JP2016195351A - Unidirectivity capacitor microphone - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a unidirectivity capacitor microphone in which the S/N is enhanced, by controlling the output level of an audio signal, depending on the correlation value in the audio signal outputted from an impedance converter.

SOLUTION: A unidirectivity capacitor microphone is configured so that an audio signal, added by capacitor microphone units connected in series, is obtained by adding the output from an impedance converter connected with one capacitor microphone unit to other capacitor microphone unit. The unidirectivity capacitor microphone includes lowpass filters 3a-3d for extracting a low band signal from the output of the impedance converter, a correlation detector 5 for obtaining a correlation value by using the low band signal via the lowpass filter, and a level control section 6 for controlling the output level of the audio signal, by a signal corresponding the correlation value from the correlation detector.

SELECTED DRAWING: Figure 1

COPYRIGHT: (C)2017,JPO&INPIT

Description

  The present invention relates to a unidirectional condenser microphone having a signal-to-noise ratio (S / N) improved by obtaining a signal corresponding to a correlation value from the sound output of the condenser microphone and controlling the output level.

The condenser microphone has a condenser microphone unit including a diaphragm and a fixed pole disposed opposite to each other, and an audio signal output unit including an impedance converter.
The unidirectional condenser microphone includes a front acoustic hole for directly applying sound waves coming from a sound source to the front side of the diaphragm, and a rear acoustic hole for acting on the back side of the diaphragm. .

  In a small condenser microphone unit having a diameter (diameter) of, for example, 20 mm or less, the distance between the front acoustic hole and the rear acoustic hole is short. That is, the distance between the acoustic terminals (front acoustic terminal and rear acoustic terminal) appearing in the vicinity of the front and rear acoustic holes, which is the center position of the air that effectively applies sound pressure to the microphone unit, is shortened. Therefore, such a small microphone unit has a stable directional frequency response up to a high frequency range. However, in a microphone having a small microphone unit, its effective capacitance is small and the inherent noise cannot be reduced. Therefore, it is difficult to improve the S / N of such a microphone.

  On the other hand, the sensitivity and S / N of a large condenser microphone unit whose diameter exceeds 20 mm, for example, are excellent because the area of the diaphragm is large. However, since the front acoustic hole and the rear acoustic hole are separated from each other, the distance between the acoustic terminals of the front acoustic terminal and the rear acoustic terminal appearing in the vicinity thereof is increased. Therefore, compared with a small microphone unit, the high frequency response is inferior.

  Therefore, the present applicant has previously proposed a condenser microphone that can secure sensitivity and S / N while securing a stable directional frequency response up to a high frequency range using a small condenser microphone unit. . This is disclosed in Patent Documents 1 and 2.

  That is, the condenser microphone disclosed in Patent Document 1 employs means for increasing the effective capacity and reducing the intrinsic noise level by connecting a plurality of small unidirectional condenser microphone units in parallel.

  In addition, the condenser microphone disclosed in Patent Document 2 employs a configuration in which the output of each impedance conversion unit of one condenser microphone unit is added to another condenser microphone unit so that the outputs of the microphone units are connected in series. . Accordingly, it is possible to provide a condenser microphone with improved sensitivity and improved S / N.

Japanese Patent No. 4336256 Japanese Patent No. 5201598

  Incidentally, the condenser microphone unit has a capacitance of about several pF to several tens of pF. Therefore, the signal source impedance of the condenser microphone unit is high. In order to transmit a signal from such a high signal source impedance, an impedance converter is used in the condenser microphone unit as described above. Noise generated in the condenser microphone includes noise due to the mechanical impedance of the microphone unit and noise due to an impedance converter using, for example, an FET, which are added to the audio signal and output.

  In this case, the signal generated by the microphone unit by receiving the sound wave has a correlation (periodicity), whereas the noise generated by the impedance converter (eg, FET) is uncorrelated. In particular, a so-called shot noise generated from the FET is dominant in the noise signal included in the small-diameter unidirectional condenser microphone. This shot noise is also called 1 / f noise because the noise level increases as the frequency decreases in the frequency domain.

