Gunshot detector

An amplitude responsive detection system analyzes the amplitude characteristic of a received noise and determines whether that characteristic conforms to the predictable audio signature of a gunshot. If a received noise reaches a predetermined amplitude level within a rise time that may be indicative of a gunshot, subsequent amplitude criteria are established representing the decay of the amplitude profile that is expected if the noise is a gunshot. The amplitude criteria are controlled as to both level and occurrence in time to provide a dynamic range that will accommodate near and far gunshots.

BACKGROUND OF THE INVENTION 
This invention relates to an improved method and apparatus for detecting 
gunshots and recognizing their characteristic waveform as separate and 
different from other common noises, particularly those encountered in a 
law enforcement environment. 
The ability to distinguish a gunshot, regardless of the type of weapon 
fired, is often difficult due to the ambient noise typically present in 
many law enforcement environments. In security applications, detecting a 
gunshot by ear is not feasible as a police officer or other person capable 
of recognizing the shot and responding in an appropriate manner is often 
not present. Therefore, remote detection and monitoring are required in 
order to adequately protect retail establishments, other public places and 
dwellings in order to prevent criminal activity and ensure a prompt 
response when such activity occurs. 
It has been found that the audio signature (amplitude envelope) of a 
gunshot has defined characteristics irrespective of whether the shot is 
produced by firing a handgun, a rifle or a shotgun. The common thread 
identifying these various types of gunshots is an extremely sharp rise 
time characteristic in all cases and a predictable decay in amplitude 
thereafter. Therefore, although the amplitude of the gunshot will, of 
course, depend upon the cartridge that is expended, the type of weapon and 
distance, the amplitude versus time format can be predicted. 
SUMMARY OF THE INVENTION 
It is, therefore, the primary object of the present invention to provide a 
method and apparatus for detecting a gunshot by analyzing the waveform of 
the noise produced to determine if it has the characteristic audio 
signature of a gunshot. 
As corollary to the foregoing object, it is an important aim of this 
invention to provide such a method and apparatus in which it is determined 
whether a received noise reaches a predetermined amplitude level within a 
rise time that may be indicative of a gunshot and, if so, subsequent 
amplitude criteria are established which, if satisfied, represent the 
expected decay of the gunshot and verify its presence. 
Another important object of the present invention is to provide a method 
and apparatus as aforesaid in which the amplitude criteria, as to both 
level and occurrence in time, are established based upon the peak 
amplitude level detected. 
Still another important object of this invention is to provide such a 
method and apparatus which relies upon the audio signature of a gunshot 
and distinguishes the gunshot from ambient noise by the amplitude 
characteristic of that signature, thereby enabling the present invention 
to be practiced by employing a reliable, relatively inexpensive detection 
system that utilizes a series of controllable amplitude level detectors to 
determine whether a received noise fits the profile of a gunshot. 
Other objects will become apparent as the detailed description proceeds.

DETAILED DESCRIPTION 
The block diagram of FIG. 1 illustrates an embodiment of the present 
invention in which the audio signature of a gunshot is verified. As 
discussed above, the common thread identifying various types of gunshots 
is the extremely sharp rise time characteristic and the predictable decay 
in amplitude. The composite waveform of a typical gunshot is illustrated 
in FIG. 2. The amplitude versus time format of the graph shows the 
following reference points: 
A: threshold for system enable (at 5 milliseconds) 
B: time=4 milliseconds after system enable 
P: variable point in time that the peak amplitude occurs 
C: time=75 milliseconds after system enable 
D: time=150 milliseconds after system enable 
E: time=225 milliseconds after system enable 
These time references and corresponding relative amplitude levels establish 
amplitude criteria which, if satisfied in the example illustrated in FIG. 
2, identify the audio signatures of gunshots and also discriminate against 
other sources of noise expected to be encountered in a law enforcement 
operating environment. Such expected noises are, for example, a passing 
semi-tractor/trailer truck, FIG. 3; a passing automobile, FIG. 4; 
automobile horns, FIG. 5; emergency vehicle sirens, FIG. 6; and wind 
noise, electrical system noise, thunder, etc. (not shown). Referring to 
FIG. 2, if the amplitude criteria at points A and B are satisfied, the 
waveform then peaks at P and begins a predictable decay. By analyzing the 
amplitude at points C, D and E, the present invention determines the 
goodness of fit of the waveform along its expected curve. If any of the 
subsequent points are not valid, then the system is disabled and resets. 
