AU613812B2 - Device for preventing unlawful telephonic communication by parallel telephone - Google Patents

Description

AUSTRALIA

PATENTS ACT 1952 COMPLETE SPECIFICATION 6A '-p Form

(ORIGINAL)

FOR OFFICE USE Short Title: Int. Cl: Application Number: Lodged: Complete Specification-Lodged: Accepted: Lapsed: Published: Priority: Related Art: TO BE COMPLETED BY APPLICANT Name of Applicant: ANRITSU CORPORATION Address of Applicant: 5-10-27, MINAMIAZABU

MINATO-KU

TOKYO 106

JAPAN

Actual Inventor: Address for Service: GRIFFITH HACK CO., 601 St. Kilda Road, Melbourne, Victoria 3004, Australia.

Complete Specification for the invention entitled: DEVICE FOR PREVENTING UNLAWFUL TELEPHONIC COMMUNICATION BY PARALLEL TELEPHONE.

The following statement is a full description of this invention including the best method of performing it known to me:- -1 II .e this part DECLARED at Tokyo, Japan on this 2nd day of November, 1989 ANRITSU CORPORATION Shinji -UGAT, President -1-I lA- Background of the Invention 1. Field of the Invention The present invention relates to a telephone set and, more particularly, to a device for preventing unlawful telephonic communications performed by connecting a telephone in parallel with a legal or regular telephone set.

2. Description of the Related Art In general, in telephone sets, when a handset is lifted (off-hook), telephone lines form a closed loop within a telephone set to permit a call. In public or pay telephone sets as well, when the off-hook and throwing of coin or the balance of a telephone card (a prepaid card for telephone) inserted is detected, a closed loop is formed, whereby a line connection is detected by an exchange and a user can talk with a called party via the enchange.

There is a case where, when a regular telephone set is not busy (in the on-hook state), an unlawful call may be made by connecting another telephone in parallel with the regular telephone set. Where a closed loop is formed by such an unlawful call, it is conventionally impossible to determine whether the set (for example, a public telephone set).

Even if a central examines the parallel telephone set, it cannot easily be detected because it seems to be normal when it is in the on-hook and off-hook states.

vJ 2 Summary of the Invention It is accordingly an object of the present invention to provide an unlawful telephone call preventing levice for preventing unlawful calls.

According to one aspect of the present invention, there is provided a device for preventing unlawful calls by a parallel telephone connected with telephone lines used for a regular telephone comprising: state detecting means for detecting the state of the regular telephone to produce a,, first detect signal when detecting that the regular telephone is not busy; impedance detecting means for detecting the impedance between the lines to produce a second detect signal when the impedance is lowered to a value less than a predetermined value; determination means responsive to the first detect signal from the state detecting means and the second detect signal from the impedance detecting means for determining the parallel telephone to be in use, thereby to output a determination signal; and control means responsive to the determination signal from the determination means for lowering the impedance between the telephone lines to a level vre U e n -t enough to dla-b" the parallel telephone from making a call unlawfully. ~0 The invention also provides a device for veti unlawful calls by a parallel telephone connected with telephone lines used for a regular telephone comprising: state detecting means for detecting the state of said regular telephone to produce a f.rst detect signal when detecting that said regular telephone is not busy; impedance detecting means for detecting the impedance between said lines to produce a second detect signal when the impedance is lowered to a value less than a predetermined value; A I /1'P T 0 fo, El 2a determination means responsive to the first detect signal from said state detecting means and the second detect signal from said impedance detecting means for determining said parallel telephone to be in use, thereby to output a determination signal; and transmitting means responsive to the determination signal for outputting a signal for informing a telephone office of the connection of said parallel telephone.

According to another aspect of the present invention, there is provided a device for preventing unlawful calls by a parallel telephone connected with telephone lines used for a regular telephone comprising 9

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3 state detecting means for detecting the state of the regular telephone to produce a first detect signal when detecting that the regular telephone is not busy, impedance detecting means for detecting the impedance between the lines to produce a second detect signal when the impedance is lowered to a value less than a predetermined value, determination means responsive to the first detect signal from the state detecting means and the second detect signal from the impedance detecting means for determining the parallel telephone to be in use, thereby to output a determination signal, 0, 1e r; n and,interrupting means responsive to the determination signal from the determination means forinte~ruptng- a dual tone multi-frequency (DTMF) signal output from the parallel telephone.

