WO2000056043A1

WO2000056043A1 - Caller identity display unit

Info

Publication number: WO2000056043A1
Application number: PCT/GB2000/000675
Authority: WO
Inventors: Thilaganathan Segaran
Original assignee: Tele-Products Limited
Priority date: 1999-03-12
Filing date: 2000-02-25
Publication date: 2000-09-21
Also published as: GB9905590D0; AU2814300A

Abstract

A caller identity display unit for connection to a telephone line on which caller identity request signals are transmitted to the telephone network from a subscriber and caller identity signals are transmitted from the telephone network to the subscriber. Call waiting signals transmitted on the line to indicate that an attempt to call a subscriber is being made when the line is busy are detected, and a shunt circuit is connected to the line for a period of time in response to detection of the call waiting signal. The shunt circuit reduces the line voltage to a level sufficiently low to mute the subscriber's equipment.

Description

CALLER IDENTITY DISPLAY UNIT

The present invention relates to a caller identity display unit.

Caller identity display units are well known and are categorised generally as type I and type II. In a type I unit, if a call is made to a particular subscriber, before the subscriber picks up the handset a caller LD signal is transmitted to the subscriber so that the subscriber knows before the telephone is picked up the identity of the caller. The caller ID information is transmitted in the form of an FSK (Frequency Shift Keying) signal which is decoded using available integrated circuits.

Type II circuits are operative to identify a caller attempting to make a connection to a subscriber when that subscriber is already engaged in a call. Type II units are also referred to as CIDCW devices, that is Caller Identity Display during Call Waiting.

In a call waiting system, if a caller tries to make a call to a line which is in use, the caller is advised that the line is in use by receiving an appropriate voice message and the called subscriber is alerted by for example a tone signal transmitted on the line. The called subscriber then has the option of switching to speak to the caller either before or after terminating the current call. In a type II caller ID system, a device is provided which can display the identity of the waiting caller.

In conventional type II caller ID systems, a CIDCW device is connected in series between the telephone line and a telephone. When the line is busy and a third party makes a call to the line, as in a type I system the called party is advised of the attempted incoming call. In addition however the telephone of the called party is temporarily disconnected from the line. Whilst the telephone is disconnected, a caller identity request signal (generally referred to as a DTMF tone) is transmitted from the subscriber to the telephone network and the FSK caller identity signal is transmitted back to the subscriber in response to receipt of the DTMF tone. After this exchange of signals, the telephone is reconnected to the line. This means that a user of the telephone can review the identity of the attempted incoming caller to decide whether or not to take the call.

The conventional CIDCW devices have limitations. In particular, where a line is connected to a number of different telephones arranged in parallel, the CIDCW device can only be connected in series with one of those telephones. If that telephone is in use, then the device will work and mute speech from and to the telephone which is in use. If one of the telephones which is not in series with the CIDCW device is in use, the CIDCW device will simply not work. The only way to overcome this problem would be to provide a CIDCW device in series with each telephone connected to a single subscriber's line. Generally this will not be acceptable for cost reasons.

It is an object of the present invention to obviate or mitigate the problem outlined above.

According to the present invention there is provided a caller identity display unit for connection to a telephone line on which caller identity request signals are transmitted to the telephone network from a subscriber and caller identity signals are transmitted from the telephone network to the subscriber, comprising means for detecting a call waiting signal transmitted on the line to indicate that an attempt to call the subscriber is being made when the line is busy, and means for connecting a shunt circuit to the line for a period of time in response to detection of the call waiting signal, the shunt circuit reducing the line voltage to a level sufficiently low to mute the subscriber's equipment.

The shunt circuit may comprise a transistor circuit which is selectively connected across terminals that are connected to the telephone line. The transistor circuit may be controlled by a timer so as to be connected to the line for a pre-set period after detection of the call waiting signal. The transistor circuit may be connected to the terminals through a diode bridge.

An embodiment of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a schematic representation of a type II caller identification system;

Figure 2 is a schematic representation of the interconnection of equipment at a subscriber's premises in a conventional type II caller identification system;

Figure 3 is an illustration similar to that of Figure 2 but illustrating a subscriber's equipment in a type II caller identification system in accordance with the present invention; Figure 4 illustrates changes in line voltage using the equipment shown in Figure 3; and

Figure 5 is a detailed circuit diagram of a device in accordance with the present invention which produces the line voltage characteristics illustrated in Figure 4.

Referring to Figure 1, a telephone network 1 interconnects a large number of users three of which are represented as users A, B and C and each of which is connected to a respective telephone line 2. The system is set up so as to provide type II caller identification, that is the ability to advise a subscriber already using a telephone line connected to the system that an attempt is being made to call that subscriber and to advise the called subscriber of the identity of the calling subscriber. For example, if user A and user B are interconnected via the network 1 and are speaking to each other, it may be that user C will attempt to call user A. The system advises user C that the line to user A is already busy by transmitting an appropriate voice message to user C, advises user A that a third party is calling by transmitting a tone on the line to user A, and transmits to user A's subscriber equipment a signal which enables that equipment to display the identity of user C. This is achieved in a series of steps which can be summarised as follows:

1. Users A and B are in conversation and user C is inactive.

2. User C attempts to ring user A.

3. The network detects that user A's line is busy and transmits a voice message to user C advising user C that user A's line is busy, but that user A is aware that a call is waiting.