  On the other hand, when recording music or the like using this type of condenser microphone, signal processing for intentionally adjusting the dynamic range is performed through a so-called compressor. Therefore, especially when there is no sound or the audio signal level is low, the 1 / f noise is also subjected to a predetermined amplification action. For this reason, there has been a problem that 1 / f noise is manifested in music recorded as residual noise. Therefore, it is important to improve the S / N by suppressing the 1 / f noise when there is no sound or when the sound signal level is small.

  In order to improve the S / N of the audio signal, there is means for A / D converting the audio signal from the microphone unit and outputting only the correlation signal by a DSP or the like. However, when the sound collection band is widened and the dynamic range is set large, it is necessary to increase the sampling frequency. When such a digital signal is converted back to an analog signal, the time delay of the signal increases. Such a time delay may affect sound quality and is not preferable.

As described in Patent Documents 1 and 2, the present invention is configured such that a plurality of unidirectional condenser microphone units are connected in parallel, or the output of an impedance converter of one condenser microphone unit is connected to another condenser microphone. A basic configuration is adopted that improves the S / N of the signal by adding it in series to the unit.
In addition to the above configuration, a unidirectional condenser microphone that further improves S / N by controlling the output level of the audio signal in accordance with the correlation value in the audio signal output from the impedance converter. Is intended to provide.
In addition, an object of the present invention is to provide a unidirectional condenser microphone in which an audio signal is processed in the state of an analog signal, thereby eliminating the influence of the delay of the signal.

  A unidirectional condenser microphone according to a first aspect of the present invention, which has been made to solve the above-described problems, includes a plurality of unidirectional condenser microphone units, and an impedance converter connected to each of the condenser microphone units. The output of the impedance converter connected to one condenser microphone unit is added to another condenser microphone unit, and the audio signal added by the condenser microphone units connected in series is obtained. A low-pass filter that extracts a low-frequency signal from the output of the impedance converter, and a correlation detection unit that obtains a correlation value using the low-frequency signal via the low-pass filter, The correlation value from the correlation detector By a signal, characterized in that a level control unit for controlling the output level of the audio signal.

  In this case, the unidirectional microphone of the first form includes two sets of capacitor microphone units connected in series, and the audio signal output from each set of capacitor microphone units is balanced via the level control unit. A configuration in which the output is output as a hot-side signal and a cold-side signal can be suitably employed.

  In addition, the unidirectional condenser microphone of the second embodiment according to the present invention made to solve the above-described problem has a plurality of unidirectional condenser microphone units, and each condenser microphone unit is connected in parallel. A unidirectional condenser microphone connected to the impedance converter and configured to obtain an audio signal through the impedance converter, wherein the low-pass filter extracts a low-frequency signal from the output of the impedance converter; A correlation detection unit that obtains a correlation value using a low-frequency signal that has passed through the low-pass filter, and a level control unit that controls the output level of the audio signal using a signal corresponding to the correlation value from the correlation detection unit. It is characterized by having.

  The capacitor microphone of the second form includes two sets of capacitor microphone unit groups in which a plurality of microphone units are connected in parallel and connected to an impedance converter, and an audio signal of each group of capacitor microphone unit groups is transmitted to the level control unit. It is possible to suitably employ a configuration in which a balanced output hot side signal and a cold side signal are output via the.

  In both the first and second condenser microphones, it is desirable that the diaphragms of the respective condenser microphone units are arranged on the same plane.

  In a preferred embodiment of the correlation detection unit, a multiplication circuit that receives two different low-frequency signals via the low-pass filter and obtains respective multiplication outputs, and a signal obtained by the multiplication circuit over a predetermined period And an integration circuit that integrates, and the output of the integration circuit is output as a signal corresponding to the correlation value from the correlation detection unit.

