GB2124454A - Cordless telephone system - Google Patents

Abstract

A cordless telephone system has a base station for connection to a telephone line, and a handset arranged for duplex radio intercommunication. Both base station and handset are microprocessor controlled and each has a non-erasable non-volatile read only memory for storing a unique digital access code. The base station has a receiver section 30, and a transmitter section 10 arranged automatically to transmit the code stored in its memory in the form of a FSK modulated signal when an incoming call is present on the telephone line, the handset having means for comparing the transmitted code with its own stored code, so that, if the two codes match, an audible ring signal is emitted. Similarly, the handset (Figs. 3,4) is arranged automatically to transmit the code stored in its memory either when the user switches it on to make a user- originating call, or when it is switched on by the user in reply to the ring signal. In this case, the base station detects the transmitted code, compares it with its own internally stored code, and if the codes match, transmitter and receiver circuitry in the base station is connected to the telephone line. <IMAGE>

Description

SPECIFICATION Cordless telephone system This invention relates to a cordless telephone system in which a base station coupled to a standard telephone line is in communication with one or more remote handsets associated with that base station.

In a known cordless telephone system access from the handset to a base station is established by the user manually dialling a code number to activate the base station. Once contact with the base station has been established and the base station accesses a telephone line connected to it, the handset user then dials the normal telephone number belonging to the intended recipient of the call. This system has the disadvantage that the user must know and remember the access code of the base station and must spend time dialling it.

This also leaves the system open to tampering in that another, unauthorised user having a similar handset needs only to know the access code number in order to gain access to the first user's telephone line.

According to this invention a cordless telephone system comprises: a base station for connection to a telephone line, and having a radio transmitter and receiver; a handset including a radio transmitter and receiver for remote communication with the base station, means for generating a dialling signal, and means for emitting an audible ring signal; wherein the base station and the handset each include respective programmable storage means for storing an access code or a matched pair of access codes; wherein the base station is operable automatically to transmit the code stored therein when an incoming call reaches the base station via the telephone line, the handset being operable automatically to emit the audible ring signal in response to the incoming call only when the said transmitted code has been received and recognised; and wherein the handset is operable automatically to transmit the code stored therein when switched on for a user-originated call, the base station being operable automatically to assume a state in which signals received from the handset can be fed to the telephone line in response to the handset being switched on only when the code transmitted by the handset is received and recognised. In this way contact between the base station and the handset or vice-versa is established automatically when an incoming call reaches the base station via the telephone line or when the handset user switches on his handset to make a call.This is achieved in the preferred embodiment of the invention by microprocessor controlled circuits in both the base station and the handset, both microprocessors being coupled to respective non-erasable nonvolatile programmable access code memories containing the same code or a matched pair of access codes. As part of the programme sequence of operations for connecting a call received on the telephone line a checking process is carried out whereby the handset checks the access code transmitted by the base station and the base station checks the access code transmitted by the handset before voice communication can pass between them.

When the base station detects an incoming call from the telephone line, it transmits its access code. This is recognised by the handset having the matching code, and a "ringing" tone is generated in the handset to alert the handset user. Any other handsets in close proximity operating on the same radio frequency channel but with unmatched access code will not produce the 'ringing tone'.

The handset user activates the handset by operating a 'talk' switch causing the handset to transmit its own access code which is recognised by the base station as valid, causing the telephone line to be enabled, i.e. the telephone line is then connected to transmitter and receiver circuitry in the base station.

The invention will now be described by way of example with reference to the drawings in which Figure 1 is a block diagram of a base station; Figure 2 is a circuit diagram of access code transmitting and receiving circuitry in the base station; Figure 3 is a block diagram of a handset; Figure 4 is a circuit diagram of access code transmitting and receiving circuitry in the handset; Figure 5 is a flow chart of a base station microprocessor program; Figure 6 is a flow chart of a handset microprocessor program; and Figure 7 is a waveform diagram of the code signal transmitted by the base station in order to access the handset.

Referring to Figure 1 of the drawings, a base station for a cordless telephone system in accordance with the invention has a transmitter section 10 for transmitting speech and code signals on a 1.6 MHz carrier to a remote handset.

The transmitter section includes an r.f. oscillator and modulator 12, a driver and switching circuit 14, and an output stage 16 coupled to a tuned wire aerial 1 8. The modulator 12 has inputs 20 and 22 for a speech signal fed from an isolating transformer 23 via a pre-amp 24, audio limiter circuit 26 and buffer 27, and for a digital code signal, the latter being transmitted in the form of an FSK (frequency shift keying) signal, as will be described hereinafter. A tuning indicator 28 comprises an l.e.d. 'in use' indicator driven by an amplifier having its input placed close to the transmitter output.

