CA2183868C - Siren detector - Google Patents
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- G08—SIGNALLING
- G08G—TRAFFIC CONTROL SYSTEMS
- G08G1/00—Traffic control systems for road vehicles
- G08G1/07—Controlling traffic signals
- G08G1/087—Override of traffic control, e.g. by signal transmitted by an emergency vehicle
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Abstract
An improved siren detector for detecting siren sounds which process at known warble rates, such as yelp, wail, and high-low, within a selected frequency band. A transducer detects the siren sounds and produces a corresponding electrical output signal. This electrica l output signal is filtered to reject signals outside of the selected siren frequency band. The signal is processed to determine the amplitud e of the electrical signal, and hence the sound level of the siren sound at the trans ducer input. This signal is also processed by an amplitu de limiter and frequency discriminator to determine the instantaneous frequency of the siren sound. This discriminator is followed by a non-lin ear filter to remove the FM clicks characteristic of siren sounds having a low s ignal to noise ratio. Selection filters are used to anal yze the precession rates, maximum frequency, minimum frequency, and shape of the pro cession characteristic to classify the siren as to its t ype, such as yelp, wail, and high-low. A sound which meets the selection criteria and has a sound level above a predetermined threshold c auses the siren detector to trigger signal which drives a preempt output. This pre empt output signal is input to a traffic light control s ystem. This alerts the traffic light control system to control thePedestrian Walk/D on't Walk and traffic lights to cause pedestrians to cl ear the intersection and to provide a preemptive traffic control signal to a vehicle eqquipped with the appropriate siren.
Description
SIREN DETECTOR
Field of the Invention This application pertains to an improved siren detector for detecting siren sounds which precess with known characteristics within a selected frequency band. By detecting siren sounds emitted by an emergency vehicle, the siren detector facilitates preemptive control of traffic lights to enable a vehicle equipped with the appropriate siren to pass through an appropriately equipped intersec-tion on a priority basis.
Background of the Invention The prior art has evolved various ways of con trolling or ~~pre-empting~~ vehicle traffic lights to stop traffic at an intersection so that an emergency vehicle may pass unimpeded through the intersection on a priority basis. One technique involves the placement of a special transmitter on each emergency vehicle which is to be allowed priority passage through intersections. The traffic light controllers at each preemptable intersection are equipped with a receiver which receives signals trans-mitted by the transmitter and there upon actuates the traffic lights to stop the normal flow of traffic. However this technique is relatively expensive and is cumbersome in that the personnel in the emergency vehicle must manually actuate the transmitter in order to control the traffic light.
Traf f is light controllers at preemptable inter sections have also been equipped with detectors capable of detecting flashing lights (normally special strobe lights) mounted on each emergency vehicle which is to be allowed priority passage through the preemptable intersections. In essence, this is similar to the system mentioned in the preceding paragraph, in that the emergency vehicle light replaces the special transmitter. The system does however enjoy something of a cost and utility advantage over the system mentioned in the previous paragraph, since emergency vehicles are normally equipped with flashing lights which 2i 838b8 _ - 2 -are actuated in emergency situations. However, the cost advantage diminishes if special lights must be provided in order to actuate the detector circuitry which interfaces with the traffic signal controller. Moreover, the inven-tors believe that such systems are susceptible to false alarm triggering because, so far as the inventors are aware, there are no regulations regarding the use of flashing lights on non-emergency vehicles. Accordingly, private vehicles may disrupt such systems by equipping their vehicles with flashing lights for the express purpose of actuating the detectors which interface with the traffic light controllers. Perhaps a more serious situation is one in which flashing lights used in advertising signs, commer-cial window displays, and decorative lighting may falsely trigger the detector. This is most prominent in dense urban areas, which is precisely the area that the preemp-tive traffic light signalling system is meant to provide reliable triggering and afford the emergency vehicle the shortest possible response time to its destination.
In the inventors' view a better solution is to devise circuitry capable of detecting the sounds produced by emergency vehicle sirens. There is clear cost advantage to this approach, in that emergency vehicles are conven-tionally equipped with sirens (ie. the emergency vehicles do not need to be equipped with additional special purpose equipment) and a utility advantage in that such sirens are normally activated in emergency situations (i.e. no separ-ate manual actuation of additional special purpose equip-ment is required). A further advantage is that regulations do exist which prohibit the use of sirens on non-emergency vehicles.
The prior art has evolved a number of circuits for detecting siren sounds (see for example published European Patent Application No. 318,668 and United States Patent No. 4,956,866). However, the inventors consider these to be problematic in that they are susceptible to false alarm triggering by sounds emanating from sources other than emergency vehicle sirens. They also provide unreliable detection of siren signals that have a relative-ly long period as well as very long detection times. The present invention provides an improved siren detector for reliably detecting siren sounds within a selected frequency band and having superior immunity to false alarm triggering by sounds emanating from sources other than emergency vehicle sirens, and having superior ability to detect siren sounds in the presence of high ambient noise levels, and detecting siren signals which have a relatively long period in a short period of time.
The invention is based on the observation that the majority of siren sounds are characteristic of a frequency modulated (or FM) waveform in which the frequency is modulated with a very characteristic and periodic waveform. By using techniques common to radio receiver engineering, it is possible to use traditional FM detection schemes to obtain a very accurate estimate of the frequency modulation waveform. This allows simple pattern recogni-tion to be applied to this modulation waveform and accurate recognition of various waveform patterns to be made. In addition, the ability of the FM detection scheme yields a great increase in the ability of this invention to detect sirens in very high noise levels. With the low cost, high degree of functional integration, and ease of reprogramming for different algorithms and parameters, Digital Signal Processing (DSP) techniques lend themselves to such a siren detection system.
Summary of the Invention The invention provides a siren detector for detecting siren sounds which precess at known rates within a selected frequency band to facilitate preemptable control of traffic light signals to enable an emergency vehicle to pass through a traffic intersection on a priority basis.
A transducer detects the siren sounds and produces a corresponding electrical sound output signal. The sound output signal is then filtered to produce an antialiased output signal which prevents aliasing in a subsequent analog to digital conversion process. The antialiased output signal is then bandpass filtered to reject signals outside the selected frequency band. A limiter-discrimina-tor then performs a constant amplitude scaling of the amplitude components of the filtered antialiased output signal to produce an output signal having frequency and amplitude-scaled components and subsequently removes the amplitude-scaled component, leaving only the desired fre-quency component.
A click filter removes impulsive noise components from the frequency component output by the limiter-discriminator, to produce a filtered discriminator output signal. A sound level detector responsive to the bandpass filtered signal produces a sound level signal indicating that a sound level within the selected frequency band at the input transducer exceeds a selected sound intensity level. A squelch detector is provided to indicate whether a signal to noise level within the selected frequency band at the input transducer exceeds a selected signal to noise level.
A detector responsive to the frequency component output by the limiter-discriminator measures the siren sound's period and indicates whether the period is within a selectable range. Another detector responds to the frequency component output by the limiter-discriminator by measuring the siren sound's frequency and indicating whether the measured frequency is within a selectable range. A means is also provided for measuring the siren sound's rate of change of frequency and for indicating whether the measured rate of change of frequency is within a selectable range.
A means is also provided to determine a correla tion coefficient providing a measure of correlation between the precession rate of the siren sound and a straight line, and for indicating whether the correlation coefficient exceeds a selectable value.
A preempt control means produces a preempt output signal for preempting control of the traffic light signals when the siren sound increases in level above a selectable threshold and for deactivating the preempt output signal when the siren sound decreases in level below a selectable threshold. The preempt output signal is held in an enabled state for a selectable period of time.
The invention may be implemented as a program-mable signal processor operated according to a computer program. The programmable signal processor may have a com-munications port allowing the computer program or user selectable operating parameters to be externally loaded from an external source, which may be remotely located.
Brief Description of the Drawings Figure 1 is a block diagram illustrating the basic operation of the siren detector according to the invention.
Figure 2a and 2b are diagram illustrating the basic configuration of a four channel siren detector at a street intersection, and the configuration of a plurality of siren detectors.
Figure 3 is a block diagram illustrating the limiter discriminator of the siren detector according to the invention.
Figure 4a, 4b, and 4c are diagrams illustrating the ideal characteristic signals of three of the many common types of siren sound which are detected when pro cessed in accordance with the preferred embodiment of the invention.
