GB2265530A - Public address system. - Google Patents

Abstract

A public address system comprising a central unit 10 and a number of remote units 20 from which a message is to be broadcast, the system incorporating a memory 26 from which a message is to be read, the readout of the message being inhibited or interrupted when background noise above a certain threshold level is detected. The system also provides for a repeat of a message to be initiated at will at a remote unit. <IMAGE>

Description

PUBLIC ADDRESS SYSTEM The invention relates to a public address system and in particular a public address system having a central control unit and a plurality of remote units, each having means for broadcasting a message.

Public address systems having the facility for broadcasting a message in a number of areas are well known and are widely used in, for example, bus stations, railway stations, airports and department stores. They also find common use in vehicles, such as trains.

Known public address (P.A.) systems, however, have a number of drawbacks, among them: (a) Messages broadcast by such systems are often rendered unintelligible by the presence of noise in the listening environment, and as a consequence their impact can be lost on the public; (b) Listener interaction is largely lacking, especially interaction in the form of the listener's being able both to request repetition of an already broadcast message and also to inititiate the broadcast, locally, of a desired piece of information.

It is the aim of the present invention to provide a P.A. system which overcomes the above-mentioned drawbacks.

Accordingly, the invention provides a public address system, comprising a central unit and a plurality of remote units for broadcasting a message, each remote unit being associated with a memory means for storing a message to be broadcast and including a readout influencing means for influencing the readout of the memory means in response to local commands.

The memory means may be located in either the remote unit or the central unit.

The readout influencing means may include a monitoring means for monitoring local noise conditions, said monitoring means serving to control readout of the message to be broadcast in response to ambient noise.

The monitoring means may either inhibit the readout of the message to be broadcast, if the message has not started to be read out, or it may interrupt readout of the message, if the message has started to be read out.

The readout influencing means may include a message repeat means for enabling the message whose readout has been interrupted due to ambient noise conditions to be repeated.

The readout influencing means may include a message continuation means for causing a message whose readout has been interrupted for only a short time due to ambient noise conditions to be continued from the point at which it was interrupted.

The readout influencing means may include an external command means for generating, under the control of a listener to the public address system, a command signal initiating the repeat of a message held in the memory means.

A live message transmitting means may be provided in the central unit, for transmitting to the remote units a signal representing a live message to be broadcast.

The live message transmitted to the remote units may be simultaneously read into the memory means and broadcast from the remote units, the broadcasting of the message from the units being influenceable by the monitoring means in the event of excess background noise being detected.

The central unit may include a preset message storing means, a preset message selecting means and a message type selecting means, said preset message storing means consisting of a bank of preset messages and said preset message selecting means serving to select, under the control of the user of the central unit, one of the preset messages stored in the preset message storing means, the message type selecting means being employed by the user to select which of a live message and a preset message is to be transmitted to and broadcast by the remote units.

Each remote unit may include a preset message storing means and the central unit may include a preset message selecting means and a message type selecting means, said preset message storing means consisting of a bank of preset messages and said preset message selecting means serving to select, under the control of the user of the central unit, one of the preset messages stored in the preset message storing means, the message type selecting means being employed by the user of the central unit to select which of a live message and a preset message is to be transmitted to and broadcast by the remote units.

The memory means and the preset message storing means may be realised as different storage areas within the same storage device, the memory means being employed as a live memory for storing a live message transmitted by the central unit, and the contents of either the live memory or the preset memory bank being selectable from the central unit for broadcast by the remote units.

The contents of either the live memory or the preset memory bank may also be selectable from the remote unit by a listener to the public address system.

The specific remote units from which a message is to be broadcast may be selectable out of the total number of remote units in the system.

The remote units may be addressable either singly or in groups.

The central unit may include an address code generating means for generating and transmitting to all the remote units in the system a digital address code word, said code word being received in each of the remote units and correlated in a correlating means with an in-unit code word specific to that remote unit, a match between the received and in-unit code words resulting in the switching of that remote unit from a standby to a message-receive condition.

The preset message select signal may be in the form of a digital code word, transmission of the address code word from the address code generating means and of the preset message select code word from the preset message selecting means being effected in that order and by means of the same transmitting means.

The transmission from the central unit to the remote units of any or all of the live message, the address code word and the preset message or preset message select signal may be effected by electromagnetic radiation.

The transmission may be effected by radio frequency, infra red or microwave radiation; the latter may take place by satellite link.

The transmission from the central unit to the remote units of any or all of the live message, the address code word and the preset message or preset message select signal may be effected by conduction in wiring.

Transmission from the central unit to the remote units may be effected by means of a bus.

