JPH07221828A - Key telephone system - Google Patents

Abstract

PURPOSE:To allow the telephone set to output a back tone of a DTMF signal stably at a telephone terminal equipment and to allow the user to listen to the back tone in an excellent way. CONSTITUTION:A DTMF signal source 15 generates a DTMF signal under the control of a control circuit 17 and the signal is sent to a trunk line 1 via a transmission signal path comprising a time switch 14, a codec 13, a back tone generating circuit 12, and a 2/4-wire conversion circuit 11. The back tone generating circuit 12 attenuates the DTMF sent through the transmission signal path at a prescribed attenuation before reaching the 2/4-wire conversion circuit 11 to adjust the level to be a prescribed level and the DTMF signal is given to a telephone terminal equipment 20 as a back tone via a reception signal path comprising the codec 13, the time switch 14 and a transmission IF circuit 16.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a key telephone device which gives a back tone to a terminal of a caller when a DTMF signal is transmitted.

[0002]

2. Description of the Related Art In a conventional button telephone device, a back tone of a DTMF signal is given to a terminal by utilizing a wraparound caused by impedance mismatch in a 2-wire 4-wire conversion circuit. FIG. 10 is an equivalent circuit diagram showing a general configuration of a 2-wire / 4-wire conversion circuit. As shown in this figure, the 2-wire / 4-wire conversion circuit includes a voice transformer 2, resistors 3, 4 connected to the office line 1.
And the balance circuit 5 constitutes a Wheatstone bridge circuit. The connection point between the resistor 3 and the resistor 4 is point A, and the connection point between the resistor 3 and the audio transformer 2 is point B.
Assuming that the connection point between the resistor 4 and the balance circuit 5 is C point and the connection point between the voice transformer 2 and the balance circuit 5 is D point, a transmission signal to be transmitted to the station line 1 is applied between AD points. In addition, the received signal component that arrives between the BC and the station line 1 appears.

The impedance seen from the station line 1 corresponds to the resistor 3 because the impedance between AD is small. Assuming that the impedance of the station line 1 is Z _L , the resistance value of the resistor 3 is R ₃ , the resistance value of the resistor ₄ is R ₄ , and the impedance of the balance circuit 5 is Z ₃ , R ₄ · Z _L = R ₃ If the relationship of Z ₃ is established, the balance circuit 5 and the impedance Z _L can be matched, and the signal applied between AD does not wrap around between BC.

Incidentally, the impedance Z _L has various values depending on various circumstances such as the length of the office line 1 and the diameter of the cable used as the office line 1. Moreover, since the balance circuit 5 is simply realized by several impedance elements in order to reduce the circuit scale and reduce the cost, the above relationship is not actually established, and some impedance mismatch occurs. For this reason, a part of the transmission signal applied between AD appears between BC and wraparound occurs.

Therefore, by utilizing this wraparound and connecting the terminal to the reception signal path at the time of transmitting the DTMF signal, the back tone of the DTMF signal can be given to the terminal.

However, with such a configuration, the amount of wraparound of the transmission signal also changes according to the change of the impedance Z _L , so that the size of the back tone changes when the impedance Z _L changes. Therefore, D
During transmission of the TMF signal, if the impedance of the station line changes during the process in which the exchange on the station side connects the line,
The size of the back tone changes, and the back tone becomes hard to hear.

[0007]

As described above, in the conventional button telephone apparatus, the back tone is obtained by utilizing the wraparound due to the impedance mismatch in the 2-wire / 4-wire conversion circuit composed of the Wheatstone bridge circuit. There was a problem that the size of the back tone also changed according to the change in the impedance of the line.

The present invention has been made in consideration of such circumstances, and an object thereof is to enable a terminal to stably output a back tone of a DTMF signal, and the user can use the back tone. It is to provide a key telephone device that can be heard well by the user.

[0009]

To achieve the above object, a first invention is to provide a DTMF signal generating means such as a DTMF signal source for outputting a DTMF signal to a transmission signal path, and the DTMF signal generating means. And a back tone generating means for supplying a DTMF signal of a constant level to the terminal via the reception signal path in synchronization with the output of the DTMF signal in the above.