Figures 5, 6, and 7 are diagrams illustrating the typical actual characteristics of three of the many common types of siren sound which are detected when processed in accordance with the preferred embodiment of the invention.
These are the yelp, high-low and wail respectively.
Figure 8 is a diagram illustrating the effect of the click filter in removing the FM clicks from the re-ceived signal when processed in accordance with the pre-ferred embodiment of the invention.
Figure 9 is a diagram illustrating the operation of the median filter, used as the click filter.
Figure 10 is a detailed diagram of a generalized siren detector used for classifying a sound as being one of a number of desired siren types.
Figure 11 is a block diagram of a noise operated squelch detector.
Figure 12 is a diagram depicting the means for measurement of the waveform period for yelp and high-low sirens.
Figure 13 is a diagram depicting an alternate means for measurement of the high-low siren.
Figure 14 is a diagram depicting the means by which a wail siren sound is detected using the linear least squares fit of a short line segment to the sampled siren data.
Figure 15 is linear correlation coefficient plot for a linear least squares fit to a wail siren. This is the "linearity coefficient" output of the slope detector.
Figure 16 is the signal slope output of the slope detector, which~gives the rate of change of frequency of the siren signal, for a wail siren.
Figure 17 is a block diagram of the siren de-tector showing the preferred embodiment.
Detailed Description of the Preferred Embodiment Emergency vehicle sirens commonly emit sounds which precess between two frequencies, the minimum and maximum frequencies, with known repetition rates and characteristics. Three of the more common siren sound are commonly referred to as the yelp, high-low, and wail. The ideal characteristics are shown in Figures 4a, 4b, and 4c respectively. Ideally, the siren has a constant intensity ~":..s.
~ia3a6a -as the signal precesses according to these siren character-istics, and others. A yelp siren sound typically has a minimum frequency of 400 Hz, a maximum frequency of 1400 Hz, and a repetition rate of about 3 Hz. A high-low siren sound typically has a minimum frequency of 400 Hz, a maximum frequency of 600 Hz, and a repetition rate of about 1 Hz. A Wail siren sound typically has a minimum frequency of 400 Hz, a maximum frequency of 1400 Hz, and a repetition rate of about 0.25 Hz. Other siren sounds exist, and new ones may be defined, which may also be detected by this invention using the method described in this invention.
Fig. 1 is a block diagram which illustrates the basic operation of a siren detector constructed in accord ance with the invention. A brief overview of the invention will first be provided with reference to Fig. 1. A de tailed description of the preferred embodiment will then be provided.
With reference to Fig. 1, the siren detector utilizes an input transducer 1 to detect sound energy and convert those to electrical signals suitable for processing by the siren detector. These electrical signals are am-plified to some nominal level for processing. The preamp-lifier 2 is followed by an anti-aliasing filter 3 prior to the analog to digital convertor 4 which converts these analog electrical signals to a digital form for subsequent processing. An analog to digital convertor with a resol-ution of 12 to 16 bits and a sampling rate of 8.0 kHz has been found to be suitable for processing the wail, yelp, and high-low sirens described so far. A band pass filter 5 with a passband from about 300 Hz to 1800 Hz has been found to suitable for wail, yelp, and high-low sirens. The sampling rate would have to be increased above 8.0 kHz if sirens with maximum frequencies much higher than those discussed so far are to be sampled without aliasing. The digital bandpass filter 5 is used to remove spectral energy outside of the band found in the wail, yelp, and high-low detectors. A passband of 300 Hz to 1800 Hz has been found - 21~386~
-8_ to suitable for these sirens . Those skilled in the art will realize that the bandpass filter 5 can be combined with the phase splitter required for the limiter-discrim-inator 6 described in Fig. 3, thus reducing the overall complexity of these two functions. The limiter-discrim-inator 6 measures the instantaneous frequency of the received signal and the magnitude of that signal. Because the spectral components of the frequency output of the limiter-discriminator, representing the precession of the siren signal, are so low for wail, yelp, and high-low sirens, the output sample rate of the limiter-discriminator vastly exceeds that required. For this reason, the limit-er-discriminator output signal sampling rate is reduced by the decimator 7 to a much lower sample rate. A decimation of 8.0 kHz to 40 Hz has been found to be suitable. Since the actual spectral content of the sirens variation of frequency with time, as shown in Figs. 5, 6, and 7, is typically less than about 15 Hz, the sample rate after the low pass filter in the decimator need only really be greater than about 30 Hz. This sample rate reduction greatly reduces the processing demands of the subsequent steps.
Another key advantage of this low pass filter operation is that it allows the limiter-discriminator detector to be operated essentially as a wideband frequency modulation detector. This allows the great improvement in siren detectability over conventional means. As is the case with conventional FM receivers of the type discussed by Jakes in "Microwave Mobile Communications" (John Wiley & Sons, 1974 ISBN 0-471-43720-4) it can be shown as the ratio of the input signal bandwidth at the input transducer 1 to the baseband output of the limiter discriminator 6 increases, the baseband output signal to noise ratio increases for the same input signal to noise ratio. The input bandwidth of the detector is defined by the input signal bandpass filter, which is about 1500 Hertz, and the low pass filter following the limiter-discriminator, which is about 15 Hertz. The performance gains of wideband versus narrowband FM detection are discussed in great deal by Jakes (supra). It is this detection scheme which allows sirens to be detected reliably in condition with signal to noise ratios as low as -2 dB, whereas conventional detec-tion means typically require a signal to noise ratio of about 6 dB or higher. This invention provides approximate-ly 8 dB gain over conventional means.
It is a characteristic of discriminator type detectors that an FM modulated waveform, such as the siren sounds, produce impulse noise or "clicks" when the signal to noise ratio of the sound is low. This occurs when a siren sound is some considerable distance from the input transducer, or the background sound level in the vicinity of the input transducer is very high. In any case, these "clicks" create a problem when trying to classify siren sounds belong to one class of a number of classes of sirens. In Fig. 7, the actual limiter-discriminator fre-quency output signal for a wail siren with a low signal to noise ratio is shown. The clicks are clearly evident at about 1.5 seconds and 6.3 seconds elapsed time in the figure. A click filter 8 as shown in Fig. 1 can very effectively remove these clicks from the limiter-discrim-inator frequency output signal. The same input signal in Fig. 7 when processed by this click filter results in a median filter output as shown in Fig. 8, where the clicks are seen to be removed. It has been found that a "Median Filter" with a length of 9 samples or about 0.225 seconds time duration is quite effective at removing these clicks.
Longer duration Median filters could be used, but they show no substantial improvement in performance.
The output of the click filter 8 in Fig. 1 serves as an input to a plurality of detectors. In this case, they are yelp detector 9, High-Low detector 10, and wail detector 11. One of more "Other Siren Detectors" 12 may be added to detect additional siren types, or replace any or all of the yelp, high-low, and wail siren detectors. These detectors determine if the variation of the signal fre-quency with time meets a number of criteria which classify it as one of a number of siren types which the siren detector has been configured to detect. The outputs) of these detectors serve as one of a number of inputs to the Preempt Detection Logic 15. The preempt detection logic uses the outputs from the siren detectors 9, 10, 11, 12, the squelch detector 13, and the sound level detector 14 to determine if the sound detected meets the siren detec-tion criteria. If they do meet the selection criteria, then the PREEMPT signal to the traffic light controller is enabled.
The output of the Bandpass Filter 5 in Fig. 1, typically with a passband from about 300 Hz to about 1500 Hz., is a signal whose amplitude is a function of the siren loudness or level at the input transducer 1. Since sirens maintain an approximately constant output level and the sound level at 1 increases with decreasing distance between the siren and the input transducer, the signal level at 5 is a function of the distance between the input transducer and the siren. The signal at 5 is input to the Sound Level Detector 14 which measures the magnitude of the that signal and compares it against a preset level threshold. If the magnitude of the signal at 5 exceeds the level threshold, it enables the output of the Sound Level Detector. If the magnitude of the signal at 5 does not exceeds the level threshold, it disables the output of the Sound Level Detector. The output of the sound level detector serves as one of the inputs to the Preempt Detection Logic 15.
