Abstract:
An audible alarm detector constituted of: a microphone generating an electronic signal from an audible signal; a phase locked loop locking onto a frequency component present in the generated electronic signal to output a demodulated signal; and a pattern detector for comparing the demodulated signal against each template of a known set of templates, each template representing a standard pulse stream, wherein upon detection that the demodulated signal matches one of the known templates, the audible alarm detector is arranged to output an alarm detected signal indicating a presence of one of the standard pulse streams.

Description:
FIELD OF THE INVENTION 
     This invention relates to the field of acoustic signal detection, and in particular to a method and apparatus for detecting specific acoustic signals indicating certain events, such as the presence of fire or carbon monoxide. 
     BACKGROUND OF THE INVENTION 
     In recent years audible fire alarm signals have standardized patterns, set by the American National Standards Institute (ANSI). For example, the pattern used for smoke alarms, in accordance with ANSI S3.41, is a three-pulse pattern, known as T3, which comprises three half second on pulses, each followed by a half second off period, the set followed by a one and a half second pause, with the cycle repeated for a minimum of 180 seconds. Carbon monoxide detectors use a similar pattern using four pulses, as defined by the National Fire Protection Association (NFPA) referred to as T4, where the signals consist of four 100 milliseconds on pulses, each followed by a 100 millisecond off period, the set followed by a 5 second pause. The alarms may use the older 3100 Hz sine wave or the newer 520 Hz square wave. 
     The purpose of the acoustic alarm is to alert personnel on site to evacuate, but it is desirable to automatically detect the existence of the acoustic alarm signal so that appropriate action can be taken, such as alerting off site personnel, without requiring integration with the smoke of carbon monoxide detector. Such acoustic detectors exist, but are limited in detection distance and noise suppression, and are prone to false alarms. Examples of prior art detection systems include U.S. Pat. Nos. 7,015,807 and 8,269,625, the entire contents of each of which are incorporated herein by reference. 
     SUMMARY OF THE INVENTION 
     According to an aspect of the present invention there is provided an audible alarm detector comprising: a microphone generating an electronic signal from an audible signal; a phase locked loop locking onto a frequency component present in the generated electronic signal to output a demodulated signal; and a pattern detector for comparing said demodulated signal against each template of a known set of templates, each template representing a standard pulse stream, wherein upon detection that said demodulated signal matches one of the known templates, said audible alarm detector is arranged to output an alarm detected signal indicating a presence of one of the standard pulse streams. 
     According to another aspect of the present invention there is provided a method of generating an alarm signal from and audible alarm, comprising: detecting an audible signal and generating an electronic signal; using a phase locked loop to lock onto a frequency component present in the generated electronic signal and output a demodulated signal; comparing said demodulated signal against each template of a known set of templates and producing a matching score, each template representing a standard pulse stream; and outputting an alarm detected signal indicating a presence of one of the standard pulse streams upon detection that said demodulated signal matches one of the known templates. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described in more detail by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a system block diagram of an audible alarm detector in accordance with an embodiment of the invention; and 
         FIG. 2  is a system block diagram of the audible alarm detector of  FIG. 1  showing details of an embodiment of the phase-locked loop and an embodiment of the out-of-band energy qualifier. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  shows a block diagram showing the top level functionality of the audible alarm detector  100  in accordance with an embodiment of the invention. The detector  100  comprises a microphone interface  110  which detects an audible alert signal, as well as other ambient sounds. These audible alert signals can comprise an industry standard T3 pulse stream emitted by a smoke/fire detector and an industry standard T4 pulse stream emitted by a carbon monoxide alarm. The T3/T4 alarm may be of the older 3100 Hz sine wave alarm or the newer 520 Hz square wave alarm. The microphone interface  110  converts the sensed acoustic energy from the audible alert signals into electromagnetic energy. The microphone interface can include a digital microphone which can comprise an analog-to-digital converter. The invention is not limited to digital microphones, however, and an analog microphone could also be implemented. An analog-to-digital converter would preferably be provided to convert the audible alert signal into a digital signal. The detected signal is preferably sampled at 8 KHz or 16 KHz for conversion into a digital signal. Next the digital signal outputted from the microphone interface  110  is input into front end