  Furthermore, in another preferable embodiment of the correlation detection unit, a multiplication circuit for obtaining a multiplication output of a low-frequency signal that has passed through the low-pass filter and a signal obtained by delaying the low-frequency signal for a predetermined time, and the multiplication An integration circuit that integrates a signal obtained by the circuit over a predetermined period is used, and an output of the integration circuit is output as a signal corresponding to the correlation value from the correlation detection unit.

The unidirectional condenser microphone according to the first aspect of the present invention is configured to add the output of an impedance converter connected to one condenser microphone unit in series to another condenser microphone unit. Thus, a condenser microphone with improved S / N can be provided.
Further, the unidirectional condenser microphone of the second form is configured by connecting a plurality of unidirectional condenser microphone units in parallel. According to the unidirectional condenser microphone of the second form, a condenser microphone with improved S / N can be provided as in the first form.

  In addition to the configuration described above, the condenser microphone according to the present invention includes a correlation detection unit for a low-frequency signal having a large uncorrelated noise level, and an output of the audio signal by a signal corresponding to a correlation value from the correlation detection unit. A configuration including a level control unit for controlling the level is employed. Thereby, when no sound is being collected, since the output signal including noise has a small correlation value, the output level of the audio signal is cut off or suppressed. With this action, it is possible to provide a unidirectional condenser microphone with further improved S / N.

1 is a block diagram showing a first form of a unidirectional condenser microphone according to the present invention. FIG. It is the block diagram which similarly showed the 2nd form. It is the block diagram which showed the 1st example of the correlation detection part mounted in the condenser microphone of the 1st and 2nd form. It is the block diagram which showed the 2nd example of the correlation detection part mounted in the condenser microphone of the 1st and 2nd form.

A first embodiment of a unidirectional condenser microphone according to the present invention will be described with reference to FIG.
FIG. 1 shows an example using four unidirectional microphone units 1a to 1d as an example using a plurality of unidirectional microphone units. For these four microphone units 1a to 1d, small condenser microphone units having the same configuration and excellent in high frequency directivity response are used.

  And although not shown in figure, the synthetic resin film which has a metal vapor deposition film is used for each diaphragm in each capacitor | condenser microphone unit 1a-1d. Although not shown, the fixed pole facing the diaphragm is provided with an electret dielectric film. These vibrating plates and fixed poles constitute an electret condenser microphone unit of the back electret method. In this embodiment, the diaphragms of the four condenser microphone units 1a to 1d are arranged on the same plane.

  Impedance converters 2a to 2d having FETs are connected to the first to fourth condenser microphone units 1a to 1d, respectively. Low-pass filters 3a to 3d are connected to the output terminals of the impedance converters 2a to 2d, respectively. Further, analog / digital converters 4a to 4d are connected downstream of the low-pass filters 3a to 3d. The signals digitally converted by the analog / digital converters 4 a to 4 d are supplied to the correlation detection unit 5.

  More specifically, in the first condenser microphone unit 1a, the diaphragm is grounded, and the fixed pole is connected to the impedance converter 2a. The output end of the impedance converter 2a is connected to the diaphragm of the second condenser microphone unit 1b. The fixed pole of the second condenser microphone unit 1b is connected to the impedance converter 2b. In other words, the first and second condenser microphone units 1a and 1b are connected in series, and the audio signal of the first condenser microphone unit 1a is added to the audio signal of the second condenser microphone unit 1b and output. The added audio signal is output via the impedance converter 2b. The audio signal output from the impedance converter 2 b is supplied to the level control unit 6. With the above connection configuration, the first and second condenser microphone units 1a and 1b form a first set of condenser microphone unit groups.

  In the third condenser microphone unit 1c, the fixed pole is connected to the ground, and the diaphragm is connected to the impedance converter 2c. The output end of the impedance converter 2c is connected to the fixed pole of the fourth condenser microphone unit 1d, and the diaphragm of the fourth condenser microphone unit 1d is connected to the impedance converter 2d. Thereby, the third and fourth condenser microphone units 1c and 1d are connected in series, and the audio signal of the third condenser microphone unit 1c is added to the fourth condenser microphone unit 1d and output. The added audio signal is output via the impedance converter 2d. The audio signal output from the impedance converter 2d is supplied to the level controller 6. With the above connection configuration, the third and fourth condenser microphone units 1c and 1d form a second set of condenser microphone unit groups.