Brief Description of the Drawings The object and features of the invention may be understood with reference to the following detailed description of illustrative embodiments of the invention, taken together with the accompanying drawings in which: Fig. 1 is a block diagram of a first embodiment of the present invention; Fig. 2 is a flowchart illustrating the operation of the control section of Fig. 1; Figs. 3A to 3D are timing diagrams for illustrating N how unlawful calls are prevented; ~TN -I_ 4 Fig. 4 is a block diagram of a second embodiment of the present invention; Fig. 5 is a block diagram of a third embodiment of the present invention; Fig. 6 is a block diagram of the dial tone sending circuit of Fig. Fig. 7A is a detailed circuit diagram of the dial switch and decoder of Fig. 6; Fig. 7B illustrates an example of the dial switch; Fig. 8 illustrates exemplary frequencies set for effective inputs of Fig. 7A for each of high and low groups; Fig. 9 illustrates a waveform of a DTMF signal generated when a key of is depressed; Fig. 10 is a block diagram of a DTMF signal receiving circuit; Figs. 11A and liB illustrate a waveform of a DTMF signal from a parallel telephone set and a waveform of a DTMF signal from the dial tone sending circuit; and Fig. 12 is a block diagram of another embodiment of Fig. 1.

Detailed Description of the Preferred Embodiments In Fig. 1, there is illustrated a block diagram of a public telephone set connected to an exchange 1 via telephone lines L1 and L2, which is provided with an unlawful call or talk preventing device according to a first embodiment of the present invention.

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5 In Fig. 1, of lines LI and L2 pulled in public telephone set 10 from line terminals A and B, line LI is connected to a talking circuit 12 via a talking loop forming circuit 11 formed of a photocoupler or relay.

The other line L2 is directly connected to talkingcircuit 12.

A control circuit 13 is responsive to a detect signal from an on-hook detecting circuit 14 for detecting the on-hook state of public telephone set 10 to close talking loop forming circuit 11 and charge a telephone rate on the basis of a charging signal from the exchange 1.

The circuitry indicated at 20 constitutes an unlawful call preventing circuit. Between lines Li and L2 is inserted an alternating-current impedance lowering circuit 21 formed of a series combination of a capacitor C and a switch P. Lines LI and L2 are connected to an impedance sensing circuit 22 via impedance lowering circuit 21.

In impedance sensing circuit 22, a photocoupler PC2 senses a current (which is supplied from exchange 1) flowing through a resistor R1 connected in series between lines Li and L2, a photocoupler PC1 (the output side) and a resistor R3 to supply a sense output to a control section 23 described later. Photocoupler PC1 receives a sensing pulse from control section 23 via an inverter INV to close its output side periodically.

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-6- 6 It is to be noted that the values of resistors R1 to R3 are set as large as possible so that the output of photocoupler PC2 may be held at a HIGH level when the output of photocoupler PC1 is closed.

Control section 23 carries out a predetermined process base,' on a flowchart shown in Fig. 2 in accordance with an operation mode set by a dial mode setting section 24 which may set a first mode representing that the telephone is of a decadic p-lse dial type and a second mode representing that the telephone is of a DTMF (dual tone multi-frequency) dial type.

The operation of the above circuit will be described with reference to the flowchart of Fig. 2.

First, in step 1, on the basis of a control signal of control circuit 13 on the side of talking circuit 12 a determination is made as to whether the public telephone set is in the on-hook state or not. When the telephone set is in the on-hook state, sensing pulses are applied to photocoupler PC1 at a constant period in step 2.

Next, in step 3, a comparison is made between logical states of the sensing pulse and an output pulse of photocoupler PC2. When a coincidence occurs, the operation returns to step 1. When no coincidence occurs, on the other hand, step 4 is carried out. In step 4, a determination is made as to whether the mode of the telephone set is the decadic pulse dial mode or 1MO 7 DTMF dial mode. The determination is made by dial mode setting section 24. For the decadic pulse dial mode a decadic pulse is generated, while for the DTMF dial mode a DTMF signal is transmitted to the lines. Where the result of the determination is the decadic pulse dial mode, a looping signal is applied to talking loop forming circuit 11 for a predetermined time in step 6.

