Abstract:
In a glass break detector, noise is removed by averaging the signal before it is analyzed for the characteristics that typically accompany glass breaking events. Low frequency signals, associated with glass flexing before it shatters, are isolated from repetitive or symmetrical noise in the same frequency range to increase the signal-to-noise ratio of the detector, improving sensitivity to valid glass breaking events and reducing false alarms.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of Invention 
     The invention relates to glass break detectors and more specifically to such detectors that convert acoustic and other atmospheric waves into electrical signals for analyzing characteristics that represent breaking glass. Still more specifically, the invention relates to detectors that react to initiate an alarm only when the frequency, amplitude and temporal sequence of the electrical signals correspond to those associated with breaking glass. 
     2. Description of the Prior Art 
     Recent improvements in glass break detectors rely on the presence of selected sonic and subsonic frequencies expected to occur in a predetermined temporal sequence representing the events that accompany breaking glass. 
     One approach relies on the occurrence of a low frequency thump at the moment the glass breaks, followed by a brief silence and a high frequency tinkling caused by the broken fragments hitting each other and falling to the floor. False alarms are reduced by requiring detection of high and low frequency components in the expected order and separated by a short time interval. This approach is disclosed, for example, in Davenport U.S. Pat. No. 4,668,941, issued May 26, 1987. 
     Other approaches sense structurally transmitted vibrations in combination with sound and other atmospheric waves. The structurally transmitted component is combined with the atmospheric component in a time-dependent function to reduce false alarms. Marino et al,. U.S. Pat. No. 5,117,220, issued May 26, 1992, discloses an example. 
     Still other approaches translate energy developed by breaking glass into electrical signals having low and high frequency components. The respective components must occur within specified time windows and above predetermined amplitudes before the detector will sound an alarm. Yanagi et al. U.S. Pat. No. 4,091,660, issued May 30, 1978, discloses one example using a piezoelectric element mounted on the glass. Smith et al. U.S. Pat. No. 5,192,931, issued Mar. 9, 1993, discloses another example substituting an acoustic transducer, such as a microphone, for the glass mounted piezoelectric element. The microphone senses atmospheric waves including a low frequency positive wave generated by an inward flex of the glass and high frequency waves generated by the glass breaking. The signals must occur in a predetermined order, and the alarm is inhibited if the high frequency waves are preceded by negative-going low frequencies that typically would accompany the opening of a door. 
     PROBLEM SOLVED BY THE INVENTION 
     Prior art detectors frequently require special mounting positions to take advantage of their most important features. In many instances the mounting surface must be capable of sensing structural vibrations, or on the glass itself. Such requirements severely restrict the areas where protection is practical. 
     A reputation for false alarms has limited the popularity of glass break detectors. Although modern approaches reduce false alarms, they often do so by imposing stringent alarm conditions that are not the best match for the mechanics of breaking glass. While false alarms might be reduced by requiring a positive pressure wave, for example, the application of such a requirement prevents use of the detector in applications where the glass is broken by a force applied away from the detector, such as a gun rack in the same room or a wall between two interior rooms. 
     Many of the more recent approaches to glass break detection process signals in high and low frequency bands selected to include frequencies typical of glass breaking events. Unfortunately, common sources of noise, such as electric motors and other sources of vibration, produce signals in the same frequencies. Since motors and similar sources are on for long periods, the resulting signal, which is sinusoidal or repetitive, frequently exceeds the threshold levels used by the detector. The problem is particularly troublesome in detectors that use frequencies characteristic of glass flexing before it shatters. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to overcoming one or more of the problems set forth above in a glass break detector with high reliability and reduced false alarms. Briefly summarized, the invention recognizes that background noise, not associated with glass breaking events, often is repetitive, producing a signal having similar positive and negative going components. Such repetitive noise is removed by averaging the signal before it is analyzed for the characteristics that typically accompany glass breaking events. The invention further recognizes that low frequency signals, associated with glass flexing before it shatters, can be isolated from repetitive or symmetrical noise in the same frequency range to increase the signal-to-noise ratio of the detector, improving sensitivity to valid glass breaking events and reducing false alarms. 