Embodiments of the invention will now be described solely by way of example and with reference to the accompanying figures, in which: Figure 1 is a block diagram of a first embodiment of the public address system according to the invention; Figure 2 is a diagram showing the action of the Repeat and Noise Control facilities of the first embodiment of the invention; Figure 3 is a block diagram of a second embodiment of the public address system according to the invention; Figure 4 is a schematic diagram of one embodiment of the Interlock and Override section of the second embodiment of the invention; Figure 5 is a block diagram of a third embodiment of the public address system according to the invention; Figure 6 is a block diagram of a remote unit for a fifth embodiment of the public address system according to the invention;; Figure 7 is a diagram showing a first method of reducing the number of transmitters and receivers needed in a public address system according to the invention; Figure 8 is a diagram showing a second method of reducing the number of transmitters and receivers needed in a public address system according to the invention; Figure 9 is a diagram showing the second method of Figure 8 used within a hardwired-transmission environment; Figure 10 is a diagram showing a first method of digital address coding in a public address system according to the invention; and Figure 11 is a diagram showing a second method of digital address coding in a public address system according to the invention. Items common to the various diagrams are allotted the same reference numerals.

Referring now to the drawings, there is shown in Figure 1 a block diagram of a first embodiment of the public address system according to the invention, in which reference numeral 10 designates a central control and transmission unit designed to transmit a message to one, several or all of a plurality of remote units 20.

While the invention is not limited to use in environments demanding the selectable addressing of remote units, in practice in most situations in which it is envisaged that the invention will be used it will be necessary to be able to discriminate between various geographical areas in the total area covered, and therefore the embodiments described hereinafter refer exclusively to a P.A. system having centrally controllable addressing of remote units.

The central unit 10 includes means (not shown) for converting mainly verbal messages into electrical signals and means (14, 11, 12, 13) for selecting which of a number of separate receiving units 20 are to receive these electrical signals. Thus a unit selector 14 receives message signals at its input 15 and outputs these, under the control of a "select remote unit" signal at input 16, to one, some or all of a number of transmitters (11, 12, 13), of which only three are here shown.

The source of the message signals may be an announcer's voice, working ; in conjunction with a microphone, or it may be a source of recorded speech or even music, e.g. a tape player, compact disk player or the like.

The transmitted message signals are received by the corresponding receivers 21, 22, 23 in the remote units 20, and are subsequently demodulated in a demodulator 24 (one per remote unit) and sent as baseband message data to a memory 26. The message can then be read out of the memory 26 under the control of suitable readout signals (not shown) and then output - after conditioning in conditioning interface 28 - from one or more transducers 30, which would normally take the form of loudspeakers.

Transmission may be effected on any of a number of carriers, among them radio frequency, infra red or microwave, and it is envisaged that satellite transmission could also be employed. Alternatively, as is the case with the majority of known P.A. systems in use, transmission may be by simple electrical cabling.

Where electromagnetic transmission is considered, the selective addressing of the remote units by the central unit could be achieved by the use of transmitters and receivers tuned to different carrier frequencies: thus transmitter 11 and receiver 21 could be tuned to one carrier frequency, transmitter 12 and receiver 22 to another, and transmitter 13 and receiver 23 to yet another.

The memory 26 in remote units 20 can be realised by a number of known means, among them magnetic tape for either analogue or digital recording, digital semiconductor technology, and so on, all with their associated control logic for reading in and reading out the message to be broadcast. Clearly, where a digital medium is employed any analogue signal produced by, for example, a microphone must be converted into digital form, and this would require the inclusion of an analogue-to-digital converter between the demodulator 24 and the memory 26. Similarly, because the memory 26 must ultimately drive an analogue transducer, such as the loudspeaker(s) 30, a digital-to-analogue converter would also have to be interposed between these two components, in the case where the memory is an digital storage device.

Digitisation of an input analogue message waveform could conveniently be effected by the well known pulse code modulation technique, in which the input waveform is sampled and its instantaneous amplitude then converted into a digital word. An entire message would then consist of a string of such digital words, each word being of such a length (i.e. number of bits) as to ensure adequate fidelity of the broadcast message vis-a-vis the input message. This digital word string would lend itself to direct storage in a digital realisation of the memory 26.

Modern semiconductor chip technology is particularly appropriate in this context, allowing up to 20 or 30 seconds of speech to be accomodated on one chip, and making it therefore economically feasible to equip each remote unit with its own memory.

The invention provides for control of the memory 26 in two ways supplementary to the conventional reading in and reading out of a message. The first is the use of an "inhibit/hold" control, shown entering the memory 26 at its input 25 in Figure 1; the second is the presence of a "repeat last message" control, shown interfacing with the memory at the input 27.

These two aspects of the invention are illustrated in more detail in Figure 2. Figure 2 contains in addition to the basic items shown in the units 20 of Figure 1 a microphone 110, or a similar sound transducer, the output of which is amplified and detected in an amplifier and detector 104 and taken to one input 109 of a comparator 107. A second input, 139, of the comparator 107 is fed with a reference voltage, Vref. The output of comparator 107 is connected to one input 103 of an AND block 98 and to the "Inhibit/Hold" input 97 of a memory readout control block 92. Input 105 of AND block 98 is connected to a subsidiary output 93 of the memory 26. The other input, "Repeat" input 101, of the memory readout control block 92 is supplied by the output of AND block 98 via an OR block 96. The other input 100 of the OR block 96 is taken to a user switch 108, which may carry the legend: "Repeat Last Message", for example. Finally, the output of the memory readout control block 92 is used to govern the readout timing of the memory 26.