When the result of the determination is the DTMF dial mode, on the other hand, the looping signal is applied to impedance lowering circuit 21 so that switch P is closed for a predetermined time in step 7.

With reference to timing diagrams of Figs. 3A to 3E, a description will be given below of how calls by an irregular telephone is prevented.

It is assumed that the public telephone is in the on-hook state as shown in Fig. 3A and sensing pulses are continuously output from control section 23 as shown in Fig. 3B. If a parallel telephone 110 is connected to line terminals A and B and placed in the off-hook state under such a circuit condition, output pulses of photocoupler PC2, which were output in synchronization with the sensing pulses up to this point, will go low.

At this point, such a looping signal as shown in Fig. 3E is applied from control section 23 to talking loop forming circuit 11 or AC impedance lowering circuit 21 for a predetermined time (for example, several seconds) in accordance with the dial mode set by dial mode r: r" 1~ 1 1C- -r 8 set by dial mode setting section 24. Hence, the impedance between the lines is extremely lowered by talking circuit 12 or AC impedance lowering circuit 21 with the result that the decadic pulse or the DTMF signal from parallel telephone 110 is not sufficiently transmitted to exchange 1. In this way unlawful calls by the use of an irregular telephone can be prevented.

A second embodiment of the present invention will be described below with reference to Fig. 4.

In the first embodiment described above, the impedance between the lines is lowered down to such a level as makes telephone calls impossible at the time of the use of an irregular telephone. Alternatively, as shown in Fig. 4, call preventing device 20' may be arranged such that AC impedance lowering circuit 21 is excluded, and transmit circuit 30 sends a predetermined code signal to lines L1 and L2 via capacitors C1 and C2 in response to a control signal from control section 23 when impedance detecting circuit 22 detects a decrease in the impedance between lines Ll and L2.

In this case, the code signal is adapted to inform the central of the fact that an irregular telephone has been connected to the lines. The code signal represents the code number of a public telephone or an alarm code at a specific frequency which can be discriminated from the dial pulse and DTMF. Responsive to the code signal, the central may prevent a call by the irregular I 9 telephone and contact other agencies to crack down on an illegal person.

The connection of transmit circuit 30 to the lines may be effected by transformer coupling in place of such capacitor coupling as shown in Fig, 4.

In the above embodiment, the state of a loop is examined through the sensing pulses from control section 23 at regular intervals. This is to decrease the dissipation of current from the exchange. Alternatively, an impedance detecting circuit that is decreased in current dissipation may be connected between the lines to examine the loop state at all times.

Also, in the embodiment of Fig. 1, the DC impedance between the lines is lowered by closing the loop of the public telephone. Alternatively, another type of impedance lowering circuit, for example, a direct-current impedance lowering circuit 21A, as shown in Fig. 12, may be used as in the DTMF dial mode.

Being constructed as above, the irregular call preventing device of the present invention can easily prevent irregular calls which may be performed by connecting an irregular telephone to a telephone set, a public telephone set, through an extreme decrease of the impedance between telephone lines or notice to the central.

A third embodiment of the present invention will be described below with reference to Figs. 5 to 11.

I 10 In Fig. 5 there is shown a block diagram of a telephone set according to the third embodiment of the present invention, in which the circuit arrangement is the same as that of the first embodiment except for dial tone sending circuit 25 attached and AC impedance lowering circuit 21 deleted.

Fig. 6 is a block diagram of dial tone sending circuit 25 of Fig. 5, to which an dial switch 101 is connected. Also, sending circuit 25 is integrated into a single chip. When a given key is pressed on dial switch 101, corresponding row and column are selected in a decoder 102. The selected decoder outputs are applied to a high-group frequency divider 105 and a low-group frequency divider 106, respectively. A clock generator 104 to which a quartz resonator 103 is connected provides a reference clock signal of 3.58 MHz which is applied to frequency dividers 105 and 106. In frequency dividers 105 and 106, the reference clock signal is frequency divided by factors corresponding to the decoder outputs. The frequency-divided outputs are converted to corresponding analog signals by D/A converters 107 and 108. The analog signals are combined by an output circuit 109 into a DTMF signal. Fig. 9 shows a waveform of the DTMF signal obtained when a key of digit 8 is pressed.