signal conditioning block  120 . The front end signal conditioning block  120  removes constant (i.e. DC) and low frequency components from the digital signal. The front end signal conditioning block  20  also levels the frequency response and amplifies the digital signal. The front end signal conditioning block  120  can comprise, but is not limited to, filters such as high-pass filters  122  for removing DC and low frequency components. The front end signal conditioning block  120  can also comprise amplifier  124  for signal amplification. The amplified signal can then be passed through an equalizer  126  to stabilize or flatten the frequency response. The equalized signal is then stored in buffer  128 . The conditioned digital signal is then output from the front end signal conditioning block  120  and input to digital phase-locked loop (PLL)  130 . The PLL  130  is used for pulse demodulation. The PLL  130  locks onto the largest fundamental frequency present within either the 520 Hz or 3100 Hz band which simplifies frequency tuning compared to other methods such as using filter banks or Fast Fourier Transform (FFT). Since each PLL will lock onto a particular frequency, at least two PLLs would be required for the detection of 520 HZ and the 3100 Hz carrier frequencies. The T3 and T4 signals each have a carrier frequency of 3100 Hz which can vary by +/−10%. Similarly, at 520 Hz, the carrier frequency can vary by +/−10%. As such, the PLL must be able to lock to those range frequencies. The largest fundamental frequency corresponds to the frequency having the strongest signal strength or amplitude. The output of the PLL  130  is the baseband demodulated pulse corresponding to the envelope of the in band modulated signal. According to an embodiment of the invention, the PLL  130  uses continuous frequency domain sampling for demodulating the 520 Hz or 3100 Hz carrier frequency which avoids sampling tied to expected input duration. This is in contrast to certain prior art systems such as the discrete sampling in the Fast Fourier transform (FFT) method used in U.S. Pat. No. 7,015,807 where quantization errors and aliasing may be of concern. Furthermore, the use of a PLL, in place of FFT is advantageous since demodulation is performed without requiring any a-priori information since the PLL  130  locks onto the fundamental frequency having the strongest signal strength. After demodulation, the signal is input into pattern detector  140 . In the pattern detector  140 , the demodulated pulse output from the PLL  130  is decoded to determine if the target T3 and/or T4 pulse stream exists. Detection of the target T3 and/or T4 pulse stream is performed by correlation against a known set of templates of the T3/T4 pulse streams  142 . In some embodiments of the present invention, pattern detection can be achieved using a correlator such as a matched filter. The pattern detector  140  is not limited to a correlator, and other implementations may be used. In the present embodiment, the set of T3/T4 templates  142  are stored in on-chip memory (not shown). In other embodiments, an external memory may be used to store a wider array of templates. The output of the pattern detector  140  is a matching score which is a numerical representation of the strength of the match between the output of the PLL  130  and the T3/T4 templates. 
     In some cases, a rich signal (often music or a similarly pulsed non T3 alarm) can cause a false positive detection. To keep those situations from causing a false trigger, the energy out of band may be tested in accordance with an embodiment of the invention. In this embodiment, the signal power including the total power and the power in the desired band (3100 Hz and/or 520 Hz) is monitored in parallel to the PLL  130  and pattern detector  140  by out-of-band energy qualifier  150 . A wideband-to-narrowband ratio is determined and output from out-of-band energy qualifier  150 . The ratio represents a value between 0 and 1 and is used to adjust the output of the pattern detector  140 . In a situation where there is little wideband noise, the output of out-of-band energy qualifier  150  will be closer to 1. Conversely, in a situation where a lot of wideband noise is present, the output of out-of-band energy qualifier  150  will be closer to 0 and thus will significantly lower the matching score output from pattern detector  140 . This has the effect of requiring the detected signal to be very exact if there is a lot of out of band noise. The output of the out-of-band energy qualifier  150  is input into multiplier  160  along with the output of the pattern detector  140 . The output of multiplier  160  represents an adjusted output of the pattern detector in view of background noise or a non T3/T4 alarm. 
     The output of multiplier  160  is input into comparator  170 . The comparator  170  compares the output of the pattern detector  140  with a threshold value  172  to qualify the result of the pattern detector  140 . If the output of the pattern detector  140  meets and/or exceeds the threshold value  172 , the audible alert signal detected by microphone interface  110  is determined to be an actual T3/T4 pulse stream and the comparator  170  outputs an active high signal. However, if the output of the pattern detector  140  is lower than the threshold value  172 , the audible alert signal is determined not to be a T3/T4 pulse stream and the comparator  170  outputs an active low signal. 
