JP2024090298A - Audio input device - Google Patents

Abstract

To provide a voice input unit capable of suppressing noise output by reducing the noise.SOLUTION: A voice input device comprises a signal terminal that outputs an audio signal inputted from the outside, an audio terminal that has a ground terminal to be connected to the ground, a differential amplifier circuit that comprises a first input terminal and a second input terminal, a first circuit that connects the signal terminal and the first input terminal, and a second circuit that connects the ground terminal and the second input terminal. The second circuit is connected to the ground via a first pull-down resistor.SELECTED DRAWING: Figure 4

Description

The present invention relates to a voice input device.

Patent document 1 discloses an amplifier device (audio input device) that amplifies an input audio signal and outputs it to a speaker.

JP 2017-17657 A

In the amplifier device disclosed in Patent Document 1, if the signal detection circuit does not detect an audio signal with a volume equal to or greater than a predetermined volume, the control circuit is configured to turn off a switch connected to the AC power source and supply voltage from the battery. With this configuration, it is possible to prevent hum noise from being output as audio if an audio signal with a volume equal to or greater than a predetermined volume is not detected.

However, the amplifier device disclosed in Patent Document 1 does not reduce the hum noise itself.

The objective of this disclosure is to provide a voice input device that can reduce noise and suppress noise output.

An audio input device according to one aspect of the present disclosure includes an audio terminal having a signal terminal for outputting an audio signal input from the outside and a ground terminal connected to ground, a differential amplifier circuit having a first input terminal and a second input terminal, a first circuit connecting the signal terminal and the first input terminal, and a second circuit connecting the ground terminal and the second input terminal, and the second circuit is connected to ground via a first pull-down resistor.

1 is a block diagram showing an example of a main configuration of an audio reproduction system according to a first embodiment of the present disclosure. 2 is a perspective view showing an example of a wireless transmitter and an electro-acoustic transducer included in the audio reproduction system shown in FIG. 1. 2 is a block diagram showing an example of a configuration of a main part of a wireless transmitter included in the audio reproduction system shown in FIG. 1 . 4 is a circuit diagram showing an example of a configuration of a main part of an audio input unit included in the wireless transmitter shown in FIG. 3. 10 is a circuit diagram showing an example of a configuration of a main part of an audio input unit included in a wireless transmitter according to a modified example of the first embodiment of the present disclosure. FIG. 11 is a circuit diagram showing an example of a configuration of a main part of an audio input unit included in a wireless transmitter according to a second embodiment of the present disclosure. FIG.

The embodiments of the present disclosure will be described with reference to the drawings. In the following, the same or corresponding elements will be given the same reference numerals throughout all the drawings, and duplicated descriptions will be omitted. Furthermore, the embodiment described below is merely one example of the present disclosure, and the present disclosure is not limited to the embodiment. Even if it is not this embodiment, various modifications can be made according to the design, etc., as long as they do not deviate from the technical concept of the present disclosure.

First Embodiment
(Audio playback system)
An audio reproduction system 100 according to a first embodiment of the present disclosure will be described with reference to Fig. 1 and Fig. 2. Fig. 1 is a block diagram showing an example of a main configuration of the audio reproduction system 100 according to the first embodiment of the present disclosure. Fig. 2 is a perspective view showing an example of a wireless transmitter 2 and an electro-acoustic transducer 3 included in the audio reproduction system 100 shown in Fig. 1.

As shown in FIG. 1, the audio reproduction system 100 is configured to include a television receiver 1, a wireless transmitter 2 (audio input device), and an electroacoustic transducer 3. An example of the electroacoustic transducer 3 is a so-called wearable neck speaker, which is a speaker worn by the user around the neck, as shown in FIG. 2. A wearable neck speaker has speakers (not shown) provided near both ends of a main body that is bent to fit the shape of the neck. The electroacoustic transducer 3 is not limited to this neck speaker, and may be an earphone-type speaker, a headphone-type speaker, or a stationary speaker.