Figs. 7A and 7B illustrate detailed circuit arrangements of dial switch 101 and decoder 102. Dial i _-ROL a.

II~L9PUry **~III*II IPIZ IDIPItllCI i LIE) r*l~i- 11 switch 101 comprises ten digit keys 0 to 9, alphabetic keys A to D, an asterisk key and a sharp key. Decoder 102 is an integrated circuit and has row input terminals R1 to R4 for negative logic and column input terminals C1 to C4 for negative logic. The row input terminals and column input terminals are connected to ends of pull-up resistors R the other ends of which are connected together to power supply voltage VDD. The value of pull-up resistor R may be ranged from kiloohms to 150 kiloohms. Each of the row and column input terminals is connected to an inverter INV. With such an circuit arrangement, when a key corresponding to a specific row and column is depressed, ground voltage (VSS) may be applied to a corresponding input of decoder 102, thereby permitting an effective key input.

Fig. 8 illustrates a table representing a relationship between nominal reference frequencies for each of row effective inputs R1 to R4 and column effective inputs C1 to C4. Upon depression of a key, effective row and column inputs are selected and then combined into a DTMF signal.

Fig. 10 is a block diagram of a receive circuit to receive the DTMF signal transmitted over the line from the dial tone sending circuit.

The DTMF signal is first amplified by an amplifier 201 and then applied to a tone bandpass filter 202, which rejects unwanted frequencies, for example, 12 commercial power frequency, to provide only the DTMF signal component. The output signal of the filter i applied to a high-group bandpass filter 203 and a lowgroup bandpass filter 204. In high-group bandpass 3 filter 203, only high-frequency components are extracted and then applied to a signal determination circuit On the other hand, only low-frequency components are extracted and then applied to signal determination circuit 205 likewise. Signal determination circuit 205 determines which of four frequencies in the high group corresponds to the frequency of the applied signal.

Likewise the determination is made for four frequencies in the low group. The determination outputs for the high and low grouw,, are applied to a matrix latch circult 206, which provides a corresponding binary code.

A description will be given of how unlawful calls by a third person is prevented by the third embodiment of the present invention with reference to Fig. First, it is assumed that a third person dials a number including on parallel telephone 110 and hence DTMF signals corresponding to the digits are sent to the telephone line. At this point, the offhook state of the parallel telephone is detected as in the first embodiment. And talking loop forming circuit 11 receives a looping signal from control section 23 and forms a talking loop. Responsive to the detection result from control section 23, control circuit 13 IL -OW sl 13 supplies a control signal to dial tone sending circuit Responsive to this control signal, dial tone sending circuit 25 selects, for example, input terminals R4 and C4 of decoder 102 of Fig. 8 which correspond to asterisk to send a corresponding DTMF signal (hereinafter referred to as an asterisk signal) to the telephone line via talking circuit 12.

In this case, depending on the timing of sending the asterisk signal and signals of resulting from the unlawful call, there may be considered a case where the asterisk signal is superimposed upon any one of the signals of as shown in Fig. 11B and a case where the asterisk signal appears between two of the signals of as shown in Fig. 11A. In the case where such signals as shown in Fig. 11A are applied to DTMF signal receiving circuit (Fig. 10), signal determination circuit 205 identifies the signals as and therefore the dial numbers can not be output therefrom.

In the case where such signals as shown in Fig. 11B are sent, on the other hand, signal determination circuit 205 will be supplied with not a two-frequency signal but a three-frequency or four-frequency signal because the astirisk signal is superimposed upon the signal of by way of example, and therefore cannot correctly determine the signal of Hence, no j. 46 e 14 correct binary code is output from matrix latch circuit 206.

As described above, unlawful calls by a third party can be prevented by detecting an irregular DTMF signal which is transmitted from a parallel telephone set and sending out a regular DTMF signal, for example, an asterisk signal.

