Abstract:
An alarm system comprises: an audio interface comprising: a microphone; a state sensor for sensing a state at the audio interface; a signal generator for generating an inaudible state signal indicative of the state; a multiplexer for combining an audio signal from the microphone and the state signal; a control panel for monitoring sensed conditions at the audio interface; a conductor interconnecting the audio interface to the control panel carrying audio signal and the state signal as combined from the audio interface to the control panel.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to alarm systems, and more particularly to an alarm including an audio interface, and the detection of a state or tampering at the audio interface. 
     BACKGROUND OF THE INVENTION 
     Home and business alarms have become commonplace. Such alarms (often referred to as “security systems” or “alarm systems”) typically include several sensors used to monitor unauthorized entry and other conditions at monitored premises, such as fire, smoke, toxic gases, high/low temperature (e.g. freezing) or flooding, at a premises. In response to sensing an alarm condition, a sensor provides a signal to an alarm panel that in turn may sound and notify the occurrence of the alarm to occupants of the premises and remotely signal a monitoring center or other third party. 
     Typically the occurrence of an alarm is signalled to a remote monitoring center that may then dispatch capable authorities to intervene at the premises. For example, in the case of sensing an unauthorized entry to the premises, the monitoring center may dispatch security personnel, typically in the form of private security guards or police officers. 
     Additionally, many alarm systems now allow audio communication with the monitoring center. Such audio communication may be one-way (e.g. from the premises to the station) or two-way (i.e. from and to the premises/monitoring center). In this way, the alarm system may be used to audibly monitor the premises during an alarm condition, or it may be used to dispatch a verbal request for help by an occupant at the monitored premises. To this end, some alarm systems are equipped with an audio interface, including a microphone and optionally a speaker. 
     Communication between the premises and the monitoring station typically takes place over a communications network, for example by way of the public switched telephone network (PSTN). 
     Unfortunately, audio interfaces are susceptible to unauthorized modification, or disconnection by intruders or others. Tampering can be deterred by encasing the audio interface. This however, is costly and cumbersome. Alternatively, tampering can be detected. 
     Accordingly there is a need for alarm systems and audio interfaces that allow tamper detection. 
     SUMMARY OF THE INVENTION 
     In accordance with an aspect of the present invention, there is provided an alarm system comprising; an audio interface comprising: a microphone; a state sensor for sensing a state at the audio interface; a signal generator for generating an analog state signal indicative of the state; a combiner for forming a combined signal from an electrical signal corresponding to audio sensed at the microphone, and the state signal so that the state signal does not interfere with reproduction of audio from the electrical signal; a control panel for monitoring sensed conditions at the audio interface; a conductor interconnecting the audio interface to the control panel carrying the electrical and the state signal as combined from the audio interface to the control panel. 
     In accordance with an aspect of the present invention, there is provided a method of operating an alarm system comprising a control panel and an audio interface. The method comprises sensing an audio signal at the audio interface; sensing a state at the audio interface panel, and generating an inaudible electric state signal in response thereto; multiplexing the audio signal and the state signal to form a combined signal; and providing the combined signal to the control panel, for sensing the state at the control panel. 
     In accordance with an aspect of the present invention, there is provided an audio interface comprising: a microphone; a state sensor for sensing a state at the audio interface; a signal generator for generating an analog state signal indicative of the state; a combiner for forming a combined signal from an electrical signal corresponding to audio sensed at the microphone, and the state signal so that the state signal does not interfere with reproduction of audio from the electrical signal. 
     Conveniently, the state signal and audio signal may be carried on a single conductor, thereby reducing the number of physical interconnections between the control panel and audio interface. 
