JP2698653B2 - Loudspeaker intercom system - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a loud-speaking intercom system, and more particularly, to a doorphone with a television camera as an intercom slave unit, and a handset on the side of the intercom master unit at home. The present invention is suitable for an intercom system that enables a so-called hands-free loudspeaker call without using a telephone.

[Prior Art] Conventional Example 1 FIG. 5 is a schematic configuration diagram of a conventional intercom system. In the figure, 100a and 100b are intercom indoor units having a handset, 500 is a doorphone adapter comprising a bidirectional amplifier, and 200a is a doorphone arranged outside the house. The conventional intercom system consists of home units 100a, 100b
When talking with each other, since each handset has a handset Ha, Hb, the transmission / reception level is high, and the home units 100a, 1
The call can be directly made by the call circuit between 00b. However, when talking between the indoor unit 100a (or 100b) and the intercom 200a, the transmission and reception level of the intercom 200a is low, so it is necessary to provide the intercom adapter 500 with a built-in bidirectional amplifier, which increases costs. I was invited.

Conventional Example 2 On the other hand, FIG. 6 is a schematic configuration diagram of a conventional intercom system with a television. In the figure, l ₁ is 2-wire wire, 200a for sending a coaxial cable, l ₂ is a call signal for sending a video signal is a door phone with a TV camera, 100a is a home device with a monitor television. Door phone 200a
It has a TV camera TVC, a speaker SPB and a microphone MB. The indoor unit 100a includes a monitor television MTV that displays video information from the door phone 200a with a television camera, and a handset H for telephone calls. This indoor unit 100a is the fifth
It also includes the function of the door phone adapter 500 shown in the figure.
This system requires a coaxial cable l ₁ and a two-wire l ₂ , and the wiring is complicated, and the coaxial cable l ₁
There is a problem in that wiring is required to be skilled because of the use of the wire. Further, since a call is made using the handset H, it is necessary to always hold the handset H during a call, and it is necessary to use at least one hand. In addition, the home unit 100a in this type of intercom system with a television is generally of a wall-mounted type, and has a drawback that the handset H comes off and falls off due to unexpected contact. Therefore, there has been proposed a hands-free intercom system capable of talking with both hands apart.

Conventional Example 3 FIG. 7 is a schematic configuration diagram of a conventional hands-free intercom system. In the figure, reference numerals 1 and 2 denote microphones and speakers for loudspeakers, 3 to 6 are amplifiers, 7 to 10 are level detection circuits of respective communication paths, and 11 is level detection circuits 7 to 10.
Is a control circuit for controlling the transmitting state and the receiving state by the output of. 12, 13 are variable loss circuits whose loss is controlled by the output of the control circuit 11, and 14 is a sidetone prevention circuit,
Perform line transformation. 1 is a two-wire transmission line, 500 is a doorphone adapter with a bidirectional amplifier, 200a is a doorphone, and SPB and MB are doorphone speakers and a microphone.

In such an intercom system, a sound emitted from a distant position is received by the microphone 1 and a two-wire transmission line l is received.
It is necessary to output the received voice from the speaker 2 at a large level.
Therefore, it is desired to set the gains of the amplifiers 3 to 6 to be high. However, if the audio signal leaks due to the poor impedance matching of the sidetone prevention circuit 14, the amplifiers 3 and 4 and the sidetone prevention circuit 14 Circuit 14, amplifiers 6 and 5,
Howling occurs in a loop returning to the microphone 1 via the speaker 2. To prevent this, the level detection circuit 7
The transmission level at the microphone 1 and the reception level from the transmission line 1 are constantly compared by means of the control circuit 11, and under control of the control circuit 11, the loss of the higher-level communication path is reduced. Variable loss circuits 12,1 so as to increase the
Controlling three. Thereby, the howling is prevented by setting the loop gain of the above-mentioned loop to 1 or less. More specifically, at the time of transmission, the transmission voice input to the microphone 1 is detected by the level detection circuit 7, the loss of the variable loss circuit 12 is controlled to be low, and the two-wire type is transmitted through the sidetone prevention circuit 14. The speaker SPB of the door phone 200a is amplified by the door phone adapter 500 on the transmission line l.
Output. Conversely, during reception, the input from the microphone MB of the intercom 200a is amplified by the intercom adapter 500, received through the sidetone prevention circuit 14, detected by the level detection circuit 10, and the loss of the variable loss circuit 13 is controlled to be low. ,
Output from the speaker 2.

