GB2068685A

GB2068685A - Selective message broadcasting system

Info

Publication number: GB2068685A
Application number: GB8003003A
Authority: GB
Original assignee: British Broadcasting Corp
Current assignee: British Broadcasting Corp
Priority date: 1980-01-29
Filing date: 1980-01-29
Publication date: 1981-08-12
Also published as: EP0033218B1; EP0033218A1; DE3161447D1

Description

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SPECIFICATION

A selective message broadcasting system

5 The present invention relates to a selective message-broadcasting system.

Selective message broadcasting systems are used to broadcast interrupting messages, e.g. traffic information messages, to specific geographical 10 areas, whereby listeners hear only those messages relevant to the area within which they are situated.

In this specification, the invention is described largely in relation to a motoring service but the invention would be applicable to other services, ,15 such as selective paging systems or aircraft or other speech or data communications, where it is desirable to direct messages to particular areas.

Known selective message-broadcasting systems can either be frequency-division multiplex systems 20 (FDM), in which a number of messages are broadcast each having a different carrier frequency, or time division multiplex systems (TDM), in which the messages have a common carrier frequency and are broadcast one at a time.

25 Several methods for selecting a desired area of reception have been used or proposed in the past, some being discussed in the specification of our cognate applications 49919/76 and 14367/77, Serial No.

30 A further method disclosed in the said specification relies upon the comparison of the levels of signals coming from a master station in the area of desired reception and from one or more inhibiting or slave stations outside the area. If the ratio of the 35 master level to the or each slave level is high enough, the receiver will respond to the message; if the ratio is not high enough, the receiver will not respond. A disadvantage of this system is that any obstructions of differences in the propagation paths, 40 which cause one signal to be attenuated more than the others, will cause variations in the relative signal levels, resulting in a displacement of the expected service area boundary. Although the displacement is negligible when operating in the low to medium fre-45 quency range, this is not the case when using very high frequencies where propagation effects will cause large and random variations in the levels of the received signals.

The need to use a system which operates at very 50 high frequencies may occur if an acceptable channel in or near the medium frequency broadcast band is not available or if sky-wave interference at night, possiblyfrom other transmissions, results in incorrect operation of the broadcasting system. 55 The object of the present invention is to provide a system which is suitable for use, inter alia, at very high frequencies.

According to the present invention there is provided a message broadcasting and receiving system 60 comprising a network of transmitters, control means arranged to select a master transmitter or transmitters to transmit a message signal and to cause the or each selected transmitterto preface its message signal with a master code signal, and to cause one or 65 more other transmitters of the network to radiate a slave code signal which is distinct from the master code signal, and a fixed or mobile receiver including a detecting circuit which detects whetherthe master code signal is received before or after an instant 70 determined by the reception of a slave code signal and allows the reception of the message signal, if, and only if, the master code signal is received before the said instant.

According to the present invention there is also 75 provided a message-broadcasting system comprising a network of transmitters which, in operation, transmit messages in time division multiplex, a control means arranged to select a master transmitter or transmitters to transmit a message signal and to 80 cause the or each selected transmitterto preface its message with a master code signal and to cause one ormoreothertransmittersofthe network to radiate a slave code signal, wherein the master code signal and slave code signal are carrier signals displaced by 85 different amounts from the carrier frequency of the message signal.

The receiver is able to determine whether or not it is in the master service area by recognising whether or not the code signal which it receives first origi-90 nated from the or a master transmitter or from the or a slave transmitter. If the master code signal, originating from the nearest master transmitter, is the first signal to be received, the receiver is inside or on the boundary of the master service area and will 95 be demuted, i.e. it will respond to the message following the master signal; if the slave code signal, originating from the nearest slave transmitter, is the first signal to be received, the receiver is outside the master service area and will remain muted, i.e. it will 100 not respond to the message following the master signal. Subsequent arrivals from more distant transmitters during the period in which the code signals are evaluated will have no further effect on the receiver. Such receivers may be fitted in vehicles 105 or may be at fixed locations such as in the homes or elsewhere.