Signals from the handset are processed by a receiver section 30 operating in this embodiment at approximately 47 MHz. The receiver section comprises an aerial 32, r.f. amplifier 34, a band pass filter 36, a local oscillator 38 and first mixer 40 for converting incoming signals to a first l.F. of 10.7 MHz, and an integrated circuit l.F. circuit 42 which has an internal oscillator and mixer for conversion to a second I.F. of 455 kHz and an f.m.

demodulator. The outputs of the l.F. circuit 42 include a broadband 'carrier detect' output 44 and an f.m. demodulator output 46, where speech signals, dialling signals and access code signals are provided. The handset, as will be described below, transmits a 4.5 kHz sub-carrier for the dialling signals. Speech signals appearing at the demodulator output 46 are fed to a 4.5 kHz notch filter 48 to remove the sub-carrier before being fed via an isolating transformer 50 to an integrated circuit line interface stage 52.

Dialling signals on the demodulator output 46 are detected by a 4.5 kHz tone detector 54, fed to a pulse shaping stage 56, and thence to a first input 58 of a microprocessor 60 for operating a relay 62 via a LINE ENABLE output 64. This relay 62 connects a resistor across a telephone line connector 66 to simulate the on-hook or dialling states of a conventional telephone.

A digital code signal transmitted by the handset appears as a multiple pulse signal at the demodulator output 46, and is also fed to the first input 58 of the microprocessor 60.

The microprocessor 60 is programmed to store a received code signal in an internal register and compare it with a code stored in a programmable read only memory (PROM) 68, the latter code being first read out of the PROM 68 serially and stored in a second internal register prior to comparison. If the received code andlune code stored in the PROM 68 are identical, the relay 62 is operated via output 64 to connect the line connector 66 to the line interface stage 52. The sequence of operations carried out by the microprocessor will be described in more detail below.

The code stored in the PROM 68 is also used by the base station for accessing the handset. A ring detector circuit 70 coupled to the line connector 66 feeds a ring signal via an opto-coupler 72 to a second RING DETECT input 74 of the microprocessor 60. In response to this, the microprocessor reads the code in the PROM 68, and transmits it in serial form from a CODE OUT output 75 to the modulator 12 for transmission as an FSK signal to the handset.

Further features of the base station include the provision of a call button 76 which alters a final bit of the transmitted code signal to provide a paging facility, the handset detecting the presence of the final bit to aiter the audible signal emitted by the handset. Secondly, the base station has a power supply 78 coupled to power output terminals 80 for charging batteries in the handset when it is placed in a handset receptacle in the base station casing.

The base station circuitry for transmitting the code stored in PROM 68 is shown in more detail in Figure 2. In the preferred circuitry of Figure 2, the microprocessor 60 is a control-oriented processor type no. COP31 1 L manufactured by National Semiconductors, which has its own internal program memory. The same device is used in the handset, and for convenience, devices used for the base station and the handset are identically programmed with a base station program and a handset program. The lack of or presence of a ground connection on pin 18 of the device determines which of the two programs it executes.

The memory 68 is a 16 x 8 bit non-erasable PROM with its four address lines 82 and one of its data lines 84 connected to the microprocessor 60.

Thus, in this embodiment, only one bit is accessed for each of the 1 6 addresses. When the base station transmits the stored code, the PROM 68 is accessed bit by bit and the bits fed sequentially into an internal register. The code is then read out of the register in serial form to a varicap 86 in the modulator 12, where each '1'frequency shifts the transmitted 1.6 MHz carrier to produce an FSK modulated signal.

Referring still to Figure 2, detection and comparison of a code signal received from the handset involves firstly the sensing of the presence of a received carrier via the connection of the broadband detector outDut 44 of the l.F.

circuit 42, this being connected to a HANDSET CARRIER DETECT input 88 on the microprocessor 60. The microprocessor is programmed to wait for a demodulated code signal on its first input 58 on receipt of the CARRIER DETECT signal, this code signal being coupled from the demodulator output 46 via an amplifier and integration stage 90, a pulse squarer 92, a Schmitt trigger 94, and an inverter 96 to the input 58 of the microprocessor.

The received signal is stored in an internal register ready for comparison with the base station code.

Once the code has been recognised as valid, the LINE ENABLE output 64 goes high, the base station transmitter is enabled via TX ENABLE output 97, the code signal path is disabled by transistor 98, and the microprocessor then waits for a dialling signal from the handset. The dialling signal is presented to the same first input 58 of the microprocessor as the code signal. The signal appears as a pulse modulated 4.5 kHz subcarrier on output 46 of the l.F. circuit 42 and, as described above, is detected by tone detector 54.