By the way, the transmission signal output from the amplifier 4 at the time of transmission goes around the amplifier 6 due to signal leakage from the sidetone prevention circuit 14, is detected by the level detection circuit 10, and is similar to the case where the reception signal is input. The transmission signal is attenuated and controlled
In some cases, you are in a listening state while trying to transmit. This is called "transmission blocking". In order to prevent such a malfunction, it is necessary to set the sensitivity of the level detection circuit 10 lower than the optimum value. In that case, especially the door phone 200a where the other party of the call does not have a handset
, The call level is generally lower than the call level of the handset.
If the sensitivity of 10 is lower than the optimum value, it is difficult for the control circuit 11 to switch to the receiving state with the signal from the door phone 200a. Also, for example, even if the intercom adapter 500 is inserted to amplify the signal from the intercom 200a, the intercom adapter 500 inserted into the two-wire transmission line l is made of a bidirectional amplifier, so that there is no limit to the amplification. If the rate is increased, howling will occur inside the doorphone adapter 500, so there is a limit on the amplification rate. Generally, the amplification rate of about 10dB in both directions is the limit.
We cannot expect the signal level from 200a to be too high. Therefore, the signal from the doorphone 200a uses the control circuit
10 is hard to switch to the receiving state. As a result, there has been a problem that a call failure such as a wrong head disconnection, a busy disconnection, and an end disconnection is likely to occur.

[Problem to be Solved by the Invention] The present invention has been made in view of the above-mentioned drawbacks, and
The purpose of the present invention is to prevent the occurrence of transmission blocking, allow the gain to be freely set, and eliminate the need for a bidirectional amplifier and sidetone prevention circuit in a loud-speaking intercom system using a wire transmission line. In addition, it is an object of the present invention to add a voice call function at low cost in an intercom system with a television.

[Means for Solving the Problems] In the present invention, in order to solve the above-described problems, as shown in FIG. 1, an intercom master unit 100 including a microphone 301 and a speaker 302 for a loudspeaker call; Intercom cordless handset 200 with microphone MB and speaker SPB for loudspeakers
And a two-wire transmission line 1 for connecting the intercom master unit 100 and the intercom slave unit 200, the intercom master unit 100 modulates an audio signal obtained from the microphone 301 to form a two-wire transmission line. 1 to send to
(FM modulation circuit 101) and a first demodulation means (FM) for demodulating the modulation output of the first modulation means obtained from the two-wire transmission line l and outputting it to the speaker SPB of the intercom slave 200. Demodulation circuit 202) and microphone MB of intercom slave 200
Modulation means (FM modulation circuit 203) for modulating the audio signal obtained from the second transmission line l and transmitting it to the two-wire transmission line l, and the modulation output of the second modulation means obtained from the two-wire transmission line l And demodulates the signal to the speaker 302 of the intercom master 100.
(FM demodulation circuit 102), a first variable loss means 312 provided on the input side of the first modulation means, and a second variable loss means 312 provided on the output side of the second demodulation means. Means of loss
313, level detection means 307 to 310 for detecting the level of the input / output signal of each variable loss means, and the loss amount of the variable loss means such that the loss amount in the call direction is reduced according to the detection value of the level detection means. The modulation output of the control means 311 for controlling and the modulation outputs of the first and second modulation means have different frequency bands.

If the present invention is applied to an intercom system with a television that multiplexes and transmits at least a video signal and an audio signal by frequency division, the present invention can be implemented simply by replacing the handset of the intercom master unit 100 with the loudspeaker unit 300. be able to.

[Operation] According to the present invention, as described above, in the intercom system in which the intercom master unit 100 and the intercom slave unit 200 are connected by the two-wire transmission line l, the intercom master unit 100 is connected to the intercom slave unit. The audio signal to the intercom slave 200 is transmitted by the first modulation signal.
Since the audio signal to the intercom master 100 is transmitted by the second modulation signal, and the frequency bands of the first modulation signal and the second modulation signal are made different, the transmission / reception speech signal is transmitted in a pseudo four lines. As a result, there is no influence of signal wraparound, so that transmission blocking and howling do not occur. Therefore, the amplification factor can be set optimally, and the loudspeaker-type intercom handset 200 Can be clearly communicated to the same extent as when a handset is used.

Embodiment 1 FIG. 1 is a block diagram of a first embodiment of the present invention. The intercom system according to the present embodiment includes an intercom master unit 100 located at home and an intercom slave unit 2 located outside home.
00, which are connected by a transmission line 1 made of a coaxial cable.