In some situations the ambient sound level from sources other than sirens, such as that due to traffic noise from tires, engine noise, industrial noise, aircraft engine noise, etc., may be so loud that these levels exceed the detection level threshold of the Sound Level Detector 14. In this situation, the output of the Sound Level Detector 14 would always be enabled and the siren would cause the Preempt Detection Logic 15 to cause a PREEMPT
s signal sooner than is desired. By utilizing a conventional squelch detector, an additional signal which is a function of the signal to noise ratio is available. The squelch detector is configured such that a threshold signal to noise ratio must be exceeded before the squelch detector output is enabled to indicate this detection criteria has been met.
The PREEMPT detection logic 15 uses combinations of the squelch detector 13 output in addition to the siren detector functions, shown in 9, 10, 11, and 12 and the sound level detector 14 of Fig. 1. In normal urban and suburban situations, the PREEMPT detection logic 15 would only enable the PREEMPT output to the traffic light con-troller when; (a) the sound reaching then input transducer 1 meets one of the valid siren selection criteria of siren detector functions shown in 9, 10, 11, and 12, and (b) the sound reaching then input transducer 1 exceeds the detec-tion threshold criteria of the sound level threshold detector 14. For very noisy environments, the PREEMPT
detection logic 15 would only enable the PREEMPT output to the traffic light controller when; (a) the sound reaching then input transducer 1 meets one of the valid siren selection criteria of siren detector functions shown in 9, 10, 11, and 12, and (b) the sound reaching then input transducer 1 exceeds the detection threshold criteria of the sound level threshold detector 14, and (c) the signal to noise ratio measured at the output of the limiter-dis-criminator 6 measured by the squelch detector 13 exceeds a squelch detection threshold.
Fig 2 (a) shows a typical installation with a traffic light 26, four input transducers 21, 22, 23, and 24 mounted such that they are optimized for detection of sound from one of the four streets which approach the traffic signal 26. The output signals from these transducers go to a four channel siren detector 20 which processes the signals from the input transducers. If an emergency vehicle 25 approaches in the direction of input transducer 24, the channel in the siren detector processing that signal will indicate a PREEMPT signal to the traffic Light Controller 30 for that direction of the traffic light 26 using the traffic light preempt line 31, and/or the pedes-trian control preempt line 32. The Traffic Light Control-ler could then be configured to give the emergency vehicle 25 priority access to the intersection. As indicated in Fig. 2(b), the siren detector can consist of a plurality of siren detector channels ranging from 1 to many. However, 4 channels is the most common. Single channel detectors could be to control lights at the driveway to fire halls, police compounds, pedestrian controlled lights. etc.
Fig. 3 shows one means for realizing a lim iter-discriminator. The input signal is split into its real and imaginary components by the phase sputter 40.
The complex conjugate and first derivative of the phase splitter output are formed by 41 and 42 respectively. The product of the complex conjugate and first derivative is taken, as well as multiplied by -j - -~-1. The real part of this product is taken by 44. The power of the input signal is determined by taking the magnitude of the phase splitter output in 46, and then squaring this signal in 47.
The frequency of the input signal is then calculated by dividing in block 45 the output of 44 by the output of 47.
The output of 47 also serves as the input to the sound level detector 14 in Fig. 1.
Fig. 4 (a), (b), and (c) show the ideal frequency versus time characteristics of the three most common sirens, these being the yelp siren, high-low siren, and wail siren respectively. In actual practice, the sirens characteristics are quite different. Fig. 5 shows the frequency versus time characteristic of a yelp siren. Fig.
6 shows the frequency versus time characteristic of a high-low siren. Fig. 7 shows the frequency versus time characteristic of a wail siren. In these three examples, the frequency was measured with actual sirens using the limiter-discriminator shown in Fig. 3.
2j83868 The Median filter is commonly used in image processing to remove impulsive noise. It operates by assembling an odd number of sequential data samples, sorting the samples in ascending or descending order, and then extracting the medial value. It operates in much the same way as sliding window finite impulse response filter, except that it is quite non-linear in nature. The use of the click filter is necessary for the detection of siren sounds where the signal to noise ratio is low. Fig 8 shows the effect of the median filter on an actual wail siren signal having a low signal to noise ratio. The input signal is shown in Fig 7. Using the example of the median filter shown in Fig. 9, the operation of the median filter can be easily demonstrated. The input samples 50 are serially shifted into the input shift register 51. They are sorted in ascending ( or descending ) order by the sorter 52 and reassembled in ascending ( or descending ) into the output register 53. From the output register 53, the medial value is taken and used as the output. In the example shown, the sampled data sequence in the register 51 is 1, 4, 6, 2, 9, 8, 5, 7, and 3. From this sequence, the median filter selects 5 as the medial value. If a new input sample with a value 11 was input into the shift register 51, the end value 3 would be discarded and the input shift register 51 contents would become 11, 1, 4, 6, 2, 9, 8, 5, and 7. These would result in the output shift register contents becoming 1, 2, 4, 5, 6, 7, 8, 9, il after sorting. The medial value output by the filter 54 would be 6 in this case.
Three basic types of sirens detectors are used for the detection of most sirens . The main obj ective of these schemes is to provide a low probability of false detection, fairly fast detection and classification time of about 2 to 3 seconds maximum, and sufficient flexibility to accommodate variations in the siren characteristics. A
common core siren detector is shown in Fig. 10, serving as the basis for the detection of yelp, wail, high-low, and other siren types.
The first of these is the most general and is suitable for yelp siren, although other siren types could also be detected. It simply sets a frequency threshold comparator 61 with a frequency threshold f~.esh midway between the minimum and maximum frequencies expected for a yelp siren, which is about 900 to 1000 Hertz. The period between times when the increasing frequency wave shape crosses the threshold for two successive threshold crossing is measured by 62. If this period falls within the user selected range for valid yelp sirens which is typically 0.27 seconds to 0.40 seconds, and the frequency of the siren signal is greater than a selectable minimum frequency f";", and less than a selectable maximum frequency f~x, a counter is incremented. The frequency comparators 63 and 64 are used for the purpose of frequency comparison. If the next period is measured to be within the user selected region, the counter is incremented again. If the next period is measured to be outside of the user selected range, the counter is decremented. The counter minimum value is 0. If the counter level exceeds a user selected threshold, typically 3 or 4 for reliable detection, then the yelp detector output is enabled to indicate that a siren meeting the yelp detection has been detected. It should be apparent that the sense of the change in fre-quency from an increasing in time sense to a decreasing in time sense in relation to the frequency threshold crossings is also possible within the context of this invention.
This means may also be used for the high-low siren type, since this siren type is characterized by its periodic two frequency characteristic. The period measurement technique is shown in Fig. 12.
The second of these is also suitable for high-low siren, although other siren types could also be detected.
It simply sets a frequency difference threshold midway between the difference of the minimum and maximum fre quencies expected for a high-low siren, which is about 100 to 150 Hertz. The frequency comparator 61 is then used to determine if the step in frequency between the low tone and the high tone exceeds some threshold f~,iesh. The period between times when the increasing frequency wave shape crosses the threshold for two successive increasing fre-quency crossings is measured. If this period falls within the user selected range for valid yelp sirens which is typically 1.00 seconds to 1.3 seconds, and the frequency of the siren signal is greater than a selectable minimum frequency f,n;" and less than a selectable maximum frequency fm~, a counter is incremented. The frequency comparators 63 and 64 are used for the purpose of frequency comparison.
If the next period is measured to be within the user selected region, the counter is incremented again. If the next period is measured to be outside of the user selected range, the counter is decremented. The counter minimum value is 0 and typically has a maximum value of less than 20. If the counter level exceeds a user selected thresh-old, typically 3 or 4 to reliable detection, then the high-low detector output is enabled to indicate that a siren meeting the high-low detection has been detected. It should be apparent that the sense of the change in fre-quency from an increasing in time sense to a decreasing in time sense in relation to the frequency threshold crossings is also possible within the context of this invention. The period measurement technique is shown in Fig. 13.
The third siren detector type is for the wail siren. This siren type is characterized by a very long period of between 4.8 and 7.2 seconds. It is readily apparent that if three to four complete cycles of a wail waveform were to be detected before the wail detect output were enabled, a detection time of about 15 or 20 seconds to 22 to 29 seconds would be required. This greatly exceeds the desired 2 to 3 seconds detection time. In fact, a siren equipped vehicle could easily be passed the intersec-tion before the siren would have been detected. This ~1838b8 highly undesirable situation is alleviated by observing the fact that the frequency characteristic is more or less a triangle wave with fairly straight portions to the curve.