The present invention further provides a message broadcast receiver comprising a detecting circuit arranged to detect whether a first signal, having a 110 given frequency, is received before an instant determined by the reception of a second signal, having a frequency different to that of the first signal, and allows the reception of a message signal if, and only if, the first signal is received before the said 115 instant.

After the message signal has been received the receiver is muted, i.e. it is prevented from receiving further messages. This may be accomplished by using the mastertransmitterto transmit a signal at 120 the frequency of the slave signals, afterthe message signal.

The invention will be described in more detail, by way of example, with reference to the accompanying drawings, wherein

The drawings originally filed were informal and the print here reproduced is taken from a later filed formal copy.

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Figure 1 illustrates a network of transmitters indicating a master service area of a particular master transmitter when surrounded by six equally spaced slaves,

5 Figure 2 is a diagram showing one possible format ofthe transmitted signals.

Figure 3 is a block diagram of a receiver-decoder.

Figure 4 is a block diagram of a master controller and one of the transmitters, and 10 Figure 5 is a circuit diagram of a basic decoder showing voltage and timing comparator circuits.

Referring to Figure 1, a network of transmitters T1, T2 etc., are arranged on an idealized equilateral triangular lattice. A practical transmitterto transmit-15 ter spacing may be 50km, although the lattice will be distorted to allow for the geographical attributes of the region served by the network.

In this particular embodiment T7 is selected as a mastertransmitterand T1 to T6 are selected as slave 20 transmitters, defining a master service area 4. The mastertransmitterT7 transmits a master signal, one possible format of which is shown in Fig. 2a, consisting of an initial burst of an unmodulated carrier6ata frequency fo-100 kHz. The message signal 8, follow-25 ing the initial burst, uses a carrier frequency fo which is frequency modulated by ±50 kHz. The slave transmitters T1 to T6 (Fig. 1) transmit a slave signal, one possible format of which is shown in Figure 2b, consisting of an initial burst of an unmodulated car-30 rier 10 at a different frequency, i.e. fo + 100 kHz, to that of the initial burst ofthe master signal 6.

The duration ofthe master and slave transmission bursts 6 and 10 is made longerthan the time taken for a signal to travel between the master transmitter 35 T7 and the slave transmitters T1 to T6, e.g., a transmission burst of 2ms for a master/slave transmitter separation of 60 km. This prevents erroneous operation ofthe receiver.

Referring now to Figure 3 a receiver 11 includes an 40 aerial 12 and V.H.F. crystal controlled RF stages 14 which receive the transmitted signals 6,8 and 10. The output from the RF stages Misapplied to an FM discriminator 16. The discriminator 16 is so designed that, on receiving the master transmission burst 6 45 (Figure 2a), at a frequency of fo-100 kHz, it wilt produce a positive d.c. voltage shift of say 1 volt whereas, on receiving the slave transmission burst 10 (Figure 2b), at a frequency of fo + 100 kHz, it will produce a negative d.c. voltage shift of similar amp-50 litude. The presence ofthe message signal 8 atthe input to the discriminator 16 will not cause the discriminator output to vary by more than say ±0.5 volts.

The output ofthe FM discriminator 16 divides 55 along two separate paths 30 and 36. The path 36 forms the input to a voltage comparator 18 which recognises the ±1 volt shifts ofthe FM discriminator 16 but ignores the ±0.5 volt variations caused by the message signal 8. Referring now to Figure 5 the vol-60 tage comparator 18 receives an input from the discriminator 16. The input is applied to one terminal of two comparator amplifiers 38 and 40. The other terminal of each comparator amplifier 38 and 40 is connected to a potentiometer 42 and 44 respectively. 65 Transistor amplifiers 46 and 48 are connected to the respective outputs ofthe comparator amplifiers 38 and 40.