     Apparatus for detecting breaking glass is provided, including a filter that removes signal components typical of repetitive noise, before processing the signals for characteristics of breaking glass. The apparatus includes first and second channels for segregating low signal frequencies that are characteristic of glass flexing, from high signal frequencies that are characteristic of glass shattering. Repetitive signals that have alternating positive and negative peaks are nulled from the low frequency channel by averaging the signals to remove signal components that are not characteristic of a glass breaking event. 
     According to certain specific features of the invention, the low frequency channel includes a band-pass amplifier centered at a frequency between ten hertz and fifty three hertz, and the high frequency channel includes a band-pass amplifier centered at a frequency between thirteen kilohertz and nineteen kilohertz. According to other specific features of the invention, the noise filter includes a by-pass for signal frequencies at the high end of the frequencies characteristic of glass flexing. 
     According to still other specific features of the invention, a glass break detector includes a transducer for converting sound and pressure waves into electrical signals; a circuit responsive to the signals from the transducer for averaging symmetrical low frequency signals atypical of glass breaking events; a detection circuit responsive to said averaging circuit for detecting signals characteristic of glass flexing from an impact; and, an alarm circuit responsive to said detecting circuit for generating an alarm when the characteristic signals are detected. 
     The invention also includes methods for detecting glass breaking including method steps similar to those carried out by the apparatus summarized above. 
     ADVANTAGEOUS EFFECTS OF THE INVENTION 
     The invention does not rely on structurally transmitted vibrations or mounting on the glass surface. The transducer can be located almost anywhere in the vicinity of the protected glass where sound and other atmospheric waves will be detected. 
     The apparatus and method use frequency, amplitude and temporal characteristics of events that accompany breaking glass and, therefor, permit the relaxing of other mechanisms often used in prior art devices to reduce false alarms. Since the sound of the glass hitting the floor is not a discriminating factor in the alarm, carpets and other floor coverings or padding will not defeat the alarm. Similarly, negative pressure waves that might occur from breaking glass in a cabinet or interior wall, are sufficient for a valid alarm. 
     Signal components from motors and other repetitive or symmetrical vibrations, are removed from the signal before it is processed to identify glass breaking events. This increases the sensitivity of the detector to valid glass breaking events and reduces false alarms. 
     These and other features and advantages of the invention will be more clearly understood and appreciated from a review of the following detailed description of the preferred embodiments and appended claims, and be reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram depicting apparatus, according to a preferred embodiment of the invention, for detecting high and low frequency events characteristic of glass breaking from an impact. 
     FIG. 2 is a schematic representation of a temporal sequence characteristic of glass breaking and carried out by the apparatus of FIG. 1 to determine appropriate conditions for initiating an alarm. 
     FIG. 3(A&amp;B) is a flow diagram representing a method carried out by the apparatus of FIG. 1. 
     FIGS. 4(A-E) are schematic diagrams of an electrical circuit according to the preferred embodiment. FIG. 4B includes a noise filter in the low frequency channel, according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, apparatus according to a preferred embodiment of the invention is depicted including an signal input section 12, low and high frequency channels 14 and 16, respectively, a high-frequency signal analysis section 18, a microcontroller 20, and an alarm and output section 22. 
     The input section 12 converts acoustic and other atmospheric pressure waves into electrical signals that are segregated by frequency range into the low and high frequency channels 14 and 16. The high frequency channel 16 detects signals characteristic of two events that occur when glass breaks: the sound of the impact that initiates the break, and the glass shattering. Analysis section 18 analyzes the pulse widths of the detected signal to make sure it is in fact characteristic of the impact and shattering events. The low frequency channel 14 detects signals characteristic of a third event that occurs between the two events already mentioned, and that is the flexing of the glass after the impact and before the shatter. Although certain glasses, particularly laminated glass, may continue to flex even after the glass shatters, flexing begins after the first sound of the impact and continues for a minimum time period indicative of the flexing event. The microcontroller 20 generates timing signals and makes decisions based on inputs representing the frequencies, amplitudes and temporal sequence of the signals in both channels. When the inputs are indicative of breaking glass, it issues one or more alarm signals, according to instructions, that can drive audible and visible alarms associated with output section 22. 