The Wail siren detector uses this fact, and uses a short duration sliding window of about 1.0 seconds in duration to perform a linear least squares fit to the sampled frequency data. A linear equation of the form:
f = mt + b is fit to a 1.0 second sequence of data samples, number 40 for the siren detector being discussed. In this equation, f is the frequency, t is the time, m is the slope of the line or rate of change of frequency, and b is the intercept frequency at t - 0.0 seconds. Also calculated is the linear correlation coefficient of the fit between the straight line segment and the samples of data. One way of calculating this linear correlation coefficient for N
samples of data, with N being 40 in this case, is using the following equation:
N~t; fi - Frtq ~. f;
r =
[ NF.ti2 - (F~t;) 2 ] 1~ [ NF.f;2 - (Efi) 2 ]''~
where f; is the frequency taken at time ti and N is the number of samples used in the linear fit. The value of r ranges from 0 where there is no correlation, to ~1 where there is complete correlation. The sign of r in this case is the same as that of the slope m, but it is only the magnitude r that is important and not the sign.
This linear least squares fit to the waveform and the frequency at any part of the waveform provide three classification criteria for the wail siren. These criteria are; (1) the frequency of the waveform must be with the user specified minimum and maximum frequencies as deter-mined by comparators 63 and 64, (2) the rate of change of the frequency with time or slope of the straight line portion of the curves must fall within two user defined ranges, typically between ~300 Hz/sec to ~ 500 Hz/sec, as determined by the slope detector 65, and (3) the goodness of fit or correlation coefficient of the piecewise linear line segment to the frequency waveform as determined by the slope detector 65, with the magnitude of a good linear correlation coefficient typically being between 0.95 and 1Ø If the siren meets all three of these criteria, it can be reliably classified as a wail siren type. Typical detection times using this technique are the order of 2 to 3 seconds, making it as reliable as the yelp siren detec-tion technique. The slope measurement technique is shown in Fig. 14. The slope m of the wail siren sound shown in Fig 8 is shown in Fig. 15, and the linear correlation coefficient r is shown in Fig. 16. In this example, the sample rate was 40 Hertz and 40 sample points were used for the linear fit. This fit was performed at a rate of 40 Hertz.
One common type of squelch detector is based on a noise operated squelch detector. This detector provides a signal which is a function of the baseband SNR of the limiter-discriminator output. It is described in detail by Rhode and Ulrich in "Communications Receivers: Principles & Design", McGraw-Hill Book Company, 1988. The operation of these noise detectors is based on the fact that as the carrier to noise ratio increases, the baseband noise energy density decreases. This detector used for this purpose is shown schematically in Fig. 11. The output of the 1.5 kHz to 1.8 kHz bandpass filter is "full-wave rectified" by the Absolute value block. This output is then filtered by a simple low pass filter with a bandwidth of about 10 Hertz.
The output of this filter is then decimated to a rate of 40 Hertz, reducing the subsequent processing rates. The decimated output, which is a function of the signal to noise ratio of the squelch input signal, is then compared against a user selected threshold and the threshold de tector output enabled when the input signal is below the threshold level.
Those skilled in the art will recognize that the siren detector described in this invention is ideally ~ ~ a3a6a suited for implementation in a programmable computing device or digital signal processor. This has the many advantages over analog implementations, such as little if any effect of temperature on the performance, ease of adapting the siren detector to new siren sounds by repro-gramming rather than modifications to the hardware, the ability to~remotely reprogram the siren detector for new siren sounds, the ability to remotely control the siren detector, etc. This preferred implementation is shown in Fig 17. The input signals from the input transducers are input to the Analog Input Signal Protection, Amplification, and Filtering section 80 to provide electrical transient protection and signal conditioning. The signal processor 81 performs the analog to digital conversions and all of the processing functions described in this invention.
Status indicators provide feedback to users as to the performance of the siren detector, detection of valid siren sounds, siren type, channel number activated, etc. Parame-ter input selectors 84 are provided to allow adjustment of the siren detection parameters locally. An External Programming and Control Input Port 85 is provided to allow local or remote reprogramming of the siren detector to update the software control program, or to locally or remotely change the siren detection parameters.
As will be apparent to those skilled in the art in the light of the foregoing d~i.sclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.
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Field of the Invention This application pertains to an improved siren detector for detecting siren sounds which precess with known characteristics within a selected frequency band. By detecting siren sounds emitted by an emergency vehicle, the siren detector facilitates preemptive control of traffic lights to enable a vehicle equipped with the appropriate siren to pass through an appropriately equipped intersec-tion on a priority basis.
Background of the Invention The prior art has evolved various ways of con trolling or ~~pre-empting~~ vehicle traffic lights to stop traffic at an intersection so that an emergency vehicle may pass unimpeded through the intersection on a priority basis. One technique involves the placement of a special transmitter on each emergency vehicle which is to be allowed priority passage through intersections. The traffic light controllers at each preemptable intersection are equipped with a receiver which receives signals trans-mitted by the transmitter and there upon actuates the traffic lights to stop the normal flow of traffic. However this technique is relatively expensive and is cumbersome in that the personnel in the emergency vehicle must manually actuate the transmitter in order to control the traffic light.
Traf f is light controllers at preemptable inter sections have also been equipped with detectors capable of detecting flashing lights (normally special strobe lights) mounted on each emergency vehicle which is to be allowed priority passage through the preemptable intersections. In essence, this is similar to the system mentioned in the preceding paragraph, in that the emergency vehicle light replaces the special transmitter. The system does however enjoy something of a cost and utility advantage over the system mentioned in the previous paragraph, since emergency vehicles are normally equipped with flashing lights which 2i 838b8 _ - 2 -are actuated in emergency situations. However, the cost advantage diminishes if special lights must be provided in order to actuate the detector circuitry which interfaces with the traffic signal controller. Moreover, the inven-tors believe that such systems are susceptible to false alarm triggering because, so far as the inventors are aware, there are no regulations regarding the use of flashing lights on non-emergency vehicles. Accordingly, private vehicles may disrupt such systems by equipping their vehicles with flashing lights for the express purpose of actuating the detectors which interface with the traffic light controllers. Perhaps a more serious situation is one in which flashing lights used in advertising signs, commer-cial window displays, and decorative lighting may falsely trigger the detector. This is most prominent in dense urban areas, which is precisely the area that the preemp-tive traffic light signalling system is meant to provide reliable triggering and afford the emergency vehicle the shortest possible response time to its destination.
In the inventors' view a better solution is to devise circuitry capable of detecting the sounds produced by emergency vehicle sirens. There is clear cost advantage to this approach, in that emergency vehicles are conven-tionally equipped with sirens (ie. the emergency vehicles do not need to be equipped with additional special purpose equipment) and a utility advantage in that such sirens are normally activated in emergency situations (i.e. no separ-ate manual actuation of additional special purpose equip-ment is required). A further advantage is that regulations do exist which prohibit the use of sirens on non-emergency vehicles.
The prior art has evolved a number of circuits for detecting siren sounds (see for example published European Patent Application No. 318,668 and United States Patent No. 4,956,866). However, the inventors consider these to be problematic in that they are susceptible to false alarm triggering by sounds emanating from sources other than emergency vehicle sirens. They also provide unreliable detection of siren signals that have a relative-ly long period as well as very long detection times. The present invention provides an improved siren detector for reliably detecting siren sounds within a selected frequency band and having superior immunity to false alarm triggering by sounds emanating from sources other than emergency vehicle sirens, and having superior ability to detect siren sounds in the presence of high ambient noise levels, and detecting siren signals which have a relatively long period in a short period of time.
The invention is based on the observation that the majority of siren sounds are characteristic of a frequency modulated (or FM) waveform in which the frequency is modulated with a very characteristic and periodic waveform. By using techniques common to radio receiver engineering, it is possible to use traditional FM detection schemes to obtain a very accurate estimate of the frequency modulation waveform. This allows simple pattern recogni-tion to be applied to this modulation waveform and accurate recognition of various waveform patterns to be made. In addition, the ability of the FM detection scheme yields a great increase in the ability of this invention to detect sirens in very high noise levels. With the low cost, high degree of functional integration, and ease of reprogramming for different algorithms and parameters, Digital Signal Processing (DSP) techniques lend themselves to such a siren detection system.
