JPH05291877A - Displacement proportional converter pre-attenuator - Google Patents

Abstract

(57) [Summary] [Purpose] Attenuate a large audio input signal without deteriorating the S / N ratio. [Structure] A phase inversion circuit 23 for phase-converting the output of the impedance conversion circuit 21 of the microphone unit 1 and feeding back to the microphone unit 1 is connected to the microphone unit 1, and a signal whose phase is inverted by the phase inversion circuit 23 is input to the microphone unit. 1 is applied. As a result, the sensitivity of the microphone unit 1 can be reduced and can be substantially attenuated.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a displacement proportional converter of the type in which an output voltage is proportional to the displacement of a vibrator, and more specifically, the S / N ratio deteriorates even for a large input signal. The present invention relates to a pre-attenuator of a displacement proportional converter that can be attenuated without any damage.

[0002]

2. Description of the Related Art A displacement proportional acoustic transducer, such as a condenser microphone or an electret condenser microphone, which obtains an output voltage proportional to the displacement amplitude of a vibrator, can be made compact and lightweight, and can be used for commercial or consumer use. Widely used. In a condenser microphone, a DC voltage is applied between the electrodes of the microphone unit via a high resistance, and the change in the voltage generated across the high resistance due to the change in the capacitance of the unit is taken out as an electric signal. The fixed pole or diaphragm of the unit is composed of a thin film having the electret phenomenon, and the electrode on the signal output side is connected with a high impedance, and the charge Q of the electrode does not move but becomes constant, but the unit capacitance C Change of Q = C × V
Since a voltage change appears according to the relationship of (V is the voltage of the bipolar sense of the capacitor), this voltage change is taken out as an electric signal. Generally, the capacitance of such a microphone unit is several tens of pF, and a high resistance value of several hundreds MΩ or more is required, but the output impedance of the microphone needs to be a low impedance of several hundreds Ω. Therefore, in order to convert high input impedance to low output impedance, FET
An impedance changing circuit composed of etc. is provided.

This impedance changing circuit will be described with reference to FIG. 6 by taking the case of an electret condenser microphone as an example. One of the fixed pole and the diaphragm of the microphone unit 1 is a resistance for extracting the capacitance change as a voltage change. 2 and the FET 3 for impedance conversion are connected. The drain of the FET 3 is connected to the power supply terminal 5 via the FET 4. This F
The ET4 makes the input capacitance of the FET3 substantially zero to improve the signal attenuation due to this, and makes the voltage between the drain and source of the FET3 almost unchanged with respect to the input signal to improve the distortion rate. It is for. FE
The source of T3 is connected to the ground terminal 7 via the FET 6 forming a constant current circuit, and the FET 6 improves the signal distortion and attenuation due to its high internal impedance like the FET 4. The output terminal 8 is connected to the source of the FET 3, and the gate of the FET 4 and the resistor 2 are connected to the output terminal 8.
Is also connected to the drain of FET6.
The other side of the microphone unit 1 is also connected to the ground terminal 7.

Thus, the electret condenser microphone needs the impedance changing circuit as described above. However, when the voice input signal becomes large, the distortion of the impedance changing circuit increases and the impedance changing circuit may be clipped. is there. Therefore, as shown in the figure, a preattenuator composed of a capacitor 9 of several tens to several 100 pF or a preattenuator called a pad is interposed between the microphone unit 1 and the impedance changing circuit,
Attenuating the audio input signal. If it is predicted in advance that the voice input signal is low, a switch 10 for opening the capacitor 9 is connected in series to the capacitor 9 in order to cancel the operation of the preattenuator.

[0005]

However, the interposition of such a pre-attenuator is less than that of the case without interposition.
There is a drawback that the / N ratio deteriorates. In order to turn off the pre-attenuator, the switch 10 may be turned off and the capacitor 9 may be opened. However, the impedance of the part to which the capacitor 9 is connected is several 100 MΩ or more, and in some cases, several 10 GΩ. It is a very high value and may pick up extraneous noise. In order to protect from external noise, the switch 10 needs to be strictly shielded, which has a drawback that the structure is complicated and the size becomes large.

Therefore, an object of the present invention is to provide a pre-attenuator of a displacement proportional converter which can attenuate a large voice input signal without deteriorating the S / N ratio.

[0007]

According to the present invention, there is provided a preattenuator of a displacement proportional converter, which comprises an impedance conversion circuit, wherein an output voltage of a converter unit is proportional to a displacement amount of a vibrator. It is characterized in that it has a phase inverting circuit for phase-converting the output and feeding it back to the converter unit.

In the pre-attenuator of the displacement proportional converter of the present invention, the connection between the phase inversion circuit and the converter unit is turned on / off by a switch.