     Other aspects and features of the present invention will become apparent to those of ordinary skill in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In the figures which illustrate by way of example only, embodiments of the present invention, 
         FIG. 1  is a block diagram of a premises monitored by an alarm system and associated monitoring center, exemplary of an embodiment of the present invention; 
         FIG. 2  is a block diagram of a control panel of the alarm system of  FIG. 1 ; 
         FIG. 3  is a schematic block diagram of a portion of an audio interface and control panel of the alarm system of  FIG. 1 ; and 
         FIG. 4  is a schematic diagram of a portion of the control panel of the alarm system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  illustrates premises  10 , monitored by an alarm system  40  exemplary of an embodiment of the present invention. As illustrated, alarm system  40  includes a central control panel  20 , in communication with a plurality of sensors  18 . Sensors  18  may be entry sensors, motion sensors, flood sensors, smoke sensors, gas sensors or any other sensors being monitored as appreciated by those of ordinary skill. Sensors  18  may be in communication with control panel  20 , wirelessly or by dedicated wires, by the electrical wiring of premises  10  or otherwise. 
     Alarm system  40  further includes an audio interface  25 . Audio interface  25  may take the form of an audio station remote from panel  20 , and may include a microphone and may optionally further include a camera, keypad, display, speaker, electronic ringer or the like. 
     Control panel  20  is interconnected with a communications network to a monitoring center  14 . In the depicted embodiment, control panel  20  is interconnected by a subscriber line  13  to the public switch telephone network “PSTN”  12 . Subscriber line  13  terminates in a central office (not shown) of PSTN  12 . A monitoring center  14  is also interconnected with PSTN  12 . Of course panel  20  could be interconnected with monitoring center  14  by way of another communications network, like a cellular telephone network, packet switched data network (e.g. the internet), or the like. 
     Monitoring center  14  is depicted as a single monitoring station. However, as is appreciated by those of ordinary skill, monitoring center  14  could be multiple monitoring stations, each in different physical locations. Monitoring center  14  is associated with one or more PSTN telephone numbers that would be used by telephone control panel  40  to contact monitoring center  14 . 
     As will become apparent, audio interface  25  allows two way audio communication between alarm system  10  and monitoring center  14 . 
     Sensors  18  and control panel  20  interact in a conventional manner. As particular sensor  18  is tripped, signifying a sensed condition, the sensor provides a signal to a sensor interface  26  of control panel  20 . Control panel  20  initiates a communication to monitoring center  14 . For example, control panel  20  may place a telephone call to a pre-programmed telephone number to contact monitoring center  14 , typically by way of PSTN  12 . 
     Control panel  20  is more particularly illustrated in  FIG. 2 . As illustrated, control panel  20  includes a central processor  32  in communication with memory  30 . Input/output interface  24 , sensor interface  26 , and network interface  28 . In the depicted embodiment, network interface  28  is interconnected through PSTN subscriber line  13 . 
     Input/output interface  24  may communicate with audio interface  25  ( FIG. 1 ), and may bridge audio signals from and to audio station over network interface  28 . In the depicted embodiment, audio interface  25  allows two-way full-duplex audio communication between control panel  20  and audio interface  25 . As such, audio interface  25  is interconnected to control panel  20  by at least two conductors  27   a ,  27   b  (typically in the form of wires). The two conductors  27   a ,  27   b  are used to carry audio signals from and to audio interface  25  to control panel  20 . As will become apparent, the audio signals may be bridged to monitoring center  14  so that sound at premises  10  may be monitored during alarm condition or otherwise at monitoring center  14 . Likewise center  14  may originate a signal to be heard at audio interface  25 . 
     A further block diagram of portion of audio interface  25  and input output interface  24  is depicted in  FIG. 3 . As illustrated, audio interface  25  includes a speaker  50 , one or more amplifiers  52 , a microphone  54 , a microphone preamplifier  56 , a signal generator  58 , a state sensor  60 , an optional signal conditioner  62  and a mixer/multiplexer  64 . 
     Signal generator  58  generates an analog signal. Signal generator  58  may be a low frequency oscillator, which may for example generate a square wave or other wave form. Signal conditioner  62  may further condition the signal generated by signal generator  58 . For example, if signal generator  58  generates a square wave, conditioner  62  may be a filter to generate a sine wave, or a saw tooth wave. 
     Mixer/multiplexer  64  may be an analog multiplexer or mixer that be capable of mixing two analog input signals. Mixer/multiplexer  64  may be a summer formed of passive components, or a sampling amplifier, or may take the form of a signal modulator or the like, or may be any other suitable signal combiner. 