Summary of the Invention The invention provides a siren detector for detecting siren sounds which precess at known rates within a selected frequency band to facilitate preemptable control of traffic light signals to enable an emergency vehicle to pass through a traffic intersection on a priority basis.
A transducer detects the siren sounds and produces a corresponding electrical sound output signal. The sound output signal is then filtered to produce an antialiased output signal which prevents aliasing in a subsequent analog to digital conversion process. The antialiased output signal is then bandpass filtered to reject signals outside the selected frequency band. A limiter-discrimina-tor then performs a constant amplitude scaling of the amplitude components of the filtered antialiased output signal to produce an output signal having frequency and amplitude-scaled components and subsequently removes the amplitude-scaled component, leaving only the desired fre-quency component.
A click filter removes impulsive noise components from the frequency component output by the limiter-discriminator, to produce a filtered discriminator output signal. A sound level detector responsive to the bandpass filtered signal produces a sound level signal indicating that a sound level within the selected frequency band at the input transducer exceeds a selected sound intensity level. A squelch detector is provided to indicate whether a signal to noise level within the selected frequency band at the input transducer exceeds a selected signal to noise level.
A detector responsive to the frequency component output by the limiter-discriminator measures the siren sound's period and indicates whether the period is within a selectable range. Another detector responds to the frequency component output by the limiter-discriminator by measuring the siren sound's frequency and indicating whether the measured frequency is within a selectable range. A means is also provided for measuring the siren sound's rate of change of frequency and for indicating whether the measured rate of change of frequency is within a selectable range.
A means is also provided to determine a correla tion coefficient providing a measure of correlation between the precession rate of the siren sound and a straight line, and for indicating whether the correlation coefficient exceeds a selectable value.
A preempt control means produces a preempt output signal for preempting control of the traffic light signals when the siren sound increases in level above a selectable threshold and for deactivating the preempt output signal when the siren sound decreases in level below a selectable threshold. The preempt output signal is held in an enabled state for a selectable period of time.
The invention may be implemented as a program-mable signal processor operated according to a computer program. The programmable signal processor may have a com-munications port allowing the computer program or user selectable operating parameters to be externally loaded from an external source, which may be remotely located.
Brief Description of the Drawings Figure 1 is a block diagram illustrating the basic operation of the siren detector according to the invention.
Figure 2a and 2b are diagram illustrating the basic configuration of a four channel siren detector at a street intersection, and the configuration of a plurality of siren detectors.
Figure 3 is a block diagram illustrating the limiter discriminator of the siren detector according to the invention.
Figure 4a, 4b, and 4c are diagrams illustrating the ideal characteristic signals of three of the many common types of siren sound which are detected when pro cessed in accordance with the preferred embodiment of the invention.
Figures 5, 6, and 7 are diagrams illustrating the typical actual characteristics of three of the many common types of siren sound which are detected when processed in accordance with the preferred embodiment of the invention.
These are the yelp, high-low and wail respectively.
Figure 8 is a diagram illustrating the effect of the click filter in removing the FM clicks from the re-ceived signal when processed in accordance with the pre-ferred embodiment of the invention.
Figure 9 is a diagram illustrating the operation of the median filter, used as the click filter.
Figure 10 is a detailed diagram of a generalized siren detector used for classifying a sound as being one of a number of desired siren types.
Figure 11 is a block diagram of a noise operated squelch detector.
Figure 12 is a diagram depicting the means for measurement of the waveform period for yelp and high-low sirens.
Figure 13 is a diagram depicting an alternate means for measurement of the high-low siren.
Figure 14 is a diagram depicting the means by which a wail siren sound is detected using the linear least squares fit of a short line segment to the sampled siren data.
Figure 15 is linear correlation coefficient plot for a linear least squares fit to a wail siren. This is the "linearity coefficient" output of the slope detector.
Figure 16 is the signal slope output of the slope detector, which~gives the rate of change of frequency of the siren signal, for a wail siren.
Figure 17 is a block diagram of the siren de-tector showing the preferred embodiment.
Detailed Description of the Preferred Embodiment Emergency vehicle sirens commonly emit sounds which precess between two frequencies, the minimum and maximum frequencies, with known repetition rates and characteristics. Three of the more common siren sound are commonly referred to as the yelp, high-low, and wail. The ideal characteristics are shown in Figures 4a, 4b, and 4c respectively. Ideally, the siren has a constant intensity ~":..s.
~ia3a6a -as the signal precesses according to these siren character-istics, and others. A yelp siren sound typically has a minimum frequency of 400 Hz, a maximum frequency of 1400 Hz, and a repetition rate of about 3 Hz. A high-low siren sound typically has a minimum frequency of 400 Hz, a maximum frequency of 600 Hz, and a repetition rate of about 1 Hz. A Wail siren sound typically has a minimum frequency of 400 Hz, a maximum frequency of 1400 Hz, and a repetition rate of about 0.25 Hz. Other siren sounds exist, and new ones may be defined, which may also be detected by this invention using the method described in this invention.
Fig. 1 is a block diagram which illustrates the basic operation of a siren detector constructed in accord ance with the invention. A brief overview of the invention will first be provided with reference to Fig. 1. A de tailed description of the preferred embodiment will then be provided.
With reference to Fig. 1, the siren detector utilizes an input transducer 1 to detect sound energy and convert those to electrical signals suitable for processing by the siren detector. These electrical signals are am-plified to some nominal level for processing. The preamp-lifier 2 is followed by an anti-aliasing filter 3 prior to the analog to digital convertor 4 which converts these analog electrical signals to a digital form for subsequent processing. An analog to digital convertor with a resol-ution of 12 to 16 bits and a sampling rate of 8.0 kHz has been found to be suitable for processing the wail, yelp, and high-low sirens described so far. A band pass filter 5 with a passband from about 300 Hz to 1800 Hz has been found to suitable for wail, yelp, and high-low sirens. The sampling rate would have to be increased above 8.0 kHz if sirens with maximum frequencies much higher than those discussed so far are to be sampled without aliasing. The digital bandpass filter 5 is used to remove spectral energy outside of the band found in the wail, yelp, and high-low detectors. A passband of 300 Hz to 1800 Hz has been found - 21~386~
-8_ to suitable for these sirens . Those skilled in the art will realize that the bandpass filter 5 can be combined with the phase splitter required for the limiter-discrim-inator 6 described in Fig. 3, thus reducing the overall complexity of these two functions. The limiter-discrim-inator 6 measures the instantaneous frequency of the received signal and the magnitude of that signal. Because the spectral components of the frequency output of the limiter-discriminator, representing the precession of the siren signal, are so low for wail, yelp, and high-low sirens, the output sample rate of the limiter-discriminator vastly exceeds that required. For this reason, the limit-er-discriminator output signal sampling rate is reduced by the decimator 7 to a much lower sample rate. A decimation of 8.0 kHz to 40 Hz has been found to be suitable. Since the actual spectral content of the sirens variation of frequency with time, as shown in Figs. 5, 6, and 7, is typically less than about 15 Hz, the sample rate after the low pass filter in the decimator need only really be greater than about 30 Hz. This sample rate reduction greatly reduces the processing demands of the subsequent steps.
Another key advantage of this low pass filter operation is that it allows the limiter-discriminator detector to be operated essentially as a wideband frequency modulation detector. This allows the great improvement in siren detectability over conventional means. As is the case with conventional FM receivers of the type discussed by Jakes in "Microwave Mobile Communications" (John Wiley & Sons, 1974 ISBN 0-471-43720-4) it can be shown as the ratio of the input signal bandwidth at the input transducer 1 to the baseband output of the limiter discriminator 6 increases, the baseband output signal to noise ratio increases for the same input signal to noise ratio. The input bandwidth of the detector is defined by the input signal bandpass filter, which is about 1500 Hertz, and the low pass filter following the limiter-discriminator, which is about 15 Hertz. The performance gains of wideband versus narrowband FM detection are discussed in great deal by Jakes (supra). It is this detection scheme which allows sirens to be detected reliably in condition with signal to noise ratios as low as -2 dB, whereas conventional detec-tion means typically require a signal to noise ratio of about 6 dB or higher. This invention provides approximate-ly 8 dB gain over conventional means.