The television receiver 1 and the wireless transmitter 2 are connected, for example, by a wired audio cable 5, and an audio signal is input from the television receiver 1 to the wireless transmitter 2 via the audio cable 5. The wireless transmitter 2 is also configured to receive power from the television receiver 1 via a wired USB cable 4, for example, and can be a box-type transmitter as shown in FIG. 2.

The wireless transmitter 2 and the electroacoustic transducer 3 are capable of wireless communication, and the wireless transmitter 2 can transmit audio signals input from the television receiver 1 to the electroacoustic transducer 3. The wireless transmitter 2 can establish communication with the electroacoustic transducer 3 using a wireless communication standard such as Bluetooth (registered trademark).

In addition, the wireless transmitter 2 is configured so that the USB cable 4 and the audio cable 5 are connected to the television receiver 1 and the wireless transmitter 2, respectively, to form a loop between the ground (GND) of the USB cable 4 and the ground of the audio cable 5, thereby suppressing the hum noise that occurs. The detailed configuration of the wireless transmitter 2 is described below.

(Radio transmitter)
A detailed configuration of the wireless transmitter 2 will be described with reference to Fig. 3 and Fig. 4. Fig. 3 is a block diagram showing an example of a configuration of a main part of the wireless transmitter 2 provided in the audio reproduction system 100 shown in Fig. 1. Fig. 4 is a circuit diagram showing an example of a configuration of a main part of the audio input unit 10 included in the wireless transmitter 2 shown in Fig. 3.

As shown in FIG. 3, the wireless transmitter 2 includes a USB terminal 24, an audio input unit 10, and a control circuit 30. When a USB digital signal transmitted from the television receiver 1 is input to the USB terminal 24 via the USB cable 4, a differential signal is output from the USB terminal 24 to, for example, the control circuit 30, and power is supplied to each part of the wireless transmitter 2. In this way, the audio reproduction system 100 is configured to be able to supply power from the power supply Vcc to the wireless transmitter 2 via the USB cable 4.

The audio input unit 10 is a circuit to which the audio signal transmitted from the television receiver 1 is input via the audio cable 5. If the television receiver 1 is the output side of the audio signal, the wireless transmitter 2 is the input side of the audio signal.

The control circuit 30 is a circuit that performs various controls on each part of the wireless transmitter 2. The control circuit 30 can be, for example, a SoC (System on a Chip) that has the functions required for various controls on each part of the wireless transmitter 2 implemented on a single semiconductor chip. As shown in FIG. 3, the control circuit 30 includes a transmission unit 31 that controls the transmission of, for example, an audio signal to the electro-acoustic transducer 3 wirelessly via an antenna. It also includes a differential amplifier circuit 12 that receives an audio signal transmitted from the television receiver 1, and an ADC 33 that converts the audio signal from an analog signal to a digital signal.

Although not shown in FIG. 3, the wireless transmitter 2 may further include an audio selector and be configured to be able to accept audio signals from multiple different sound sources.

The television receiver 1 and the wireless transmitter 2 are connected to an audio cable 5 that transmits audio signals and a USB cable 4 that transmits USB digital signals. As shown in FIG. 4, in the wireless transmitter 2, the audio terminal 21 to which the audio cable 5 is connected and the USB terminal 24 to which the USB cable 4 is connected are both connected to ground.

For this reason, in the wireless transmitter 2, ground noise generated by the loop between the ground of the USB cable 4 and the ground of the audio cable 5 via the television receiver 1 may become hum noise and be mixed into the audio signal. Therefore, in the wireless transmitter 2 according to the first embodiment of the present disclosure, the audio input unit 10 is configured as shown in FIG. 4.

That is, the audio input unit 10 includes a USB terminal 24 to which power is supplied from a power source Vcc via a USB cable 4, a power supply circuit 23 that supplies power from the USB terminal 24 to each part of the wireless transmitter 2, and a signal supply circuit 26 that supplies a differential signal from the USB terminal 24 to the control circuit 30, etc. The USB terminal 24 is connected to ground via a power ground circuit 25.