     The input section 12 includes a transducer 24 and a pre amplifier 26. The transducer is an omnidirectional microphone that responds to a broad band of sound and other atmospheric pressure waves, including those having frequencies between ten hertz and twenty kilohertz (10Hz.-20kHz.). It converts the pressure waves into electrical signals having frequencies and amplitudes corresponding to the converted waves. The pre-amplifier gain, which is adjustable, is used to amplify the microphone signal and to correct for variability in the sensitivity of the microphone. 
     Low frequency channel 14 includes a band-pass amplifier 28, a noise riding circuit 29, a threshold detector 30 and a threshold level generator 32. The amplifier has a gain of approximately eighty at its center frequency, and passes signals having frequencies in a range between ten and fifty three hertz (10Hz.-53Hz.). 
     The noise riding circuit 29 is illustrated most clearly on FIG. 4B, and includes three signal paths 200, 202 and 204, coupled between low frequency amplifier 28 and threshold detector 30. Signal paths 200 and 202 are essentially identical, except path 200 includes a transistor Q6, substituted for diode D8, to compensate for a difference in the ability of amplifier 28 to source and sink current. The paths 200 and 202 also differ in that the transistor Q6 and capacitor C30 in path 200 detect and hold positive voltage peaks, while diode D8 and capacitor C31 in path 202 detect and hold negative voltage peaks. Since paths 200 and 202 end at a common junction at 206, they act to average the positive and negative peak components of signals from the amplifier, canceling repetitive signals that have substantially equal positive and negative peak voltages. Paths 200 and 202 thus act as a filter that removes or nulls signals having alternating positive and negative peaks, such as electric motors or vibrations, that amount to noise atypical of a glass breaking event. 
     Center path 204 serves three functions. It couples the leading edge of strong low frequency signals through capacitor C32 directly to output junction 206. Strong leading edge signals contain frequency components well above the cut-off frequency of the band pass amplifier, but have sufficient strength for detection. Typical post-amplifier signal frequencies that result from glass breaking events exceed one hundred and sixty hertz (160 Hz), for example. These high end frequencies are coupled to junction 206, through capacitor C32, and are used to initiate timing cycles, described hereinafter, that begin when the signal in the low frequency channel exceeds a predetermined quarter volt (0.025 v) threshold. Center path 204, through resister R69, also determines the rest voltage at junction 206 when the signals are below the junction voltages of the transistor Q6 or diode D8. The third function of the center path, again through resistor R69, is to pass or couple through to junction 206 approximately one tenth of the signal from the output of the amplifier. This permits detection of signals with sufficient amplitude and some imbalance that otherwise would be nulled in the channels 200 and 202. 
     The threshold detector 30 is inverting and is settable by command from the microcontroller 20 acting through the threshold level generator 32. Eight signal level thresholds are provided: plus or minus signal levels of a quarter volt (0.25 v.); fifty four hundredths of a volt (0.54 v.); seventy eight hundredths of a volt (0.78 v.); and ninety hundredths of a volt (0.90 v.). When the positive low frequencies exceed the active threshold, the output of the detector is a logical zero. When the negative low frequencies exceed the active threshold, the output is a logical one. In both cases, the logical output becomes the low frequency input to the microcontroller 20. It will become more apparent from the description associated with the flow diagram of FIG. 3 that the low-frequency signal threshold starts at a quarter volt (0.25 v.) until sometime after high frequencies are detected. The level is then raised progressively to the other values, i.e. 0.54 v, 0.78 v or 0.90 v. 
