GB2164523A - Electronic alarm sounder - Google Patents

Abstract

A system particularly for use with a rocking armature type of alarm sounder (1), enables remotely generated speech or other sound signals to be emitted in addition to normal alarm sounds. A remote transmitter (9) superimposes amplified speech or other sound signals onto a D.C. carrier and transmits them over unscreened wires (7,11) to a receiving circuit (10), the signals being maintained within a frequency bandwidth centred around a centre frequency of natural resonance of the sounder (1). A.C. coupling and attenuation of the received signals is effected in the receiver (10) which also includes a differential amplifier (17). The speech signals after amplification are emitted by the sounder (1). The wire (7) is common to the power supply and the speech or other sound signal transmission. The arrangement minimises problems due to electrical noise and radio-frequency pick-up on the unscreened wires (7,11). The receiving circuit (10) and a generator (2) for the sounder (1) are arranged to be activated by voltages of opposite polarity and the required operating mode is selected by reversing the polarity of the supply voltage. <IMAGE>

Description

SPECIFICATION Electronic alarm sounder This invention relates to electronic alarm sounders particularly, but not exclusively, of rocking armature type and more particularlyto a system for use with alarm sounders whereby remotely generated speech or other sound signals may be emitted in addition to normal alarm sounds generated in the sounders.

Electronic alarm sounders ofthe rocking armature receiver type are well-known and have been used worldwide in fire and intrusion alarm applications for a number ofyears. Rocking armature transducers such as are used in these sounders are described for example on pages 116 to 123 of S.T.C. Monograph entitled "ELECTROACOUSTICS Microphones, Earphones and Loudspeakers", by M. L. Gayford, and were developed originallyfor use as telephone receivers. They have the advantage of being reliable, efficient and of low power consumption.The sound pressure level produced by a basic rocking armature telephone receivertransducer in free field is not sufficient to make an effective alarm sounder and the sound level is increased bythe addition of a suitable horn for matching the output to the acoustic impedance of the air and by eliminating the internal acoustic resistance so so as to allow the transducerto resonate ata frequency to which the human ear is particularly sensitive.

Requirements existfora system which is suitable for issuing verbal commands throughout a building, in additiontothenormal alarm sounds, when an emergency arises. In particular, such an emergency may resultfrom a break-out offire in the building and it would be advantageous to be able to issue verbal instructionsfortheevacuation ofthe building and also to issue any other information relevant to the situation. Hitherto, the only method availablefor issuing verbal commandsthroughouta building has been by way of a conventional public address system, usually involving remotely positioned loudspeaker units of the paper cone or other moving coil type, non fireproofconnecting wiring and relatively high a.c.

operating voltages. A system of this type cannot be relied upon for issuing evacuation instructions because of its extreme vulnerability to damage byfire.

By contrast, alarm sounding systems are generally provided with fire-proof wiring, e.g. involving metalclad mineral insulated cable, and alarm sounders, e.g.

ofthe rocking armature type, are generally con structed to high standards offire-resistance and operate at relatively low voltages (e.g. 24 volts).

We have discoveredthatan alarm sounder, particu larly of the type involving a rockingarmaturetrans- ducer can be employed to emit remotely-generated speech orothersound signals in addition tothe normal alarm sounds which it is designedto generate.

Several problems arise, however, in the practical implementation ofthis idea. Firstly, a rocking arma ture type of sounder is not suitable for transmitting wide bandwidth speech at high power levels. When using low frequencies at high power levels the rocking armature overshoots and strikes pole facets incorporated therein,thus causing severe distortion and, in some cases, mechanical damage. Secondly, fire-proof cable used in alarm systems incorporates unscreened conductor wires and a pair of such wires is required to transmit an audio signal to the appropriate alarm sounderunit(ortoa numberofsuch units, e.g. upto twenty, connected in parallel and located throughout a building). Large cable lengths may be required.A problem occurs as a result of noise and radiofrequency pick-up on an unscreened pairof connecting wires and this can be severe when considering a receiving circuit, forthe speech or other sound signals, located atthe alarm sounder and exhibiting necessarily high input impedance and driven from a lowamplitudealternating current source.

Thirdly, since fire-proof cable is expensive, the number ofconductorwires required to implement the idea in addition to those employed fora normal alarm sounder system must be kept to a minimum.