First, the intercom master unit 100 includes an FM modulation circuit 101 that FM-modulates an audio signal, an FM demodulation circuit 102 that demodulates the FM modulation signal transmitted from the slave unit 200 and extracts an audio signal, and
There is provided a balanced input / output circuit 104 for inputting the FM modulation signal output from the modulation circuit 101 and outputting the FM modulation signal transmitted from the slave 200 through the transmission line l to the FM demodulation circuit 102. The FM modulation circuit 101 is configured to FM-modulate the transmission voice signal from the loudspeaker communication circuit 300 and send it out to the transmission line 1 through the balanced input / output circuit 104.
In the configuration 2, the FM modulation signal from the balanced input / output circuit 104 is demodulated, and the sound of the slave 200 is output from the speaker 302 of the loudspeaker communication circuit 300.

On the other hand, intercom slave device 200 includes an FM demodulation circuit 202 that demodulates the FM modulation signal sent from master device 100 and extracts an audio signal, an FM modulation circuit 203 that FM-modulates an audio signal from microphone MB, and an FM It has a balanced input / output circuit 205 that sends out a signal output from the modulation circuit 203 to the transmission line 1 and outputs an FM modulation signal to the FM demodulation circuit 202. FM modulation circuit 2
In 03, the audio signal from the microphone MB is FM-modulated and sent to the parent device 100 via the balanced input / output circuit 205, and the FM demodulation circuit 202
In this configuration, the FM modulation signal from the balanced input / output circuit 205 is demodulated and the sound is output from the speaker SPB.

In this intercom system, the parent device 100 and the child device 200
Are connected by a transmission line 1 made of a coaxial cable, and the connection terminals T1 and TG provided on the master device 100 side and the connection terminals T2 and TG provided on the slave device 200 side are connected to the transmission line 1 formed of the coaxial cable. They are connected by a core wire and a shield sheath wire, respectively. An FM-modulated audio signal is multiplex-transmitted between the two via the transmission line l.

Hereinafter, the operation of the present embodiment will be described. The voice signal from the microphone 301 in the voice communication circuit 300 of the base unit 100 is
FM modulation is performed by the FM modulation circuit 101, and the slave 200 is transmitted via the transmission line l.
The demodulation is performed by the FM demodulation circuit 202, and the resident's voice is output from the speaker SPB. The visitor's voice signal input by the microphone MB is FM-modulated by the FM modulation circuit 203 and transmitted via the transmission line l. Transmitted to master unit 100, FM demodulation circuit 10 of master unit 100
The signal is demodulated in 2 and the speaker 302 in the voice communication circuit 300
Output a voice signal of the visitor, and a call is made between the visitor and the resident.

In the present embodiment, since the frequency of the modulation signal output from the FM modulation circuit 101 is different from the frequency of the modulation signal demodulated by the FM demodulation circuit 102, the sound signal does not wrap around in the balanced input / output circuit 104. Therefore, the microphone
301, amplifier 303, variable loss circuit 312, amplifier 304, FM modulation circuit 101, balanced input / output circuit 104, FM demodulation circuit 102, amplifier 3
06, the gain of the howling loop including the variable loss circuit 313, the amplifier 305, the speaker 302, and the microphone 301 is zero. However, microphone 301, amplifier 303, variable loss circuit 31
2, amplifier 304, FM modulation circuit 101, balanced input / output circuit 104, transmission line 1, balanced input / output circuit 205, FM demodulation circuit 202, speaker
SPB, microphone MB, FM modulation circuit 203, balanced input / output circuit 205,
Transmission line 1, balanced input / output circuit 104, FM demodulation circuit 102, amplifier
Since there is a howling loop including the 306, the variable loss circuit 313, the amplifier 305, the speaker 302, and the microphone 301, the loss of the variable loss circuits 312 and 313 does not need to be set as large as in the conventional example, but is required to some extent. Needless to say, if the acoustic coupling between the speaker 302 and the microphone 301 or the acoustic coupling between the speaker SPB and the microphone MB is structurally or electrically removed, the variable loss circuits 312 and 313 become unnecessary.