     The high frequency channel 16 includes a narrow band-pass amplifier 34, a threshold detector 36 and a threshold and integrator reset device 38. The amplifier has a gain of approximately one hundred and ten, and passes signals having frequencies in a range between approximately thirteen kilohertz and nineteen kilohertz (13kHz.-19kHz.), centered at fifteen and seven tenths kilohertz (15.7 kHz). When the high frequencies exceed a signal threshold of half of a volt (0.5 v), the output 40 of threshold detector 36 is a logical zero, which becomes the high frequency input to the microcontroller 20. It is this detected high frequency signal that is used to start a timing signal generator in the microcontroller for determining the timing and temporal sequence of the high frequency and low frequency signals. The timing signal generator also is used, as will be described more fully hereinafter, for analyzing the high frequency spectrum during different stages of detection. 
     The high-frequency signal analysis section 18 includes two pulse-width discriminators 42 and 44, and a pulse-width integrator 46. The term pulse width, as used in this specification, refers to the length of time starting when the signal crosses a predetermined threshold in one direction and ending when the signal next crosses the same threshold in the opposite direction. The threshold axis is determined by reset device 38. In the case of threshold detector 36, the threshold starts at half of a volt (0.5 v), but is reset soon after the high frequency signal is detected, to correspond to a signal level that is substantially zero, but slightly positive to eliminate noise. 
     The discriminator 42 passes for further analysis only frequencies having pulse widths longer than thirty seven microseconds (37 μs). It also has the effect of subtracting thirty seven microseconds (37 μs) from the duration of the pulses it does pass. Since the sound of the impact produces frequencies having short pulse widths, many will be removed by this discriminator. The discriminator has a threshold of one and five hundred thirty four thousandths volts (1.534 v), which corresponds to a the pulse width of thirty seven microseconds (37 μs). 
     The second pulse width discriminator 44, and the integrator 46, process the signal passed by discriminator 42, to identify the impact and shattering stages of a glass breaking event. The discriminator 44 identifies any pulse widths greater than one and one tenth milliseconds (1.1 ms). Glass breaking does not generate amplified signals having such long pulse widths. The integrator 46 sums the pulse widths passed by discriminator 42, and identifies when the sum exceeds a predetermined minimum. In a valid glass shattering event, the sum of the lows during the pertinent period, approximately thirty five milliseconds (35 ms), should not exceed six and eighty six hundredths milliseconds (6.86 ms) (after subtracting the 37 μs at discriminator 42). The outputs of discriminator 44 and integrator 46 are logically coupled as an OR circuit 50, which provides a low (or logical zero) input to the microcontroller if the discriminator 44 detects a single pulse width greater than one and one tenth milliseconds (1.1 ms) or the integrator 46 determines that the sum of the pulse widths exceeds six and eighty six hundredths milliseconds (6.86 ms). The purpose of these components in the high-frequency analysis section, and the time periods they monitor, will become more clear from the following description of FIG. 2. 
     The output section 22 includes conventional alarm drivers for local and remote annunciators. Driver 52 operates a remote alarm through relay 54. Alarm circuit 56 is a local alarm indicator. 
     The operation of the apparatus described in connection with FIG. 1 is represented in temporal sequence in FIG. 2. When a signal above the initial high frequency threshold, one half volt (0.5 v), is first detected in the high frequency channel 16, the microcontroller checks low frequency channel 14 for signals exceeding the initial low frequency threshold, one quarter volt (0.25 v). In the case of glass breaking from an impact, the high frequencies should occur first, and will be detected first. The low frequencies result from flexing of the glass due to the impact, and should not be present when the high frequencies are first detected. Assuming high frequencies are detected in the absence of low frequencies, the microcontroller initiates a timing signal, beginning at t 0 . These events are depicted beginning at block 60. 
