JPH1127378A - Howling prevention circuit for radio telephone set - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent howling with a simple circuit configuration in an on-vehicle hand-free device or the like. SOLUTION: The circuit is provided with an external speaker switch 9a, an external microphone switch 9b, an external microphone timer 4 that opens the external microphone switch 9b for a setting time when an output is received from a receiver, and an external speaker timer 5 that opens the external microphone switch 9b for only a setting time when an input comes from the external microphone. When either of the external microphone timer 4 and the external speaker timer 5 is closed, the other is reset.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a howling prevention circuit for preventing howling that occurs when transmitting and receiving signals using an external microphone and an external speaker in a wireless telephone such as a portable telephone.

[0002]

2. Description of the Related Art The number of traffic accidents caused by using a mobile phone while driving a car is increasing rapidly with the rapid spread of mobile phones. Therefore, an in-vehicle hands-free device that enables a user to transmit and receive a signal while driving using an external microphone and an external speaker without holding a mobile phone in his hand has been used as a useful device, and the number of such devices has been increasing. I am following.

Conventionally, such an in-vehicle hands-free device is preferably installed as close to the driver's head as possible without interfering with driving, so that an external microphone and an external speaker are close to each other. It is provided, for example, so as to be detachably attached to a sun visor in a vehicle and used. FIG. 3 shows an example of the use, in which a mobile phone 50 and a hands-free device 51 are connected by a lead wire 52, and the hands-free device 51 is detachably attached to a sun visor 53. The hands-free device 51
An external speaker 54 and a required circuit are provided in a housing 55,
A small external microphone 56 is connected to a flexible rod 57
And is detachably attached to the outside of the housing 55.

[0004]

However, in the conventional hands-free device, howling occurs between the external microphone and a receiver output (external speaker) from the main body of the mobile phone, and especially in the case of a digital mobile phone. ,
There was a case where conversation became impossible due to a specific echo phenomenon (it was impossible to hear at the other end of the call).

[0005] Although a general telephone is provided with an anti-howling circuit, the conventional anti-howling circuit is generally complicated and expensive.
Moreover, it is unsuitable for application to an in-vehicle hands-free device that is merely for external expansion.

An object of the present invention is to provide a howling prevention circuit that can prevent howling with a simple circuit configuration in, for example, the above-described hands-free device for a vehicle.

[0007]

According to the present invention, an anti-howling circuit includes an external speaker switch 9a provided on a path from a receiver output terminal of a radio telephone body to an external speaker, and an external microphone of the radio telephone main body. An external microphone switch 9b provided on the path leading to the terminal 11, an external microphone timer 4 for turning off the external microphone switch 9b for a set time when an input is received from a receiver output terminal, and an input from the external microphone. And the external speaker timer 5 for turning off the external speaker switch 9b for a set time when there is an error. The external microphone timer 4 and the external speaker timer 5 reset the other when one is on. Connected to be.

Such a howling prevention circuit can be built in a hands-free device having an external microphone and an external speaker.

[0009]

Next, embodiments of the present invention will be described in detail with reference to the drawings.

FIG. 1 shows an example of a howling prevention circuit according to the present invention. This howling prevention circuit is provided in the housing when applied to an in-vehicle hands-free device as shown in FIG. In FIG. 1, the left side is the input side, and the right side is the output side.

The receiver input terminal 1 is connected to a receiver output terminal (not shown) of the mobile phone main body, and an output Rx from the receiver of the mobile phone main body is input to this terminal. The external microphone input terminal 2 is connected to an external microphone (the external microphone 56 in the case of the hands-free device 51 shown in FIG. 3).
An output MIC from an external microphone is input.

Operational amplifiers 3a and 3b formed into ICs are connected to the receiver input terminal 1 and the external microphone input terminal 2, respectively.
It is input to the external microphone timer 4 and the external speaker timer 5 which are converted into C. The external microphone timer 4 includes a resettable monostable multivibrator 6
a, a logic circuit 7a, and an RC circuit 8a. Similarly, the external speaker timer 5 is also capable of performing a reset operation, and is a monostable multivibrator 6b, a logic circuit 7b, and an RC circuit 8b.
It is composed of

The operation time of the monostable multivibrator 6a of the timer 4 for the external microphone can be arbitrarily set by the resistor R1 and the capacitor C1 of the RC circuit 8a, and the operation time of the monostable multivibrator 6b of the timer 5 for the external speaker. Can be arbitrarily set by the resistor R2 and the capacitor C2 of the RC circuit 8b.

As for the monostable multivibrators 6a and 6b, the output from the output terminal (Q bar terminal) of the monostable multivibrator 6a is input to the reset terminal of the monostable multivibrator 6b. Are connected so that the output from the output terminal (Q-bar terminal) of the monostable multivibrator 6a is inputted to the reset terminal.

The output Rx from the receiver amplified by the operational amplifier 3a is input to the input terminal of the logic circuit 7a.
The output MIC from the external microphone is input to the input terminal of the logic circuit 7b.

The output Rx from the receiver amplified by the operational amplifier 3a is supplied to an analog switch 9 formed as an IC.
When “a” is on, the signal is input to the speaker driving IC amplifier 10, and the external speaker 54 is driven by the IC 10. On the other hand, the output MIC from the external microphone amplified by the operational amplifier 3b is output from the microphone output terminal 11 to the external microphone connection terminal of the mobile phone when the analog switch 9b formed into an IC is turned on. Signal.

