  11  and  12  are determined depending on a position of the second wiper  154 . The output signals having phases opposite to each other are combined. 
     As illustrated in  FIG. 1 , when the first and second wipers  153  and  154  are positioned at the middle points of the first and second variable resistors  151  and  152 , respectively, the first variable resistor  151  provides the same resistance values for respective output signals from the first and second condenser microphone units  11  and  12 . The output signal from the first condenser microphone unit  11  therefore offsets the output signal from the second condenser microphone unit  12 . As a result, a composite signal does not flow through the first wiper  153 . This does not supply the diaphragm  111  with either an in-phase signal or an opposite phase signal relative to the output signal of the first condenser microphone unit  11 . 
     Similarly, the second variable resistor  152  provides the same resistance values for respective output signals from the second and first condenser microphone units  12  and  11 . The output signal from the second condenser microphone unit  12  therefore offsets the output signal from the first condenser microphone unit  11 . As a result, a composite signal does not flow through the second wiper  154 . This does not supply the fixed electrode  122  with either an in-phase signal or an opposite phase signal relative to the output signal of the second condenser microphone unit  12 . 
     In this way, when the first and second wipers  153  and  154  are positioned at the middle points of the first and second variable resistors  151  and  152 , respectively, the output signals from the first and second condenser microphone units  11  and  12  do not either increase or decrease, are inputted to the first and second impedance converters  13  and  14 , and are provided as a balanced output from the hot and cold terminals. 
       FIG. 6  illustrates typical frequency response characteristics of the condenser microphone  10  when the first and second wipers  153  and  154  are positioned at the middle points of the first and second variable resistors  151  and  152 , respectively. 
     The output signal of the condenser microphone  10  will now be explained after the first and second wipers  153  and  154  are shifted from the middle point.  FIGS. 2 and 3  illustrate exemplary states of the condenser microphone  10  after the first and second wipers  153  and  154  are shifted to the positive end. 
     The output signal  16  of the first condenser microphone unit  11  and the output signal  17  of the second condenser microphone unit  12  are combined after being attenuated in proportion to the resistance value of the first variable resistor  151  into a composite signal  161  to be applied to the diaphragm  111  through the first wiper  153 . As illustrated in  FIG. 2 , after the first wiper  153  is shifted to the positive end, the first variable resistor  151  applies a minimum resistance value to the output signal  16  and a maximum resistance value to the output signal  17 . At this time, the composite signal  161  to be applied to the diaphragm  111  from the first wiper  153  contains a maximum proportion of signal component in phase with the output signal  16  of the first condenser microphone unit  11 . This increases the output level from the fixed electrode  112  of the first condenser microphone unit  11 . 
     The output signal  16  of the first condenser microphone unit  11  and the output signal  17  of the second condenser microphone unit  12  are combined after being attenuated in proportion to the resistance value of the second variable resistor  152  into a composite signal  171  to be applied to the fixed electrode  122  through the second wiper  154 . As illustrated in  FIG. 3 , after the second wiper  154  is shifted to the positive end, the second variable resistor  152  applies a maximum resistance value to the output signal  16  and a minimum resistance value to the output signal  17 . At this time, the composite signal  171  to be applied to the fixed electrode  122  from the second wiper  154  contains a maximum proportion of signal component in phase with the output signal  17  of the second condenser microphone unit  12 . This raises the output level from the diaphragm  121  of the second condenser microphone unit  12 . 
     In this way, the shift of the first wiper  153  toward the positive end decreases the resistance value of the first variable resistor  151  in accordance with the output of the first condenser microphone unit  11 , leading to the shift of the second wiper  154  toward the positive end together. This increases the resistance value of the first variable resistor  151  in accordance with the output of the second condenser microphone unit  12 , increases the resistance value of the second variable resistor  152  in accordance with the output of the first condenser microphone unit  11 , and decreases the resistance value of the second variable resistor  152  in accordance with the output of the second condenser microphone unit  12 . That is, a change in the resistance value of the twin variable resistor  15  can continuously increase the levels of the input signal to the first impedance converter  13  and the input signal to the second impedance converter  14 . 