     The low frequency signal, to represent flexing in a glass breaking event, should not exceed the one quarter volt (0.25 v) threshold before one hundred and twenty microseconds (120 μs). It should, however, exceed quarter volt (0.25 v) threshold during a time window that opens, in this preferred embodiment, at one hundred and twenty microseconds (120 μs) and closes at one and a half milliseconds (1.5 ms). This is depicted at box 62. Assuming the low frequency signal exceeds the threshold within the window, then it must continue for a minimum predetermined duration. Although the minimum duration might be as low as half of a millisecond (0.5 ms) in some embodiments, in this preferred embodiment the minimum duration is approximately one millisecond (1 ms), and further depends on the following sliding parameters. If the signal does not exceed fifty four hundredths of a volt (0.54 v) before four milliseconds (4 ms), then it should remain above one quarter volt (0.25 v) for nine milliseconds (9 ms). If the signal does exceed fifty four hundredths of a volt (0.54 v) within four milliseconds (4 ms), but not seventy eight hundredths of a volt (0.78 v), then it should remain above a quarter volt for at least five milliseconds (5 ms) from the time it crosses fifty four hundredths of a volt (0.54 v). If the signal exceeds seventy eight hundredths of a volt (0.78 v) before four milliseconds (4 ms), but not nine tenths of a volt (0.9 v), then it should exceed fifty four hundredths of a volt (0.54 v) for at least four milliseconds (4 ms) from the time it crosses seventy eight hundredths of a volt (0.78 v). If the signal exceeds nine tenths of a volt (0.9 v) before four milliseconds (4 ms), it should remain above seventy eight hundredths of a volt (0.78 v) for one and one tenth milliseconds (1.1 ms) from the time it crosses nine tenths of a volt (0.9 v). During the above time periods, the low frequencies also are checked to make sure there are no transients of opposite polarity. The low frequency signal must last for the predetermined minimum duration, as established above, before it is considered a qualified signal that is characteristic of glass flexing after an impact. The first detection of low frequencies is indicated at t=x 1  on FIG. 2. Qualification occurs at t=x 1  +x 2  on FIG. 2, provided the low frequency signal meets the above requirements. 
     Detection of the low frequency signal at t=x 1  also represents a time that is chosen to approximate the transition between stages in the glass breaking sequence. Although the stages are not precise, and overlap somewhat, in this preferred embodiment the entire glass breaking event is approximated from t 0  until t=35 ms, at least as far as the detector is concerned. Shattering of the glass is approximated by a second time period from t=x 1 , until t=35 ms. Analysis of the glass flexing is approximated by the period from t=x 1  until t=x 1  +x 2 . The impact is approximated by the period from t 0  until t=x 1 . Again the above selections are only approximations, since the actual events occur quickly and overlap. 
     After detection of the low frequency signal, the microcontroller to analyzes the high frequency signal for characteristics of the sound of the impact and the glass shattering. The microcontroller looks at the output of discriminator 44 and integrator 46. As already mentioned, the high frequency spectrum should contain almost all highs of short pulse widths, many of which are removed from the signal by discriminator 42. Discriminator 44 and integrator 46 look at the remaining signal after it is modified by the discriminator 42. If the sum of the pulse widths determined at integrator 46 exceeds six and eighty six hundredths milliseconds (6.86 ms), during the time period from t 0  until t=35 ms, then the high frequencies are not characteristic of glass breaking, and there is no alarm. Similarly, if any pulse width detected by discriminator 44 exceeds one and one tenth milliseconds (1.1 ms), during the time period from t=x 1 , until t=35 ms, there is no alarm. 
     If all of the requisite conditions are met, the detector issues an alarm signal for three seconds. In summary, an appropriate high frequency signal must be detected first, before the low frequency signal is detected; the low frequency signal must start within a predetermined window, and last beyond the predetermined minimum duration; and the pulse widths of the high frequency signal must meet individual and summed criteria during the selected time periods. 