The arrangement shown in Figure 2 functions as follows: The P.A. announcer selects the remote units through which it is desired to broadcast a message, and inputs the message at the central unit 10 (see Figure 1). The selected remote units 20 (see Figure 2) receive the transmitted signal, demodulate it and store it in memory 26.

Background noise is picked up by the microphone (or similar transducer) 110 and its level is compared with a threshold level, as set by the reference voltage Vref at the input 139 of comparator 107. This threshold level represents the amount of noise that can be tolerated before a broadcast message becomes inaudible. If now a greater level of background noise than this is picked up, the output of comparator 107 will deliver a signal to the input 97 of memory readout control block 92, thereby inhibiting the readout of the memory 26. Then, when the ambient noise level has subsided to within acceptable limits, the comparator output signal at input 97 of the memory readout control block 92 will change level, instructing the memory readout control 92 to commence readout of the memory contents. This describes the INHIBIT function.

A second mode of operation in this noise-monitoring area is the "HOLD" or "INTERRUPT" function.

The sole difference between the "inhibit" function and the "hold" or "interrupt" function is that in the latter case the message has already started to be read out of the memory 26 when the microphone 110 picks up the start of excess noise in the listening environment. Now, since the contents of memory 26 are already in the process of being read out, the invention provides for a signal to appear as a result of this at the subsidiary output 93 of the memory 26. This signal enables the AND function 98 and allows the signal at the output of comparator 107, entering the AND block 98 at input 103, to produce a signal at the output of the AND block 98, this signal in turn producing, via OR function 96, a signal at the repeat input ("R") 101 of the memory readout control block 92. A signal at this input 101 then causes a repeat readout of the message from the memory 26.

Under certain circumstances it may be desirable, instead of causing a repeat of a message that has already been started to be read out of the memory 26, to allow the message to carry on from where it left off under the above "hold" process. This is most likely to be required where the excess background noise is comparatively short-lived, e.g. 2 to 3 seconds, and where it would be tiresome to the listener to have a message always repeated under these circumstances. To cater for this some form of timing of the excess noise signal (i.e. the length of time the output of comparator 107 provides a signal to the memory readout control block 92) may be carried out and used then to override the effect of the "message readout already started" signal at the output 93 of memory 26. (This feature is not illustrated in the drawings).

The above description of the first embodiment assumes that a message is only broadcast after it has been loaded into the memory 26. In practice this could well lead to unacceptable delays, especially if the message is a long one, and so the invention also allows for the direct broadcast of a message at the same time as the message is entering the memory. This process is represented by a dotted line 350 between the output of demodulator 24 and the conditioning block 28. Since such a direct broadcast would require to be inhibited or held in the event of excess noise, in the same way as the readout of the same message from memory 26 is inhibited and held as explained above, a connection must be made between the "inhibit/hold" input 97 of memory readout control 92 and the conditioning block 28. This is shown as dotted line 351.This facility is, for simplicity, omitted in the description and drawings of subsequent embodiments.

The invention also provides for triggering of the "repeat message" function by a user. This could be a passenger, in the case where the invention is employed in, for example, a railway station or even a train itself.

To this end a switch or button 108 is provided on or in association with each remote unit, and this may be marked, for example: "Repeat Last Message". Switch 108 when operated sets up a signal at input 100 of OR block 96, which acts in the same way as that at input 99 from the noise detector stage (104, 107, 110) and causes a repeat readout of the message stored in memory 26 by issuing a signal to the "R" input 101 of the memory readout control block 92.

This facility is useful where a passenger may have just missed an announcement, or may have arrived at the station and wished to know if any important information had been given out over the P.A. system.

Whatever the initiator of a message repeat, whether a user (passenger) or the lessening of background noise during a "hold" process, any message repeated will, like a message given first time, be subject to the "inhibit"/ "hold" process described above.

In many situations it is desirable not only to be able to broadcast live messages in a P.A. system, but also prerecorded messages. Typical prerecorded messages in a railway station environment may include details of the next train to arrive at a particular platform, services on board the train, and so on, as well as perhaps standard apologies for late arrival. A similar message content would apply to a system used on board a train; here, however, messages might also include details of the next station to be stopped at, platform to be used, and so on.

A further advantage of employing a bank of fixed, preset messages is that these might also include versions of standard messages in several languages. In this case there will normally be a trade-off between the number of different messages accomodated and the number of languages in which they are broadcast. Thus, in a bank having a storage capacity of, say, 36 messages, it could be arranged to have 12 different messages in 3 languages, or 9 different messages in 4 languages, or, in very cosmopolitan areas, 6 different messages in 6 languages.