A second aspect of the present invention is characterized in that the back tone generating means according to the first aspect of the present invention attenuates the transmission signal transmitted through the transmission signal path only for a predetermined period in which the DTMF signal should be transmitted. It is assumed that the signal is output to the reception signal path at a predetermined position before reaching the 2-wire / 4-wire converting means.

According to a third aspect of the present invention, the back tone generating means according to the first aspect of the present invention has the same signal content as the DTMF signal output from the DTMF signal generating means, and the DTMF.
The DTMF signal having a predetermined level lower than the level of the DTMF signal output by the signal generating means is output to the reception signal path.

According to a fourth aspect of the present invention, the back tone generating means of the first aspect of the invention guides the DTMF signal output from the DTMF signal generating means to the reception signal path, and
Attenuated and given to the terminal.

[0013]

[Operation] By taking such a means, DTMF
When the signal generating means outputs the DTMF signal to the transmission signal path, the DTM in the DTMF signal generating means
In synchronization with the output of the F signal, the DTMF signal of a constant level is given from the back tone generating means to the terminal via the reception signal path.

[0014]

【Example】

(First Embodiment) A first embodiment of the present invention will be described below with reference to the drawings. FIG. 1 is a functional block diagram showing a main configuration of a key telephone device according to this embodiment. 10 in the figure
Is a main unit and 20 is a telephone terminal.

The main unit 10 includes a 2-line to 4-line conversion circuit 11, a back tone generation circuit 12, a codec 13, a time switch 14, a DTMF signal source 15, a transmission interface circuit (transmission IF circuit) 16 and a control circuit 17. It is configured to include.

The two-wire to four-wire conversion circuit 11 is connected to the office line 1 and supplies the received signal arriving via the office line 1 to the codec 13 via the backtone generation circuit 12 and back from the codec 13. The transmission signal provided via the tone generation circuit 12 is transmitted to the office line 1.

The back tone generation circuit 12 includes the control circuit 1
The back tone of the DTMF signal is returned by returning the transmission signal given from the codec 13 to the 2-wire to 4-wire conversion circuit 11 to the codec 13 only in a predetermined period during which the DTMF signal is transmitted under the control of 7. To generate.

The codec 13 is a 2-wire / 4-wire conversion circuit 11
The PCM encoding of the reception signal given from the above and the PCM decoding of the transmission signal given from the time switch 14 are performed. The time switch 14 includes a codec 13 and a DTM.
The F signal source 15 and the transmission IF circuit 16 are respectively connected, and they are arbitrarily exchanged and connected under the control of the control circuit 17. Although not shown, the main device 10
In addition to the station line 1, another station line is connected to, and another telephone is connected to the telephone terminal 20. A circuit corresponding to the 2-line to 4-line conversion circuit 11, the back tone generation circuit 12, and the codec 13 is provided for one station line, and a circuit corresponding to the transmission IF circuit 16 is provided for one telephone terminal. The time switch 14, including these, makes an exchange connection.

Under the control of the control circuit 17, the DTMF signal source 15 generates a DTMF signal to be sent to the office line 1. A telephone terminal 20 is connected to the transmission IF circuit 16, and a transmission signal, a reception signal, and control data are exchanged with the telephone terminal 20.

The control circuit 17 has, for example, a microcomputer as a main control circuit, and generally controls each unit in the main device 10. On the other hand, the telephone terminal 20 is the handset 2
1, codec 22, transmission interface circuit (transmission IF circuit) 23, transmission transformer 24, function button 2
5, a dial pad 26 and a control circuit 27 are included.

The handset 21 includes a transmitter 21a for converting a voice uttered by a user into a transmission signal and a receiver 21 for outputting a reception signal given from a codec 22 as a voice.
b.

The codec 22 PCM-encodes the transmission signal output from the transmitter 21a of the handset 21 and PCMs the reception signal supplied from the transmission IF circuit 23.
Decrypt.

The transmission IF circuit 23 is a transmission transformer 2.
4 is connected to the transmission IF circuit 16 of the main device 10 via the communication unit 4, and a transmission signal, a reception signal, and control data are exchanged with the transmission IF circuit 16.

The function button 25 is used by the user to select an office line and answer an incoming call. The dial pad 26 is used by the user to dial the telephone number of the callee when making a call.