     Microphone  54  may sense a signal in the audible range at audio interface  25 . For example, microphone  54  may sense audio signals in the frequency range of 200-2000 Hz. Of course, a broader frequency range signals could be sensed by microphone  54 . The audio signal may be amplified by microphone pre-amplifier  56 . 
     Conductors  27   a ,  27   b  act as speaker and microphone wires to panel  20 , respectively. 
     At panel  20 , microphone conductor  27   b  feeds a filter  84  of input/output interface  24 . Filter  84  is a low pass filter, and acts as a detector that detects a low frequency signal on conductor  27   b . Filter  84  provides an output to a signal conditioner  85  in communication with a debounce filter  86  to provide digital signal output to processor  32  ( FIG. 2 ). 
     Debounce filter  86  filters noise and transients that may interfere with a few cycles of the state signal, so that a digital tamper indication is not presented to the processor  30  prematurely. Electrostatic discharge at the audio station or a surge pulse induced into the conductor  27   b  are examples of noise signals that may be filtered by debounce filter  86 . 
     Microphone conductor  27   b  similarly feeds one or more audio amplifiers  88  and buffers that, in turn, amplify any audible signal on conductor  27   b  to provide an audio signal that may be passed to network interface  28 . 
     Likewise an audio signal from network interface  28  may be passed to conductor  27   a  for provision to audio interface  25 . 
     Power may be provided to audio interface  25  by power supply conductors  29 . These may originate at panel  20 , or elsewhere, such as at a battery, AC adapter or the like. Conveniently, one of conductors  29  may provide a common ground between panel  20  and audio interface  25 . 
     At panel  20 , as illustrated in  FIG. 4 , filter  82  forms a hi-pass filter with a 3 dB point near 250 Hz to allow the audio signal from microphone  54  to be passed to the audio section of panel  20  and blocks the subsonic signal. Transistor  90  is used to shut down the microphone input when required. The subsonic and audio signals are presented to the low-pass filter  84  which has a 3 dB roll-off at near 30 Hz. This allows the subsonic signal to be passed to signal conditioner  85  (formed of transistor  84 ) then debounce filter  86 , while attenuating the audio signals. 
     In operation, signal generator  58  generates an inaudible signal, indicative of a sensed state at audio interface  25 . In the depicted embodiment, the oscillator generates a low frequency electrical signal (which would produce a subsonic or near subsonic audio signal), for example having a frequency of less than 50 Hz (e.g. 25 or 20 Hz or less). As noted, this signal may be conditioned by signal conditioner  62 , and provided to modulator/mixer  64 . This low frequency state signal is carried by microphone conductor  27   b . As required, the low frequency signal may be amplified in order to provide a desired signal strength at panel  20 . 
     The sensed state may be an indication of whether or not audio interface is powered, whether it has been tampered with, or any other sensed condition, sensed at audio interface  25 . 
     Audio interface  25  may be activated by user interaction at audio interface  25  (e.g. by way of button press, or the like), or by panel  20  in the presence of a sensed condition (e.g. a tripped entry sensor, fire, or the like). 
     In any event, once audio interface  25  is activated, an audio signal detected by microphone  54  may also be provided to multiplexer/mixer  64 . Multiplexer/mixer  64  combines an electrical signal corresponding to audio sensed at the microphone from microphone  54  and the state signal from signal generator  58  to form a combined signal. As will become apparent, the state signal does not interfere with reproduction of audio from the electrical signal once extracted from the combined signal. 
     Thus, microphone conductor  27   b  carries the combined signal including both the audio signal and the state signal generated by signal generator  58 . 
     Input/output interface  24  demultiplexes the audio and state signals on conductor  27   b . Specifically hi-pass filter  82 , low-pass filter  84  and signal conditioner  85  act as the de-multiplexer for the complex signal of different frequencies. Low-pass filter  84  allows any subsonic signal on conductor  27   b  to pass and feed signal conditioner  85  to create a digital signal that signals the presence or absence of the state signal on conductor  27   b . This digital signal is applied to debounce filter  86  which filters transients, and thereby delays the application of a sudden change in the digital signal to the processor due, for example, to missing subsonic pulses on conductor  27   b  caused by electrostatic discharge at audio station  25  or the like. 