The above problems have been minimised or overcome in the system ofthe present invention.

The present invention provides a system for use with an electronic alarm sounder whereby remotely generated speech or other sound signals may be emitted by said sounder in addition to normal alarm sounds generated in said sounder, said system comprising: a transmitter, remote from said alarm sounder, adapted to transmit said speech or other sound signals within a frequency bandwidth compatible with frequency characteristics of said sounder; a receiving circuit for location at, and electrical connection to, said sounder and which is adapted to receive and condition said signals from saidtransmitterfor emission by said sounder; and means to select operation of said sounder between a mode for emitting said normal alarm sounds and a mode for emitting said speech or other sound signals.

Preferablythesaid alarm sounderincorporatesa rocking armature transducer.

The said transmitter is suitably adapted to transmit said signals within a frequency bandwidth compatible with a natural resonance of the said alarm sounder, said bandwidth being preferably centred around a centre frequency of said resonance. By restricting the bandwidth in this way, sound distortion and mechanical damage to the sounder is thereby avoided, particularly when a rocking armatu retransducer is employed, overshoot ofthe rocking armature and striking of pole pieces therein being prevented.

A pair ofconductorwirns, which may be unscreened, is suitably used to transmit said signals between the transmitter and the receiving circuit the receiving circuit being arranged to exhibit a high input impedance. In orderto minimise or reduce problems of noise and/or radio-frequency pick-up, means are provided associated with said transmitterto amplify speech or other sound signals applied thereto and to superimpose the amplified signals onto a direct current carrier. The amplified signals superimposed on the carrier are arranged to be transmitted overthe said conductorwiresto the receiving circuit. Means are provided in the receiving circuitto remove the directcurrentcarrierfrom the signls, said means suitably providing alternating current coupling ofthe signals.Further means in the receiving circuit attenu atesthesignalstoa required level.

To furtherdeal with the problem of noise,the receiving circuit preferably incorporates a differential amplifier which operates to amplifythespeech or othersoundsignals received from the transmitter and which are actually transmitted in only one of the wires inthe pair, while atthe sametime substantially cancelling noise signals which occur in both wires in the pair. This arrangement is effective since the pair of conductor wires will generally be provided running alongside each other and hence any induced noise or other induced signals will be ofsubstantiallythe same amplitude and phase in each ofthewires.

One ofthe conductorwires in thesaid pairand one oftwo wires essentially required to provide a supply voltageto an alarm sound generator incorporated in the alarm sounderfor operating the sounder in its normal alarm emitting mode may be provided as a single common wire. By this means, only one additional conductor wire may be required in implementing the system ofthe invention.

Preferably the said means to select operation of said sounder between a normal alarm emitting mode and a speech or other sound emitting mode comprises an arrangementforreversing polarity of a voltage provided for driving the said receiving circuitand an alarm sound generatorin the alarm sounder,the said alarm sound generator being arranged to be activated by said voltagewhen of a first polarityandthe said receiving circuit being arranged to be activated by said voltagewhen of a second polarity, said first polarity being the reverse of said second polarity.

Preferably the said conductor wires are provided in the form of one or more fire-proof cables.

The system ofthe invention may be used to transmit speech commands by way of a microphone; synthesized speech commands,tape-recorded messages, or additional remotely generated alarm sounds, may also be transmitted.

The invention also provides an electronic alarm sounder provided withthe above system.

The invention is now described by way of example with reference to the accompanying drawings in which: Figure lisa schematic representation of a rocking armature alarm sounder provided with a system according to the invention whereby remotely generated speech or other sound signals may be emitted in addition to normal alarm sounds.

Figure 2 is a schematic representation of a network of rocking armature alarm sounders provided with the system of Figure 1.

A known rocking armature alarm sounder is provided in a building and operating to providewarning of a fire or other emergency situation. The sounder comprises a rocking armature transducer 1 incorporating a matchir.g horn and designed to resonate ata setfrequencytoemitalarm sounds. The sounder incorporates an alarm sound generator2 which activatesthe rocking armature transducer 1 through a power amplifier 3. Such an alarm sounder is commercially available, an example being the Banshee (Registered Trade Mark) manufactured by A. P.

Besson. The alarm sounder is operated from a power supply4 remotely located in a control centre 5, in the building, the alarm sounder and power supply being linked by a fire-proof cable incorporating a pair of unscreened conductor wires 6 and 7.