The control circuit 27 has, for example, a microcomputer as a main control circuit, and generally controls each unit in the telephone terminal 20. FIG. 2 shows a 2-wire / 4-wire conversion circuit 1 of the main device 10.
2 is a circuit diagram showing a specific configuration of 1 and a back tone generation circuit 12. FIG. The same parts as those in FIG. 10 are designated by the same reference numerals.

The 2-wire / 4-wire conversion circuit 11 is mainly composed of a Wheatstone bridge circuit composed of an audio transformer 2, resistors 3, 4 and a balance circuit 5, and additionally has resistors 11a, 11b, 11c, 11d and a DC cut. The capacitor 11e and the suction circuit 11f are included.

The back tone generating circuit 12 is composed of switching circuits 12a and 12b and a resistor 12c. The connection point A between the resistors 3 and 4 is the codec 1
3 is connected to the output end of the transmission output amplifier 13a built in the device.

Connection point B between the audio transformer 2 and the resistor 3
Is connected to the inverting input terminal of the reception operational amplifier 13b incorporated in the codec 13 via the switching circuit 12a and the resistor 11a. The connection point C between the resistor 4 and the balance circuit 5 is connected to the receiving operational amplifier 13 via the resistor 11b.
It is connected to the non-inverting input terminal of b. In addition, resistor 1
1c is inserted between the inverting input terminal and the output terminal of the reception operational amplifier 13b. The resistor 11d has one end connected to the non-inverting input terminal of the reception operational amplifier 13b and the other end grounded. Thus, resistor 1
1a, 11b, 11c and 11d are reception operational amplifiers 13b
Together with this, a differential amplifier circuit that differentially amplifies the potential at the point B and the potential at the point C is configured.

The DC-cutting capacitor 11e is used for the office line 1
The direct current component arriving via the is prevented from flowing into the audio transformer 2. The suction circuit 11f absorbs the loop current of the office line 1. Thus, the DC cutting capacitor 11e
The suction circuit 11f forms a circuit that supplies only the audio component to the audio transformer 2.

Under the control of the control circuit 17, the switching circuit 12a selectively connects the inverting input terminal of the reception operational amplifier 13b to the point B or the ground. Under the control of the control circuit 17, the switching circuit 12b selectively connects the inverting input terminal of the reception operational amplifier 13b to the ground or the output end of the transmission output amplifier 13.

The resistor 12c is inserted between the switching circuit 12b and the inverting input terminal of the reception operational amplifier 13b, and when the switching circuit 12b selects the output end of the transmission output amplifier 13, the transmission output is obtained. Received from the amplifier 13 Operational amplifier 13b
Attenuate the signal flowing to.

Next, the operation of the key telephone device configured as described above will be described. First, the back tone generation circuit 1
2 is in the state shown in FIG. 2 in the normal state. That is, the selection circuit 12a is on the B point side, and the selection circuit 12b is also
Selects the ground side respectively.

In this state, the equivalent circuit of the portion shown in FIG. 2 is as shown in FIG. As can be seen from this figure, in the normal state, the transmission signal output from the transmission output amplifier 13 is applied between AD of the Wheatstone bridge circuit included in the 2-wire 4-wire conversion circuit 11, and BC The potential difference between the two is extracted and amplified by the differential amplifier including the reception operational amplifier 13b and the resistors 11a, 11b, 11c and 11d to obtain the reception signal. That is, it is in a state of performing an operation similar to that of the general 2-wire / 4-wire conversion circuit shown in FIG.

On the other hand, in the DTMF signal transmission state,
The selection circuit 12a selects the ground side, and the selection circuit 12b selects the output end side of the transmission output amplifier 13a. In this state, the equivalent circuit of the part shown in FIG. 2 is that shown in FIG. As can be seen from this figure, in the DTMF signal transmission state, the transmission signal output from the transmission output amplifier 13 is applied between AD of the Wheatstone bridge circuit included in the 2-wire 4-wire conversion circuit 11. Before reaching the Wheatstone bridge circuit, the reception operational amplifier 13b is directly applied (but via the resistor 12c).

Based on the above points, the operation when the DTMF signal is transmitted will be described in detail. First, the back tone generation circuit 12 is in the normal state described above when the station line 1 is unused.