     In certain embodiments, after a single T3/T4 alarm period is detected at the output of comparator  170  by an active high signal, the alarm can be further qualified by checking if subsequent alarms are present by multi-pulse qualifier  180 . For example, in some embodiments of the invention, N audible alarms must be detected within a predetermined time window determined by timer  182  before outputting an alarm detected signal. In the event that only a single alarm period is detected, with no subsequent alarm period within the predetermined time window, the multi-pulse qualifier  180  does not assert an alarm detected signal. This adds to the general robustness of the alarm detection accuracy. This process looks to see if more than a predetermined number of frames in a given interval resulted in assertion of an active high signal by comparator  170 . Since the output of the pattern detector  140 , before comparator  170 , is a score corresponding to the probability a T3/T4 alarm was detected, these scores may be summed over time to provide a continuous multiple pulse qualification. If so, the host/user is alerted that a T3/T4 alarm was detected responsive to an output alarm detected signal from the multi-pulse qualifier  180 . In block  190 , an interrupt or a notification is generated and output, responsive to output alarm detected signal from the multi-pulse qualifier  180 , preferably to a host system so that an action can be taken. The interrupt or notification is thus generated responsive to the asserted signal at the output of comparator  170 . In certain embodiments neither multi-pulse qualifier  180  nor out-of band energy qualifier  150  are provided. Alternately, in other embodiments of the present invention, the output of pattern detector  140 , appropriately buffered or amplified if required, is used as the interrupt or notification output, without requiring comparator  170 , or multi-pulse qualifier  180 . 
       FIG. 2  shows the detector  100  of  FIG. 1 , with details of the PLL  130  and out-of-band energy qualifier  150 . As shown in  FIG. 2 , microphone interface  110  is connected to front end signal conditioning block  120 , the details of which are shown in  FIG. 1 . The conditioned signal then is input to PLL  130  and out-of-band energy qualifier  150 . The structure of the PLL  130  generally comprises a phase detector  132 , a loop filter  134  and an oscillator  136 , such as a numerically-controlled oscillator (NCO) or a voltage-controlled oscillator. Other oscillator configurations can also be implemented. The conditioned signal is input into the phase detector  132  along with the feedback from the oscillator  136 . The phase detector can be thought of as a multiplier, such that the output of the phase detector contains both sum and difference frequency components. The loop filter  134  removes the high frequency components and the output from the loop filter  134  is the demodulated signal. This demodulated signal output from loop filter  134  is then fed into pattern detector  140 . In parallel to the PLL, the out-of-band energy qualifier  150  functions to qualify the detected audible alert signal to avoid false positive detection of the T3/T4 stream due to background noise or a non T3/T4 alarm. Out-of-band energy qualifier comprises filter  152 , which is generally a band-pass filter to narrow the band of interest which can either be the 520 Hz band or the 3100 Hz band. Power estimator  154  is then used to determine the power of the band of interest. Concurrently, power estimator  156  is used to determine a total power of the entire frequency band of the conditioned signal which corresponds generally to the frequency band of the detected audible alert signal. In block  158 , the wideband-to-narrowband ratio of the output of power estimator  154  (power of the band of interest, or narrowband) to the output of power estimator  156  (power of entire spectrum of detected audible alert signal) is determined. The result is a value which ranges between 0 and 1 and is used as an input to multiplier  160  to adjust the output or matching score of the pattern detector  140  as described above. 
     It should be appreciated by those skilled in the art that any block diagrams herein represent conceptual views of illustrative circuitry embodying the principles of the invention. For example, a processor may be provided through the use of dedicated hardware as well as hardware capable of executing software in association with appropriate software. When provided by a processor, the functions may be provided by a single dedicated processor, by a single shared processor, or by a plurality of individual processors, some of which may be shared. Moreover, explicit use of the term “processor” should not be construed to refer exclusively to hardware capable of executing software, and may implicitly include, without limitation, digital signal processor (DSP) hardware, network processor, application specific integrated circuit (ASIC), field programmable gate array (FPGA), read only memory (ROM) for storing software, random access memory (RAM), and non-volatile storage. Other hardware, conventional and/or custom, may also be included. The functional blocks or modules illustrated herein may in practice be implemented in hardware or software running on a suitable processor.