It is a characteristic of discriminator type detectors that an FM modulated waveform, such as the siren sounds, produce impulse noise or "clicks" when the signal to noise ratio of the sound is low. This occurs when a siren sound is some considerable distance from the input transducer, or the background sound level in the vicinity of the input transducer is very high. In any case, these "clicks" create a problem when trying to classify siren sounds belong to one class of a number of classes of sirens. In Fig. 7, the actual limiter-discriminator fre-quency output signal for a wail siren with a low signal to noise ratio is shown. The clicks are clearly evident at about 1.5 seconds and 6.3 seconds elapsed time in the figure. A click filter 8 as shown in Fig. 1 can very effectively remove these clicks from the limiter-discrim-inator frequency output signal. The same input signal in Fig. 7 when processed by this click filter results in a median filter output as shown in Fig. 8, where the clicks are seen to be removed. It has been found that a "Median Filter" with a length of 9 samples or about 0.225 seconds time duration is quite effective at removing these clicks.
Longer duration Median filters could be used, but they show no substantial improvement in performance.
The output of the click filter 8 in Fig. 1 serves as an input to a plurality of detectors. In this case, they are yelp detector 9, High-Low detector 10, and wail detector 11. One of more "Other Siren Detectors" 12 may be added to detect additional siren types, or replace any or all of the yelp, high-low, and wail siren detectors. These detectors determine if the variation of the signal fre-quency with time meets a number of criteria which classify it as one of a number of siren types which the siren detector has been configured to detect. The outputs) of these detectors serve as one of a number of inputs to the Preempt Detection Logic 15. The preempt detection logic uses the outputs from the siren detectors 9, 10, 11, 12, the squelch detector 13, and the sound level detector 14 to determine if the sound detected meets the siren detec-tion criteria. If they do meet the selection criteria, then the PREEMPT signal to the traffic light controller is enabled.
The output of the Bandpass Filter 5 in Fig. 1, typically with a passband from about 300 Hz to about 1500 Hz., is a signal whose amplitude is a function of the siren loudness or level at the input transducer 1. Since sirens maintain an approximately constant output level and the sound level at 1 increases with decreasing distance between the siren and the input transducer, the signal level at 5 is a function of the distance between the input transducer and the siren. The signal at 5 is input to the Sound Level Detector 14 which measures the magnitude of the that signal and compares it against a preset level threshold. If the magnitude of the signal at 5 exceeds the level threshold, it enables the output of the Sound Level Detector. If the magnitude of the signal at 5 does not exceeds the level threshold, it disables the output of the Sound Level Detector. The output of the sound level detector serves as one of the inputs to the Preempt Detection Logic 15.
In some situations the ambient sound level from sources other than sirens, such as that due to traffic noise from tires, engine noise, industrial noise, aircraft engine noise, etc., may be so loud that these levels exceed the detection level threshold of the Sound Level Detector 14. In this situation, the output of the Sound Level Detector 14 would always be enabled and the siren would cause the Preempt Detection Logic 15 to cause a PREEMPT
s signal sooner than is desired. By utilizing a conventional squelch detector, an additional signal which is a function of the signal to noise ratio is available. The squelch detector is configured such that a threshold signal to noise ratio must be exceeded before the squelch detector output is enabled to indicate this detection criteria has been met.
The PREEMPT detection logic 15 uses combinations of the squelch detector 13 output in addition to the siren detector functions, shown in 9, 10, 11, and 12 and the sound level detector 14 of Fig. 1. In normal urban and suburban situations, the PREEMPT detection logic 15 would only enable the PREEMPT output to the traffic light con-troller when; (a) the sound reaching then input transducer 1 meets one of the valid siren selection criteria of siren detector functions shown in 9, 10, 11, and 12, and (b) the sound reaching then input transducer 1 exceeds the detec-tion threshold criteria of the sound level threshold detector 14. For very noisy environments, the PREEMPT
detection logic 15 would only enable the PREEMPT output to the traffic light controller when; (a) the sound reaching then input transducer 1 meets one of the valid siren selection criteria of siren detector functions shown in 9, 10, 11, and 12, and (b) the sound reaching then input transducer 1 exceeds the detection threshold criteria of the sound level threshold detector 14, and (c) the signal to noise ratio measured at the output of the limiter-dis-criminator 6 measured by the squelch detector 13 exceeds a squelch detection threshold.
Fig 2 (a) shows a typical installation with a traffic light 26, four input transducers 21, 22, 23, and 24 mounted such that they are optimized for detection of sound from one of the four streets which approach the traffic signal 26. The output signals from these transducers go to a four channel siren detector 20 which processes the signals from the input transducers. If an emergency vehicle 25 approaches in the direction of input transducer 24, the channel in the siren detector processing that signal will indicate a PREEMPT signal to the traffic Light Controller 30 for that direction of the traffic light 26 using the traffic light preempt line 31, and/or the pedes-trian control preempt line 32. The Traffic Light Control-ler could then be configured to give the emergency vehicle 25 priority access to the intersection. As indicated in Fig. 2(b), the siren detector can consist of a plurality of siren detector channels ranging from 1 to many. However, 4 channels is the most common. Single channel detectors could be to control lights at the driveway to fire halls, police compounds, pedestrian controlled lights. etc.
Fig. 3 shows one means for realizing a lim iter-discriminator. The input signal is split into its real and imaginary components by the phase sputter 40.
The complex conjugate and first derivative of the phase splitter output are formed by 41 and 42 respectively. The product of the complex conjugate and first derivative is taken, as well as multiplied by -j - -~-1. The real part of this product is taken by 44. The power of the input signal is determined by taking the magnitude of the phase splitter output in 46, and then squaring this signal in 47.
The frequency of the input signal is then calculated by dividing in block 45 the output of 44 by the output of 47.
The output of 47 also serves as the input to the sound level detector 14 in Fig. 1.
Fig. 4 (a), (b), and (c) show the ideal frequency versus time characteristics of the three most common sirens, these being the yelp siren, high-low siren, and wail siren respectively. In actual practice, the sirens characteristics are quite different. Fig. 5 shows the frequency versus time characteristic of a yelp siren. Fig.
6 shows the frequency versus time characteristic of a high-low siren. Fig. 7 shows the frequency versus time characteristic of a wail siren. In these three examples, the frequency was measured with actual sirens using the limiter-discriminator shown in Fig. 3.
2j83868 The Median filter is commonly used in image processing to remove impulsive noise. It operates by assembling an odd number of sequential data samples, sorting the samples in ascending or descending order, and then extracting the medial value. It operates in much the same way as sliding window finite impulse response filter, except that it is quite non-linear in nature. The use of the click filter is necessary for the detection of siren sounds where the signal to noise ratio is low. Fig 8 shows the effect of the median filter on an actual wail siren signal having a low signal to noise ratio. The input signal is shown in Fig 7. Using the example of the median filter shown in Fig. 9, the operation of the median filter can be easily demonstrated. The input samples 50 are serially shifted into the input shift register 51. They are sorted in ascending ( or descending ) order by the sorter 52 and reassembled in ascending ( or descending ) into the output register 53. From the output register 53, the medial value is taken and used as the output. In the example shown, the sampled data sequence in the register 51 is 1, 4, 6, 2, 9, 8, 5, 7, and 3. From this sequence, the median filter selects 5 as the medial value. If a new input sample with a value 11 was input into the shift register 51, the end value 3 would be discarded and the input shift register 51 contents would become 11, 1, 4, 6, 2, 9, 8, 5, and 7. These would result in the output shift register contents becoming 1, 2, 4, 5, 6, 7, 8, 9, il after sorting. The medial value output by the filter 54 would be 6 in this case.
Three basic types of sirens detectors are used for the detection of most sirens . The main obj ective of these schemes is to provide a low probability of false detection, fairly fast detection and classification time of about 2 to 3 seconds maximum, and sufficient flexibility to accommodate variations in the siren characteristics. A
common core siren detector is shown in Fig. 10, serving as the basis for the detection of yelp, wail, high-low, and other siren types.
The first of these is the most general and is suitable for yelp siren, although other siren types could also be detected. It simply sets a frequency threshold comparator 61 with a frequency threshold f~.esh midway between the minimum and maximum frequencies expected for a yelp siren, which is about 900 to 1000 Hertz. The period between times when the increasing frequency wave shape crosses the threshold for two successive threshold crossing is measured by 62. If this period falls within the user selected range for valid yelp sirens which is typically 0.27 seconds to 0.40 seconds, and the frequency of the siren signal is greater than a selectable minimum frequency f";", and less than a selectable maximum frequency f~x, a counter is incremented. The frequency comparators 63 and 64 are used for the purpose of frequency comparison. If the next period is measured to be within the user selected region, the counter is incremented again. If the next period is measured to be outside of the user selected range, the counter is decremented. The counter minimum value is 0. If the counter level exceeds a user selected threshold, typically 3 or 4 for reliable detection, then the yelp detector output is enabled to indicate that a siren meeting the yelp detection has been detected. It should be apparent that the sense of the change in fre-quency from an increasing in time sense to a decreasing in time sense in relation to the frequency threshold crossings is also possible within the context of this invention.