From this state, when the user designates the capture of the station line 1 by the function button 25, the control circuit 27 accepts this designation, and to that effect, the transmission IF circuit 23, the transmission transformer 24, and the transmission IF circuit. Control circuit 1 through 16
Notify to 7. Then, the control circuit 17 causes the time switch 14 to output the DTM output from the DTMF signal source 15.
F signal is given to codec 13 and codec 1
Switching connection is performed so that the reception signal output from the terminal 3 is applied to the transmission IF circuit 16.

After the user designates the acquisition of the office line 1, the dialing pad 26 dials the telephone number of the callee, which is accepted by the control circuit 27, and the IF circuit 23 for transmission and the transformer 24 for transmission 24. And transmission IF
The control circuit 17 is notified via the circuit 16. Then, the control circuit 17 switches the switching circuits 12a and 12b, respectively, and causes the back tone generation circuit 12 to perform the above-mentioned DT.
The MF signal is sent (at TA in FIG. 5).

Then, the control circuit 17 starts driving the DTMF signal source 15 at a time point (time point TB in FIG. 5) after a lapse of a short time t1 from the time point TA, and the DTMF corresponding to the designated telephone number. Start signal generation.
In response to this, the DTM output from the DTMF signal source 15
The F signal is sent to the office line 1 through the transmission signal path consisting of the time switch 14 → codec 13 → back tone generation circuit 12 → 2 line / 4 line conversion circuit 11.

At this time, the back tone generation circuit 12 outputs D
The TMF signal is in the transmission state, and equivalently, the configuration shown in FIG. 4 is formed. Therefore, the DTMF signal output from the transmission output amplifier 14a is given to the reception operational amplifier 13b. At this time, the 2-line to 4-line conversion circuit 11
The point B in the inside is the reception operational amplifier 13 by the selection circuit 12a.
Since it is separated from b, the reception signal from the office line 1 is cut by the selection circuit 12a and is not given to the reception operational amplifier 13b.

Thus, as the reception signal output from the codec 13 to the time switch 14, the DTMF signal folded in the back tone generation circuit 13 appears. And this reception signal (DTMF signal)
Is given to the handset 21 through a reception signal path of time switch 14 → transmission IF circuit 16 → voice transformer 24 → transmission IF circuit 23 → codec 22, and is output as a back tone of the DTMF signal from the handset 21b of the handset 21. To be done.

Now, the equivalent circuit of the part centering on the reception operational amplifier 13b is shown in FIG.
When the MF signal is transmitted, it becomes as shown in FIG. 7, respectively. Focusing on FIG. 7, the level of the DTMF signal output from the reception operational amplifier 13b is R _{11a for} the resistance value of the resistor _11a , R 11b for the resistance value of the resistor _11b , and R 11c for the resistance value of the resistor _11c. , The resistance value of the resistor 11d is R
_11d and the resistance value of the resistor _12c is R _12c , the output of the transmission output amplifier 13a is amplified by a multiplication factor G calculated by the following equation.

[0042]

[Equation 1]

That is, the level of the DTMF signal output from the reception operational amplifier 13b is the resistance value R of the resistor 11a.
_11a , the resistance value R _{11b of} the resistor _11b , the resistance value R _{11c of} the resistor _11c , the resistance value R _11d of the resistor _11d, and the resistance value R _12c of the resistor _12c , which are constant regardless of the impedance of the station line 1. Is. The resistor 12c acts as an attenuator, and the level of the DTMF signal output from the reception operational amplifier 13b can be arbitrarily set by setting the resistance value of the resistor _12c to R _12c to an appropriate value. Thus, the handset 21b of the handset 21 outputs the back tone of the DTMF signal at a constant optimum level.

Then, the control circuit 17 uses the DTMF signal source 1
5 has finished outputting the DTMF signal (TC in FIG. 5).
When a certain time t2 has elapsed from the time point (time point) (T in FIG. 5)
At time D), the switching circuits 12a and 12b are respectively switched to return the back tone generating circuit 12 to the normal state, and the time switch 14 is controlled so that the transmission signal output from the transmission IF circuit 16 is given to the codec 13. Change the connection so that As a result, the received signal coming through the station line 1 is given to the telephone terminal 20, and the transmitted signal output from the telephone terminal 20 is sent to the station line 1 so that the telephone call can be performed.

