JPH098926A - Doorphone slave set - Google Patents

Abstract

PURPOSE: To realize a doorphone slave set with a howling prevention function not causing reception blocking by means of a comparatively inexpensive circuit by providing an impedance adjustment circuit selectively increasing only the impedance at a prescribed frequency band so as to prevent howling. CONSTITUTION: A intercom slave set 1 is provided with a speaker 12 transducing an electric signal into a sound wave and providing an output of the wave and a microphone 10 picking up the sound wave and transducing it into an electric signal and is connected to an interphone master set 2 by a speech line Lt. Then an impedance adjustment circuit 14 that increases the impedance only at a prescribed frequency band selectively to prevent howling is provided to the speech line Lt. The impedance of the impedance adjustment circuit 14 has a frequency characteristic, and increases selectively the impedance at the frequency band matching the frequency of a howling tone specific to the system to reduce the loop gain, thereby preventing the production of howling.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an intercom subunit used in an intercom system, and more particularly to improvement of an intercom subunit for preventing howling of the intercom system.

[0002]

2. Description of the Related Art A conventional intercom system is shown in FIG. This system is configured by connecting a doorphone subunit 1 and an intercom master unit 2 via a communication line Lt. The intercom subunit 1 is installed at the entrance, etc., converts the voice of the visitor into an electric signal, transmits it to the intercom master unit 2 installed in the room, and also transmits the voice of the dwelling unit resident transmitted from the intercom master unit 2. The electric signal shown is converted into voice and output.

The intercom subunit 1 is a microphone 10
A microphone 12, a speaker 12, and a receiver circuit 13. The microphone 10 converts a voice of a visitor into an electric signal, and the transmitter circuit 11 transmits the transmission line Lt.
While receiving the voice signal from the intercom master unit 2, the speaker 12 converts the voice signal into voice and outputs the voice signal. The above audio signals are both transmitted between the intercom subunit 1 and the intercom master unit 2 via the communication line Lt.

Here, a part of the sound wave emitted from the speaker 12 is picked up by the microphone 10, converted into an electric signal and transmitted to the intercom master unit 2 again. Due to the acoustic coupling between the microphone 10 and the speaker 12, a closed loop of the audio signal is formed between the intercom subunit 1 and the intercom master unit 2. If the gain of the closed loop exceeds 1, a phenomenon called so-called howling occurs, which causes oscillation.

As a circuit for preventing this howling, there has been a circuit conventionally called a voice switch. This circuit configuration is shown in FIG. The voice switch 3 is composed of variable attenuators 31 and 32 and a control circuit 30. The variable attenuators 31 and 32 are provided in the transmission line L1 and the reception line L2, respectively, and the amount of attenuation thereof is controlled by the control circuit 30.

The control circuit 30 monitors the signal levels of the transmission line L1 and the reception line L2, and increases the attenuation amounts of the variable attenuators 31 and 32 when the signal levels exceed a predetermined level. By using this voice switch 3, the transmission / reception line L
It is possible to prevent howling from occurring by adjusting the loop gain of the closed loop formed by 1 and L2 so that it is always 1 or less.

[0007]

However, the conventional howling prevention circuit using the voice switch has a large circuit scale because the circuit configuration is complicated, and the door phone subunit is downsized and the price is reduced. There was a problem that I could not. In addition, the voice switch dynamically changes the gain of the transmitter circuit and the gain of the receiver circuit to prevent howling.Therefore, if the loop gain increases during a call, the gain decreases sharply. There is a problem that a phenomenon called reception blocking occurs in which the reception voice is interrupted.

On the other hand, it is known from experience that the howling sound generated in the transmitting / receiving circuit when the howling occurs occupies a specific frequency band of each system. The present invention has been made in view of the above circumstances, and realizes an intercom subunit having a howling prevention function that does not cause reception blocking by suppressing the gain in a specific frequency band with a relatively inexpensive circuit. The purpose is to

[0009]

An intercom subunit according to the present invention as defined in claim 1 comprises a speaker for converting an electric signal into a sound wave and outputting the sound wave, and a microphone for collecting the sound wave and converting the sound wave into an electric signal. An intercom subunit that is provided and is connected to the intercom master unit via a communication line, and selectively increases only the impedance in a predetermined frequency band,
An impedance adjusting circuit for preventing howling is provided on the communication line.

In the intercom subunit according to the present invention described in claim 2, the impedance adjusting circuit of the intercom subunit according to the present invention described in claim 1 is configured as a resonance circuit in which a resistor, a capacitor and a coil are connected in parallel. It

[0011]

In the intercom subunit according to the present invention as defined in claim 1, the impedance adjusting circuit regulates the impedance of the communication line between the intercom subunit and the intercom master. The impedance of this impedance adjustment circuit has a frequency characteristic, and this circuit selectively increases the impedance of the frequency band in accordance with the frequency of the howling sound peculiar to the system to reduce the loop gain in the frequency band. This prevents howling from occurring.

In the intercom subunit according to the second aspect of the present invention, the impedance increases in the vicinity of the resonance frequency of the resonance circuit in which the resistor, the capacitor and the coil are connected in parallel, which makes use of the predetermined frequency band. It is possible to prevent howling from occurring by selectively reducing only the gain.

[0013]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 shows an example of the construction of an intercom subunit according to the present invention as defined in claims 1 and 2. This doorphone handset 1
Is a microphone 10, a transmission circuit 11, and a speaker 1.
2, the reception circuit 13, and the resonance circuit 14 are provided. The resonance circuit 14 is an impedance adjustment circuit provided on the communication line Lt. This impedance adjustment circuit is a resonance circuit configured by connecting a resistor R, a capacitor C, and a coil L in parallel, and has a unique resonance frequency f.
have o.