This means may also be used for the high-low siren type, since this siren type is characterized by its periodic two frequency characteristic. The period measurement technique is shown in Fig. 12.
The second of these is also suitable for high-low siren, although other siren types could also be detected.
It simply sets a frequency difference threshold midway between the difference of the minimum and maximum fre quencies expected for a high-low siren, which is about 100 to 150 Hertz. The frequency comparator 61 is then used to determine if the step in frequency between the low tone and the high tone exceeds some threshold f~,iesh. The period between times when the increasing frequency wave shape crosses the threshold for two successive increasing fre-quency crossings is measured. If this period falls within the user selected range for valid yelp sirens which is typically 1.00 seconds to 1.3 seconds, and the frequency of the siren signal is greater than a selectable minimum frequency f,n;" and less than a selectable maximum frequency fm~, a counter is incremented. The frequency comparators 63 and 64 are used for the purpose of frequency comparison.
If the next period is measured to be within the user selected region, the counter is incremented again. If the next period is measured to be outside of the user selected range, the counter is decremented. The counter minimum value is 0 and typically has a maximum value of less than 20. If the counter level exceeds a user selected thresh-old, typically 3 or 4 to reliable detection, then the high-low detector output is enabled to indicate that a siren meeting the high-low detection has been detected. It should be apparent that the sense of the change in fre-quency from an increasing in time sense to a decreasing in time sense in relation to the frequency threshold crossings is also possible within the context of this invention. The period measurement technique is shown in Fig. 13.
The third siren detector type is for the wail siren. This siren type is characterized by a very long period of between 4.8 and 7.2 seconds. It is readily apparent that if three to four complete cycles of a wail waveform were to be detected before the wail detect output were enabled, a detection time of about 15 or 20 seconds to 22 to 29 seconds would be required. This greatly exceeds the desired 2 to 3 seconds detection time. In fact, a siren equipped vehicle could easily be passed the intersec-tion before the siren would have been detected. This ~1838b8 highly undesirable situation is alleviated by observing the fact that the frequency characteristic is more or less a triangle wave with fairly straight portions to the curve.
The Wail siren detector uses this fact, and uses a short duration sliding window of about 1.0 seconds in duration to perform a linear least squares fit to the sampled frequency data. A linear equation of the form:
f = mt + b is fit to a 1.0 second sequence of data samples, number 40 for the siren detector being discussed. In this equation, f is the frequency, t is the time, m is the slope of the line or rate of change of frequency, and b is the intercept frequency at t - 0.0 seconds. Also calculated is the linear correlation coefficient of the fit between the straight line segment and the samples of data. One way of calculating this linear correlation coefficient for N
samples of data, with N being 40 in this case, is using the following equation:
N~t; fi - Frtq ~. f;
r =
[ NF.ti2 - (F~t;) 2 ] 1~ [ NF.f;2 - (Efi) 2 ]''~
where f; is the frequency taken at time ti and N is the number of samples used in the linear fit. The value of r ranges from 0 where there is no correlation, to ~1 where there is complete correlation. The sign of r in this case is the same as that of the slope m, but it is only the magnitude r that is important and not the sign.
This linear least squares fit to the waveform and the frequency at any part of the waveform provide three classification criteria for the wail siren. These criteria are; (1) the frequency of the waveform must be with the user specified minimum and maximum frequencies as deter-mined by comparators 63 and 64, (2) the rate of change of the frequency with time or slope of the straight line portion of the curves must fall within two user defined ranges, typically between ~300 Hz/sec to ~ 500 Hz/sec, as determined by the slope detector 65, and (3) the goodness of fit or correlation coefficient of the piecewise linear line segment to the frequency waveform as determined by the slope detector 65, with the magnitude of a good linear correlation coefficient typically being between 0.95 and 1Ø If the siren meets all three of these criteria, it can be reliably classified as a wail siren type. Typical detection times using this technique are the order of 2 to 3 seconds, making it as reliable as the yelp siren detec-tion technique. The slope measurement technique is shown in Fig. 14. The slope m of the wail siren sound shown in Fig 8 is shown in Fig. 15, and the linear correlation coefficient r is shown in Fig. 16. In this example, the sample rate was 40 Hertz and 40 sample points were used for the linear fit. This fit was performed at a rate of 40 Hertz.
One common type of squelch detector is based on a noise operated squelch detector. This detector provides a signal which is a function of the baseband SNR of the limiter-discriminator output. It is described in detail by Rhode and Ulrich in "Communications Receivers: Principles & Design", McGraw-Hill Book Company, 1988. The operation of these noise detectors is based on the fact that as the carrier to noise ratio increases, the baseband noise energy density decreases. This detector used for this purpose is shown schematically in Fig. 11. The output of the 1.5 kHz to 1.8 kHz bandpass filter is "full-wave rectified" by the Absolute value block. This output is then filtered by a simple low pass filter with a bandwidth of about 10 Hertz.
The output of this filter is then decimated to a rate of 40 Hertz, reducing the subsequent processing rates. The decimated output, which is a function of the signal to noise ratio of the squelch input signal, is then compared against a user selected threshold and the threshold de tector output enabled when the input signal is below the threshold level.
Those skilled in the art will recognize that the siren detector described in this invention is ideally ~ ~ a3a6a suited for implementation in a programmable computing device or digital signal processor. This has the many advantages over analog implementations, such as little if any effect of temperature on the performance, ease of adapting the siren detector to new siren sounds by repro-gramming rather than modifications to the hardware, the ability to~remotely reprogram the siren detector for new siren sounds, the ability to remotely control the siren detector, etc. This preferred implementation is shown in Fig 17. The input signals from the input transducers are input to the Analog Input Signal Protection, Amplification, and Filtering section 80 to provide electrical transient protection and signal conditioning. The signal processor 81 performs the analog to digital conversions and all of the processing functions described in this invention.
Status indicators provide feedback to users as to the performance of the siren detector, detection of valid siren sounds, siren type, channel number activated, etc. Parame-ter input selectors 84 are provided to allow adjustment of the siren detection parameters locally. An External Programming and Control Input Port 85 is provided to allow local or remote reprogramming of the siren detector to update the software control program, or to locally or remotely change the siren detection parameters.
As will be apparent to those skilled in the art in the light of the foregoing d~i.sclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.
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Claims (24)
1. A siren detector for detecting siren sounds which precess at known rates within a selected frequency band to facilitate preemptable control of traffic light signals to enable an emergency vehicle to pass through a traffic intersection on a priority basis, said detector comprising transducer means (1) for detecting said sounds and for producing an electrical sound output signal representative thereof, first filter means (3) for filtering said sound output signal to produce an antialiased output signal to prevent aliasing in a subsequent analog to digital conversion process, and second filter means (5) for producing a band-limited signal by filtering said antialiased output signal to reject signals outside said selected frequency band; said detector character-ized by limiter-discriminator means for producing an indication of the frequency of said band-limited signal.
2. A siren detector as defined in claim 1, further comprising a click filter (8) responsive to said frequency indication for removing impulsive noise components therefrom to produce a filtered discrimina-tor output signal.
3. A siren detector as defined in claim 1, further comprising sound level detection means (14) responsive to said band-limited signal to produce a sound level signal indicating that a sound level within said selected frequency band at said transducer means exceeds a selected sound intensity level.
4. A siren detector as defined in claim 1, further comprising squelch detection means (13) responsive to said frequency indication for indicating that a signal to noise level within said selected frequency band at said transducer means exceeds a selected signal to noise level.
5. A siren detector as defined in claim 1, further comprising means (62) responsive to said frequency indication for measuring a period of said siren sound and for providing an indication that said period is within a selectable range.
6. A siren detector as defined in claim 2, further comprising means (62) responsive to said frequency indication for measuring a period of said siren sound and for providing an indication that said measured period is within a selectable range.