As described above, according to this embodiment, the DTMF is
At the time of signal transmission, a part of the transmission signal is circulated as a reception signal before reaching the 2-wire to 4-wire conversion circuit 11, so that the back tone of the DTMF signal can be stably supplied to the telephone terminal 20. . As a result, the telephone terminal 20
Then, a certain amount of back tone is output from the handset 21b, and the user can hear the back tone well. Moreover, in this embodiment, the level of the DTMF signal fed as the back tone is set to the resistor 12c.
Since it is attenuated and adjusted appropriately, a back tone that is easy for the user to hear can be output.

(Second Embodiment) Next, a second embodiment of the present invention will be described. FIG. 8 is a functional block diagram showing the main configuration of the key telephone device according to the present embodiment. The same parts as those in FIG. 1 are designated by the same reference numerals, and detailed description thereof will be omitted.

The key telephone device of this embodiment is a main device 30.
And a telephone terminal 20. The main device 30 includes a 2-wire / 4-wire conversion circuit 11, a codec 13, a time switch 14, and D.
It is configured to include a TMF signal source 15, a transmission IF circuit 16, a back tone signal source 31, and a control circuit 32.

The back tone signal source 31 outputs a DTMF signal having the same signal content and a low predetermined level as a back tone in synchronization with the DTMF signal output of the DTMF signal source 15.

The control circuit 32 has, for example, a microcomputer as a main control circuit, and generally controls each unit in the main device 30. Next, the operation of the key telephone device configured as described above will be described.

First, when the user designates the acquisition of the station line 1 by the function button 25, the control circuit 27 accepts this designation, and to that effect, the transmission IF circuit 23 and the transmission transformer 24.
And the control circuit 32 via the transmission IF circuit 16. Then, the control circuit 32 causes the time switch 14
, The DTMF signal output from the DTMF signal source 15 is given to the codec 13, and the back tone signal source 3
Switching connection is performed so that the back tone output from 1 is given to the transmission IF circuit 16.

After the user specifies the capture of the office line 1, the dialing pad 26 dials the telephone number of the destination, which is accepted by the control circuit 27, and the IF circuit 23 for transmission and the transformer 24 for transmission 24. And transmission IF
The control circuit 32 is notified via the circuit 16. Then, the control circuit 32 starts driving the DTMF signal source 15 and the back tone signal source 31 to start generating the DTMF signal and the back tone corresponding to the designated telephone number.

Then, the DTMF signal source 15 becomes DTMF.
The output of the signal is started, and in synchronization with this, the backtone signal source 31 generates a backtone. DTMF signal source 15
The DTMF signal output by the is transmitted to the office line 1 through the transmission signal path of the time switch 14 → codec 13 → back tone generation circuit 12 → 2 wire / 4 wire conversion circuit 11. The back tone generated by the back tone signal source 31 is time switch 14 → transmission IF circuit 16 → transmission transformer 24 → transmission IF circuit 23 → codec 22.
It is given to the handset 21 via the following reception signal path, and is output from the handset 21b of the handset 21 as a back tone of the DTMF signal.

The control circuit 32 uses the DTMF signal source 1
5 and the back tone signal source 31 have finished outputting the DTMF signal and the back tone, the time switch 14
So that the transmission signal output from the transmission IF circuit 16 is supplied to the codec 13,
The reception signal output by the above is reconnected so as to be given to the transmission IF circuit 16. As a result, the reception signal coming through the station line 1 is given to the telephone terminal 20, and
The transmission signal output from the telephone terminal 20 is in a state of being transmitted to the central office line 1, and the telephone call can be performed.

As described above, according to this embodiment, the DTMF is
At the time of transmitting the signal, the back tone output from the back tone signal source 31 is directly applied to the telephone terminal 20 without passing through the 2-wire to 4-wire conversion circuit 11. Therefore, the back tone is not affected by the impedance of the office line 1, and the back tone of the DTMF signal can be stably supplied to the telephone terminal 20. As a result, the telephone terminal 20 outputs a certain amount of back tone from the handset 21b, and the user can hear the back tone well.