Thereafter, it is inverted by transistor stage 100, and used to trigger a monostable 102 (type 74 LS 221 N) which controls the width of dialling pulses eventually fed to the telephone line. The output of the monostable 102 is connected to the first input 58 of the microprocessor 60. During dialling, signals between the line and the speech circuits are muted by a mute relay (not shown) controlled from the MUTE output 104.

Referring now to Figure 3, the handset is similar in many ways to the base station. It has a microprocessor 106 and PROM 108 which are identical to those of the base station (also identically programmed), it has a transmitter section 110 which transmits f.m. speech signals, dialling signals and an FSK modulated handset code signal, and a receiver section 112 for receiving f.m. speech signals and an FSK modulated code signal from the base station.

The main components of the transmitter section 110 are a microphone 114, microphone preamp 116, limiter 11 8, an r.f. oscillator and f.m.

modulator 120, an r.f. driver stage 122, an output stage 124, and an aerial 126 tuned to 47 MHz.

The handset casing has a keyboard 128 for dialling, this being connected to an integrated circuit dialler 130, which in turn gates a 4.5 kHz subcarrier oscillator 132 for amplitude modulating the r.f. oscillator. It should be noted that the 4.5 kHz oscillator is normally on whenever the transmitter is on. Dialling pulses merely interrupt the oscillator.

The receiver section 112 comprises a ferrite rod aerial 134 tuned to 1.6 MHz, a local oscillator 1 36 and mixer 138 for converting the received signals to an l.F. frequency of 455 kHz, an I.F. filter and amplifier stage 140, an a.m. carrier detector 142, an f.m. ratio detector 144, an audio preamp 146, an audio output amplifier 148, and an earphone 1 50.

The code signal received from the base station appears at the output of the ratio detector 144 as a multiple pulse signal which requires shaping in squarer stage 1 52 and amplifying in amplifier 1 54 before being fed to the logic circuits controlling the handset.

As in the base station, the microprocessor 106 receives the demodulated code signal on a CODE IN input 1 56. Matching of the received code and the handset access code stored in PROM 108 results in a ring signal being produced at RING output 158, the type of ring signal depending on whether the last bit of the received code signal indicates an outside telephone call or operation of the call button at the base station. The ring signal is fed to the audio output amplifier 148 and earphone 150.

To reply to the ring signal or to initiate a call, the user moves a talk switch 1 60 from its STANDBY or OFF position to a TALK or ON position. This turns on the transmitter section 110, and causes the microprocessor to access the PROM 108 again and transmit the handset code signal via output 1 58.

The handset is powered by a rechargeable battery 1 62 coupled to charging contacts 1 64. To save battery power, the microprocessor 106 is powered up only for code signal reception or transmission, and for generating the ring signal. At all other times it, and the PROM are switched off by switching devices 1 66 and 168, the former being controlled by a latch circuit 170. Thus the stimuli for switching on the microprocessor are (i) switching of the talk switch 1 60 to the TALK position, and (ii) detection of an incoming code signal. A 500 kHz clock 172 is also gated by the latch 170.

The pin connections to the microprocessor 106 and the associated circuitry for detecting and transmitting code signals in the handset are shown in more detail in Figure 4.

Operation of the base station in accordance with the base station program stored in the base station microprocessor will now be described with reference to the flow chart of Figure 5.

In the absence of a telephone call on the telephone line or from the handset, the base station microprocessor cycles around a standby loop 200 in which three inputs are periodically monitored. These are the RING DETECT input, the CARRIER DETECT input, and the CALL BUTTON input.

As soon as a ringing tone is detected on the input from the telephone line the program enters a 'ring handset' loop 202 in which the transmitter driver 14 and output stage 1 6 are enabled, and the base station code signal is read out from the base station PROM and transmitted serially. The code signal is preceded by a 'wake-up' pulse for activating the handset microprocessor, and includes as its final bit, a 'call' bit which in this case indicates that the base station has received a call on the line, not an input from the call button. The program follows the loop 202 until the user operates the talk switch on the handset, causing the handset carrier and subcarrier to be transmitted (provided the handset has recognised the base station code signal).

The handset responds by transmitting its own code which the microprocessor in the base station then compares with the stored base station code.

If the codes match, the line relay in the base station is operated and the telephone line is connected to the speech circuits (ENABLE LiNE).

If, on the other hand, the handset code does not match, or is not received within a predetermined time, the program goes back to the standby loop 200. This is the connection sequence for a call originating from the line. If, however, a useroriginating call, i.e. a call initiated by the handset, is detected (HANDSET CARRIER ON in loop 200) the program enters the above described code reception and comparison sequence before the base station transmitter is enabled. If the codes match, only then does the base station respond by switching on its transmitter and connecting the telephone line.