With the above-described connection configuration, the audio signal output from the first set of capacitor microphone unit groups and the audio signal output from the second set of capacitor microphone unit groups are output in a reverse phase relationship with each other.
And the said signal of a reverse phase relationship is output as a balanced output signal to a hot side output terminal (HOT) and a cold side output terminal (COLD) via the variable resistors VRa and VRb provided in the level control unit 6, respectively. Is done.

  The resistance values of the variable resistors VRa and VRb provided in the level control unit 6 are varied according to the correlation value detected by the correlation detection unit 5. As a result, the output level of the audio signal by the first and second sets of condenser microphone units is varied by the variable resistors VRa and VRb. Therefore, audio signals of the same level are output to the hot side output terminal (HOT) and the cold side output terminal (COLD), respectively.

  As described above, the sound output from the impedance converters 2a to 2d is supplied to the correlation detection unit 5 via the low-pass filters 3a to 3d and the analog / digital converters 4a to 4d. Therefore, according to this embodiment, the correlation detection unit 5 performs correlation detection using a low-frequency signal having a large uncorrelated noise level. At this time, each signal is supplied as a correlation detection digital signal to the correlation detector shown in FIG. 3 disposed in the correlation detection unit 5 by the analog / digital converters 4a to 4d, respectively.

FIG. 3 shows a first basic configuration example of a correlation detector that can be used in the condenser microphone according to the present invention.
For example, when correlation detection is performed in the first microphone unit group, low-frequency signals that have passed through the low-pass filters 3a and 3b are input from the input terminals IN1 and IN2 and multiplied by the multiplication circuit 5a. Then, the multiplication signal obtained by the multiplication circuit 5a is supplied to the integration circuit 5b. Further, the integration circuit 5b integrates the multiplied signal over a predetermined time, whereby the correlation values of the respective audio signals from the first and second condenser microphone units 1a and 1b can be obtained. That is, the signal obtained from the integrating circuit 5c is output to the output terminal OUT as the correlation value of the audio signals from the first and second condenser microphone units 1a and 1b. The absolute value of the output correlation value generally falls within the range of 0 to 1. In general, when the absolute value of the correlation value is 0, the signal is uncorrelated.

  According to the correlation detector shown in FIG. 3, the periodicity (correlation) of the sound given to the input terminals IN1 and IN2 can be captured. In general, a sound wave signal input to a condenser microphone unit has a correlation, whereas a noise caused by the impedance converter is uncorrelated. Therefore, the presence or absence of noise can be detected from the correlation value of the signal.

  In the above-described correlation detection example, the correlation value is obtained from the respective signals (outputs of 3a and 3b) from the first and second condenser microphone units 1a and 1b. For example, the third and fourth condenser microphones are used. A configuration in which a correlation value is obtained from the respective signals (outputs of 3c and 3d) from the units 1c and 1d may be employed. Similarly, it is possible to adopt a configuration in which a correlation value is obtained from the respective signals (outputs of 3a and 3c) from the first and third condenser microphone units 1a and 1c. Furthermore, it is possible to adopt a configuration in which the correlation value is obtained from the respective signals (outputs of 3b and 3d) from the second and fourth condenser microphone units 1b and 1d.

Thus, a signal corresponding to the correlation value output from the output terminal OUT of the correlation detector shown in FIG. 3 is supplied from the correlation detection unit 5 shown in FIG. 1 to the level control unit 6. Become.
Therefore, the resistance values of the variable resistors VRa and VRb provided in the level control unit 6 are controlled by a signal corresponding to the correlation value supplied from the correlation detection unit 5, and the hot side output terminal (HOT) and the cold side output terminal The output level of the audio signal output to (COLD) is controlled. The level control unit 6 determines that there is no correlation when the absolute value of the input correlation value (signal corresponding to the correlation value) is from 0 to a predetermined value (for example, 0.2), and is larger than the predetermined value. In the case, it is determined that there is a correlation.