Choice of languages installed would be dictated by the kind of area in which the P.A. system was to be used, and its geographical location.

This additional feature of a prerecorded message bank is part of a second embodiment of the invention and is illustrated in Figure 3.

The central control unit 10 in Figure 3 contains a "preset message bank" 46 as well as a "live message" input 49. Now the announcer can choose either to broadcast a live message via the input 49 or to send out a preset (prerecorded) message by selecting one of the messages present in the memory bank, via input 45.

The act of selecting a preset message has the effect of causing immediate readout of that message and its transmission to the selected remote units; this is shown by the presence of the control input 41 on the memory readout control block 42, the input 41 being fed directly from the "preset message select" line 55.

In normal operation it may be arranged for either message source, live or preset, to be allowed to run its full course before a message from the other source is transmitted. To achieve this, some form of interlocking (not shown in Figure 3) would have to be incorporated into the live message input line 49 and the memory readout control block 42.

In addition to this, however, either the preset message readout or the live message transmission may be interrupted by an "override" input on line 67. This is necessary when it is desired to override an existing message in the interest of a more important message. The operation of this is as follows: Assuming the announcer has just selected one of the preset messages from the message bank 46 and the message is in the process of being read out, if now she wishes to make a live announcement she may press a switch (not shown), putting a suitable signal onto an "override" line 67, and having thereby the effect of triggering an interrupt circuit 60 coupled to the memory readout control circuit 42. Interrupt circuit 60 in turn feeds an interrupt signal into the input 58 of memory readout control circuit 42 and causes readout of the message to cease.At the same time the signal on the "override" line 67 has the opposite effect on the transmission of the live message, allowing this message - via the presence of the interrupt signal on input 43 of the interrupt block 44 - to be passed through to one or more of the transmitters 61-63. In this way the live message has been given priority over the preset message.

The same applies mutatis mutandis to the opposite case, where it is desired to override a live message in favour of a preset message. This situation, however, is less likely to occur in practice than the situation described above.

A scheme capable of carrying out the interlocking and overriding functions described above, and involving actual logic components, is illustrated in Figure 4.

Figure 4 shows a message source interlock 200 in association with a message source override 300. Both sections centre around a JK flip-flop, 210 and 310 respectively.

Looking at the interlock section 200 first, two switches 240 and 250 are shown feeding the J-input and K-input respectively of flip-flop 210 and also inputs 221 and 222 of exclusive-OR (XOR) gate 220. Switch 240 is associated with the preset message source, while switch 250 is associated with the live message source. These switches when closed feed a voltage +V to the J and K-inputs of flip-flop 210, these inputs being grounded by resistors 252 when the switches are open. The output 223 of XOR gate 220 is taken via a delay generator 230 to the clock input 211 of flip-flop 210. The Q and not-Q outputs of flip-flop 210 are used to control the interrupt circuits 44 and 60 associated with the live and preset message sources respectively.

The Q and not-Q outputs of flip-flop 210 are also taken to the J and K-inputs respectively of the flip-flop 310 in the override circuit 300. The Q and not-Q outputs of flip-flop 310 are connected respectively to the reset (R) and set (S) inputs of flip-flop 210. Finally, the clock input 311 of flip-flop 310 is supplied by the "override " line 67.

The interlock and override circuit operates as follows (assuming all inputs to be high-active, i.e. the flip-flops 210, 310 are reset, for example, when their reset inputs (R) go high (logic "1"), and they are clocked when a low-to-high transition appears on their clock inputs 211, 311): If we assume a preset message is currently being output, the switch 240, which may be a switch which is always operated when a preset message is being transmitted, will be closed and the switch 250, which may be a microphone switch for live transmission, will be open.

Flip-flop 210 then has a "1" on its J-input and a "0" on its K-input, setting - assuming a clock signal to have been generated at the output of XOR gate 220 at the moment the switches 250 and 240 assumed the position shown - its Q output 213 to "1" and its not-Q output 214 to "o".

Assuming also that a "1" on either of the interrupt circuits 60 and 44 causes interruption of their associated message sources, this causes an interruption (or in this case a disabling, since no message is yet forthcoming) of the live message input on line 49 (see Figure 3), and an enabling of the preset message readout via the memory readout control circuit 42.

If now the announcer tries to make a live announcement - without triggering an "override" signal - by operating (microphone) switch 250, this will put a "1" on the K-input of flip-flop 210 and a "1" on input 222 of XOR 220. This latter, however, merely sends the clock input 211 low, since like inputs on an XOR gate always produce a low output; the result is that the outputs of flip-flop 210 remain the same.

As soon however as the preset message ceases, switch 240 opens, sending the J-input of flip-flop 210 low and the output of XOR 220 high, since the inputs of the XOR are unlike again. This produces a low-to-high transition on the clock input 211 of the flip-flop 210, causing the not-Q output 214 of this flip-flop to go high while the Q- output 213 goes low. This causes interruption of the preset message readout via interrupt circuit 60 and transmission of the live message which follows, via interrupt circuit 43.