An outline of the frequency characteristic of the resonance circuit 14 is shown in FIG. 2A shows the frequency characteristic of the reactance component, and FIG. 2B shows the frequency characteristic of the impedance component. The resonance frequency fo of this circuit is

[0015]

[Equation 1]

Is given by At this resonance frequency fo, the inductive reactance due to the coil L and the capacitive reactance due to the capacitor C cancel each other out, and its impedance becomes the maximum value, that is, the resistance R. Here, the intercom system can know the frequency component of the howling sound by measuring the transmitted / received signal when the howling is generated and analyzing the frequency component thereof. As a result of this analysis, if the frequency of the main component of the frequency components that make up the howling sound can be matched with the resonance frequency fo, the gain of the closed loop at that frequency can be reduced, and howling occurs. Can be suppressed.

Here, the frequency component of the howling sound is the frequency characteristic of the closed loop formed in the intercom slave unit 1 and the intercom master unit 2, that is, the frequency characteristic of the amplifier constituting this closed loop and the sound between the speaker and the microphone. It depends on the frequency characteristics of the coupling, etc., and varies from system to system. Therefore, each system can be known by measuring the frequency characteristic of the transmission / reception signal at the time of howling.

In general, the frequency band in which howling occurs is often higher than the human voice band. Therefore, even if the impedance of such a frequency band is increased and its gain is reduced. , The effect on the transmitted / received voice is small. Next, such a resonance circuit 14 is connected to the communication line L.
Call line L when connected to t and when not connected
The frequency characteristic of t was measured. The result is shown in FIG. The characteristic indicated by the broken line in the figure is the characteristic of the conventional intercom subunit 1 that does not include the resonance circuit 14, and the characteristic indicated by the solid line in the figure is the intercom subunit according to the present invention that includes the resonant circuit 14. This is a characteristic of 1.

It has been confirmed by measurement that the intercom system used in this experiment produces howling having a main component near 2.5 kHz. In this experiment, a 0.57 μF capacitor C, an 8 mH coil L,
A resonant circuit 14 consisting of a resistance R of 680Ω was used. As can be understood from the result of this measurement, the resonance frequency f
o is about 2.45 kHz, and the impedance at that time is about 1.13 kΩ.

In the intercom subunit without the conventional resonance circuit 14, the impedance at around 2.5 kHz is about 700Ω, and the main band of the audio signal is 1 kHz.
There is no significant difference between the impedance of 600 Ω to 700 Ω at ˜2 kHz, while in the case of the intercom subunit according to the present invention including the resonance circuit 14, the resonance frequency is 2.5 kHz.
The impedance around z is about 1.13 kΩ,
It has a larger value than other frequency bands. For this reason, it is possible to suppress howling which causes a howling sound at around 2.5 kHz.

Since the resonance frequency fo is a frequency determined by the values of the capacitor C and the coil L as shown in the equation (1), the main frequency of the howling sound of each system is measured and the resonance frequency fo is resonated in the frequency band. If these values are determined so as to have the frequency fo, howling can be prevented with a simple circuit. Further, unless the resonance frequency fo is within the call band, the call voice is not attenuated, so that there is no inconvenience that reception blocking occurs to prevent howling unlike the case where a conventional voice switch is used. .

[0022]

The intercom subunit according to the present invention described in claim 1 can prevent the occurrence of howling by providing an impedance adjusting circuit in the communication line with the intercom master. That is, by reducing the loop gain of the specific frequency component that contributes to howling, howling can be suppressed without affecting the communication voice.

In the intercom subunit according to the present invention as set forth in claim 2, howling can be prevented by providing a resonance circuit in which a capacitor, a coil and a resistor are connected in parallel, in the communication line. Therefore, it is possible to realize an intercom subunit having a howling prevention function with a simple circuit, and it is possible to provide an inexpensive and small intercom subunit.

[Brief description of drawings]

FIG. 1 is a diagram showing a configuration example of an intercom subunit according to the present invention described in claims 1 and 2.

FIG. 2 is a diagram showing an example of a schematic frequency characteristic of a resonance circuit shown in FIG.

FIG. 3 is a diagram showing experimental results in comparison with a conventional intercom subunit.

FIG. 4 is a diagram showing a configuration of a conventional intercom subunit.

FIG. 5 is a diagram showing a configuration of a voice switch.

[Explanation of symbols]

 1 ・ ・ ・ Doorphone slave 2 ・ ・ ・ Intercom master 10 ・ ・ ・ Microphone 12 ・ ・ ・ Speaker Lt ・ ・ ・ Call line 14 ・ ・ ・ Impedance adjusting circuit R ・ ・ ・ Resistor C ・ ・ ・ Capacitor L ・ ・·coil

Claims (2)

[Claims] 1. An intercom subunit that includes a speaker that converts an electric signal into a sound wave and outputs the sound wave, and a microphone that collects the sound wave and converts the sound wave into an electric signal, which is connected to an intercom master unit through a communication line. An intercom subunit, wherein an impedance adjusting circuit for selectively raising only impedance in a predetermined frequency band to prevent howling is provided on the communication line. 2. The intercom subunit according to claim 1, wherein the impedance adjusting circuit is configured as a resonance circuit in which a resistor, a capacitor and a coil are connected in parallel.