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Claims (14)

1. A device for preventing unlawful calls by a parallel telephone connected with telephone lines used for a regular telephone comprising: state detecting means for detecting the state of said regular telephone to produce a first detect signal when detecting that said regular telephone is not busy; impedance detecting means for detecting the impedance between said lines to produce a second detect signal when the impedance is lowered to a value less than a predetermined value; determination means responsive to the first detect signal from said state detecting means and the second detect signal from said impedance detecting means for determining said parallel telephone to be in use, thereby to output a determination signal; and control means responsive to the determination signal from said determination means for lowering the impedance between said telephone lines to a level enough p r eu en to said parallel telephone from making a call unlawfully.

2. A device according to claim 1, further comprising loop forming means for forming a talking loop seco v d and cintrol means responsive to the first detect signal from said state detecting means for closing said loop forming means.

3. A device according to claim 1, further ZV 1' j 1,h 16 comprising means for setting one of a first mode representing a decadic pulse type and a second mode representing a DTMF dial type according to the type of said regular telephone.

4. A device according to claim 1, in which said impedance detecting means includes a first photocoupler, and said control means supplies detect pulses at a constant period to said first photocoupler when said state detecting means detects that said regular telephone is not busy.

A device according to claim 4, in which said impedance detecting means includes a second photocoupler for detecting current flowing through resistors connect- ed in series between said telephone lines.

6. A device according to claim 1, further comprising an alternating-current impedance lowering circuit responsive to a control signal from said control means to lower the impedance between said telephone lines to a level enough to disable said parallel telephone from making a call unlawfully.

7. A device according to claim 6, in which said alternating-current impedance lowering circuit includes a series combination of a capacitor and a switch.

8. A device according to claim 1, further comprising a talking loop forming circuit responsive to a control signal from said control means for lowering -1 J, op 17 the impedance between said telephone lines to a level enough to disable said parallel telephone from making a call unlawfully.

9. A device according to claim 1, further comprising a direct-current impedance lowering circuit.

A device for 1-4-unlawful calls by a parallel telephone connected with telephone lines used for a regular telephone comprising: state detecting means for detecting the state of said regular telephone to produce a first detect signal when detecting that said regular telephone is not busy; impedance detecting means for detecting the impedance between said lines to produce a second detect signal when the impedance is lowered to a va'.ue less than a predetermined value; determination means responsive to the first detect signal from said state detecting means and the second detect signal from said impedance detecting means for determining said parallel telephone to be in use, thereby to output a determination signal; and transmitting means responsive to the determination signal for outputting a signal for informing a telephone office of the connection of said parallel telephone.

11. A device according to claim 10, further comprising means for setting one of a first mode representing a decadic pulse dial type and a second mode representing a DTMF dial type according to the type of 18 said regular telephone.

12. A device according to claim 10, further comprising an alternating-current impedance loweri circuit responsive to a control signal from sad control means to lower the impedance between said lephone lines to a level enough to disable sai parallel telephone from making a call.

13. A device according t claim 12, in which said alternating-current impeda ce lowering circuit includes a series combination o a capacitor and a switch.

14. A devic according to c'aim 10, further comprising a lking loop forming circuit responsive to a control ignal from said control means for lowering the i edance between said telephone lines to a level e ough to disable said parallel telephone from making a call unlawfully. A device according to claim 10, further comprising a direct-current impedance lowering circuit. A device for preventing unlawful calls by a parallel telephone connected with telephone lines used for a regular telephone comprising: state detecting means for detecting the state of said regular telephone to produce a first detect signal when detecting that said regular telephone is not busy; impedance detecting means for detecting the impedance between said lines to produce a second detect signal when the impedance is lowered to a value less j T' oL- _I I CT 19 than a predetermined value; determination means responsive to the first detect signal from said state detecting means and the second detect signal from said impedance detecting means for determining said parallel telephone to be in use, thereby to output a determination signal; and altering means responsive to the determination signal from said determination means for altering a dual tone multi-frequency (DTMF) signal output from said parallel telephone. 14. A device according to claim 13, in which said interrupting means includes means for transmitting a predetermined DTMF signal to said telephone lines for interrupting said DTMF signal output from said parallel telephone. A device for preventing unlawful telephonic communication by parallel telephone, substantially as hereinbefore described with reference to the accompanying drawings. DATED THIS 29th DAY OF May 1991 ANRITSU CORPORATION By Its Patent Attorneys GRIFFITH HACK CO. Fellows Institute of Patent Attorneys of Australia fr~z~ I