Note that a delay stage 230 is included in the XOR output circuit, in order to ensure that the J and K inputs of flip-flop 210 are given time to settle before the flip-flop is clocked by the output of XOR 220.

The operation of the override circuit 300 is simple, in that whatever state the outputs of flip-flop 210 have, a positive-going pulse on the "override" line 67 reverses it.

Thus, assuming in the above case the announcer wished to override the preset message being transmitted, she could press the override switch, which would put a high on the "override" line 67 and cause clocking of the flip-flop 310 via its clock input 311 Since Q-output 213 was high before this point and not-Q output 214 low, clocking of flip-flop 310 has the effect of sending the Q-output 309 of flip-flop 310 high and its associated not-Q output (312) low, thereby resetting flip-flop 210 and sending therefore the Q-output 213 and not-Q output 214 low and high, respectively. This allows the live message through via interrupt circuit 43 and disables the preset message via interrupt circuit 60.

Interrupt circuit 60 acts not only to interrupt the readout of a message from the message bank 46, it is also arranged to initiate a repeat of the message once the overriding live message has finished. This is represented by the connection between the interrupt circuit 60 and the "R" input 59 of memory readout control circuit 42 in Figure 3.

Both preset message and live message are interfaced to the remote unit selector and to the succeeding transmitters (where a non-conductive transmission system is used) by conditioning stages 48 and 50, respectively.

Where the live message is in analogue form and the preset message digital, one message must be converted into the other form before transmission can be effected. Thus, where transmitters 61-63 accept analogue modulating signals (the carrier inputs are not shown) and where the preset message bank 46 is realised as digital storage, conditioning stage 48 must include a D/A converter in order for the digital preset message signal read out from the message bank 46 to be able to modulate these transmitters.

The OR block 52 in Figure 3 simply indicates that either a preset message or a live message is broadcast at any one time.

A feature characterising this embodiment as opposed to the first embodiment shown in Figure 1 is the transmission of a "memory clear" signal as well as of the message itself. The purpose of transmitting a "memory clear" signal is to ensure that, when a message already being broadcast is interrupted or overridden by the announcer, the memory 26 in the remote units 20 is erased or cleared in preparation for the receipt of the new message. This is achieved by the inclusion of a second set of transmitters 81, 82, 83, i.e. one per remote unit, in the central control unit 10, and by the inclusion of a corresponding receiver 84 (see Figure 3) in the remote units.Thus, when an interrupt signal is initiated by the operator this signal is transmitted to the same remote unit(s) as the message itself, is picked up by the receiver 84 in each of these units and, after demodulation in demodulator 88, is used as an erasing signal at input 91 of memory 26 to clear the memory.

A demodulation process also occurs after receipt of the message in receiver 21, and depending upon whether analogue modulation or a form of pulse code modulation, for example, is being used, either an analogue baseband signal or a series of digital words is input into the memory 26.

The remaining features of the remote unit 20 shown in Figure 3 have already been described in connection with Figure 2, their function being exactly the same in both cases.

A third embodiment of the invention is shown in Figure 5, where the bank of preset messages is accomodated in each of the remote units instead of centrally in the central control unit 10. This in turn means that instead of transmitting the actual preset message from the central unit to the remote units, the "preset message select" signal 55, shown earlier in Figure 3, must be transmitted.

This would normally take the form of a digital address for the message bank and so the most obvious form of transmission in this case is digital rather than analogue.

A disadvantage of this embodiment is the fact that three sets of transmitters 77 to 79, 71 to 73 and 81 to 83 respectively are needed in order to transmit all the signals now required, namely the "message select" signal, the "live" message and the "clear" signal for the memories located in the remote units.

The "message select" signals are received by a receiver 64 in each remote unit 20 and are demodulated and used as an address word for the preset message bank 46.

Similarly, any live message transmitted is received by the receiver 74 and demodulated in the demodulator 124. By incorporating in the central control unit 10 the same sort of interlocking and override arrangement that has already been described in connection with Figures 3 and 4, it can be ensured that the remote units 20 need only deal with one incoming message signal at a time. Hence the outputs of the message bank 46 and the demodulator 124 are shown entering an OR block 118, after first being interfaced/conditioned in a manner already described in connection with Figure 3, and the output of this OR block 118 is then taken to the data input 89 of the memory 26.

An advantage of this third embodiment over the first two already described is that, since the preset message bank is now situated in the remote unit rather than the central unit, it is possible to arrange for a preset message to be selected locally by the user (e.g.

passenger) as well as centrally by the announcer. To illustrate this a further connection 121 is shown in Figure 5 between the user interface 120 and the message bank 46.

Now a passenger may approach the interface 120 and press a button, say, marked with a particular message, in order to have announced purely locally in his particular area information in which he is particularly interested.