[0009]

With this configuration, the output of the converter unit converted to the low impedance by the impedance conversion circuit is applied to the converter unit as the signal whose phase is inverted by the phase inversion circuit, so that the sensitivity of the converter unit 1 is lowered. Can be done and will be substantially dampened.

This attenuation can be on / off controlled by a switch connected between the phase inverting circuit and the converter unit, and the output of the phase inverting circuit can be fed back to the converter unit only when attenuation is required. it can. Since this switch is connected to the output side of the phase inversion circuit, it has a low impedance and can be switched without picking up external noise even if the shield is insufficient.

[0011]

The following is an example of applying the pre-attenuator of the present invention to an electret condenser microphone.
An embodiment shown in the drawings will be described. In the above-mentioned conventional example and each embodiment, substantially the same constituent elements are designated by the same reference numerals, and detailed description thereof will be omitted.

First, the first embodiment shown in FIG. 1 will be described. In the preattenuator, an impedance conversion circuit 21 having the same configuration as that of the above-described conventional example, and an output side of the impedance conversion circuit 21 having an input side thereof are provided. The buffer circuit 22 is connected, and the phase inverting circuit 23 is connected to the output side of the buffer circuit 22 at its input side. The buffer circuit 22 includes two transistors 24 and 2 connected to form a complementary push-pull circuit.
5 and a transistor 26 connected as a bias for the push-pull circuit so that the signal is not distorted even when the input impedance of the phase inverting circuit 23 is low. The phase inversion circuit 23 includes a transistor 27 for phase inversion, two transistors 28 and 29 connected to form a complementary push-pull circuit like the transistor of the buffer circuit 22, and a transistor for this push-pull circuit. Transistor 30 connected for bias and resistor 31 for setting input impedance
And the output of this phase inversion circuit 23
3 and a resistor 32 for determining the level of the inverting output by feeding back to 3. The ratio of the output to the input of the phase inversion circuit 23 is determined by the value obtained by dividing the resistance value of the resistor 32 by the resistance value of the resistor 31. The output side of the phase inversion circuit 23 is connected to the fixed pole or the diaphragm of the microphone unit 1. Reference numeral 33 is an output terminal for taking out the output of the phase inverting circuit 23.

Therefore, the impedance conversion circuit 21
The output signal of the microphone unit 1 converted to low impedance by the buffer circuit 22 is applied to the phase inverting circuit 23 after being prevented from being distorted even by the low input impedance of the phase inverting circuit 23 of the next stage. The input impedance of the phase inversion circuit 23 is set low because the resistor 31 is connected in series to the signal input,
If the resistance value is high, thermal noise increases and the S / N ratio of the circuit deteriorates. Therefore, the resistance value is set to a relatively low value, whereby a phase-inverted signal without distortion is generated by the microphone unit. Since it is applied to the fixed pole or the vibration plate of No. 1, the sensitivity of the microphone unit 1 is lowered and it is substantially attenuated. The attenuation amount can be set by changing the ratio of the input impedance setting resistors 31 and 32.

In the above embodiment, the phase inversion transistor 27, the two transistors 28 and 29 forming the push-pull circuit, and the bias transistor 3 are provided.
0, the input impedance setting resistor 31 and the feedback resistor 32 constitute the phase inverting circuit 23. However, as in the second embodiment shown in FIG. 2, a transformer 34 is used to obtain a reverse phase signal. The phase inverting circuit 23 may be configured as described above, and the inverted signal thereof may be applied to the fixed pole or the diaphragm of the microphone unit 1. That is, the transformer utilizes the fact that the in-phase output and the anti-phase output are obtained from its two output terminals, connects the anti-phase output terminal side of the transformer 34 to the phase inversion output terminal 33, and outputs the output of the microphone unit 1. It is returned to the fixed pole or the diaphragm side.

Further, when the preattenuator of the first embodiment is turned on and off, the fixed pole or the diaphragm side of the microphone unit 1 is connected to the phase inverting circuit as in the third embodiment shown in FIG. It is sufficient to provide a switch 35 for switching to connect to 23 or to connect to the ground terminal 7. The connection point of the switch 35 has an impedance of several tens of Ω.
Since it is as low as several KΩ, it is possible to switch without picking up external noise even if the shield is insufficient, and there is an advantage that the shield process is simple.