4. The network briefly interrupts communication between users A and B.

5. A CAS tone signal is transmitted from the network to the terminal equipment of user A to alert user A to the fact that a third party is trying to call.

6. The terminal equipment of user A temporarily mutes speech on the line to user A so that any signals transmitted on that line are inaudible.

7. Whilst speech is muted on the line to user A, the terminal equipment of user A transmits a DTMF acknowledgement signal back to the network. 8. Whilst speech is still muted on the line to user A, the network transmits to the user A terminal an FSK caller identification signal.

9. The user A terminal terminates muting of speech on the line to user A and users A and B can then converse again.

The period for which speech is muted on the line to user A is sufficiently short for users A and B to continue to converse as if no signals had been muted.

Referring to Figure 2, a line 2 such as that shown in Figure 1 linking user A to the telephone network 1 is shown as being connected to three telephone handsets 3, 4 and 5. Telephone 3 is connected to the line 2 via a CIDCW circuit 6 of conventional design which is capable of providing the type II caller ID functionality described with reference to Figure 1.

With the conventional circuit 6 as shown in Figure 2, the handset 3 is connected in series with the CIDCW circuit 6 whereas the handsets 4 and 5 are connected directly to the line 2. If when an incoming call is made telephone 3 is in use, the CIDCW device will work to mute speech on the line 2 as signals are transmitted between the network and the circuit 6 so as to deliver to the circuit 6 a caller identity signal which enables for example the caller's telephone number to be displayed on the device 6. If however when an incoming call is made telephone 4 or telephone 5 is in use, the CIDCW circuit cannot work as it is not connected in series with the telephone 4 or 5 which is in use. This is because the CIDCW device 6 is only operational when the telephone 3 is in use. The circuit 6 must not function if only telephone 4 or 5 is in use as if it did the exchange of signals between the network and the CIDCW circuit would be audible to the subscriber connected to the line 2 via telephone 4 or 5. There is also a risk of corruption of the FSK tones by speech or room noise. Thus with a conventional CIDCW circuit, it is only functional if it is connected in series with the particular telephone handset which is in use at the time.

Referring now to Figure 3, this illustrates the layout of a system incorporating a caller identity display unit in accordance with the present invention. The same reference numerals are used where appropriate in Figures 2 and 3. It will be seen that in Figure 3 the CIDCW circuit 6 is connected in parallel with each of the telephone handsets 3, 4 and 5 rather than being in series with one of those handsets. The CJDCW circuit 6 of Figure 3 is arranged to detect a CAS tone on line 2 and when such a tone is detected to connect a shunt circuit to the line 2 so that the voltage on the line 2 drops. This is effective to mute speech to all of the three telephones 3, 4 and 5. Once this has been achieved DTMF and FSK signals can be exchanged between the subscriber terminal and the telephone network without those signals being heard by a person using one of the telephones 3, 4 or 5. There is also no risk of corruption of the FSK tones by speech or room background noise. The FSK caller identity signal can be displayed on for example a type I C-D circuit 7. Of course, the circuit 6 could be provided with an integral display device for displaying the caller identity represented by the received FSK signals.

With the arrangement illustrated in Figure 3, it will be appreciated that the CIDCW circuit operates regardless of which of the three telephones 3, 4 and 5 is in use at the time an attempt is made to call the subscriber. Thus the problem encountered with the prior art arrangement as illustrated in Figure 2 is avoided.

Figure 4 schematically represents the DC voltage appearing on the line 2 shown in Figure 3. The vertical axis represents line voltage but is not drawn to scale. The horizontal axis represents time.

When none of the telephones 3, 4 and 5 is off hook, the DC line voltage on line 2 will typically be 50 volts. When any one of the telephones is lifted off hook, for example at the time represented by line 8, the line voltage will drop to approximately 8 volts. If a CAS tone is detected at the time indicated by line 9, then at the time indicated by line 10 the line voltage will be reduced to 3 volts. At the time indicated by line 11 the DTMF acknowledgement signal will be transmitted back to the telephone network. In the period between lines 12 and 13 the FSK caller identity signal will be detected, and at the time indicated by line 14 the line voltage will be returned to 8 volts. Any telephone connected to the line 2 will be muted in the interval between the times indicated by lines 10 and 14 as a result of the line voltage being reduced for that period to 3 volts.

Referring to Figure 5, a circuit is illustrated which is capable of producing the voltage versus time characteristic illustrated in Figure 4.

Referring to Figure 5, the illustrated circuit would be connected to a telephone line via terminals 15 and 16. An integrated circuit 17, for example the MT8843 from Mitel, provides line reversal, ring, CAF and FSK detection. Data signals on the line connected to terminals 15 and 16 are monitored by a high impedance network formed by capacitors 18 and 19 and resistors 20 and 21. On detection of a CAS tone, two 555 timers 22 and 23 are triggered. Timer 22 is triggered for about 400ms and generates a line loop signal on output 24. Timer 23 is triggered for 80ms and produces a latch output on terminal 25.