     FIG. 3 is a flow diagram that represents the method steps carried out by the apparatus of FIG. 1. HF and LF are abbreviations for high frequency and low frequency, respectively. The detector actually looks at both positive and negative signals, but only the positive is shown to simplify the presentation. 
     Decision blocks 70, and 72 require a high frequency start in the absence of low frequencies. The high frequency signal must exceed one half volt while the low frequency signal is below a threshold of one quarter volt (0.25 v), and the low frequency signal must remain below that threshold for one hundred and twenty microseconds (120 μs). When these conditions are met, the high frequency signals may represent the sound of an impact on glass. A timing signal generator is initiated at time t 0 , corresponding to block 70. 
     Decision blocks 72 and 74 require a low frequency signal exceeding the quarter volt (0.25 v) threshold starting during a time window that opens at one hundred and twenty microseconds (120 μs.), and closes at one and one half milliseconds (1.5 ms.), measured from t 0 . The lows may represent flexing of the glass from the impact. To qualify as a glass breaking event, however, the low frequency signal also must meet the following criteria, including a minimum duration, represented by decision blocks 76, 77, 78, 80, 82, 83 and 84. If the signal at four milliseconds (4ms) has not reached fifty four hundredths of a volt (0.54), then it should remain above a quarter volt (0.25 v) for nine milliseconds (9 ms) from the time it exceeded a quarter volt (0.25 v). If the signal at four milliseconds (4 ms) has not reached seventy eight hundredths of a volt (0.78 v), then it should remain above a quarter volt for at least five milliseconds (5 ms) from the time it exceeded fifty four hundredths of a volt (0.54 v). If the signal at four milliseconds (4 ms) has not reached nine tenths of a volt (0.9 v), then it should remain above fifty four hundredths of a volt (0.54 v) for at least four milliseconds (4 ms) from the time it exceeded seventy eight hundredths of a volt (0.78 v). If the signal exceeds nine tenths of a volt (0.9 v), then it should remain above seventy eight hundredths of a volt (0.78 v) for one and one tenth milliseconds (1.1 ms) from the time it exceeded nine tenths of a volt (0.9 v). 
     The high frequency signals are analyzed, during two of the previously mentioned time periods: the first, which represents the sound of the impact and the glass shattering, from t 0  until t=35 ms, and the second, which represents the shattering of the glass, from t=x 1  until t=35 ms. 
     During the first or overall time period, from t 0  until t=35 ms, the sum of the pulse widths, less the thirty seven microseconds (37 μs.) subtracted by discriminator 42, should not exceed the threshold of six and eighty six hundredths milliseconds (6.86 ms), blocks 86 and 88. 
     During the second time period, from t=x 1  until t=35 ms, the high frequency signal is analyzed for individual pulse widths greater than one and one tenth milliseconds (1.1 ms), block 86,. Again the signal is analyzed after removal of thirty seven microseconds (37 μs.) by discriminator 42. Exceeding the threshold again rejects the signal because the high frequency signal is not indicative of shattering. 
     Assuming all of the noted conditions are met, the events indicative of glass breaking have occurred, and the detector will issue an alarm signal lasting three seconds, block 90. 
     FIG. 4(A-E) is a schematic diagram of the preferred embodiment including circuits and components for carrying out the invention. FIG. 4 is described here in connection with FIG. 1. 
     The input section 12 and low frequency channel 14 are illustrated in FIG. 4A and 4B. The microphone 24 and preamplifier 26 define the input section, while amplifier 28, threshold detector 30 and threshold level generator 32 define the low frequency channel. The amplifier 28 has two stages that are inverting operational amplifiers 102 and 104 coupled in series and configured to amplify signals in a frequency range from approximately ten hertz to approximately fifty three hertz (10-53 Hz), thereby acting as a band-pass filter. The noise Riding circuit 29 (FIG. 4B) reduces false alarms from cyclical or repetitive background noise, as described earlier. Its components are selected to achieve detect peaks within a tenth of a millisecond (0.1 ms) and have a time constant of twenty two milliseconds (22 ms). The threshold detector 30 provides a logical zero as an output signal in lead 108 when the input signal exceeds the active threshold. The active threshold includes eight values, as described above, and is set by the threshold level generator 32 under the control of the microcontroller 20 through leads 110 and 112. 