The audio input unit 10 also includes an audio terminal 21 to which an audio signal is input, a differential amplifier circuit 12, a first circuit 19, and a second circuit 20.

The audio terminal 21 has a signal terminal 21a that outputs an audio signal input via an audio cable 5 from an external device such as a television receiver 1, and a ground terminal 21b that is connected to ground.

The first circuit 19 connects the signal terminal 21a of the audio terminal 21 to the non-inverting input terminal (+) (first input terminal) of the differential amplifier circuit 12, and transmits an audio signal. In the first circuit 19, the audio signal is input to the non-inverting input terminal (+) of the differential amplifier circuit 12 via the second resistor 15 (first signal level adjustment resistor) and the first capacitor 13. In this specification, the circuit from the signal terminal 21a of the audio terminal 21 through the second resistor 15 and the first capacitor 13 to the non-inverting input terminal (+) of the differential amplifier circuit 12 is referred to as the first circuit 19.

The second circuit 20 connects the ground terminal 21b of the audio terminal 21 to the inverting input terminal (-) (second input terminal) of the differential amplifier circuit 12. In this specification, the circuit from the ground terminal 21b of the audio terminal 21 to the inverting input terminal (-) of the differential amplifier circuit 12 via the third resistor 16 (second signal level adjustment resistor) and the second capacitor 14 is referred to as the second circuit 20. The second circuit 20 is also connected to the ground via the first resistor 18 (first pull-down resistor). Note that noise caused by providing the first resistor 18 in front of the ground is input to the inverting input terminal (-) of the differential amplifier circuit 12 via the third resistor 16 and the second capacitor 14 in the second circuit 20, and is input to the non-inverting input terminal (+) of the differential amplifier circuit 12 via the fourth resistor 17, the second resistor 15, and the first capacitor 13. The differential amplifier circuit 12 is an amplifier circuit that amplifies the difference between the signal input to the non-inverting input terminal (+) and the signal input to the inverting input terminal (-) by a constant coefficient, and includes an operational amplifier. In the wireless transmitter 2 according to the first embodiment of the present disclosure, the inverting input terminal (-) of the differential amplifier circuit 12 is connected to ground via the second circuit 20 and the first resistor 18, as described above.

As shown in FIG. 4, the first circuit 19 and the second circuit 20 are connected via a fourth resistor 17. The fourth resistor 17 is a resistor provided to adjust the input level of the audio signal input from the television receiver 1, which is the input side of the audio signal, to an appropriate value.

In the wireless transmitter 2 according to the first embodiment of the present disclosure, the resistance value of the first resistor 18 provided in front of the ground in the second circuit 20 is set to have the same potential as the ground. Since the second circuit 20 is not directly connected to the ground in this way, the current carrying hum noise from the USB terminal 24 side is blocked from flowing to the second circuit 20, thereby suppressing the effect of hum noise on the audio signal.

In the first circuit 19, the first capacitor 13 and the second resistor 15 are provided in front of the non-inverting input terminal (+) of the differential amplifier circuit 12, as described above. In the second circuit 20, the second capacitor 14 and the third resistor 16 are provided in front of the inverting input terminal (-) of the differential amplifier circuit 12. Here, the first capacitor 13 and the second capacitor 14 have the same specifications. Also, the second resistor 15 and the third resistor 16 have the same specifications, i.e., the same resistance value.

In this way, since the first circuit 19 is provided with the first capacitor 13 and the second circuit 20 is provided with the second capacitor 14, only the analog signal with the DC component removed can be input to the non-inverting input terminal (+) and the inverting input terminal (-) of the differential amplifier circuit 12.

In addition, a second resistor 15 is provided in front of the non-inverting input terminal (+) of the differential amplifier circuit 12 in the first circuit 19, and a third resistor 16 is provided in front of the inverting input terminal (-) of the differential amplifier circuit 12 in the second circuit 20. This allows adjustment so that signals containing noise that are the same layer and have the same signal level are input to each of the non-inverting input terminal (+) and the inverting input terminal (-) of the differential amplifier circuit 12.