(Third Embodiment) Next, a third embodiment of the present invention will be described. FIG. 9 is a functional block diagram showing the main configuration of the key telephone device according to the present embodiment. The same parts as those in FIG. 1 are designated by the same reference numerals, and detailed description thereof will be omitted.

The key telephone unit of this embodiment is a main unit 40.
And a telephone terminal 20. The main device 40 includes a 2-wire 4-wire conversion circuit 11, a codec 13, a time switch 14, and a D
The TMF signal source 15, the transmission IF circuit 16, the digital attenuation circuit 41, and the control circuit 42 are included.

The digital attenuating circuit 41 is inserted in the reception signal path from the time switch 14 to the transmitting IF circuit 16 and is inserted from the time switch 14 to the transmitting IF circuit 16.
The received signal given to is attenuated by a certain amount of attenuation.
The digital attenuation circuit 41 can turn on / off the execution of the attenuation process under the control of the control circuit 42.

The control circuit 42 has, for example, a microcomputer as a main control circuit, and totally controls each unit in the main device 40. Next, the operation of the key telephone device configured as described above will be described.

First, when the user designates the capture of the station line 1 by the function button 25, the control circuit 27 accepts this designation, and to that effect, the transmission IF circuit 23 and the transmission transformer 24.
And the control circuit 42 via the transmission IF circuit 16. Then, the control circuit 42 causes the time switch 14
Switch connection so that the DTMF signal output from the DTMF signal source 15 is given to the codec 13 and the digital attenuation circuit 41.

After the user designates the capture of the office line 1, the dialing pad 26 dials the telephone number of the callee, which is accepted by the control circuit 27, and the IF circuit 23 for transmission and the transformer 24 for transmission. And transmission IF
The control circuit 42 is notified via the circuit 16. Then, the control circuit 42 starts driving the DTMF signal source 15 to start the generation of the DTMF signal corresponding to the designated telephone number.

Then, the DTMF signal source 15 becomes DTMF.
Start outputting signals. The DTMF signal output from the DTMF signal source 15 is the time switch 14 → codec 13
→ back tone generation circuit 12 → 2 line to 4 line conversion circuit 11 is transmitted to the office line 1 via the transmission signal path.

The DTMF signal output from the DTMF signal source 15 is the time switch 14 → the digital attenuation circuit 41 →
It is also given to the handset 21 via the reception signal path of the transmission transformer 24 → the transmission IF circuit 23 → the codec 22, and the handset 21 receives the DTMF from the handset 21b.
It is output as the back tone of the signal. At this time, the control circuit 42 causes the digital attenuating circuit 41 to perform the attenuating operation, and in the digital attenuating circuit 41, the DT is performed with a constant attenuation amount.
The MF signal is attenuated and the DTMF signal is adjusted to an optimum level as a back tone.

The control circuit 42 uses the DTMF signal source 1
When 5 finishes outputting the DTMF signal, the time switch 14 is controlled so that the transmission signal output by the transmission IF circuit 16 is given to the codec 13, and the reception signal output by the codec 13 is a digital attenuation circuit. Reconnect as given to No. 41. At this time, the control circuit 42 stops the execution of the attenuation operation of the digital attenuation circuit 41. As a result, the received signal coming through the station line 1 is given to the telephone terminal 20, and the transmitted signal output from the telephone terminal 20 is sent to the station line 1 so that the telephone call can be performed.

As described above, according to this embodiment, the DTMF is
The DTMF signal source 15 outputs the DT when the signal is transmitted.
The MF signal is directly applied to the telephone terminal 20 as a back tone without passing through the 2-wire / 4-wire conversion circuit 11. Therefore, the back tone is not affected by the impedance of the office line 1, and the telephone terminal 20 receives the DT signal.
The back tone of the MF signal can be stably supplied. As a result, the telephone terminal 20 outputs a certain amount of back tone from the handset 21b, and the user can hear the back tone well. The present invention is not limited to the above embodiments, and various modifications can be made without departing from the gist of the present invention.

[0065]

The first aspect of the present invention provides a DTM to the transmission signal path.
An DTMF such as a DTMF signal source that outputs an F signal
Signal generating means and DTM in this DTMF signal generating means
Back tone generating means for providing a DTMF signal of a constant level to the terminal via the reception signal path is provided in synchronization with the output of the F signal.

