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.
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.
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.
The invention will now be described in more detail by way of example only, with reference to the accompanying drawings, in which:
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.
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.
Number | Date | Country | |
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62190282 | Jul 2015 | US |