In the differential amplifier circuit 12, the noise superimposed on the audio signal input to the non-inverting input terminal (+) and the noise input to the inverting input terminal (-) cancel each other out and are removed. The audio signal from which the noise has been removed is then supplied from the differential amplifier circuit 12 to the ADC 33, where it is converted into a digital signal.

Therefore, the wireless transmitter 2 according to the first embodiment can reduce noise and suppress noise output.

In the example shown in FIG. 4, the second resistor 15 is provided in front of the first capacitor 13, and the third resistor 16 is provided in front of the second capacitor 14. However, the second resistor 15 may be provided in front of the non-inverting input terminal (+) in the first circuit 19 and in the rear of the first capacitor 13. Similarly, the third resistor 16 may be provided in front of the inverting input terminal (-) in the second circuit 20 and in the rear of the second capacitor 14.

Also, as shown in FIG. 5, the first circuit 19 may be configured to be connected to ground via a fifth resistor 22 (second pull-down resistor) in the same manner as the second circuit 20. FIG. 5 is a circuit diagram showing an example of a main configuration of an audio input unit 10 included in a wireless transmitter 2 according to a modified example of the first embodiment of the present disclosure.

As shown in FIG. 5, the first circuit 19 is connected to ground via the fifth resistor 22 before the second resistor 15, and the fifth resistor 22 has the same specifications as the first resistor 18.

When the first circuit 19 is configured to be grounded in the same way as the second circuit 20, the signal level of noise generated at the ground of the first circuit 19 is adjusted by the second resistor 15. Also, noise generated at the ground of the second circuit 20 is adjusted by the third resistor 16. Then, the noise superimposed on the signal input to the non-inverting input terminal (+) of the differential amplifier circuit 12 through the first circuit 19 and the noise input to the inverting input terminal (-) of the differential amplifier circuit 12 through the second circuit 20 can be adjusted to be the same signal level on the same layer.

The audio input unit 10 according to the modified example of the first embodiment of the present disclosure has an advantage in that it is easier to adjust the signal level of the noise input to the differential amplifier circuit 12 than the audio input unit 10 according to the first embodiment of the present disclosure.

Second Embodiment
A wireless transmitter 50 (audio input device) according to a second embodiment of the present disclosure will be described with reference to Fig. 6. Fig. 6 is a circuit diagram showing an example of a main configuration of an audio input unit 10 included in the wireless transmitter 50 according to the second embodiment of the present disclosure.

As shown in FIG. 6, the audio input section 10 of the wireless transmitter 50 according to the second embodiment differs from the configuration of the audio input section 10 of the wireless transmitter 2 according to the first embodiment in that, instead of the fourth resistor 17, the audio input section 10 includes a first transistor 56 (first switch) and a second transistor 62 (second switch) that function as switches so as to be ON when there is headphone input, and a third transistor 59 (third switch) and a fourth transistor 65 (fourth switch) that function as switches so as to be ON when there is line input.

Headphone input refers to the case where an audio signal output from the headphone terminal of the television receiver 1 is input to the wireless transmitter 50, and line input refers to the case where an audio signal output from the line terminal of the television receiver 1 is input to the wireless transmitter 50.

Furthermore, the audio input section 10 provided in the wireless transmitter 50 according to the second embodiment differs from the configuration of the audio input section 10 provided in the wireless transmitter 2 according to the first embodiment in that it further includes a sixth resistor 54, a seventh resistor 55, an eighth resistor 57, a ninth resistor 58, a tenth resistor 60, an eleventh resistor 61, a twelfth resistor 63, a thirteenth resistor 64, a fourteenth resistor 69, a fifteenth resistor 70, a sixteenth resistor 71, a seventeenth resistor 72, an eighteenth resistor 73, a third capacitor 66, a fourth capacitor 67, and a fifth capacitor 68.

