The present invention relates to a telephone paging system.

Large scale paging systems catering for a large number of subscribers are well-known.

A paging system previously proposed in for example US patent 4263480 (Levine) includes a transmitter which automatically sends out an identifying address code to activate a selected pager receiver in response to a telephone call from a calling party who dials the normal telephone number of the party to be reached. The transmitter station also automatically sends out a message to the activated receiver in response to a number message subsequently dialled by the calling party. The transmitter station automatically responds to the dialled identifying message and the dialled message from the caller to locate the identifying code and the message code to be transmitted that have been previously stored in a memory. At the receiver is provided a memory having a series of pre-stored limited content messages, each of which is selectively retrievable upon a receipt of

a message code corresponding to the address of that message in the memory.

When the calling party initially calls the system, the system sends to the calling party a message instructing the calling party to use the system.

The system of Levine, like other large scale paging systems, is adapted for use by many subscribers and is unsuitable for small scale, domestic and/or small office use.

US Patent 3811012 (Barber) discloses a remote telephone calling system in which, upon detection of a ringing signal on a telephone line, a tape answering device is automatically actuated to send to the caller a message requesting the caller to hold the line. In addition, a radio transmitter is automatically actuated to send to a radio receiver worn by a user of the system a signal for alerting the user to the presence of a call.

It is also known to provide auxiliary ringing devices or other indicators wired to a telephone line, on e.g. the outside of a building to indicate the presence of a call. Such devices and indicators are

effective only over a very short range e.g. within ear shot or within direct line of sight of a user and can be annoying to people other -than the intended users of the devices or indicators.

According to one aspect of the present invention there is provided a telephone paging system which lacks provision for the transmission of voice or other intelligent information and which remotely and wirelessly indicates the presence, on a single telephone line, of a ringing signal without seizing the line.

A preferred embodiment of that system comprises a base unit which includes a radio transmitter, and a remote pager which includes a radio receiver. Preferably, the ringing cadence of the ringing signal is reproduced at the remote receiver. Furthermore, it is preferred that, for security, the transmitter transmits to the receiver a coded signal.

According to a further aspect of the invention there is provided a telephone paging system which lacks provision for the transmission of voice or other intelligence information and which remotely and wirelessly reproduces the cadence of a ringing

signal. Preferably, the system detects, and remotely reproduces the cadence of a ringing signal without seizing the line on which the ringing signal occurs.

Those and other aspects of the invention are specified in the accompanying claims to which attention is directed.

For a better understanding of the present invention, and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying drawings in which:

Figure 1 is a schematic block diagram of an illustrative base unit of an illustrative paging system in the accordance with the present invention.

Figure 2 is a schematic block diagram of an illustrative pager of the system,

Figures 3 A B and C are wave-form diagrams illustrating the cadence of an illustrative ringing signal, a coded signal, and the format of a code of the coded signal,and

Figure 4 illustrates an alternative coding scheme,

Figure 5 is a schematic diagram of another illustrative paging system in accordance with the invention, and

Figure 6 is a schematic diagram of the base unit of either embodiment of the invention.

The base unit of Figure 1 is connected to a single telephone line 1. It responds, without seizing the line, to a ringing signal on the line 1 to radiantly transmit, e.g. by radio, a coded signal to the pager of Figure 2. The pager of Figure 2 compares the code represented by the coded signal with a reference code and actuates, in dependence upon the comparison, an indicator 2 for indicating to a user of the pager the presence of a ringing signal on the line 1. The paging system of Figures 1 and 2 has provision only for transmitting and receiving such a coded signal. There is no provision in the system for transmitting voice or other intelligence information from the system to the line 1, or from the base unit to the pager or from the pager to the base unit.

The illustrative system of Figures 1 and 2 is intended for use in domestic dwellings and small offices where there is a need for a cheap, simple, short range paging system. In a domestic dwelling having typically one line, the base unit plugs into a standard socket connected to the line. In a small office having several lines, it is envisaged that one base unit per line would be provided. If the office has a PBX with several extensions it is envisaged one unit per extension would be provided.

THE BASE UNIT OF FIGURE 1

The base unit is . preferably housed in a socket replicating box having a replicating socket 3, the box being adapted to plug into a standard telephone company (e.g, British Telecom Pic) socket 4. As shown in Figure 1, that socket 4 includes a capacitor 6 which is provided for DC blocking and other purposes, and also a line terminating resistor 7.

The base unit comprises a conventional ringing signal detection circuit and power supply circuit 5. The power supply powers at least some of the base unit from the telephone line, but in some circumstances an additional power source, eg. a battery, may also be

required. The ringing signal detection circuit produces, without seizing the line, a signal indicating the presence of the ringing signal on the line. Preferably, the indicating signal reproduces the ringing cadence. Thus if the ringing cadence is a repeated pattern of 0.4 seconds ring followed by a 0.2 second interval followed by 0.4 seconds ring followed by a _. 2 seconds interval (see Figure 3a) the indicating signal is a binary signal in which eg. HIGH represents the ring and LOW represents the intervals between rings as shown in Figure 3a. (alternatively, LOW may represent the ring and HIGH the interval) .