A second invention is such that the back tone generating means in the first invention attenuates the transmission signal transmitted through the transmission signal path only for a predetermined period in which the DTMF signal should be transmitted. It is assumed that the signal is output to the reception signal path at a predetermined position before reaching the 2-wire / 4-wire converting means.

According to a third aspect of the present invention, the back tone generating means according to the first aspect of the present invention has the same DTMF signal content as the DTMF signal output from the DTMF signal generating means, and the DTMF.
The DTMF signal having a predetermined level lower than the level of the DTMF signal output by the signal generating means is output to the reception signal path.

A fourth aspect of the present invention is directed to the back tone generating means according to the first aspect of the invention, wherein the DTMF signal output from the DTMF signal generating means is guided to the reception signal path, and
Attenuated and given to the terminal.

As a result, the back tone of the DTMF signal can be stably output at the terminal, and the key telephone device can be made to allow the user to hear the back tone.

[Brief description of drawings]

FIG. 1 is a functional block diagram showing a main configuration of a key telephone device according to a first embodiment of the present invention.

FIG. 2 is a circuit diagram showing a specific configuration of a 2-line to 4-line conversion circuit 11 and a back tone generation circuit 12 in FIG.

3 is a diagram showing an equivalent circuit of the circuit shown in FIG. 2 when the back tone generation circuit 12 is in a normal state.

FIG. 4 is a diagram showing an equivalent circuit of the circuit shown in FIG. 2 when the back tone generation circuit 12 is in a DTMF signal transmission state.

FIG. 5 is a timing chart showing a change timing of an operation state of the back tone generation circuit 12.

FIG. 6 is a diagram showing an equivalent circuit of a portion centered on a reception operational amplifier 13b when the back tone generation circuit 12 is in a normal state.

FIG. 7 is a diagram showing an equivalent circuit of a portion centering on a reception operational amplifier 13b when the back tone generation circuit 12 is in a DTMF signal transmission state.

FIG. 8 is a functional block diagram showing a main configuration of a key telephone device according to a second embodiment of the present invention.

FIG. 9 is a functional block diagram showing a main configuration of a key telephone device according to a third embodiment of the present invention.

FIG. 10 is a diagram illustrating a conventional technique.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 ... Station line 10, 30, 40 ... Main device 11 ... 2-wire 4-wire conversion circuit 12 ... Back tone generation circuit 12a, 12b ... Switching circuit 12c ... Resistor 13 ... Codec 14 ... Time switch 15 ... DTMF signal source 16 ... Transmission interface circuit (transmission IF circuit) 17, 32, 42 ... Control circuit 20 ... Telephone terminal 31 ... Backtone signal source 41 ... Digital attenuation circuit

Claims (4)

[Claims] 1. A transmission signal output from a terminal and transmitted through a transmission signal path is sent to a communication line via a 2-wire to 4-wire conversion means, and a reception signal arriving via the communication line is sent to the communication line. In a button telephone device which is introduced into a reception signal path through a line-to-four-line converting means and is given to the terminal through the reception signal path, a DTMF which outputs a DTMF signal to the transmission signal path.
A signal generating means and a back tone generating means for supplying a DTMF signal of a constant level to the terminal via a reception signal path in synchronization with the output of the DTMF signal from the DTMF signal generating means. Button telephone device. 2. The back tone generation means attenuates the transmission signal transmitted through the transmission signal path only for a predetermined period in which the DTMF signal should be transmitted, and then reaches the 2-wire to 4-wire conversion means. The button telephone device according to claim 1, wherein the button telephone device outputs to a reception signal path at a predetermined position. 3. The back tone generating means has the same signal content as the DTMF signal output by the DTMF signal generating means,
The button telephone device according to claim 1, wherein the DTMF signal generating means outputs a DTMF signal having a predetermined level lower than the level of the DTMF signal output to the reception signal path. 4. The button telephone apparatus according to claim 1, wherein the back tone generation means guides the DTMF signal output from the DTMF signal generation means to the reception signal path and attenuates and supplies the DTMF signal to the terminal.