     The high frequency channel 16 is depicted beginning on FIG. 4C. The high frequency amplifier 34 is coupled to the output 113 of the preamplifier 26 and includes two stages of amplification 14 and 116 acting as a band-pass filter for frequencies ranging from approximately thirteen and two tenths kilohertz to approximately eighteen and six tenths kilohertz (13.2 kHz-18.6 kHz). 
     FIG. 4D depicts the high frequency threshold detector 36, which receives signals on lead 118 from amplifier 34, threshold reset device 38, and provides an output to the signal analysis section 18. The threshold detector 120 is a comparator with a threshold level set by reset device 38 under the control of the microcontroller 20 initiated from a signal on lead 122. The signal threshold initially is set at half of a volt (0.5 v). When a high frequency signal is detected at this threshold, a timing signal generator is triggered in microcontroller 20 through lead 40. This is time t 0  depicted on FIG. 2. After the high frequency signal is detected, however, the threshold at detector 36 is lowered to substantially zero for use during signal analysis in section 18. 
     Signal analysis section 18 includes the discriminators 42 and 44 and the integrator 46. Discriminator 42 is defined by capacitor C19 (at the output of threshold detector 120, resister R50, reset device 38, and two comparators U6-4 and U6-1. Detector 36 output drops from five volts (5 v) to zero volts (0 v) when high frequencies are first detected at a threshold of half of a volt (0.5 v). The threshold is then dropped to substantially zero volts (0 v) as noted above. After the high frequency signal drops below the zero volt (0 v) threshold, capacitor C19 begins to charge through resistor RS0, with the rate of charge determined by the values of the capacitor and resistor. The charge continues to build until the high frequency signal swings positive, exceeds the substantially zero threshold and is driven to zero (0 v) again by threshold detector 36. The threshold level (1.534 v) at comparators U6-4 and U6-1 is chosen so the charge on the capacitor will exceed the threshold level in thirty seven microseconds (37 μs) at the predetermined charge rate. This process continues throughout the analysis period, and has the effect of shortening the signal pulse widths by thirty seven microseconds (37 μs) and, of course, eliminating any pulse widths shorter than thirty seven microseconds (37 μs). 
     Pulse width discriminator 44 includes capacitor C24(at the output of threshold detector U6-1), resistor R64 and comparator U6-2, and works similar to discriminator 42, except the respective capacitor, resistor and threshold values are chosen for a pulse width of one and one tenth milliseconds (1.1 MS). The output of discriminator 44 is one of the two inputs to OR gate S0. 
     Integrator 46 includes capacitor C23 (FIG. 4D at the output of threshold detector U6-4) and comparator U6-3. It sums the pulse widths for identifying a sum over six and eighty six milliseconds (6.86 ms). Its output is the other input to OR gate 50. 
     FIG. 4E illustrates the microcontroller 20, which includes the timing signal generator, relay driver 52 and alarm relay 54. Other components presented on FIG. 4, but not part of the present invention, are testing circuits 152 and low voltage drop out circuit 154. 
     It should now be apparent that a glass break detector according to the invention filters out repetitive or symmetrical signals in the same frequency bands that are used to detect glass breaking events, particularly those frequencies that are characteristic of the glass flexing. The sinusoidal signal produced by a motor or low frequency vibration, for example, is nulled before further processing. Transient signals typical of glass breaking events, on the other hand, pass through the filter for processing and detection. 
     While the invention is described with particular reference to a preferred embodiment, including specific circuits, frequencies and time durations, other modifications and applications will occur to those skilled in the art. It is intended that the claims cover all such modifications and applications that do not depart from the true spirit and scope of the invention.