7. A siren detector as defined in claim 5, further comprising means (63, 64) responsive to said frequency indication for measuring a frequency of said siren sound and for providing an indication that said measured frequency is within a selectable range.
8. A siren detector as defined in claim 6, further comprising means (63, 64) responsive to said frequency indication for measuring a frequency of said siren sound and for providing an indication that said measured frequency is within a selectable range.
9. A siren detector as defined in claim 1, further comprising means (65) responsive to said frequency indication for measuring a rate of change of frequency of said siren sound, and for providing an indication that said measured rate of change of frequency is within a selectable range.
10. A siren detector as defined in claim 2, further comprising means (65) responsive to said frequency indication for measuring a rate of change of frequency of said siren sound, and for providing an indication that said measured rate of change of frequency is within a selectable range.
11. A siren detector as defined in claim 1, further comprising means (65) for determining a correlation coefficient providing a measure of correlation between the precession rate of said siren sound and a straight line, and for providing an indication that said corre-lation coefficient exceeds a selectable value.
12. A siren detector as defined in claim 2, further comprising means (65) for determining a correlation coefficient providing a measure of correlation between the precession rate of said siren sound and a straight line, and for providing an indication that said corre-lation coefficient exceeds a selectable value.
13. A siren detector as defined in claim 4, further comprising a) sound level detection means (14) responsive to said band-limited signal to produce a sound level signal indicating that a sound level within said selected frequency band at said transducer means exceeds a selected sound intensity level; and b) preempt control means (15) for producing a preempt output signal for preempting control of said traffic light signals in response to said squelch detector, sound level detector, and siren detector signals while said pre-empt control means activates a traffic light control system.
14. A siren detector as defined in claim 7, further comprising means (15) for producing a preempt output signal for input to a traffic light controller when said siren sound increases in level above a selectable threshold and for deactivating said preempt output signal when said siren sound decreases in level below a selectable threshold.
15. A siren detector as defined in claim 8, further comprising means (15) for producing a preempt output signal for input to a traffic light controller when said siren sound increases in level above a selectable threshold, for deactivating said preempt output signal when said siren sound decreases in level below a selectable threshold, and for holding said preempt output signal in an enabled state for a selectable period of time.
16. A siren detector as defined in claim 1, wherein said siren detector is implemented in a programmable signal processor (81) operated according to a computer program, said programmable signal processor having a communications port (85) allowing said computer program to be externally loaded from an external programming source.
17. A siren detector as defined in claim 16, wherein said external programming source is remotely located.
18. A siren detector for detecting, in accordance with user selectable parameters, a siren sound which precesses at a known rate within a selected frequency band to facilitate pre-emptable control of traffic light signals controlled by a traffic light control-ler, to enable an emergency vehicle to pass through a traffic intersection on a priority basis, said de-tector comprising:
(a) transducer means (1) for detecting said sound and for producing an electrical sound output signal representative thereof;
(b) a first filter means (3) for producing an anti-aliased signal by filtering said sound output signal to prevent aliasing in a subsequent analog to digital conversion process;
(c) a second filter means (5) for producing a band-limited signal by filtering said antialiased signal to reject signals outside of the selected frequency band; and (d) a limiter-discriminator (6) for processing said band-limited signal to provide a discriminator output signal having a large number of possible signal levels each of which indicates a different frequency of said sound output signal.
(a) transducer means (1) for detecting said sound and for producing an electrical sound output signal representative thereof;
(b) a first filter means (3) for producing an anti-aliased signal by filtering said sound output signal to prevent aliasing in a subsequent analog to digital conversion process;
(c) a second filter means (5) for producing a band-limited signal by filtering said antialiased signal to reject signals outside of the selected frequency band; and (d) a limiter-discriminator (6) for processing said band-limited signal to provide a discriminator output signal having a large number of possible signal levels each of which indicates a different frequency of said sound output signal.
19. A siren detector as defined in claim 18, wherein said siren detector is implemented in a programmable signal processor (81) operated according to a computer program, said programmable signal processor having a communications port (85) allowing user selectable parameters to be externally loaded from an external programming source.
20. A siren detector as defined in claim 19, wherein said external programming source is remotely located.
21. A siren detector as defined in claim 2, wherein said click filter (8) is a median filter.
22. A siren detector as defined in claim 3, further comprising squelch detection means (13) responsive to said frequency indication for producing a squelch detector signal indicating that a signal to noise level within said selected frequency band at said input transducer exceeds a selected signal to noise level.
23. A siren detector as defined in claim 22, further comprising means (62) responsive to said frequency indication for measuring a period of said siren sound and for providing an indication that said measured period is within a selectable range.
24. A siren detector as defined in claim 23, further comprising preempt control means (15) for producing a preempt output signal for preempting control of said traffic light signals in response to said squelch de-tector, sound level detector, and siren detector signals while said preempt control means activates a traffic light control system.
Applications Claiming Priority (1)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
PCT/CA1994/000119 WO1995024028A1 (en) | 1994-03-04 | 1994-03-04 | Siren detector |
Publications (2)
Publication Number | Publication Date |
---|---|
CA2183868A1 CA2183868A1 (en) | 1995-09-08 |
CA2183868C true CA2183868C (en) | 2002-01-15 |
Family
ID=4173012
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
CA002183868A Expired - Lifetime CA2183868C (en) | 1994-03-04 | 1994-03-04 | Siren detector |
Country Status (7)
Country | Link |
---|---|
EP (1) | EP0748494B1 (en) |
JP (1) | JPH09512902A (en) |
CA (1) | CA2183868C (en) |
DE (1) | DE69411195T2 (en) |
HK (1) | HK1009352A1 (en) |
NZ (1) | NZ262083A (en) |
WO (1) | WO1995024028A1 (en) |
Families Citing this family (4)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
US6133849A (en) * | 1996-02-20 | 2000-10-17 | Unity Wireless Systems Corporation | Control signal coding and detection in the audible and inaudible ranges |
WO2009021902A1 (en) * | 2007-08-16 | 2009-02-19 | Siemens Aktiengesellschaft | Method and device for prioritizing special-purpose vehicles at a traffic light-controlled intersection |
CN102867424B (en) * | 2012-09-26 | 2014-07-30 | 杭州鼎鹏交通科技有限公司 | Area coordinating traffic control method |
EP3753003A1 (en) * | 2018-02-13 | 2020-12-23 | Premel SA | Electronic device for event detection for traffic regulation, and corresponding method |
Family Cites Families (4)
Publication number | Priority date | Publication date | Assignee | Title |
---|---|---|---|---|
GB2027243B (en) * | 1978-07-27 | 1982-10-13 | Marconi Co Ltd | Priority vehicle control of traffic signalling system |
US4864297A (en) * | 1987-10-14 | 1989-09-05 | Tekedge Development Corp. | Siren detector |
US4806931A (en) * | 1988-01-25 | 1989-02-21 | Richard W. Clark | Sound pattern discrimination system |
US4956866A (en) * | 1989-06-30 | 1990-09-11 | Sy/Lert System Ltd. | Emergency signal warning system |
-
1994
- 1994-03-04 JP JP7522583A patent/JPH09512902A/en active Pending
- 1994-03-04 NZ NZ262083A patent/NZ262083A/en unknown
- 1994-03-04 CA CA002183868A patent/CA2183868C/en not_active Expired - Lifetime
- 1994-03-04 WO PCT/CA1994/000119 patent/WO1995024028A1/en active IP Right Grant
- 1994-03-04 EP EP94908234A patent/EP0748494B1/en not_active Expired - Lifetime
- 1994-03-04 DE DE69411195T patent/DE69411195T2/en not_active Expired - Fee Related
-
1998
- 1998-08-13 HK HK98109887A patent/HK1009352A1/en not_active IP Right Cessation
Also Published As
Publication number | Publication date |
---|---|
CA2183868A1 (en) | 1995-09-08 |
AU6152694A (en) | 1995-09-18 |
EP0748494B1 (en) | 1998-06-17 |
NZ262083A (en) | 1998-05-27 |
DE69411195T2 (en) | 1999-02-04 |
WO1995024028A1 (en) | 1995-09-08 |
EP0748494A1 (en) | 1996-12-18 |
JPH09512902A (en) | 1997-12-22 |
AU681380B2 (en) | 1997-08-28 |
HK1009352A1 (en) | 1999-05-28 |
DE69411195D1 (en) | 1998-07-23 |
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