The operation of the loudspeaker communication circuit 300 during a call of this intercom system will be described in detail. The transmission voice detected by the microphone 301 is amplified by the amplifier 303, and by comparing the detection results of the level detection circuits 307 and 309, the control circuit 31
The variable loss circuits 312 and 313 are controlled, the loss of the variable loss circuit 312 is reduced, and the loss of the variable loss circuit 313 is increased, so that the microphone 301, the amplifier 303, the variable loss circuit 312, the amplifier 304, the FM modulation circuit 101, the balanced input Output circuit 104, transmission line 1, balanced input / output circuit 205, FM demodulation circuit 202, speaker SPB, microphone MB, FM
Modulation circuit 203, balanced input / output circuit 205, transmission line 1, balanced input / output circuit 104, FM demodulation circuit 102, amplifier 306, variable loss circuit 3
13, the howling loop including the amplifier 305, the speaker 302, and the microphone 301 has a loop gain of 1 or less to prevent howling and without attenuating the transmission signal detected by the microphone 301 to the FM modulation circuit 101 side. Send out.

Conversely, the audio signal input from the microphone MB of the slave 200 is input to the loudspeaker circuit 300 as a reception signal by the FM demodulation circuit 102 by the system described above, and the level detection circuit 310
The control circuit 311 compares the detection results of and 308 to increase the loss of the variable loss circuit 312,
The reception signal is output from the speaker 302 with the loss of the thirteen being reduced.

Here, the level of the received signal output from the FM demodulation circuit 102 can be amplified to some extent by the FM modulation circuit 203 and the FM demodulation circuit 102, so that the level detection circuit 310 can easily detect the received signal, and Disconnection or disconnection during talking is unlikely to occur. As described above, in the conventional hands-free intercom call, there is a limit to the increase in the gain of the intercom adapter, but in the present embodiment, the modulation frequency of the communication signal between the master unit and the slave unit on the transmission line l is different. ,
This makes it possible to amplify the call signal to an optimum level. Further, in this embodiment, since there is no sidetone prevention circuit for two-wire / four-wire conversion, there is an advantage that erroneous reception detection (transmission blocking) due to a wraparound of a transmission signal due to impedance mismatch does not occur. . That is,
Transmitting FM signals with different frequencies between the parent device and the child device means that the transmission and reception signals are transmitted over four wires in a pseudo manner, eliminating the influence of signal wraparound and causing transmission blocking. It will not be.

Second Embodiment FIG. 2 shows the appearance of an intercom system according to a second embodiment of the present invention. In this system, a call switch S on the intercom slave 200 side provided at the entrance or the like is provided.
When the WB is operated, a calling signal is transmitted to the intercom master unit 100 provided in the room through the transmission line l, and a calling sound is output from the speaker SPA. When the resident responds by pressing the response switch SWA, the television camera TV of the child device 200
Power is supplied to C, a video signal is sent to master unit 100 via transmission line l, and is displayed as an image on monitor TV MTV, and speaker SPB and microphone MB of slave unit 200 and a loudspeaker unit of master unit 100 are provided. 300 microphones 301 and speakers 3
With 02, a hands-free call is made.

FIG. 3 is a block diagram showing the configuration of the above system. This system includes an intercom master unit 100 having a monitor TV MTV installed in the house and an intercom slave unit 200 having a TV camera TVC installed at the entrance.
And both are connected by a dedicated transmission line 1 such as a coaxial cable. Power is supplied to the slave 200 from the master 100, and video, audio, and control signals are multiplexed and communicated with necessary information.

A more specific configuration will be described.
0 is a DC power supply E for supplying power to the slave 200,
A constant current circuit 105 for stabilizing the current from the DC power supply E and supplying it to the slave 200, and a call signal for demodulating the FM modulated wave of the call signal transmitted in response to the operation of the call switch SWB on the slave 200. A demodulation circuit 106 and a ringing signal amplification circuit 107 that amplifies the demodulated ringing tone and sounds a ringing tone from the speaker SPA.
And an FM modulation circuit 101 for FM-modulating the audio signal, an FM demodulation circuit 102 for demodulating the FM-modulated signal sent from the slave 200 and extracting the audio signal, and a video signal sent from the slave 200 A video signal amplification circuit 103 that amplifies and outputs the signal to the monitor television MTV, and an FM modulation signal output from the FM modulation circuit 101 are input, and the FM modulation signal transmitted from the slave 200 via the transmission line l is input. Output to the FM demodulation circuit 102,
A balanced input / output circuit 104 outputs a video signal transmitted from the slave 200 through the transmission line l to the video signal amplifier circuit 103, and further transmits power supplied from the constant current circuit 105 to the slave 200. ing. The FM modulation circuit 101 is configured to FM-modulate the transmission voice signal from the loudspeaker communication circuit 300 and send the result to the transmission line l through the balanced input / output circuit 104.
Reference numeral 2 denotes a configuration in which the FM modulation signal from the balanced input / output circuit 104 is demodulated and the sound of the slave 200 is output from the speaker 302 of the loudspeaker communication circuit 300. The video signal transmitted from the slave 200 is amplified by the video signal amplifier 103 through the balanced input / output circuit 104, and then output to the monitor television MTV. It is needless to say that it is easy to configure a single speaker as the speakers SPA and 302.