That is, when noise is generated when there is no sound to be collected, the correlation value detected by the correlation detection unit 5 is small (for example, when the absolute value of the correlation value is 0 to 0.2). At this time, the level controller 6 acts to suppress the output level of the audio signal. As a result, the noise mainly caused by the impedance converter during silence is cut off or attenuated by the level control unit 6.
On the other hand, when the correlation value detected by the correlation detection unit 5 is large (for example, when the absolute value of the correlation value is larger than 0.2), it is considered that a sound wave signal is included, and the variable resistor VRa and VRb act to output the signal as it is without being attenuated.

  FIG. 4 shows a second basic configuration example of the correlation detector that can be used in the condenser microphone according to the present invention. The correlation detector shown in FIG. 4 captures the periodicity (correlation) of one signal applied to the input terminal IN. In the configuration shown in FIG. 1, the first to fourth condenser microphone units 1a to 1a are used. Any signal 1d (outputs 3a to 3d) is supplied to the input terminal IN.

  For example, when a signal (output of 3a) from the first condenser microphone unit 1a shown in FIG. 1 is supplied to the input terminal IN, this signal is supplied to the multiplication circuit 5a and also to the delay unit 5c. Is also supplied. By passing through the delay unit 5c, a signal delayed by a predetermined time is generated and supplied to the multiplication circuit 5a. Then, the multiplication signal obtained by the multiplication circuit 5a is integrated over a predetermined time by the integration circuit 5b. As a result, the correlation value of the signal from the first condenser microphone unit 1a can be obtained. A second basic configuration example of the correlation detector uses so-called autocorrelation. In the second basic configuration example of the correlation detector, similarly to the first basic configuration, the absolute value of the correlation value falls within the range of 0 to 1. As explained so far, 1 / f noise has no periodicity and is uncorrelated. Therefore, when 1 / f noise is mixed in the signal, the original signal supplied to the input terminal IN and the signal with a time delay are not correlated with each other, and when an audio signal is included, they are correlated with each other.

  A signal corresponding to the correlation value output from the output terminal OUT of the correlation detector illustrated in FIG. 4 is supplied from the correlation detection unit 5 illustrated in FIG. 1 to the level control unit 6. The resistance values of the variable resistors VRa and VRb of the level control unit 6 are controlled by a signal corresponding to the correlation value. As a result, the noise mainly caused by the impedance converter during silence is cut off or attenuated by the level control unit 6, and the same effect as the example described above can be obtained.

According to the embodiment shown in FIG. 1, a condenser microphone having a good directional frequency response with improved S / N can be provided by obtaining an added audio signal by using two condenser microphone units. In addition, the unidirectional condenser microphone according to this embodiment is configured by adopting means for controlling the output level of the audio signal based on the correlation value by the correlation detection unit, so that a high S / N is always obtained. It can be secured.
Since the hot-side output and the cold-side output, which are balanced, are level-controlled while being analog signals, for example, there are limitations on the sound collection band caused by digital processing of audio signals, problems with signal delay, etc. can avoid.

  FIG. 2 shows a second embodiment of the condenser microphone according to the present invention. Also in the example shown in FIG. 2, a plurality of, here, four unidirectional microphone units 1a to 1d are used. For these four unidirectional microphone units 1a to 1d, small condenser microphone units having the same configuration and excellent in high frequency directivity response are used.

  As in the example shown in FIG. 1, although not shown, a synthetic resin film having a metal vapor deposition film is used for each diaphragm in each of the condenser microphone units 1 a to 1 d. In addition, an electret dielectric film is provided on the fixed pole facing the diaphragm. The pair of vibrating plates and the fixed pole constitute an electret condenser microphone unit of the back electret method. Furthermore, also in this embodiment, the diaphragms of the four condenser microphone units 1a to 1d are arranged on the same plane.