     Of course, if the state signal and audio signal were otherwise multiplexed (e.g. frequency modulated, phase modulated, or the like), input/output interface  24  could be perform complimentary demultiplexing to detect the state signal. 
     Now, state sensor  60 , in the form of a switch is interconnected with low signal generator  58 . In the presence of a tamper condition, the switch forming state sensor  60 , is opened and the state signal is no longer generated by signal generator  58  or provided to microphone conductor  27   b . Tamper switch forming state sensor  60  may, for example, be physically interconnected with the panel  20 . The tamper switch may, for example, be a leaf switch, or contact switch used to sense the integrity of a housing, housing audio interface  25 . The tamper switch may alternatively be a leaf switch or similar switch used to sense that audio panel  25  remains mounted. Alternatively, a mercury switch or other tamper sensor could be used as state sensor  60 . As will be appreciated, although only a single tamper switch is depicted, multiple tamper switches or sensors could form part of audio panel  25  to form state sensor  60 . The multiple switches could be connected in series or to multiple inputs of signal generator  58 . A person of ordinary skill will readily appreciate multiple possible tamper detection arrangements that may be used as tamper switch  60 , to detect unauthorized access, manipulation, disconnection or other tampering with audio interface  25 . Alternatively, the tamper switch could be replaced with some other state sensor at audio interface  25 . For example, a photo detector, moisture sensor, or the like could be used in place of a switch. 
     At panel  20 , the absence of the state signal (e.g. the low frequency subsonic signal) may indicate a sensed state—such as a tamper condition, which may in turn, be signaled by control panel  20  by way of PSTN  12  to monitoring center  14 . Additionally, a local alarm at premises  10  could be signalled. For example a siren may sound, or another audible or visual indicator could be signalled by panel  20 . 
     Conveniently, disconnecting power from audio interface  25  may likewise cause signal generator  58  to cease providing the low frequency tamper signal, indicating a tamper with audio panel  25 . 
     At the same time, audio signals from monitoring center  14  may be passed to audio interface  25  by network interface  28  over conductor  27   a , allowing full duplex communication with monitoring center  14 . Alternatively, half duplex communication could be permitted over a single conductor. 
     As will now be appreciated, by multiplexing both an audio signal and a low frequency (sub-audio) tamper, it may be possible to carry both audio and state signals using only one signal conductor (e.g. microphone conductor  27   b ). Conveniently, the state signal may result in an inaudible acoustic signal at panel  20  (and hence monitoring center  14 ), and need not interfere with audio monitoring at audio interface  25 . Both audio and state signals are analog signals. 
     As will further be appreciated, although only a single audio interface  25  has been depicted, panel  20  may communicate with multiple such audio interfaces like audio interface  25 , at different locations in premises  10 , as suggested by  FIG. 2 . 
     In an alternate embodiment, a high frequency electrical signal (producing a supersonic acoustic signal—e.g. a signal in a frequency range higher than about 15,000 Hz) could be used as a state signal. Of course, suitable filters could be used at audio interface  25  to sense the presence or absence of a supersonic state signal. 
     In a further alternate embodiment, the state signal could used to modulate the bias of microphone  54 . For example, the output of microphone  54  could be biased at a specific voltage level. The AC audio signal from the microphone would be AC coupled on the bias voltage and the tamper switch  60  would control the bias level. The bias level could be detected at panel  20  and if the bias level were inside a specific voltage window, no tamper condition could be sensed. Likewise if the bias level were outside the voltage window, a tamper condition/state could be detected. The bias level could be measured by an analog to digital converter at interface  24 , and the voltage window limits could be assessed by processor  30  under software control. 
     Likewise, the state signal could be a frequency or phase modulated signal modulated onto the audio signal. The frequency could be detected using a frequency discriminator to determine the presence or absence of the tamper at the audio station. A phase modulated signal could likewise be detected using a phase detector. 
     Of course, the above-described embodiments are intended to be illustrative only and in no way limiting. The described embodiments of carrying out the invention, are susceptible to many modifications of form, arrangement of parts, details and order of operation. The invention, rather, is intended to encompass all such modification within its scope, as defined by the claims.