In all other respects, the wireless transmitter 50 in the second embodiment is similar to the wireless transmitter 2 in the first embodiment, so similar components are given the same reference numerals and their description is omitted.

6, the audio input unit 10 includes a third circuit 51 that supplies a first signal (low signal) to the base of the first transistor 56 from the control circuit 30 via a pin (e.g., GPIO; General Purpose Input/Output) not shown in the control circuit 30, and a fourth circuit 52 that supplies a second signal (low signal) to the base of the third transistor 59. The audio input unit 10 also includes a fifth circuit 53 that supplies current from a power source 80 to each of the first transistor 56 and the third transistor 59.

The third circuit 51 connects the control circuit 30 and the base of the first transistor 56 via the sixth resistor 54. The third circuit 51 also branches between the sixth resistor 54 and the first transistor 56, and is connected to the fifth circuit 53 via the seventh resistor 55.

The fourth circuit 52 connects the control circuit 30 and the base of the third transistor 59 via the eighth resistor 57. The fourth circuit 52 also branches between the eighth resistor 57 and the third transistor 59, and is connected to the fifth circuit 53 via the ninth resistor 58.

The fifth circuit 53 is connected to the collector of the first transistor 56 and the collector of the third transistor 59.

The emitter of the first transistor 56 is connected to the base of the second transistor 62 via the tenth resistor 60. The emitter of the third transistor 59 is connected to the base of the fourth transistor 65 via the twelfth resistor 63.

The collector of the second transistor 62 is connected to the first circuit 19 via the eleventh resistor 61, and the emitter of the second transistor 62 is connected to the second circuit 20.

The collector of the fourth transistor 65 is connected to the first circuit 19 via the thirteenth resistor 64, and the emitter of the fourth transistor 65 is connected to the second circuit 20.

In addition, between the second transistor 62 and the fourth transistor 65, the first circuit 19 and the second circuit 20 are connected via the third capacitor 66, the fourth capacitor 67, and the fifth capacitor 68.

In addition, in the first circuit 19, a fifteenth resistor 70 is provided between the node with the fifth capacitor 68 and the node with the fourth capacitor 67, and in the first circuit 19, a fourteenth resistor 69 is provided between the node with the fourth capacitor 67 and the node with the third capacitor 66.

In addition, in the second circuit 20, a 17th resistor 72 is provided between the node with the fifth capacitor 68 and the node with the fourth capacitor 67, and in the first circuit 19, a 16th resistor 71 is provided between the node with the fourth capacitor 67 and the node with the third capacitor 66. Furthermore, in the second circuit 20, an 18th resistor 73 is provided in front of the node with the first resistor 18.

In the audio input section 10 provided in the wireless transmitter 50 according to the second embodiment, the signal level of the signal input to the differential amplifier circuit 12 through the first circuit 19 and the second circuit 20 can be adjusted by the 14th resistor 69, the 15th resistor 70, the 16th resistor 71, the 17th resistor 72, the third capacitor 66, the fourth capacitor 67, and the fifth capacitor 68.

In addition, in the wireless transmitter 50 according to the second embodiment, when a headphone input is applied, the control circuit 30 supplies a first signal to the base of the first transistor 56 through the third circuit 51. This turns the first transistor 56 ON, and a current flows between the collector and emitter of the first transistor 56 according to the voltage applied from the power source 80, and is supplied to the base of the second transistor 62.

When a current is supplied to the base of the second transistor 62, the second transistor 62 turns ON, and the collector and emitter of the second transistor 62 are electrically connected. In other words, the first circuit 19 and the second circuit 20 are electrically connected via the eleventh resistor 61.

The eleventh resistor 61 is a resistor provided to adjust the input level of the audio signal input from the television receiver 1, which is the input side of the audio signal when the headphone input is used, to an appropriate value.