Next, the operation of the howling prevention circuit will be described with reference to the timing chart of FIG.

When neither the output Rx from the receiver nor the output MIC from the external microphone is present, the terminals of the logic circuits 7a and 7b are fixed at the LOW level as shown in FIG. At this time, the CD terminals of the monostable multivibrators 6a and 6b are at the HIGH level, and the timers 4 and 5 are in the standby state.

When the output Rx from the receiver is input in the standby state, the terminal of the logic circuit 7a is triggered, the monostable multivibrator 6a of the external microphone timer 4 is turned on, and the output of the output terminal is inverted. To a LOW level. Since this output is input to the CD terminal of the monostable multivibrator 6b of the external speaker timer 5, the logic circuit 7b does not accept the output MIC from the external microphone even if it is input to the terminal. . Monostable multivibrator 6
Since the output of the output terminal b goes high, the analog switch 9a is turned on. The output Rx from the receiver is input to the speaker drive IC amplifier 10 via the turned-on analog switch 9a, so that the external speaker 54 is driven. However, since the output of the output terminal of the monostable multivibrator 6a becomes LOW level, the analog switch 9b is turned off. Therefore, at this time, even if the output MIC from the external microphone is input, it is cut by the analog switch 9b and is not output from the microphone output terminal 11.

When the output Rx from the receiver disappears and the operation time of the monostable multivibrator 6a elapses, the output of the output terminal is inverted and returns to the standby state.
As described above, the time t1 at which the output MIC from the external microphone is cut off is determined by the operation time of the monostable multivibrator 6a, and can be arbitrarily set by the resistor R1 and the capacitor C1.

On the other hand, when the output MIC from the external microphone is inputted in the standby state, the logic circuit 7b
Is triggered, the monostable multivibrator 6b of the external speaker timer 5 is turned on, and the output of its output terminal is inverted to a LOW level. This output is input to the CD terminal of the monostable multivibrator 6c of the timer 4 for the external microphone, so that the logic circuit 7
In the state a, even if the output Rx from the receiver is input to the terminal, it is not accepted. Since the output of the output terminal of the monostable multivibrator 6a becomes HIGH level, the analog switch 9b is turned on. The output MIC from the external microphone is output from the microphone output terminal 11 to the external microphone connection terminal of the mobile phone via the turned on analog switch 9b, and becomes a transmission signal of the mobile phone. However, the monostable multivibrator 6b
Since the output of the output terminal becomes low level, the analog switch 9a is turned off. Therefore, at this time, even if the receiver output Rx from the mobile phone is input, this is cut off by the analog switch 9a, so that the external speaker 54 is not driven.

When the output MIC from the external microphone disappears and the operation time of the monostable multivibrator 6b elapses, the output of the output terminal is inverted and returns to the standby state. As described above, the time t2 at which the receiver output Rx is cut off is determined by the operation time of the monostable multivibrator 6b, and can be arbitrarily set by the capacitor C2 and the resistor R2.

The time t1 at which the output MIC from the external microphone is cut off when the output Rx from the receiver comes first, and the output Rx from the receiver is cut off when the output MIC from the external microphone comes first. Time t
Appropriate selection of 2 enables howling to be suppressed even when the external microphone and the external speaker are close to each other, while maintaining a situation close to simultaneous calls, eliminating the echo phenomenon unique to digital mobile phones. Becomes possible.

[0024]

As described above, according to the present invention, it is possible to suppress howling between an external microphone and an external speaker while maintaining a situation close to a simultaneous call, and achieve this with a simple circuit configuration. Therefore, it is suitable for application to an in-vehicle hands-free device connected to a digital mobile phone.

[Brief description of the drawings]

FIG. 1 is a circuit configuration diagram of a howling prevention circuit according to an embodiment of the present invention.

FIG. 2 is a timing chart showing a part of the above operation.

[Explanation of symbols]

 Reference Signs List 1 receiver input terminal 2 external microphone input terminal 3a / 3b operational amplifier 4 external microphone timer 5 external speaker timer 6a / 6b monostable multivibrator 7a / 7b logic circuit 8a / 8b RC circuit 9a / 9b analog switch 10 speaker drive IC amplifier 11 Microphone output terminal

Claims (3)

[Claims] An external speaker switch provided on a path from a receiver output terminal of a wireless telephone body to an external speaker, and an external microphone switch provided on a path from an external microphone to an external microphone terminal of the wireless telephone body. An external microphone timer that turns off the external microphone switch for a set time when an input is received from the receiver output terminal; and an external speaker switch for only the set time when an input is received from the external microphone. A timer for an external speaker that is turned off, wherein the timer for an external microphone and the timer for an external speaker are connected so that when one of them is on, the other is reset, and a howling prevention circuit for a wireless telephone is provided. . 2. A hands-free device comprising an external microphone and an external speaker, wherein the external speaker switch, the external microphone switch, the external microphone timer, and the external speaker timer are built-in. Howling prevention circuit for wireless telephones. 3. A hands-free device for use in a vehicle.
2. A howling prevention circuit for a wireless telephone according to claim 1.