Alarm system audio interface tamper and state detection

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.

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.

DETAILED DESCRIPTION

FIG. 1illustrates premises10, monitored by an alarm system40exemplary of an embodiment of the present invention. As illustrated, alarm system40includes a central control panel20, in communication with a plurality of sensors18. Sensors18may be entry sensors, motion sensors, flood sensors, smoke sensors, gas sensors or any other sensors being monitored as appreciated by those of ordinary skill. Sensors18may be in communication with control panel20, wirelessly or by dedicated wires, by the electrical wiring of premises10or otherwise.

Alarm system40further includes an audio interface25. Audio interface25may take the form of an audio station remote from panel20, and may include a microphone and may optionally further include a camera, keypad, display, speaker, electronic ringer or the like.

Control panel20is interconnected with a communications network to a monitoring center14. In the depicted embodiment, control panel20is interconnected by a subscriber line13to the public switch telephone network “PSTN”12. Subscriber line13terminates in a central office (not shown) of PSTN12. A monitoring center14is also interconnected with PSTN12. Of course panel20could be interconnected with monitoring center14by way of another communications network, like a cellular telephone network, packet switched data network (e.g. the internet), or the like.

Monitoring center14is depicted as a single monitoring station. However, as is appreciated by those of ordinary skill, monitoring center14could be multiple monitoring stations, each in different physical locations. Monitoring center14is associated with one or more PSTN telephone numbers that would be used by telephone control panel40to contact monitoring center14.

As will become apparent, audio interface25allows two way audio communication between alarm system10and monitoring center14.

Sensors18and control panel20interact in a conventional manner. As particular sensor18is tripped, signifying a sensed condition, the sensor provides a signal to a sensor interface26of control panel20. Control panel20initiates a communication to monitoring center14. For example, control panel20may place a telephone call to a pre-programmed telephone number to contact monitoring center14, typically by way of PSTN12.

Control panel20is more particularly illustrated inFIG. 2. As illustrated, control panel20includes a central processor32in communication with memory30. Input/output interface24, sensor interface26, and network interface28. In the depicted embodiment, network interface28is interconnected through PSTN subscriber line13.

Input/output interface24may communicate with audio interface25(FIG. 1), and may bridge audio signals from and to audio station over network interface28. In the depicted embodiment, audio interface25allows two-way full-duplex audio communication between control panel20and audio interface25. As such, audio interface25is interconnected to control panel20by at least two conductors27a,27b(typically in the form of wires). The two conductors27a,27bare used to carry audio signals from and to audio interface25to control panel20. As will become apparent, the audio signals may be bridged to monitoring center14so that sound at premises10may be monitored during alarm condition or otherwise at monitoring center14. Likewise center14may originate a signal to be heard at audio interface25.

A further block diagram of portion of audio interface25and input output interface24is depicted inFIG. 3. As illustrated, audio interface25includes a speaker50, one or more amplifiers52, a microphone54, a microphone preamplifier56, a signal generator58, a state sensor60, an optional signal conditioner62and a mixer/multiplexer64.

Signal generator58generates an analog signal. Signal generator58may be a low frequency oscillator, which may for example generate a square wave or other wave form. Signal conditioner62may further condition the signal generated by signal generator58. For example, if signal generator58generates a square wave, conditioner62may be a filter to generate a sine wave, or a saw tooth wave.

Mixer/multiplexer64may be an analog multiplexer or mixer that be capable of mixing two analog input signals. Mixer/multiplexer64may 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.

Microphone54may sense a signal in the audible range at audio interface25. For example, microphone54may sense audio signals in the frequency range of 200-2000 Hz. Of course, a broader frequency range signals could be sensed by microphone54. The audio signal may be amplified by microphone pre-amplifier56.

Conductors27a,27bact as speaker and microphone wires to panel20, respectively.

At panel20, microphone conductor27bfeeds a filter84of input/output interface24. Filter84is a low pass filter, and acts as a detector that detects a low frequency signal on conductor27b. Filter84provides an output to a signal conditioner85in communication with a debounce filter86to provide digital signal output to processor32(FIG. 2).

Debounce filter86filters 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 processor30prematurely. Electrostatic discharge at the audio station or a surge pulse induced into the conductor27bare examples of noise signals that may be filtered by debounce filter86.

Microphone conductor27bsimilarly feeds one or more audio amplifiers88and buffers that, in turn, amplify any audible signal on conductor27bto provide an audio signal that may be passed to network interface28.