The lower half of the flow chart deals with the dialling and/or conversation stages of a call. It should be understood that the 4.5 kHz subcarrier from the handset may disappear for any of three reasons: (i) presence of a dialling pulse; (ii) radio link has faded; or (iii) the handset talk switch has been switched off (i.e. call finished). The lower haif of the flow chart enables the base station to distinguish between these three conditions on the basis that any break resulting from a dialling pulse is less than 77 ms in duration. Any break longer than 77 ms but less than 1.5 s is counted as a fade, and a break longer than 1.5 s is treated as an end-of-call signal.

In the case of a call received from the telephone line, no dialling pulses will normally be received from the handset in which case the subcarrier will normally be present at ali times, causing the program to follow the loop marked 204. If a fade occurs, the program will receive a NO response at box 206 and will enter firstly loop 208 for 77 ms (during which the line is disabled but the call is not lost), and will then enter a delay loop 210 during which the line is reconnected so as not to lose the call. Providing the subcarrier fades for less than 1.5 s, the program will return to loop 204, otherwise it will allow the call to be disconnected and return to RESET.

In the case of a call originating from the handset, the microprocessor is programmed to accept dialling pulses after the code from the handset has been received and validated. At the beginning of the first dialling pulse the program will enter loop 208, 'breaking' the line connection, then at the end of the pulse enter loop 204 'making' the line connection. In this way, a d.c.

path is switched periodically across the line as in a conventional telephone during dialling. The MUTE relay is also caused to operate during dialling by the MUTE ON and MUTE OFF instructions.

Referring to Figure 6, the handset program follows a simpler procedure since it is not concerned with dialling. It should be remembered that the handset transmitter and receiver remain on whenever the talk switch is on. The flow chart of Figure 7 is largely self explanatory. When a call is received from the base station the program follows the left hand side of the chart, in which the base station code is checked and either a telephone type ring or a paging signal is emitted by the earphone. When the talk switch is operated, either in response to a ring or to initiate a call, the microprocessor causes the handset code to be transmitted, and then turns itself off until it receives another code or until the talk switch is switched on again.

The code signals transmitted between the base station and handset take the form of a train of pulses of variable width. The waveform illustrated in Figure 7 is the signal transmitted by the base station, showing the 'wake-up' pulse preceding the code.

Claims (11)

1. A cordless telephone system comprising: a base station for connection to a telephone line, and having a radio transmitter and receiver; a handset including a radio transmitter and receiver for remote communication with the base station, means for generating a dialling signal, and means for emitting an audible ring signal; wherein the base station and the handset each include respective programmable storage means for storing an access code or a matched pair of access codes; wherein the base station is operable automatically to transmit the code stored therein when an incoming call reaches the base station via the telephone line, the handset being operable automatically to emit the audible ring signal in response to the incoming call only when the said transmitted code has been received and recognised; and wherein the handset is operable automatically to transmit the code stored therein when switched on for a user-originated call, the base station being operable automatically to assume a state in which signals received from the handset can be fed to the telephone line in response to the handset being switched on only when the code transmitted by the handset is received and recognised.

2. A system according to claim 1 , wherein the handset storage means is a non-volatile memory device.

3. A system according to claim 2, wherein the memory device is a non-erasable programmable read only memory.

4. A system according to any preceding claim wherein the handset is operable to transmit the code stored therein in response to reception and recognition of the code transmitted by the base station, and wherein the base station is operable to feed signals from the handset to the telephone line and vice versa in response to an incoming call only when the code transmitted by the handset is received and recognised.

5. A system according to any preceding claim, wherein the code or matched pair of codes comprises a digital word or words unique to the particular base station and associated handset or handsets.

6. A system according to claim 5, wherein the or each code is transmitted as a train of variable width pulses.

7. A system according to claim 6, wherein the base station and the handset each include a frequency shift modulator for modulating the respective radio transmitters with the respective codes to be transmitted.

8. A method of operating a cordless telephone system comprising a handset and a base station, the method including the steps of detecting an incoming call or a telephone line connected to the base station, automatically accessing a memory in the base station to read out or access code therein, transmitting the code to the handset, comparing in the handset the transmitted code with an identical or matching access code stored in a memory in the handset, and causing an audible ring signal to be generated by the handset in response to a positive comparison result.

9. A method according to claim 8, wherein the access code stored in the handset is transmitted by the handset when the handset is switched on, and wherein the transmitted code is received in the base station, compared with the access code stored in the base station, and a connection is established in the base station in response to a positive comparison result to enable signals to pass from the handset to the telephone line.

10. A cordless telephone system constructed and arranged substantially as herein described and shown in the drawings.

11. A method of operating a cordless telephone system substantially as herein described with reference to the drawings.