The diaphragm of the first condenser microphone unit 1a and the diaphragm of the second condenser microphone unit 1b constituting the first set are both connected to the ground, and the fixed poles are connected in common and the impedance converter 7a provided with the FET. It is connected to the.
Further, the fixed pole of the third condenser microphone unit 1c and the fixed pole of the fourth condenser microphone unit 1d constituting the second set are both connected to the ground, and the diaphragms are connected in common and impedance conversion provided with an FET. Connected to the device 7b. That is, the first condenser microphone unit 1a and the second condenser microphone unit 1b are connected in parallel.

  Low-pass filters 8a and 8b are connected to the output terminals of the impedance converters 7a and 7b, respectively, and analog / digital converters 9a and 9b are connected via the low-pass filters 8a and 8b. The signals digitally converted for correlation detection by the analog / digital converters 9a and 9b are each supplied to the correlation detection unit 5.

The first and second condenser microphone units 1a and 1b constituting the first set are connected in parallel as described above, thereby increasing the effective capacity and reducing the inherent noise level. The audio output from the impedance converter 7 a to which the first and second condenser microphone units 1 a and 1 b are connected is supplied to the level control unit 6.
Similarly, the third and fourth condenser microphone units 1c and 1d constituting the second set are connected in parallel as described above, thereby increasing the effective capacity and reducing the inherent noise level. The audio output from the impedance converter 7b to which the third and fourth condenser microphone units 1c and 1d are connected is supplied to the level control unit 6.

  As described above, the diaphragms of the first and second condenser microphone units 1a and 1b forming the first set are both connected to the ground, and the third and fourth condenser microphone units 1c and 1d forming the second set are connected to the ground. Both fixed poles are grounded. Therefore, the audio signals from the impedance converter 7a and the impedance converter 7b are output in a reverse phase relationship. And the said signal of a reverse phase relationship is each as a balanced output signal to a hot side output terminal (HOT) and a cold side output terminal (COLD) via the variable resistors VRa and VRb with which the level control part 6 was equipped. Is output.

  The resistance values of the variable resistors VRa and VRb provided in the level control unit 6 are varied according to the correlation value detected by the correlation detection unit 5. As a result, the output level of the audio signal by the first and second sets of condenser microphone units is varied by the variable resistors VRa and VRb. Therefore, audio signals from the first and second sets of condenser microphone units are output to the hot side output terminal and the cold side output terminal in the same level state.

  As described above, the audio output from the impedance converters 7a and 7b is supplied to the correlation detection unit 5 as the correlation detection digital signal via the low-pass filters 8a and 8b. Therefore, also in this embodiment, the correlation detector 5 performs correlation detection using a low frequency region where the uncorrelated noise level is large. The correlation detector shown in FIG. 3 or 4 is also used in the correlation detector 5.

  The configuration and operation of the correlation detector shown in FIGS. 3 and 4 are as described above, and detailed description thereof is omitted. When the correlation detector shown in FIG. 3 is used for the correlation detector 5 in FIG. 2, signals from the impedance converters 7a and 7b are supplied to the input terminals IN1 and IN2, respectively. Then, the correlation detector 5 outputs a signal whose correlation between the two signals corresponds to the determined correlation value to the level controller 6.

  When the correlation detector shown in FIG. 4 is used for the correlation detector 5 shown in FIG. 2, the signal from either the impedance converter 7a or 7b is supplied to the input terminal IN. Then, the correlation is determined from the periodicity of the signal supplied to the input terminal IN, and a signal corresponding to the correlation value is output from the correlation detection unit 5 to the level control unit 6.

Similar to the first embodiment shown in FIG. 1, the variable resistors VRa and VRb of the level control unit 6 are varied by a signal corresponding to the correlation value from the correlation detection unit 5.
Therefore, when there is noise from the impedance converters 7a and 7b during silence, the audio signal is blocked or attenuated by the level control unit 6 because the correlation value of the output correlation digital signal is small.
On the other hand, when the correlation value detected by the correlation detection unit 5 is large, it is determined that a sound wave signal is included. Therefore, the variable resistors VRa and VRb act so as to output the sound signal as it is without being attenuated.