Likewise an audio signal from network interface28may be passed to conductor27afor provision to audio interface25.

Power may be provided to audio interface25by power supply conductors29. These may originate at panel20, or elsewhere, such as at a battery, AC adapter or the like. Conveniently, one of conductors29may provide a common ground between panel20and audio interface25.

At panel20, as illustrated inFIG. 4, filter82forms a hi-pass filter with a 3 dB point near 250 Hz to allow the audio signal from microphone54to be passed to the audio section of panel20and blocks the subsonic signal. Transistor90is used to shut down the microphone input when required. The subsonic and audio signals are presented to the low-pass filter84which has a 3 dB roll-off at near 30 Hz. This allows the subsonic signal to be passed to signal conditioner85(formed of transistor84) then debounce filter86, while attenuating the audio signals.

In operation, signal generator58generates an inaudible signal, indicative of a sensed state at audio interface25. 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 conditioner62, and provided to modulator/mixer64. This low frequency state signal is carried by microphone conductor27b. As required, the low frequency signal may be amplified in order to provide a desired signal strength at panel20.

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 interface25.

Audio interface25may be activated by user interaction at audio interface25(e.g. by way of button press, or the like), or by panel20in the presence of a sensed condition (e.g. a tripped entry sensor, fire, or the like).

In any event, once audio interface25is activated, an audio signal detected by microphone54may also be provided to multiplexer/mixer64. Multiplexer/mixer64combines an electrical signal corresponding to audio sensed at the microphone from microphone54and the state signal from signal generator58to 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 conductor27bcarries the combined signal including both the audio signal and the state signal generated by signal generator58.

Input/output interface24demultiplexes the audio and state signals on conductor27b. Specifically hi-pass filter82, low-pass filter84and signal conditioner85act as the de-multiplexer for the complex signal of different frequencies. Low-pass filter84allows any subsonic signal on conductor27bto pass and feed signal conditioner85to create a digital signal that signals the presence or absence of the state signal on conductor27b. This digital signal is applied to debounce filter86which 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 conductor27bcaused by electrostatic discharge at audio station25or 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 interface24could be perform complimentary demultiplexing to detect the state signal.

Now, state sensor60, in the form of a switch is interconnected with low signal generator58. In the presence of a tamper condition, the switch forming state sensor60, is opened and the state signal is no longer generated by signal generator58or provided to microphone conductor27b. Tamper switch forming state sensor60may, for example, be physically interconnected with the panel20. The tamper switch may, for example, be a leaf switch, or contact switch used to sense the integrity of a housing, housing audio interface25. The tamper switch may alternatively be a leaf switch or similar switch used to sense that audio panel25remains mounted. Alternatively, a mercury switch or other tamper sensor could be used as state sensor60. As will be appreciated, although only a single tamper switch is depicted, multiple tamper switches or sensors could form part of audio panel25to form state sensor60. The multiple switches could be connected in series or to multiple inputs of signal generator58. A person of ordinary skill will readily appreciate multiple possible tamper detection arrangements that may be used as tamper switch60, to detect unauthorized access, manipulation, disconnection or other tampering with audio interface25. Alternatively, the tamper switch could be replaced with some other state sensor at audio interface25. For example, a photo detector, moisture sensor, or the like could be used in place of a switch.

At panel20, 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 panel20by way of PSTN12to monitoring center14. Additionally, a local alarm at premises10could be signalled. For example a siren may sound, or another audible or visual indicator could be signalled by panel20.

Conveniently, disconnecting power from audio interface25may likewise cause signal generator58to cease providing the low frequency tamper signal, indicating a tamper with audio panel25.

At the same time, audio signals from monitoring center14may be passed to audio interface25by network interface28over conductor27a, allowing full duplex communication with monitoring center14. 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 conductor27b). Conveniently, the state signal may result in an inaudible acoustic signal at panel20(and hence monitoring center14), and need not interfere with audio monitoring at audio interface25. Both audio and state signals are analog signals.

As will further be appreciated, although only a single audio interface25has been depicted, panel20may communicate with multiple such audio interfaces like audio interface25, at different locations in premises10, as suggested byFIG. 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 interface25to 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 microphone54. For example, the output of microphone54could 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 switch60would control the bias level. The bias level could be detected at panel20and 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 interface24, and the voltage window limits could be assessed by processor30under 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.