On the other hand, when there is a line input, the control circuit 30 supplies the second signal to the base of the third transistor 59 through the fourth circuit 52. This turns the third transistor 59 ON, and a current corresponding to the voltage applied from the power supply 80 flows between the collector and emitter of the third transistor 59 and is supplied to the base of the fourth transistor 65. When a current is supplied to the base of the fourth transistor 65, the fourth transistor 65 turns ON, and the collector and emitter of the fourth transistor 65 are electrically connected. In other words, the first circuit 19 and the second circuit 20 are electrically connected via the thirteenth resistor 64.

The thirteenth resistor 64 is a resistor provided to adjust the input level of the audio signal input from the television receiver 1, which is the input side of the audio signal during line input, to an appropriate value.

Here, the audio signal specifications are different for headphone input and line input, and the impedance value of the television receiver 1, which is the output side of the audio signal, is also different. For this reason, the first circuit 19 and the second circuit 20 are configured to be connected via the eleventh resistor 61 for headphone input, and the first circuit 19 and the second circuit 20 are configured to be connected via the thirteenth resistor 64 for line input, so that the input level of the audio signal input from the television receiver 1 can be set to an appropriate value according to the input method of the audio signal.

In other words, the wireless transmitter 50 according to the second embodiment has the following configuration. That is, the wireless transmitter 50 includes a control circuit 30 that outputs a first signal when an audio signal is input by a first input method (e.g., headphone input) and outputs a second signal when the audio signal is input by a second input method (e.g., line input). The wireless transmitter 50 further includes a first switch (first transistor 56) that switches from an OFF state to an ON state in response to the first signal output from the control circuit 30, through which a current flows, and a second switch (second transistor 62) that connects the first circuit 19 and the second circuit 20 via a first input level adjustment resistor (eleventh resistor 61), and switches from an OFF state to an ON state in response to the current that flows when the first switch is in an ON state, thereby conducting the first circuit 19 and the second circuit 20.

The wireless transmitter 50 also includes a third switch (third transistor 59) that switches from an OFF state to an ON state in response to a second signal output from the control circuit 30, thereby allowing a current to flow, and a fourth switch (fourth transistor 65) that connects the first circuit 19 and the second circuit 20 via a second input level adjustment resistor (thirteenth resistor 64) and switches from an OFF state to an ON state in response to the current that flows when the third switch is in the ON state, thereby connecting the first circuit 19 and the second circuit 20.

The resistance value of the first input level adjustment resistor (11th resistor 61) is set so that the input level of the audio signal input by the first input method (e.g., headphone input) is an appropriate value. The resistance value of the second input level adjustment resistor (13th resistor 64) is set so that the input level of the audio signal input by the second input method (e.g., line input) is an appropriate value. In the wireless transmitter 50 according to the second embodiment, the resistance value of the first resistor 18 provided in front of the ground in the second circuit 20 is set to the same potential as the ground, as in the wireless transmitter 2 according to the first embodiment. Since the second circuit 20 is not directly connected to the ground in this way, the current carrying hum noise from the USB terminal 24 side is blocked from flowing to the second circuit 20, thereby suppressing the effect of hum noise on the audio signal.

In addition, in the first circuit 19, a first capacitor 13, a second resistor 15, a fourteenth resistor 69, and a fifteenth resistor 70 are provided in front of the non-inverting input terminal (+) of the differential amplifier circuit 12, as shown in FIG. 6.

In the second circuit 20, the second capacitor 14, the third resistor 16, the sixteenth resistor 71, and the seventeenth resistor 72 are provided in front of the inverting input terminal (-) of the differential amplifier circuit 12, as shown in FIG. 6. Here, the first capacitor 13 and the second capacitor 14 have the same specifications. The second resistor 15 and the third resistor 16 have the same specifications, i.e., the same resistance value. The fourteenth resistor 69 and the sixteenth resistor 71 have the same resistance value. The fifteenth resistor 70 and the seventeenth resistor 72 have the same resistance value.