On the other hand, intercom slave device 200 includes a video signal amplification circuit 201 for amplifying a video signal from television camera TVC, and master device 100.
FM demodulation circuit 202 that demodulates the FM modulation signal sent from the side to extract the audio signal, FM modulation circuit 203 that FM modulates the audio signal from the microphone MB, and generates a call signal according to the operation of the call switch SWB Ringing signal generation circuit 206, a ringing signal modulation circuit 207 for FM-modulating the ringing signal, and an FM modulation circuit 203
The modulation signal output from the ringing signal modulation circuit 207 and the video signal output from the television camera TVC through the video signal amplification circuit 201 are input, and the multiplex transmission signal is sent out to the transmission line l. A balanced input / output circuit 205 for outputting a signal, and the internal power supply of the slave unit 200 and the power supply of the television camera TVC via the balanced input / output circuit 205 from the multiplex transmission signal transmitted from the master unit 100 via the transmission line l. And a power supply separation circuit 204 for separation. FM modulation circuit 2
In 03, the audio signal from the microphone MB is FM-modulated and sent to the parent device 100 via the balanced input / output circuit 205, and the FM demodulation circuit 202
In this configuration, the FM modulation signal from the balanced input / output circuit 205 is demodulated and the sound is output from the speaker SPB.

In this intercom system, the parent device 100 and the child device 200
Are connected by a transmission line 1 made of a coaxial cable, and the connection terminals T1 and TG provided on the master device 100 side and the connection terminals T2 and TG provided on the slave device 200 side are connected to the transmission line 1 formed of the coaxial cable. They are connected by a core wire and a shield sheath wire, respectively. Video, audio, and other control signals necessary for information communication between the two are multiplexed and transmitted through the transmission line l.

FIG. 4 shows a frequency division state of a signal transmitted via the transmission line 1 in the present system. As shown in the figure, the first carrier frequency of the FM modulation wave (master modulation wave) of the voice transmitted from the parent device 100 to the child device 200, and the FM modulation wave of the voice transmitted from the child device 200 to the parent device 100 The second carrier frequency of the (modulator modulated wave) and the third carrier frequency of the FM modulated wave of the calling signal are set to frequencies higher than the video signal frequency band of the television camera transmitted in baseband to prevent interference. The dedicated transmission line 1 is multiplex-transmitted.

Hereinafter, the operation of the present embodiment will be described. When the visitor operates the call switch SWB provided on the handset 200 side, the call signal is FM-modulated by the call signal modulation circuit 207, transmitted to the base unit 100 side through the transmission line l, and the call signal of the base unit 100. The paging signal is demodulated by the demodulation circuit 106, and the paging signal amplification circuit 1
With the output of 07, a ringing tone sounds from the speaker SPA.
When the resident responds to this ringing sound by pressing the answer switch SWA, power is supplied to the television camera TVC provided in the handset 200, and the video signal is supplied to the video signal amplification circuit 201.
The signal is transmitted to the parent device 100 via the transmission line 1 through the balanced input / output circuit 205, amplified through the balanced input / output circuit 104 by the video signal amplifier circuit 103, and displayed on the monitor television MTV. Further, the audio signal from the microphone 301 in the voice communication circuit 300 of the
The modulated signal is modulated by the FM demodulation circuit 202 of the slave 200 via the transmission line l, and the resident's voice is output from the speaker SPB. On the other hand, the visitor's voice signal input by the microphone MB is FM-modulated by the FM modulation circuit 203, transmitted to the parent device 100 via the transmission line l, and demodulated by the FM demodulation circuit 102 of the parent device 100. The voice signal of the visitor is output from the speaker 302 in the loudspeaker communication circuit 300. As a result, a call can be made between the visitor and the resident, and the resident can check the appearance of the visitor.