In orderto use the rocking armature transducer 1 to emit speech orothersound signals, e.g. building evacuation instructions, generated remotely in the control centre 5 using an input device 8 such as a microphone, tape recorder or speech synthesizer, the following system is provided consisting basically of a transmitter, represented generally by reference numeral 9, located in the control centre 5 and a receiving circuit, represented generally by reference numeral 10, located at the alarm sounder. A pair of conductorwires is required to be provided between thetransmitter9andthe receiving circuit 10.However, in orderto minimise the number of expensive fire-proof cables provided between the control centre 5 and the alarm sounder it is arranged for one of the conductorwires 7, in the pair of power supplying wires 6,7 to serve as a common wire for both the power supply and the speech or other sound signal transmission. This means that only one additional conductor wire 11 its required to be provided between the control centre 5 and the alarm sounder compared with the arrangement where the sounder is installed for operation in normal alarm sounding mode only.

The transmitter 9 operates as follows. Speech or other sound signals from the input device 8 (i.e.

microphone, tape recorder, speech synthesizer etc) are amplified in a pre-amplifier 12 and the signal level is then adjusted in an automatic gain control 13. As explained previously, the rocking armature transduc- er 1 is not suitable fortransmitting wide bandwidth speech at high power levels. The signalsatthe output ofthe automatic gain control 13 are therefore passed to an audio filter 14 which limits them to a desired bandwidth. This bandwidth is centred around a centre frequency ofthe resonant system incorporated in the rocking armaturetransducer 1. By so restricting the bandwidth, overshoot ofthe rocking armature in the transducer and striking of pole pieces therein when handling the speech or other sound signals, particularlythose of low frequency, is prevented.

The conductorwires 11 and 7 involved in the transmission ofthe speech or othersound signals from the transm itter 9 to the receiving circuit 10 are unscreened and will usually be of considerable length.

The receiving circuit 10 is necessarily arranged to exhibit a high input impedance and it is well known that a circuit of this nature can sufferfrom problems with noise and radiofrequency pick-up,when driven from a low amplitude alternating current source. In orderto overcome this problem, which could be serious as a result of the use ofthe unscreened conductor wires 7,11, the signals atthe output of the audio filter 14 are passed to a voltage amplifier and output driver unit 15 where they are amplified and superimposed onto a direct current carrier. The quiescent voltage of the carrier is suitably setto about 4.5 volts. The resulting signals are then transmitted by way of a low impedance driver to the receiving circuit 10, using the unscreened conductor wires.At the receiving circuit 1 0,the signals are alternatingcurrent-coupled by well known means, in section 16, to remove the direct current carrier and then attenu atedtotherequired level. The signalstransmitted from the transmitter 9 actually pass to the receiving circuit 10 only along the conductor wire 11, but any undesirable induced signals or noise will occur in both ofthe unscreened conductor wires 7 and 11 associated with the speech or other sound signal transmission circuit The conductorwires 11,6 and 7 will generally be arranged to run alongside one another and hence the induced signals or noise will be of substantially the same amplitude and phase in each of the wires 7 and 11.This enables a further method to be employed to ensurethe minimisation of the problem of such noise or induced signals.Adifferential amplifier 17 is provided which amplifies the signals passing along conductor wire 11 differentially with respectto signals on the common conductor wire 7.

By this method noise or other induced signals are substantially cancelled whereas the speech or other sound signals passing along conductorwire 11 are amplified, the differential amplifier 17 also serving as a poweramplifier in this respect. The amplified speech or other sound signals arethen applied to the rocking armature transducer 1 for emission thereby.

Means are provided for selecting operation ofthe alarm sounder between a mode for emitting normal alarm sounds generated by the sound generator2 and amplified by amplifier3, and a mode for emitting the remotely generated speech orothersoundsignals after receipt and conditioning by the receiving circuit 10. The power supply4 located in the control centre 5 is arranged to be of reversible polarity. The alarm sound generator 2 is arranged to be activated by a voltage from the supply 4 of a first polarity and the receiving circuit 10 is arranged to be activated by a voltage from the supply4 of a second polarity, the first polarity being the reverse of the second polarity.A polarity sensing circuit 18 controls the operation of the alarm sound generator2 andthe receiving circuit 10 by means of control lines 19 and 20. Thus by simply reversing the polarityof the power supply4, the alarm soundercan be changed from its normal alarm sound emitting mode to the speech orothersound signal emitting mode. This arrangement is advantageous in that it obviates the need to provide additional conduc torwiresfor use in the operating mode selection.