The fourth embodiment shown in FIG. 4 is a phase inversion circuit 23.
An example is shown in which the total amount of feedback is reduced. In the fourth embodiment, two resistors 36 and 37 connected in series are connected between the phase inversion output terminal 33 and the ground terminal 7 of the first embodiment described above, and the midpoint of the resistors 36 and 37 is connected to the microphone unit 1. It is the same as the first embodiment except that it is connected to a fixed pole or a diaphragm. Resistance 3
Reference numerals 6 and 37 are called bleeder resistances. The output of the phase inversion circuit 23 is reduced by a ratio obtained by dividing the resistance value of the resistance 37 by the total resistance value of the resistances 36 and 37, and an output is obtained from the intermediate point. The signal can be reduced by this ratio, resulting in lower overall attenuation.

The fifth embodiment shown in FIG. 5 shows a case where the attenuation amount of the phase inverting circuit 23 is increased and the overall attenuation amount is increased, contrary to the fourth embodiment. For example, the microphone unit 1 has a capacitance of several pF to
It is as small as several tens of pF, and the feedback amount may be attenuated at this portion, and the total attenuation amount may not be as large as the design value.
That is, the input impedance of the impedance conversion circuit 21 is as high as several hundreds MΩ to several tens of GΩ, but because the electrostatic capacitance of the microphone unit 1 is small, when passing through it, the same attenuation as if a high resistance is inserted will occur. .. In order to reduce the attenuation of the feedback amount, the capacitance of the microphone unit 1 may be increased. However, the microphone unit 1 itself has a microphone shape, frequency characteristics,
Due to performance such as sensitivity, it cannot be changed simply due to the amount of attenuation. Therefore, in the fifth embodiment, the condenser 38 is connected in parallel to the microphone unit 1,
The capacitance of the feedback system is increased, which increases the overall attenuation.

The amount of attenuation of the phase inverting circuit 23 can be set by the ratio of the resistors 31 and 32 as described above, but the output level of the phase inverting circuit 23 can be made constant. That is, in the circuits shown in FIGS. 4 and 5, balanced outputs can be realized without a transformer by taking out in-phase (positive phase) and reverse-phase outputs at a ratio of 1: 1.

Comparing the S / N ratios of the conventional example shown in FIG. 6 and the fifth embodiment shown in FIG. 5, the switch 10 is turned off in the conventional example shown in FIG. 6 and the preattenuator is not used. The S / N ratio was 78.2 dB, the S / N ratio when the switch 10 was turned on and the pre-attenuator was used was 75.4 dB, and the S / N deterioration degree was 2.8 dB. In contrast,
In the fifth embodiment of FIG. 5, the S / N ratio is 78.1 dB.
The S / N deterioration degree was only 0.1 dB. In the experiment, the frequency was 1 KHz and the input sound pressure level was 1P.
a, and the audibility correction was performed for measurement.

Although each of the above-described embodiments has been described by taking the case of the electret condenser micro as an example, it is needless to say that it can be applied to a contact type pickup such as a bone conduction microphone, a throat microphone or a vibration pickup.

[0021]

As described above, the pre-attenuator of the displacement proportional converter of the present invention is provided with the phase inversion circuit for phase-converting the output of the impedance conversion circuit and feeding it back to the converter unit. Since the signal whose phase has been inverted by the phase inversion circuit is applied to the converter unit, the sensitivity of the converter unit can be reduced and the signal is substantially attenuated.

If the connection between the phase inverting circuit and the converter unit is turned on / off by a switch as described in claim 2, the output of the phase inverting circuit is converted only when attenuation is required. It can also be returned to the vessel unit. Since this switch is connected to the output side of the phase inversion circuit, it has a low impedance and can be switched without picking up external noise even if the shield is insufficient.

[Brief description of drawings]

FIG. 1 is a circuit diagram showing a first embodiment of the present invention.

FIG. 2 is a circuit diagram showing a second embodiment of the present invention.

FIG. 3 is a circuit diagram showing a third embodiment of the present invention.

FIG. 4 is a circuit diagram showing a fourth embodiment of the present invention.

FIG. 5 is a circuit diagram showing a fifth embodiment of the present invention.

FIG. 6 is a circuit diagram showing an example of a conventional preattenuator.

[Explanation of symbols]

 1 Microphone unit 2 Resistance 3,4,6 FET 9 Capacitor 21 Impedance conversion circuit 22 Buffer circuit 23 Phase inversion circuit 34 Transformer 35 Switch 36,37 Resistor 38 Capacitor

Claims (2)

[Claims] 1. A preattenuator of a displacement-proportional converter comprising an impedance conversion circuit, wherein an output voltage of the converter unit is proportional to a displacement amount of a vibrator, and the output of the impedance conversion circuit is phase-converted to perform the phase conversion. A pre-attenuator for a displacement proportional converter characterized by having a phase inversion circuit for feeding back to a converter unit. 2. The displacement proportional converter front-end according to claim 1, wherein the phase inversion circuit and the converter unit are connected via a switch for turning on / off the connection. Attenuator.