As described above, when the present invention is applied to an intercom system with a television that multiplexes an audio signal, a video signal, and a control signal by frequency division through the same transmission line, without using a doorphone adapter or a sidetone prevention circuit, A hands-free intercom system can be easily configured. In addition, the intercom master unit equipped with a TV camera is often designed to be mounted on the wall, but by eliminating the handset and enabling hands-free loudspeaking, it is possible for the handset to be mistaken or accidentally touched. Troubles such as dropping the handset are eliminated. Furthermore, since a handset is not required, the design of the intercom master unit can be a flat type, and the embedding of the wall surface can be facilitated.

[Effects of the Invention] As described above, the present invention relates to an intercom system in which a loud-speaking intercom master unit and an intercom slave unit are connected by a two-wire transmission line, and an audio signal from the interphone master unit to the interphone slave unit is transmitted. Since the first modulation signal is transmitted and the audio signal from the intercom slave to the intercom master is transmitted by the second modulation signal, and the frequency bands of the first modulation signal and the second modulation signal are different, Since the transmission signal and the reception signal are transmitted independently on four wires in a simulated manner, no influence of the signal wraparound occurs, so that transmission blocking or howling does not occur. A line conversion circuit and a bidirectional amplifier are not required, and the gain can be increased to the same extent as when using a handset even when talking to the other party using a loud-talking intercom slave unit. Can be detected, the reception can be detected easily,
This has the effect of improving the clarity of the voice call.

If the present invention is applied to an intercom system with a television that multiplexes and transmits a video signal, a voice signal, and a calling signal by frequency division, the present invention is implemented simply by replacing the handset of the intercom master unit with a loudspeaker unit. Therefore, there is an effect that it is possible to realize a loudspeaker-type intercom system without adding an intercom adapter, a 2-wire 4-wire conversion circuit, etc., and without substantially increasing the cost.

[Brief description of the drawings]

FIG. 1 is a block diagram of a first embodiment of the present invention, FIG. 2 is a front view showing the appearance of a second embodiment of the present invention, FIG. 3 is a block diagram of the same, and FIG. FIG. 5 is a schematic configuration diagram of a conventional example, FIG. 6 is a schematic configuration diagram of another conventional example, and FIG. 7 is a block diagram of still another conventional example. 100 is an intercom master, 101 is an FM modulator, 102 is an FM demodulator, 200 is an intercom slave, 202 is an FM demodulator, 20
3 is an FM modulation circuit, and 1 is a transmission line.

 ──────────────────────────────────────────────────の Continuing on the front page (72) Inventor Keiichi Mizuguchi 1048 Kazuma Kadoma, Kadoma City, Osaka Prefecture Inside Matsushita Electric Works Co., Ltd. No. (72) Inventor Yoshiaki Ueno 1048 Kazuma, Kadoma, Osaka Prefecture Inside Matsushita Electric Works, Ltd. (56) References JP-A-60-69996 (JP, A) JP-A 64-27358 (JP, A)

Claims (2)

(57) [Claims] 1. An intercom master unit having a microphone and a speaker for a loudspeaker call, an intercom slave unit having a microphone and a speaker for a loudspeaker call, and a two-wire transmission line connecting the interphone master unit and the interphone slave unit. And a first modulating means for modulating an audio signal obtained from a microphone of the intercom master unit and transmitting the modulated signal to a two-wire transmission line, and a first modulation means obtained from the two-wire transmission line. First demodulating means for demodulating the modulated output of the intercom slave unit and outputting it to the speaker of the intercom slave unit, and second modulating means for modulating the audio signal obtained from the microphone of the intercom slave unit and sending it out to the two-wire transmission line A second demodulation means for demodulating the modulation output of the second modulation means obtained from the two-wire transmission line and outputting the demodulated output to a speaker of the intercom master unit;
The first variable loss means provided on the input side of the modulation means, the second variable loss means provided on the output side of the second demodulation means, and the level of the input / output signal of each variable loss means are detected. And a control means for controlling the loss amount of the variable loss means so as to reduce the loss amount in the call direction according to the detection value of the level detection means, and performing the modulation in the first and second modulation means. An intercom system of a loudspeaker type, wherein the output is in a different frequency band. 2. The intercom master includes a monitor television, the intercom slave includes a television camera, and the intercom slave includes means for multiplexing at least a video signal and an audio signal to the interphone master by frequency division.
2. The loudspeaker-type intercom system according to claim 1, wherein said intercom master unit has means for superimposing at least a voice signal on a power supply to said intercom slave unit for multiplex transmission.