The realisation of this passenger-initiated "select message" feature can take one of several forms: for example, there may be a button supplied on the interface 120 for each of the messages available in the memory bank 46, plus a similar button for each of the language options, if installed; or, there may be some form of processor-driven menu made visible on a display in the interface 120, which would enable passenger selection of messages to be achieved in a way involving less hardware at the remote unit.

Whichever method is chosen, the ability of the message bank to store information in more than one language is most useful in this context, since this feature is readily available to those members of the public who may need it. Instead of the announcer determining which language a message is to be broadcast in, something which can only be helpful to a very limited number of passengers, a foreign passenger himself can determine the language to be used.

Selection of a message by the user (passenger) may involve parallel addressing of the message bank 46, or it may involve serial addressing. In the first case the "select message" line 121 from the user interface 120 may be a multiway cable; in the second case it may be a simple cable pair.

In a fourth embodiment of the invention (not illustrated), it may be arranged to situate the memory 26 and the preset message bank 46 in the central control unit 10, rather than in the individual remote units 20. While this would constitute a saving in memory and memory bank outlay, it would make realisation of the previously described functions of local noise control and local user message repeat and message select initiation considerably more difficult, since then each remote unit would have to transmit back to the central unit signals corresponding to "noise inhibit/hold", "message repeat" and 1,message select".In addition of course, receivers to receive these signals would have to be incorporated into the central unit, although since all the memory functions would be confined to that unit, only one transmitter set (with a total number of transmitters equalling the number of remote units installed) transmitting to the remote units a signal corresponding to either a preset message or a live message, as selected by the announcer or - in the former case alternatively by a passenger, would be needed out of the three sets illustrated as part of the central control unit 10 of Figure 5.

While the preset message bank 46 may be a non-alterable device storing purely fixed information which cannot be erased or modified in any way, it may also be an erasable device allowing other messages to be written into it when desired. This is a better option, since many standard messages may contain details which need to be updated from time to time.

Where an erasable and rewritable memory unit is used for the preset message bank 46, since this would have the same characteristics as the memory 26, it would be possible to combine the functions of the two memories. This is shown in Figure 6, which is a diagram of a fifth embodiment of a remote unit according to the invention.

In this embodiment the two memories 26 and 46 of the previous embodiments are combined into one larger memory 90, which is correspondingly split into two parts: a live memory area 112, into which a demodulated live message signal is fed via data input 89, and a preset message area 114, which is addressed as in the previous embodiment by a demodulated and decoded "message select" signal at an input 45. An input 111 to the live memory area 112 may also be provided, to enable a previously announced live message stored now in the live memory portion of memory unit 90 to be reaccessed and rebroadcast, if desired, by the announcer.

Corresponding preset and live message selection inputs 116 and 115 respectively enable selection of either a preset message from the preset message area 114 or the last-announced live message from the live memory area 112 to be made by a passenger from the user interface 120.

All the embodiments so far considered have featured one transmitter per remote unit in the system per transmitted signal type. Thus, in Figure 5, for example, the central control unit 10 is seen to comprise, for a system containing three remote units, nine transmitters altogether: three (77 to 79) to transmit the "preset message select" signal (MS1 to MS3); three (71 to 73) to transmit the "live message" signal (L1 to L3); and three (81 to 83) to transmit the "memory clear" signal (C1 to C3). This represents a considerable amount of hardware, especially in a system employing a large number of remote units.

To reduce this transmitter /receiver overhead it might be possible to modulate one transmitter with two of the above signals, e.g. the "message" (M) and "clear" (C) signals shown in Figure 3. Two of the receivers so far needed in the remote units of the embodiment of Figure 3 i.e. receivers 21 and 84, could then be replaced by one receiver only, as in Figure 7. In this case one receiver 130 is shown as feeding a demodulator 132 to produce two output signals: "message" (M) and "clear" (C), which serve to input data into the memory 90 and clear the contents of memory 90, respectively. Thus the two transmitted signals have been separated out in the demodulator of the remote unit.

Another technique for reducing the number of transmitters required, this time in the central control unit 10, is to use one transmitter per signal and rely on coding to effect differential addressing of the remote units.

Such a system is shown in Figure 8 and involves the transmission of a digital identifying code from an address code generator 140 via a transmitter 146, followed by transmission, via the same transmitter, of a digital "preset message select" code originating from a message code generator 142 or, alternatively, the transmission of a live message via a transmitter 148. A switch means 144 indicates that only one of the two signals from the address code generator 140 and the message code generator 142 can be output at any one time.

Interlocking and overriding of the two message sources (live and preset) can be as in the previous embodiments, and will not be further discussed here.

The corresponding circuitry in the remote units could take the form shown in the lower half of Figure 8. Thus a remote unit 20 has three receivers 147, 149 and 151 receiving the address/message select codes (Add/MS), the live message signal (L) and the clear memory signal (C) respectively. The digital codes received by receiver 147 are decoded by a decoder 86 and the decoded output is then taken to a correlator 160, where the address code, which is the first one to be transmitted, is compared with a unique code for that remote unit.If the transmitted code and the unit code are the same, the correlator 160 sends a signal to a switching means 164 in the power supply of the remote unit, thereby supplying power to components in the remote unit which until then had been deprived of power. (The direct output of the power supply unit 162 goes only to those components which must be powered in order to be able to receive the incoming address code (i.e. the receiver 147 and the decoder 86)). In this way a remote unit in its normal quiescent state is, when addressed by its identifying code, made receptive to any subsequent message transmission.