This allows adjustments to be made so that signals containing noise that are at the same signal level in the same layer are input to the non-inverting input terminal (+) and the inverting input terminal (-) of the differential amplifier circuit 12.

Therefore, the differential amplifier circuit 12 can eliminate the noise superimposed on the audio signal input to the non-inverting input terminal (+) and the noise input to the inverting input terminal (-) by canceling each other out.

As described above, the wireless transmitter 50 according to the second embodiment can reduce noise and suppress noise output whether the input is through headphones or line input.

REFERENCE SIGNS LIST 1 Television receiver 2 Wireless transmitter 3 Electroacoustic transducer 4 USB cable 5 Audio cable 10 Audio input section 12 Differential amplifier circuit 13 First capacitor 14 Second capacitor 15 Second resistor (first signal level adjustment resistor)
16 Third resistor (second signal level adjustment resistor)
17 Fourth resistor 18 First resistor (first pull-down resistor)
19 First circuit 20 Second circuit 21 Audio terminal 21a Signal terminal 21b Ground terminal 22 Fifth resistor (second pull-down resistor)
23 Power supply circuit 24 USB terminal 25 Power ground circuit 30 Control circuit 31 Transmitter 50 Wireless transmitter 51 Third circuit 52 Fourth circuit 53 Fifth circuit 54 Sixth resistor 55 Seventh resistor 56 First transistor (first switch)
57 Eighth resistor 58 Ninth resistor 59 Third transistor (third switch)
60 Tenth resistor 61 Eleventh resistor (first input level adjustment resistor)
62 Second transistor (second switch)
63 twelfth resistor 64 thirteenth resistor (second input level adjustment resistor)
65 Fourth transistor (fourth switch)
66 third capacitor 67 fourth capacitor 68 fifth capacitor 69 fourteenth resistor 70 fifteenth resistor 71 sixteenth resistor 72 seventeenth resistor 73 eighteenth resistor 80 power supply 100 audio reproduction system

Claims (6)

an audio terminal having a signal terminal for outputting an audio signal input from an external device and a ground terminal for connecting to a ground;
a differential amplifier circuit having a first input terminal and a second input terminal;
a first circuit connecting the signal terminal and the first input terminal;
a second circuit connected to the ground terminal and the second input terminal;
The second circuit is coupled to ground through a first pull-down resistor. a first signal level adjusting resistor provided in a preceding stage of the first input terminal in the first circuit;
a second signal level adjusting resistor provided in the second circuit at a stage preceding the second input terminal;
2. The voice input device according to claim 1, wherein the first signal level adjusting resistor and the second signal level adjusting resistor have the same resistance value. The audio input device according to claim 1 or 2, wherein the first circuit is connected to ground via a second pull-down resistor. The audio input device according to claim 1 or 2, wherein the differential amplifier circuit removes noise superimposed on the audio signal input to the first input terminal and noise input to the second input terminal. a control circuit that outputs a first signal when the audio signal is input via a first input method, and outputs a second signal when the audio signal is input via a second input method;
a first switch that switches from an OFF state to an ON state in response to the first signal output from the control circuit, thereby allowing a current to flow;
a second switch that connects the first circuit and the second circuit via a first input level adjustment resistor, and that switches from an OFF state to an ON state in response to a current that flows when the first switch is in an ON state, thereby making the first circuit and the second circuit conductive;
a third switch that is switched from an OFF state to an ON state in response to the second signal output from the control circuit, thereby allowing a current to flow;
5. The voice input device according to claim 4, further comprising: a fourth switch that connects the first circuit and the second circuit via a second input level adjustment resistor, and switches from an OFF state to an ON state in response to a current that flows when the third switch is in an ON state, thereby electrically connecting the first circuit (19) and the second circuit (20). a resistance value of the first input level adjustment resistor is set so that an input level of the audio signal input by the first input method becomes an appropriate value;
6. The audio input device according to claim 5, wherein a resistance value of the second input level adjusting resistor is set so that an input level of the audio signal input by the second input method becomes an appropriate value.

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