As illustrated in Figure 2, a network of alarm sounders according totheinvention can be provided throughout a building and it can be arranged forthe speech orothersound signals to be transmitted selectivelytoalarm sounders arranged in parallel in particular zones ofthe building. Three such zones are illustrated in Figure 2. Zone 1 contains three alarm sounders Al ,A2 and A3 each of which comprisesthe components represented generally by reference numeral 21 in Figure 1. Zone 2 likewise com prises three such alarm sounders B1, B2 and B3 and Zone 4 comprises three such alarm sounders C1, C2 and C3.

Reversible polarity powersupply4in the control centre 5 serves to supply all of these alarm sounders.

Thespeech or other sound signals initiated at the input 8 (microphone, tape recorder etc) in the control centre and conditioned and transmitted by the transmitter 9, as previously described with reference to Figure 1, are relayed to a selected zone byway of a selector switch 22 located in the control centreS.

Claims (15)

1. A system for use with an electronic alarm sounderwherebyremotelygeneratedspeech orother sound signals may be emitted by said sounder in addition to normal alarm sounds generated in said sounder, said system comprising: a transmitter, remotefrom said alarm sounder, adapted to transmit said speech or other sound signals within a frequency bandwidth compatible with frequency characteristics of said sounder; a receiving circuitforlocation at, and electrical connection to, said sounder and which is adapted to receive and condition said signals from saidtransmitterforemission by said sounder; and means to select operation of said sounder between a modeforemitting said normal alarm sounds and a modeforemitting said speech or other sound signals.

2. A system according to claim 1 inwhichthesaid alarm sounder incorporates a rocking armature transducer.

3. A system according to claim 1 or 2 in which the said transmitter is adapted to transmit said signals within a frequency bandwidth compatible with a natural resonance ofthe said alarm sounder.

4. A system according to claim 3 in which said frequency bandwidth is centred around a centre frequency of said resonance.

5. A system according to any preceding claim in which a pair of conductorwires is used to transmit said signals between the transmitter and the receiving circuitthe receiving circuit being arranged to exhibit a high input impedance.

6. A system according to claim 5 in which said pair ofconductorwires is unscreened.

7. A system according to claim 5 or 6 in which in orderto minimise or reduce problems of noise and/or radio-frequency pick-up, means are provided associated with said transmitter to amplify speech or other sound signals applied thereto and to superimpose the amplified signals onto a direct current carrier, the said amplified signals superimposed onto the direct current carrier being arranged to be transmitted over the said conductor wiresto the receiving circuit.

8. A systen according to claim 7 in which means are provided in the receiving circuit to remove the direct current carrierfrom the signals.

9. A system according to claim 8 in which said means to remove the directcurrentcarrierfromthe signals provides alternating current coupling ofthe signals.

10. A system according to any preceding claim in which means are provided in said receiving circuit to attenuate said signalsto a required level.

11. A system according to any of claims 5to lOin which the receiving circuit incorporates a differential amplifier which operates to amplify the speech or other sound signals received from the transmitter and which are actuallytransmitted inonlyoneofthewires in the pair, while atthe sametime substantially cancelling noise signals which occur in both wires in the pair.

12. Asystem according to any of claims Sto 11 in which one ofthe conductorwires in the said pair and one oftwo wires essentially required to provide a supplyvoltageto an alarm sound generator incorporated in the alarm sounderforoperating the sounder in its normal alarm emitting mode are provided as a single common wire.

13. Asystem according to any preceding claim in which the said means to select operation of said sounder between a normal alarm emitting mode and a speechorothersound emitting mode comprises an arrangementforreversing polarity of a voltage provided for driving the said receiving circuit and an alarm sound generator in the alarm sounder, the said alarm sound generator being arranged to be activated by said voltage when of a first polarity and the said receiving circuit being arranged to be activated by said voltagewhenofasecond polarity, saidfirst polarity being the reverse of said second polarity.

14. A system for use with an electronic alarm sounderconstructed and arranged substantially as herein described with reference to the accompanying drawings.

15. An electronic alarm sounder provided with a system according to any ofthe preceding claims.