The basic scheme embodied in the arrangement of Figure 8 can also be used where transmission between the central control unit 10 and the remote units 20 is effected not by electromagnetic radiation but by conduction in cables. Figure 9 shows such a scheme, in which a bus 195, fed by the three signals: address/message select (Add/MS), live message (L) and clear memory (C) via drivers 170, 172, 174, supplies all the remote units in the system. Coding may be exactly as in the previous embodiment, i.e. a serial digital address code from the address code generator 140, followed if required by a similar code for selecting one of the messages stored in the preset message area of the memory 90, or it may take the form of a parallel digital code in both cases.In the former case the Add/MS bus may be a simple two-conductor cable; in the latter it may be a multi-core conductor, as illustrated in Figure 9.

Each of the three signals: Add/MS, L and C entering the remote units 20 are first interfaced (conditioned) in conditioning stages 180, 182 and 184 before being taken to the correlator 160, memory unit 90 and memory unit 90/memory readout control circuit 92, respectively.

The construction of the control unit and remote unit shown in Figure 9 corresponds in almost all other respects to that of the corresponding units shown in Figure 8, the only other difference being the omission in the hardwired embodiment of Figure 9 of the decoders 86, 124, 88 illustrated in Figure 8.

Since wiring from each of the remote units 20 does not have to run all the way back to the central unit 10, communication between the two items being effected purely by means of the bus 195, there can be a considerable saving in cable over a conventional P.A. system not using bus-based address coding in this way.

Whether coding is done in parallel or in serial form, it is possible to have the remote units arranged in various groups or subgroups. Thus, for example, a railway station layout could be divided into geographical areas, which in turn could be split into platforms. On the other hand, if the invention were used on board a train, the larger grouping might be constituted by groups of carriages, while the smaller grouping might be the carriages themselves.

Thus each platform (carriage) would be individually addressable within its particular area (group of carriages).

Figure 10 illustrates one way of organising the code bits in the address digital word. Assuming a code word of 8 bits, the first four bits, 0-3, could be used to address a maximum of 8 platforms (to choose the first of the above real-life situations) with the fourth bit, no.3, being reserved for a universal address - that is, an active level on this bit indicates that the announcer in the central unit is addressing all the platforms in a particular area.

Exactly the same situation could obtain in connection with the second four bits, the first three being used to address a maximum of 8 areas, the fourth (no.7) being reserved to address all the areas.

Clearly in this system an active level on both bits 3 and 7 would indicate that all the platforms in the system were being addressed.

If it were desired to reduce the number of bits in the address code word without reducing too much the addressing capacity of the system, the scheme of Figure 11 could be employed. In Figure 11 only six bits are used (two groups of three), but none of them is reserved for universal addressing. Instead, the number of addressable items is limited to 7 in each case, not 8, and a condition of either all "l's" or all "0's" indicates that all the members of a particular group are being addressed.

This arrangement is particularly useful where parallel addressing and message selection is employed, since then bit density is at a premium.

Other bit groupings are equally possible: it may, for example, be expedient to subdivide the lower grouping referred to above (e.g. platforms) into a number of different subgroups (i.e. platform sections), in which case the division and subdivision of the address code bits would have to be differently organised.

One disadvantage of structuring the address code word as shown in Figures 10 & 11 is that at any one time only one or all of each grouping of remote units can be addressed at any one time. To address any other number between these limits, it would be necessary to perform a number of consecutive addressing actions from the central unit until all the remote units from which it was desired to broadcast a message had been addressed and switched from standby to message-receive mode. If this is not felt to be desirable, an alternative and probably more complicated way of addressing the units would have to be employed.

While the public address system according to the invention has been assumed to possess means for selecting one or more of a number of remote units to broadcast a message, in practice this selecting means may be dispensed with in cases where blanket coverage of a total area is required. In such a situation, however, the invention still provides for local control of any message broadcast, either by allowing the last message sent centrally to be repeated at the behest of the user, or by allowing the user to initiate the broadcast of a message himself from a remote unit. In both cases inhibit or hold of a message is provided for by local monitoring of ambient noise conditions.

Also, although the public address system according to the invention has been described solely within the context of a P.A. system per se, it is also envisaged that in practice it could form part of a larger information system involving not simply audible information broadcasting but also visual information display. In such a system it would be possible and desirable to co-ordinate the functions of the two subsystems, such that the same information was both visually displayed and audibly broadcast at the same time on a central or local basis.