       FIG. 7  illustrates typical frequency response characteristics of the condenser microphone  10  during increases in the output signals of the first and second condenser microphone units  11  and  12 . This drawing illustrates that the output level is about  6  dB higher than that in  FIG. 6  illustrating the frequency response characteristics of the first and second wipers  153  and  154  at the middle position. 
       FIGS. 4 and 5  illustrate exemplary states of the condenser microphone  10  after the first and second wipers  153  and  154  are shifted to the negative ends. As illustrated in  FIG. 4 , after the first wiper  153  is shifted to the negative end, the first variable resistor  151  applies a maximum resistance value to the output signal  16  and a minimum resistance value to the output signal  17 . As a result, the output signal  16  of the first condenser microphone unit  11  is combined at a maximum proportion of the output signal of the second condenser microphone unit  12  having a phase opposite to the output signal  16 . The resultant composite signal  162  is then applied to the diaphragm  111  of the first condenser microphone unit  11  from the first wiper  153  of the first variable resistor  151 . This decreases the output level from the fixed electrode  112  of the first condenser microphone unit  11 . 
     As illustrated in  FIG. 5 , after the second wiper  154  is shifted to the negative end, the second variable resistor  152  applies a minimum resistance value to the output signal  16  and a maximum resistance value to the output signal  17 . At this time, a composite signal  172  to be applied to the fixed electrode  122  from the second wiper  154  is combined at a maximum proportion of output signal of the first condenser microphone unit  11  having phases opposite to the output signal  17  of the second condenser microphone unit  12 . This decreases the output level from the diaphragm  121  of the second condenser microphone unit  12 . 
     In this way, the shift of the first wiper  153  toward the negative end increases the resistance value of the first variable resistor  151  in accordance with the output signal of the first condenser microphone unit  11 , leading to the shift of the second wiper  154  toward the negative end together. This decreases the resistance value of the first variable resistor  151  in accordance with the output signal of the second condenser microphone unit  12 , decreases the resistance value of the second variable resistor  152  in accordance with the output signal of the first condenser microphone unit  11 , and increases the resistance value of the second variable resistor  152  in accordance with the output signal of the second condenser microphone unit  12 . That is, a change in the resistance value of the twin variable resistor  15  can continuously decrease the levels of the input signals to the first impedance converter  13  and the input signal to the second impedance converter  14 . 
       FIG. 8  illustrates typical frequency response characteristics of the condenser microphone  10  during decreases in the output signals of the first and second condenser microphone units  11  and  12 . This drawing illustrates that the output level is about 6 dB lower than that in  FIG. 6  illustrating the frequency response characteristics of the first and second wipers  153  and  154  at the middle position. 
     As mentioned above, if a low level of sound waves is inputted to the condenser microphone  10 , the shift of the first and second wipers  153  and  154  of the variable resistor  15  toward the positive end increases the levels of the input signals to the impedance converters. This increases the levels of the balanced output signals outputted from the hot and cold terminals. A high input signal level to the impedance converter does not vary the noise level inherent in the impedance converter. This can produce a suitable output level without deterioration of the signal-to-noise ratio. 
     Moreover, the twin variable resistor  15  can provide the same advantageous effect as that of conventional pads. The twin variable resistor  15  can decrease the input levels to the first and second impedance converters  13  and  14 , prevent distortion of the balanced output signals outputted from the hot and cold terminals, and produce suitable output signals. 
     According to the condenser microphone  10  in the present embodiment, the twin variable resistor  15  can continuously vary the input signal levels to the first and second impedance converters  13  and  14  within the range of, for example, −6 dB to +6 dB. This can provide a condenser microphone capable of attenuating an excess input signal level and amplifying an excessively low input signal level without a variation in signal-to-noise ratio.