A circuit 12 responds to the signal, indicating the presence of the ringing signal to supply a coded signal to a radio transmitter 9. Preferably, and as shown in Figure 3b, the coded signal is generated repeatedly during the ring periods of the ringing signal (in which the indication signal is high). Thus the coded signal reproduces the cadence 6± the ringing signal. The circuit 12 comprises an encoder 8 which is responsive to a code selector 10 to produce the coded signal in response to the signal indicating the presence of the ringing signal. The code selector defines the code to be transmitted and

is preferably preset at manufacture of the base unit although it is conceivable that the code could be set by the customer.

In the example of Figure 1, the radio transmitter 9 PSK modulates a carrier with the coded signal. Other modulation schemes could be used. PSK modulation may be advantageous because it provides good immunity against noise. The modulated carrier, produced in the oscillator 11, is radiated from an aerial 13.

As shown in Figure 3c the code which is repeatedly produced during the HIGH period of the indication signal of Figure 3a -comprises start bits, "data" bits representing the security code and stop bits. If the code is produced at a rate of 16 kilo bits per second and has a bit length of e.g. 10 bits, then the code would be repeated over 600 times within the 0.4 second ring interval. The carrier frequency may be for example 49 mega hertz.

THE PAGER OF FIGURE 2

The pager comprises a receiving aerial 20 which supplies the transmitted PSK signal to a PSK receiver

21. The signal is demodulated in the receiver 21 and

the security code is supplied to a decoder 22 where it is compared with a reference code produced by a code selector 23 similar to code selector 10. If the received and reference codes agree, or bear the correct relationship with each other, the decoder actuates, via an amplifier 24 the indicator 2. The indicator 2 comprises an audible indicator 2a such as a piezo electric resonator and/or a visual indicator 2b such as a light emitting diode. Where both indicators 2a and 2b are provided a selection switch 25 is also provided for selectively connecting them to the amplifier 24. In addition, or alternatively, a tactile device can be used. Such a device would be useful for a person.who is deaf for example.

The pager being portable, is battery powered. In order to conserve power a logic circuit 26 intermittently actuates the receiver 21 and if, when actuated, the receiver detects a transmitted coded signal or at least the carrier thereof, the logic circuit 26 responds to the detection to maintain the actuation of the receiver and to actuate the comparator amplifier and indicator. The receiver is deactuated and other parts of the pager are also deactuated when the carrier ceases.

Referring to figure 3 the standard ringing cadence in the UK is a repeated pattern of 0.4 seconds ring, 0.2 seconds interval 0.4 seconds ring 2 second interval. Many PBX's when a call is made from one internal extension to another internal extension produce a different cadence so that the user of an extension phone can distinguish between external and internal calls. Because the above described embodiment preserves the ringing cadence, the user of the system in combination with a PBX can distinguish between internal and external calls when being paged.

The code selectors 10 and 23 may be of conventional design.

The above described embodiments of the invention provide a simple paging system which acts in such a way that it appears to a caller that he is calling a normal telephone; indeed, the caller could not discern that he was calling a paging system. Furthermore, the paging system provides iirs owner with a low cost, secure paging over a short range but with greater flexibility and less potential annoyance to others than for example auxiliary ringing devices wired to the outside of buildings etc.

MODIFICATIONS

Various modifications may be made to the above-described illustrative paging systems.

Although the illustrative systems have been described as reproducing the ringing cadence, a modified embodiment may not reproduce the cadence. The coded signal which was described above as being repeated so as to indiciate the cadence may be repeated less frequently whereby it no longer indicates the cadence.

In the illustrative systems described above a security code eg. a pseudo random number, is modulated eg. PSK modulated onto a carrier. An alternative security coding scheme is illustrated in figure 4. This scheme is based on the detection of a signal at preset time after the generation and detection of carrier. Each pager and transmitter pair is allocated one of (say) 256 time slots during the complete carrier transmission time. The transmitter begins to generate carrier upon detection of ringing. It generates a short signal e.g. a _. VF (voice frequency) tone modulating the carrier at a pre-defined period determined by the encoding after

the commencement of carrier. The pager detects carrier and looks for the presence of the short signal at the pre-defined period determined by its code selection. This cycle will be repeated as ringing persists. In such a scheme the security code is the timing of the tone. With such a sheme the transmitter need not be a PSK transmitter.

A presently preferred security coding scheme will now be described with reference to figures 5A and B in which parts corresponding to parts shown in figures 1A and B bear the same reference numerals as in figures 1A and B and need no further description here.

In the system of figures 5A and B, each transmitter/receiver pair is allocated two tones out of many potentially available tones. The tones actually used are selected by a security selection device. The two tones are transmitted simultaneously upon detection of the ringing signal, being modulated onto the carrier. The receiver responds to the simultaneous presence of the two tones to indicate the presence of the ringing signal.

The two tones may be at voice frequency, the pair of

tones selected being those unlikely to occur simultaneously in other signals (eg. speech) which occupy the radio band of the transmitter. Alternatively the tones may be outside the band of voice frequences.

The transmitter may use a known DTMF (dual tone multi frequency) dialler chip for tone generation and selection, the selection being set by eg. wire links. The receiver may use I.C. tone decoders with resistors setting the tone selection.

The tones may reproduce the ringing cadence.

In such a scheme the security code is the pair of tones.

Referring to Figure 6, the base unit of either embodiment is further shown schematically. The base unit 30 comprises the box 32 containing the circuitry from the ring detector to the transmitter, shown collectively as element 34, and the aerial 13. The box 32 has a plug 36 connectable to the standard telephone socket 4 mounted on a wall 38. The box 32

also has a socket 40 to which a standard plug 42 of telephone 44 can be connected.