If all of the points are valid, then the waveform is deemed to have 
originated from a gunshot and the system output is delivered. 
Referring again to FIG. 1, the block diagram of the system, the sound 
(incoming noise) is received by an audio frequency microphone 20, 
converted to an audio signal and then fed to an audio preamplifier 22. 
From the preamp 22 it is then filtered by a bandpass filter 24 whose pass 
band, for example, is 1 kHz to 10 kHz. This filtered signal is then 
amplified at 26 to raise it to the desired level for analysis. 
The signal output of the audio amplifier 26 on line 27 is fed 
simultaneously to a peak detector 28 and to the system clock and control 
block 30. The peak detector 28 is an operational amplifier configured as a 
voltage peak detector with a reset input. The output of the peak detector 
28 is fed into the system control block 30 and serves as an initial 
reference level from which the goodness of fit curve control points are 
derived. The audio signal from the amplifier 26 is distributed by the 
control block 30 to the signal input lines of each of five level detectors 
consisting of a voltage comparator and a latch, the comparator and latch 
components of the detectors being designated A, B, C, D and E in FIG. 1 to 
correspond with the criteria points A, B, C, D and E illustrated in FIG. 
2. Comparators A and B operate as threshold detectors, while comparators 
C, D and E comprise dual window comparators. It will be appreciated that a 
greater number of window comparators may be employed to establish 
additional criteria points if desired. 
Comparator A has a fixed reference level set somewhat above the level of 
the expected ambient noise. For example if the expected ambient noise 
level is 1.5 volts, the reference level could be set at 3.0 volts. This 
establishes the minimum signal level or threshold necessary to activate 
the system. Once this threshold is exceeded, the output of comparator A 
shifts to a logic level "1" and sets latch A. The output of latch A is 
thereby set to a logic level "1" and is routed simultaneously to a FET 
switch 34 via line 32 to enable the peak detector 28 and the system clock 
to begin a timing sequence, and to the enable line 36 of latch B. The 
reference voltage level on comparator "B" is also a fixed reference and is 
set at the minimum level required to be considered for analysis, in the 
present example, 6.0 volts. Clock pulse "B" on line 38 occurs 4 
milliseconds after the system clock is started, and if the output of 
comparator B is high, indicating the 6.0 volt reference threshold has 
been crossed, then latch B is set. This timing and comparison tests the 
rise time characteristic of the waveform to determine if further analysis 
is required. If latch B fails to set, then the signal is disregarded and 
the system will cease processing until it later resets. If latch B sets, 
the waveform has met the first criteria and latch B enables latch C via 
line 40. 
Comparator C is a dual window comparator configured to provide a logic "1" 
output when the input signal voltage is between or inside the window 
established by an input reference voltage "C" and an offset reference 
voltage (discussed below). In the present example, clock pulse "C" on line 
42 occurs at approximately 75 milliseconds after the system is enabled, 
and if the voltage has peaked at 10 volts and has now decayed to a voltage 
between 4.0 and 3.6 volts, then latch C sets. If the latch does not set, 
then the system is inactive until a reset occurs. 
If latch C sets, then latch D is enabled via line 44. Comparator D is also 
a dual window comparator. The clock pulse "D" (line 46) occurs 
approximately 150 milliseconds after the system enable and if the voltage 
has decayed to a level between 1.6 and 1.2 volts, latch D sets enabling 
the latch E via line 48. If not, then the system is inactive until a reset 
occurs. 
Comparator E is another dual window comparator. Clock pulse "E" (line 50) 
occurs approximately 225 milliseconds after system enable. If the voltage 
has decayed to less than 300 millivolts, the latch E is set and all of the 
check points for goodness of fit have been deemed valid. The validating 
output of latch E is sent over line 52 to the system output logic 54. If 
latch E does not set, the system is inactive until a reset occurs. 
The output logic 54 is a conventional arrangement of gates that generates a 
resultant pulse and delivers the same to an output block 56, or to a 
system reset 58 depending on whether or not latches A through E have been 
set in their respective time constraints. If so, the resulting pulse is 
directed to the output block 56 which reports that the goodness of fit 
criteria have been met, and the waveform has been determined to fit the 
profile of a gunshot. The output block 56 may include an indicator light, 
an audible alert, or an analog or digital signal source to modulate a 
carrier or interface with a radio transmitter, telephone, cellular link, a 
GPS, or other satellite positioning and reporting system. 