According to the embodiment shown in FIG. 2, by connecting two condenser microphone units in parallel, it is possible to provide a condenser microphone with a good directional frequency response in which the effective capacity is increased and the S / N is improved. Similarly to the first embodiment, the unidirectional condenser microphone in this embodiment employs a means for controlling the output level of the signal based on the correlation value by the correlation detection unit, so that the S / S is always high. N can be secured.
The level control of the hot-side output and the cold-side output that are balanced is performed with the analog signal as it is, so that, for example, the limitation of the sound collection band caused by digital processing of the audio signal and the problem of signal delay are avoided. be able to.

In the first embodiment described above (FIG. 1), two condenser microphone units are connected in series to add an audio signal, but this is because three or more condenser microphone units are connected in series. It is possible to adopt a configuration to According to this, since the addition output increases further, it can contribute to improving S / N.
Similarly, in the second embodiment (FIG. 2), two capacitor microphone units are connected in parallel to increase the effective capacity, but this is achieved by connecting three or more capacitor microphone units in parallel. It can contribute to improving S / N.

1a to 1d Condenser microphone unit 2a to 2d Impedance converter 3a to 3d Low-pass filter 4a to 4d Analog / digital converter 5 Correlation detector 5a Multiplier circuit 5b Integrator circuit 5c Delay device 6 Level controller 7a, 7b Impedance converter 8a, 8b Low-pass filter 9a, 9b Analog / digital converter VRa, VRb Variable resistor

Claims (7)

A plurality of unidirectional condenser microphone units, and an impedance converter connected to each of the condenser microphone units,
A unidirectional condenser microphone in which condenser microphone units are connected in series by adding the output of an impedance converter connected to one condenser microphone unit to another condenser microphone unit,
A low-pass filter that extracts a low-frequency signal from the output of the impedance converter;
A correlation detection unit that obtains a correlation value using a low-frequency signal through the low-pass filter;
A level control unit for controlling an output level of the audio signal by a signal corresponding to a correlation value from the correlation detection unit;
A unidirectional condenser microphone characterized by comprising: Two sets of condenser microphone units connected in series,
The audio signal output of each set of condenser microphone units is made to be a hot side output of a balanced output and a cold side output via the level control unit,
The unidirectional condenser microphone according to claim 1. A plurality of unidirectional condenser microphone units, each condenser microphone unit is connected in parallel, and an impedance converter is connected to the subsequent stage, and a single unit configured to obtain an audio signal via the impedance converter. A unidirectional condenser microphone,
A low-pass filter that extracts a low-frequency signal from the output of the impedance converter;
A correlation detection unit that obtains a correlation value using a low-frequency signal through the low-pass filter;
A level control unit for controlling an output level of the audio signal by a signal corresponding to a correlation value from the correlation detection unit;
A unidirectional condenser microphone characterized by comprising: Two capacitor microphone units connected in parallel and connected to an impedance converter are provided.
The audio signal output of each set of condenser microphone units is made to be a hot side output of a balanced output and a cold side output via the level control unit, respectively.
The unidirectional condenser microphone according to claim 3.   The unidirectional condenser microphone according to any one of claims 1 to 4, wherein diaphragms of the plurality of condenser microphone units are arranged on the same plane. The correlation detection unit
A multiplication circuit for receiving two different low-frequency signals via the low-pass filter and obtaining respective multiplication outputs;
An integration circuit for integrating the signal obtained by the multiplication circuit over a predetermined period;
With
The integrating circuit outputs a signal corresponding to the correlation value;
The unidirectional condenser microphone according to claim 1, wherein: The correlation detection unit
A multiplication circuit for obtaining a multiplication output of the low-frequency signal passed through the low-pass filter and a signal obtained by delaying the low-frequency signal by a predetermined time;
An integration circuit for integrating the signal obtained by the multiplication circuit over a predetermined period;
With
The integrating circuit outputs a signal corresponding to the correlation value;
The unidirectional condenser microphone according to claim 1, wherein:

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