The potentiometer42 is so adjusted that the master code signal fo-100 kHz produces a logic 1 level on the output 50 ofthe transistor amplifier46. The poten-tiometer44 is so adjusted thatthe slave code signal fo + 100 kHz produces a logic 1 level on the output 52 ofthe transistor amplifier 48.

The output 50 is connected through cascaded inverters 54 and 58 to the D input of a D-type flip-flop 62 in a timing comparator 20. The outputs 50 and 52. are connected to a NOR gate 56 whose output is connected through an inverter 60 to the clock terminal CK ofthe flip-flop.

When a logic 1 level appears first on the master output 50, the D input gives true early enough for the flip-flop to set on the rising edge on CK provided by gate 56 and inverter 60. If the slave output 52 goes true first, CK is strobed while the D input is still false and the flip-flop remains un-set.

The Q output ofthe flip-flop is applied to an audio switch 22 (Fig. 3) and, when the flip-flop is set, the switch completes the audio path 30 between the discriminator 16 and AF stages 24 connected to a loudspeaker 32. The receiver is thus demuted. The switch 22 can simultaneously mute an alternative audio source normally feeding the speaker 32.

In an experimental system is was found thatthe flip-flop 62 set provided thatthe master code signal was received about 5 /zs before the slave code signal.

Afterthe message signal 8 (Fig. 2) has been received, the receiver 11 is muted, i.e. the audio switch 22 is turned off. To do this the flip-flop 62 is reset, by broadcasting a transmission burst 13 (Fig. 2), from the mastertransmitterT7, atthe frequency offo + 100 kHz afterthe message signal 8, i.e. the frequency normally considered to be that ofthe slave transmitters T1 to T6. The flip-flop is therefore clocked with the D-input false.

Thus the slave transmitters T1 to T6 provide a simple means for defining the service area 4 ofthe mastertransmitterT6. In addition, the shape of each service area 4 may be varied by delaying the switch-on ofthe master or slave transmission bursts 6 and 10. By delaying the switch-on of mastertrans- ^ mission burst 6, the boundary will be moved towards the master transmitter, i.e. the master ser-vicearea will be reduced. By delaying the switch-on 1 of aslave transmission burst 10, the boundary will be moved towards the slave transmission in question, thus increasing the master service area. The system has the further advantage that, should a particular transmitter fail, the service areas ofthe transmitter surrounding it automatically expand and thus tend to take its place. Furthermore, since the system relies on the fact that the receiver 11 makes its decision on the first signal to arrive at its input and ignores subsequent arrivals, multipath reception at the receiver is unlikery to cause difficulties.

This invention does not preclude the use of additional codes by which receivers may distinguish between various types of message. Additional coded signals may be transmitted either immediately before or immediately afterthe start code or could

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form the start code signal that could indicate, for example, particular classes of recipient for whom the message that follows is intended, and/or whether the message is a repeat of one that has previously 5 been broadcast, or other features ofthe message. Such additional signals would enable receivers to disregard irrelevant messages.

In the present state ofthe art, it is a relatively straightforward matterto control the codetransmis-10 sions ofthe various transmitters. The transmitter network could be controlled by a single master controller or by several interconnected controllers. However, an example will be given for operating the network of Figure 1 which uses a single master con-1S trailer. A master controller 64 (Fig. 4) is able to communicate with all the transmitters in the network by means of land lines, orthe equivalent, but is here shown linked to only one ofthe transmitters. The master controller distributes the messages which 20 the different transmitters are to broadcast, in conjunction with a prefixed digital address code which selects the transmitter which is to broadcast the meassage. The address code is followed by a master transmission burst 6 (Fig. 2), then the message 25 signal 8 (Fig. 2) itself and finally the second transmission burst 12 (Fig. 2). The controller 64 may be simply a preset device which emits the digital codes and messages to a predetermined time schedule or it may utilize a computerto determine the schedul-30 ing of transmitters in accordance with requirements.