The system reset logic 58 is connected to the reset inputs of the five 
latches A through E and the peak detector 28, and to a voltage comparator 
F, responsive to the output of amplifier 26, that is used to control the 
system reset. If a signal is applied to the system that fails to meet the 
goodness of fit criteria established, but is of sufficient amplitude to 
enable the system, then at the end of the clock cycle time the output of 
comparator F will be high and prevent the reset logic 58 from resetting 
the system. The clock stops on clock pulse "E" and the system shuts down 
until the amplitude of the noise falls below the comparator F reference 
level. At that point the system reset is generated and the system is ready 
to process the next waveform. If the system is tripped (output block 56 
activated), it then requires a manual reset from the operator of the 
device as illustrated at 60. 
Referring to FIGS. 9 and 10, the manner in which the peak detector 28 sets 
the amplitude criteria is shown in detail. FIG. 9 is a simplified 
illustration of the circuitry associated with each of the window 
comparators C, D and E that establishes the voltage window of the 
comparator in response to the output of the peak detector 28. The 
circuitry will be described with reference to comparator C. 
Referring to FIG. 9, the peak voltage of the audio signal from amplifier 26 
is detected by the peak detector 28 and is utilized to drive the gate 62 
of a junction field effect transistor (JFET) 64 having a source 66 and a 
drain 68. The voltage applied to the gate 62 determines the gate bias 
current which, in turn, controls the source-drain junction current. 
Varying the gate current thus causes a corresponding change in the 
source-drain current and, therefore, changes the resistance across the 
source-drain junction. A fixed resistor 70 is connected in parallel with 
source 66 and drain 68, this parallel combination comprising a voltage 
controlled resistance in series with fixed resistors 72 and 74. 
Accordingly, a series voltage divider is provided between the supply 
voltage terminal 75 and ground to establish the reference voltage "C" 
(FIG. 1) at 76 at an input of an operational amplifier 78. The result is a 
voltage at 76 having a level that is dependent upon the peak voltage of 
the input audio signal. 
A second operational amplifier 80 provides the window comparator 
configuration. A second, offset reference voltage for amplifier 80 is 
provided at 82 by the voltage divider resistors 72 and 74 to define a 
voltage window, e.g., 3.6 to 4.0 volts in the present example for 
comparator C. As this same voltage controlled resistor arrangement is 
employed for comparators D and E, they likewise set their successively 
lower voltage windows in accordance with the peak voltage level detected 
by the peak detector 28. Resistors 72 and 74 are selected for each of the 
comparators C, D and E to establish the progressively lower voltage levels 
indicative of a decaying gunshot waveform. 
The circuitry in FIG. 9 sets the levels of the reference level voltages for 
each of the comparators C, D and E, whereas the diagram shown in FIG. 10 
shows the manner in which the timing of the amplitude criteria is 
determined. The voltage output from the peak detector 28 drives a 
voltage-to-frequency converter 84 (for example, a phase locked loop) in 
the system control block 30. The output frequency from converter 84 is 
then counted by a counter decoder 86 which delivers a binary coded output 
to a clock divider 88. The clock divider 88 is a variable divider under 
the control of the decoded frequency which divides the frequency of the 
clock signal from the system clock 90 in order to produce a pulse train at 
the clock output 92 having a repetition rate which is inversely 
proportional to the level of the voltage peak detected by the peak 
detector 28. For example, if the output from the peak detector 28 is 7 
volts, the system clock frequency would be divided by 7. If the output 
voltage from the peak detector 28 is 10 volts, the system clock frequency 
would be divided by 10 to provide a lower clock frequency to lengthen the 
clock times for levels C, D and E. Therefore, the amplitude points 
established by comparators C, D and E are placed at times after system 
enable which shape the goodness of fit curve to fit the overall amplitude 
envelope of the applied audio signal. This imparts to the system the 
capability of operating on a wider dynamic range of signals thereby 
increasing its sensitivity and range. As illustrated in FIGS. 7 and 8, the 
signatures of near and far gunshots are alike but it will be appreciated 
that the amplitude and decay times are different. However, the amplitude 
at a given time is essentially proportional to the peak amplitude over a 
substantial portion of the decay period and thus is predicted in the 
system of the present invention.