Claims (29)

1. A public address system, comprising a central unit and a plurality of remote units for broadcasting a message, each remote unit being associated with a memory means for storing a message to be broadcast and including a readout influencing means for influencing the readout of the memory means in response to local commands.

2. A public address system, according to claim 1, in which the memory means is located in the remote unit.

3. A public address system, according to claim 1, in which the memory means is located in the central unit.

4. A public address system, according to any of the preceding claims, in which the readout influencing means includes a monitoring means for monitoring local noise conditions, said monitoring means serving to control readout of the message to be broadcast in response to ambient noise.

5. A public address system, according to claim 4, in which the monitoring means either inhibits the readout of the message to be broadcast, if the message has not started to be read out, or interrupts readout of the message, if the message has started to be read out.

6. A public address system, according to claim 5, in which the readout influencing means includes a message repeat means for enabling the message whose readout has been interrupted due to ambient noise conditions to be repeated.

7. A public address system, according to claim 5 or 6, in which the readout influencing means includes a message continuation means for causing a message whose readout has been interrupted for only a short time due to ambient noise conditions to be continued from the point at which it was interrupted.

8. A public address system, according to any of the preceding claims, in which the readout influencing means includes an external command means for generating, under the control of a listener to the public address system, a command signal initiating the repeat of a message held in the memory means.

9. A public address system, according to any of the preceding claims, in which a live message transmitting means is provided in the central unit, for transmitting to the remote units a signal representing a live message to be broadcast.

10. A public address system, according to claim 9, in which the live message transmitted to the remote units is simultaneously read into the memory means and broadcast from the remote units, the broadcasting of the message from the units being influenceable by the monitoring means in the event of excess background noise being detected.

11. A public address system, according to claims 9 or 10, in which the central unit includes a preset message storing means, a preset message selecting means and a message type selecting means, said preset message storing means consisting of a bank of preset messages and said preset message selecting means serving to select, under the control of the user of the central unit, one of the preset messages stored in the preset message storing means, the message type selecting means being employed by the user to select which of a live message and a preset message is to be transmitted to and broadcast by the remote units.

12. A public address system, according to claims 9 or 10, in which each remote unit includes a preset message storing means and the central unit includes a preset message selecting means and a message type selecting means, said preset message storing means consisting of a bank of preset messages and said preset message selecting means serving to select, under the control of the user of the central unit, one of the preset messages stored in the preset message storing means, the message type selecting means being employed by the user of the central unit to select which of a live message and a preset message is to be transmitted to and broadcast by the remote units.

13. A public address system, according to claim 12, in which the memory means and the preset message storing means are realised as different storage areas within the same storage device, the memory means being employed as a live memory for storing a live message transmitted by the central unit, and the contents of either the live memory or the preset memory bank being selectable from the central unit for broadcast by the remote units.

14. A public address system, according to claim 13, in which the contents of either the live memory or the preset memory bank are also selectable from the remote unit by a listener to the public address system.

15. A public address system, according to any of the preceding claims, in which the specific remote units from which a message is to be broadcast are selectable out of the total number of remote units in the system.

16. A public address system, according to claim 15, in which the remote units are addressable either singly or in groups.

17. A public address system, according to claim 15 or 16, in which the central unit includes an address code generating means for generating and transmitting to all the remote units in the system a digital address code word, said code word being received in each of the remote units and correlated in a correlating means with an in-unit code word specific to that remote unit, a match between the received and in-unit code words resulting in the switching of that remote unit from a standby to a message-receive condition.

18. A public address system, according to claim 17, in which the preset message select signal is in the form of a digital code word, transmission of the address code word from the address code generating means and of the preset message select code word from the preset message selecting means being effected in that order and by means of the same transmitting means.

19. A public address system, according to any of the preceding claims, in which the transmission from the central unit to the remote units of any or all of the live message, the address code word and the preset message or preset message select signal is effected by electromagnetic radiation.

20. A public address system, according to claim 19, in which transmission is effected by radio frequency, infra red or microwave radiation.

21. A public address system, according to claim 19, in which transmission is effected by microwave radiation via a satellite link.

22. A public address system, according to any of claims 1 to 18, in which the transmission from the central unit to the remote units of any or all of the live message, the address code word and the preset message or preset message select signal is effected by conduction in wiring.

23. A public address system, according to claim 22, in which transmission from the central unit to the remote units is effected by means of a bus.

24. A public address system substantially as shown in or as hereinbefore described with reference to Figures 1 and 2 of the drawings.

25. A public address system substantially as shown in or as hereinbefore described with reference to Figure 3 or 4 of the drawings.

26. A public address system substantially as shown in or as hereinbefore described with reference to Figure 4 or 5 of the drawings.

27 A public address system substantially as shown in or as hereinbefore described with reference to Figure 4 or 6 of the drawings.

28. A public address system substantially as shown in or as hereinbefore described with reference to Figure 4 or 8 or 10 or 11 of the drawings.

29. A public address system substantially as shown in or as hereinbefore described with reference to Figure 4 or 9 or 10 or 11 of the drawings.