Each transmitter (Fig. 4) includes a control demodulator 66 which demodulates the digital codes in a manner well known in data communication. The digital codes are fed to a control decoder 68 35 which recognises, from the address code, whether its transmitter is to radiate the mastertransmission burst 6 and the message 8, orthe slave transmission burst 10, or to remain quiescent.

Assuming that frequency modulation is used for 40 the message 8, the transmitter comprises a drive stage 70 feeding RF output stages 72 and an aerial 74 through an FM message modulator 76, an oscillator 77 providing the carrier frequency fo to the drive circuits 70.

45 A100 kHz oscillator 88 and a mixer 85 modulate fo " with 100 kHz to provide sidebands fo ±100 kHz which are separately filtered out by lower and upper sideband band-pass filters 84 and 86.

The control decoder 68 is used to control various 50 switches as follows, as commanded by demodulated control signals. If the transmitter is to transmit,

switch 78 is closed. If the transmitter is selected as master transmitter, a switch 80 is then closed for 2 msto radiate the master code signal fo - 100 kHz. A 55 switch 82 is then closed to enable the message sent overthe land line to be applied to the modulator76. Atthe conclusion ofthe message, a switch 83 is closed for 2 msto radiate the slave code signal fo + 100 kHz, which re-mutes the receivers which were 60 de-muted by the master code signal.

If the transmitter is selected as a slave, only the switch 83 is closed for 2 ms to radiate the slave code signal.

The correct relative timings can be established by 65 the times at which commands are sent to the different transmitters orthe decoder 68 can incorporate say monostable circuits to establish such relative delays as may be required.

Claims

1. A message broadcasting and receiving system comprising a network of transmitters, control means arranged to selecta mastertransmitterortransmit-ters to transmit a message signal and to cause the or each selected transmitterto preface its message signal with a master code signal, and to cause one or more othertransmitters ofthe network to radiate a slave code signal which is distinct from the master code signal, and a fixed or mobile receiver including a detecting circuit which detects whetherthe master code signal is received before or after an instant determined by the reception of a slave code signal and allows the reception ofthe message signal if, and only if, the master code signal is received before the said instant.

2. A message broadcasting system comprising a network of transmitters which, in operation, transmit messages in time division multiplex, a control means arranged to selecta mastertransmitteror transmitters to transmit a message signal and to cause the or each selected transmitterto preface its message with a master code signal and to cause one or more othertransmitters ofthe network to radiate a slave code signal, wherein the master code signal and slave code signal are carrier signals displaced by different amounts from the carrier frequency ofthe message signal.

3. A system according to claim 2, wherein the said different amounts are positive and negative displacements ofthe same magnitude.

4. A system according to claim 2 or 3, wherein the code signals start from their respective transmitters at different times.

5. A message-broadcasting system substantially as hereinbefore described with reference to and as illustrated in Fig 6 ofthe accompanying drawings.

6. A message broadcast receiver comprising a detecting circuit arranged to detect whether a first signal, having a given frequency, is received before an instant determined by the reception of a second signal, having a frequency different to that of the first signal, and allows the reception of a message signal if, and only if, the first signal is received before the said instant.

7. A message broadcast receiver according to claim 6, for use with signals such that the message signal is a frequency modulated carrier with frequency deviations restricted to a predetermined range, one ofthe first and second signals is a carrier signal at a frequency above the said range and the other ofthe first and second signals is a carrier signal at a frequency below the said range, wherein the detecting circuit comprises an f.m. discriminator which produces first and second voltage levels in response to the first and second signals respectively, and a timing comparator which determines, from the appearance ofthe first and second voltage levels, whetherthe first signal is received before or afterthe said instant.

8. A message broadcast receiver substantially as hereinbefore described with reference to and as

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illustrated in Fig 3 ofthe accompanying drawings.

Printed for Her Majesty's Stationery Office by The Tweeddale Press Ltd., Berwick-upon-Tweed, 1981.

Published at the Patent Office, 25 Southampton Buildings, London, WC2A1 AY, from which copies may be obtained.