The latch output on terminal 25 of timer 23 is applied to an IC26 which generates a DTMF output of 65ms duration on terminal 27. That output is coupled via a capacitor 28 and a resistor 29 to a diode bridge 30, the diode bridge being connected to terminals 15 and 16.

After transmission of the DTMF signal to the telephone network, an FSK signal will be returned carrying the caller identity information. This FSK signal is decoded by circuit 17 and presented on a data output terminal in digital format. This data signal can be decoded by a microprocessor or passed straight to a PC or processed in any other way to generate the required presentation of the caller ID information.

The line loop signal on terminal 24 of timer 22 is applied to the IC17 and to a shunt circuit which functions to reduce the DC voltage between terminals 15 and 16 whilst the timer 22 is triggered. The line loop signal is applied to an input 31 of the shunt circuit. The shunt circuit comprises a transistor 32 having a low impedance emitter resistor 33 such that on a typical telephone line, normally supplying about 30mA, the voltage across the resistor 33 is 0.3 volts. The voltage on the base of transistor 32 will then be about 1.7 volts. Given the voltage drop across the diode bridge 30 and the transistor 34 the base of which is connected to the line loop input 31 this results in a very low voltage of about 3 volts between the terminals 15 and 16.

As a result of the low voltage between terminals 15 and 16, any line powered telephones connected to the same line as terminals 15 and 16 will not receive sufficient current for the speech integrated circuits in those telephones to operate. Such telephones will also incorporate a diode bridge which will drop about 1.4 volts, leaving less than 2 volts for the speech integrated circuits which is insufficient for them to operate. Since in line powered telephones the speech integrated circuit is used to power the telephone handset microphone and speech amplifiers, the handset is muted. Thus irrespective of the number of off-hook telephones connected to the line, the speech output of any such telephones is muted so that there is no interference on the line which might disrupt the transmission of data signals and in particular the FSK data signal carrying the caller identity information. In addition the FSK signal is muted so that the telephone users cannot hear it. Despite muting of the telephones however the DTMF signal can be injected into the line and transmitted reliably to the telephone network. The correct AC impedance to signals on the line is also maintained by a capacitor 35 on the base of transistor 32 and the resistor 29 through which DTMF signals are coupled to the line.

In some locally powered telephones a circuit known as a gyrator similar to the shunt circuit illustrated in Figure 5 is used to drop the line current which in line powered systems is used to power components in the telephone handsets. Such locally powered telephone gyrator circuits do not drop the line voltage to a level sufficient to mute speech signals. If a shunt circuit as described in Figure 5 was used with a locally powered telephone handset incorporating a gyrator circuit, the shunt circuit would be connected in parallel with the gyrator and would have the same effect in terms of line muting as in the case of a line powered system.

It will be appreciated that the present invention makes it possible to connect any desired number of handsets in parallel with type II caller ID circuits. It is not necessary for an extension to be connected in series with a CIDCW device. This saves on connectors and avoids the cost of providing a relay for cutting off the telephone connected in series with the device. There is no need to check for conditions such as multiple extensions being off-hook. The circuit mutes all telephone handsets connected to the line. In the absence of a relay cut-off device, the power requirements are low, enabling the use of battery-powered circuits which results in significant cost reductions. Furthermore, because the muting circuit is connected in parallel with other units to the telephone line, existing type I caller identity units can be used for example at each extension connected to a line providing a single CIDCW unit in accordance with the present invention is connected to that line.

It will be appreciated that the embodiment of the invention illustrated in detail in Figure 5 has been assembled using readily available components already widely used in the telephone industry. A purpose built circuit would incorporate the functions of the illustrated components in a single integrated circuit and in particular the functionality of the timers 22 and 23 could be embodied in a suitably programmed microprocessor.

It will be appreciated that the invention may be applied in association with a simple telephone as described, or in association with a modem, a facsimile machine, a PBX or bigger system, or any other system in which CIDCW facilities are required.

Claims

1. A caller identity display unit for connection to a telephone line on which caller identity request signals are transmitted to the telephone network from a subscriber and caller identity signals are transmitted from the telephone network to the subscriber, comprising means for detecting a call waiting signal transmitted on the line to indicate that an attempt to call the subscriber is being made when the line is busy, and means for connecting a shunt circuit to the line for a period of time in response to detection of the call waiting signal, the shunt circuit reducing the line voltage to a level sufficiently low to mute the subscriber's equipment.

2. A display unit according to claim 1, wherein the shunt circuit comprises a transistor circuit which is selectively connected across terminals that are connected to the telephone line.

3. A display unit according to claim 2, wherein the transistor circuit is controlled by a timer to be connected to the line for a pre-set period after detection of the call waiting signal.

4. A display unit according to claim 2 or 3, wherein the transistor circuit is connected to the terminals through a diode bridge.

5. A caller identity display unit substantially as hereinbefore described with reference to Figures 3, 4 or 5 of the accompanying drawings.