Abstract:
The present invention may be used for detecting sound frequency signals emitted by warning devices of emergency vehicles. A sensor may sense a selected frequency signal spectrum and may be in communication with an initial detection stage to detect a frequency of approximately 600 Hz and approximately 1400 Hz within a specific time interval. A signal identification stage may be in communication with said initial detection stage and may have a digital signal processor programmed to identify a wail and yelp warning signal to compare said warning signal to a histogram stored in a memory to estimate the distance from the sound frequency signal. The signal identification stage may be maintained in a power save mode until activated by said initial detection stage upon detection of a warning signal.

Description:
BACKGROUND OF THE INVENTION 
   This invention relates to apparatus for detection of selected sound frequency signals emitted by sirens of emergency vehicles. The new apparatus may detect a selected sound frequency signal spectrum emitted by various siren or warning sound sources in an initial detection stage. When a warning sound may be detected, a signal identification stage may determine the sound signal characteristics and estimate the distance to the warning sound source. 
   Various types of emergency vehicle siren detection systems may be currently known that may tend to be complicated, overly elaborate or use a lot of electric power. Less complicated devices, for example, that disclosed in U.S. Pat. No. 7,245,232, issued on Jun. 17, 2007, and herein incorporated by reference, may be known. However, this type of apparatus may be susceptible to false detection of sounds thought to be warning signals. To achieve a balance for a lower power system that may not be overly complicated, a system may be needed that uses a lower power, simple warning signal detection stage with a power save signal identification stage that only has full power applied once a sound warning signal has been detected. 
   SUMMARY OF THE INVENTION 
   The present invention is directed to apparatus for detecting sound frequency signals emitted by warning devices of emergency vehicles. A sensor may sense a selected frequency signal spectrum and may be in communication with an initial detection stage to detect a frequency of approximately 600 Hz and approximately 1400 Hz within a specific time interval. A signal identification stage may be in communication with said initial detection stage and may have a digital signal processor programmed to identify a wail and yelp warning signal to compare said warning signal to a histogram stored in a memory to estimate the distance from the sound frequency signal. The signal identification stage may be maintained in a power save mode until activated by said initial detection stage upon detection of a warning signal. 
   These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a functional block diagram of a system according to an embodiment of the invention; 
       FIG. 2  illustrates a schematic of an initial detection stage according to an embodiment of the invention; 
       FIG. 3  illustrates a schematic of a signal identification stage according to an embodiment of the invention; 
       FIG. 4  illustrates a schematic of peripheral elements for a system according to an embodiment of the invention; 
       FIG. 5  illustrates a generic wave form of a warning signal according to an embodiment of the invention; 
       FIG. 6  illustrates a flow diagram of a stored program according to an embodiment of the invention; 
       FIG. 7  illustrates a flow diagram of a stored program according to an embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   The following detailed description represents the best currently contemplated modes for carrying out the invention. The description is not to be taken in a limiting sense, but is made merely for the purpose of illustrating the general principles of the invention. 
   Referring to  FIGS. 1 through 4 , an emergency vehicle alert system  10  may have an initial siren sound detection stage  12  that may be a low power consumption element and a signal identification or confirmation stage  14  that may require more power for operation. The initial stage  12  for siren sound detection may have a microphone  20 , that may be a condenser microphone requiring low power that may be in communication with a low pass filter  22  and gain amplifier  24  set at a cut-off frequency to reduce interfering signals and to produce sufficient signal strength for processing of siren audio frequency signals. The gain output signal  25  of the gain amplifier  24  may be communicated to two band pass filters  26 ,  28  with each in communication with a comparator  30 ,  32  to function as part of the initial siren sound detection stage  12 . 
   The gain output signal  25  may also be communicated to a digital signal processor  50  that may normally be in a power save or sleep mode unless signaled by the initial stage  12  that a siren sound has been detected. The comparators  30 ,  32  may communicate siren frequency detection to trigger devices  34 ,  36  to signal the digital signal processor  50  that a siren signal has been detected. 
   The microphone  20 , low pass filter  22 , gain amplifier  24 , band pass filters  26 ,  28 , comparators  30 ,  32 , and trigger devices  34 ,  36  may be relatively low power consumption devices as compared to a digital signal processor  50  and may be connected as illustrated in the example circuit of  FIG. 2 . Given the three most recognized guidelines and regulations for emergency vehicles and several manufactures of siren apparatus it appears that many sirens produce sounds in a frequency range of 100 Hz to 3000 Hz. Microphones with frequency sensitive range greater than the sound range may be available with a sensitivity of −42 dB to provide sufficient resolution for analog-to-digital converters (ADC) and may require current usage of less than 0.5 mA. Given the sound frequency range of interest the low pass filter  22  cut off frequency may be set at 3000 Hz. 
   The band pass filters  26 ,  28  may be set to detect a siren sound frequency using a center band frequency of 600 Hz and 1400 Hz respectively with a timed detection separation for each frequency component. With reference to  FIG. 5 , a generic wave form for a siren wail sound is illustrated. The band pass filter  26  may detect a signal frequency centered at 600 Hz and start a timer for the band pass filter  28  to detect a signal frequency centered at 1400 Hz that if not detected by comparators  30 ,  32  will not activate trigger devices  34 ,  36  to activate the digital signal processor  50 . If further reduction of false detection may be desired, a second detection of the lower frequency 600 Hz by the band pass filter  26  may be implemented using a second timed detection constraint as illustrated in  FIG. 5 . 
   If the initial stage  12  detects a siren sound and trigger devices  34 ,  36  are set, the digital signal processor  50  may be activated by communication from AND gate  38  to transition from a power save mode to process the gain output signal  25 . The digital signal processor  50  may have an analog-to-digital converter  52  as part of the device, may sample the gain output signal  25 , and may decode the frequency and amplitude patterns using fast fourier transform processing to more accurately determine whether the signal is a siren sound based signal. This processing may be done with a software program stored in a memory  54  of the digital signal processor  50 . 
   In addition to identifying a siren sound it may be useful to identify how far away the siren sound source may be located. Depending on the microphone  20  characteristics and low pass filter  22  with gain filter  24  a minimum amplitude level signal that may be processed by the digital signal processor  50  may be determined. Using this as a maximum distance baseline for a siren sound identification that may be determined by testing the device, the digital signal processor  50  may be programmed to extrapolate relative distances of a siren sound source based on the amplitude characteristics of a siren sound. 
   The digital signal processor  50  may have a 10 bit resolution analog-to-digital converter  52  and a 10 mips processing speed to process the siren sound frequency signal. A power save or stand by mode for the digital signal processor  50  to conserve power may be approximately in the microampere range. The digital signal processor  50  may also control the operation of a speaker  70 , light emitting diodes and a motor  72  to warn a user of a detected siren sound. Buffering and amplification of digital signal processor  50  signals may be included to operate these devices. The sound, light and motor vibration characteristics may be controlled to indicate an approaching or receding siren sound, for example, increasing or decreasing the frequency of a flashing light. 
   Referring to  FIGS. 1 ,  6  and  7 , in operation the emergency vehicle alert system  10  may operate as follows. When electrical power is applied the initial stage  12  may be activated for initial detection of a siren sound and the signal identification stage  14  may be initialized  100  and placed in a power save or sleep mode  102  until a siren sound may be detected by the initial stage  12 . As described earlier the receipt of a trigger device signal may be detected by the digital signal processor  50  to activate and be controlled by a stored program. The gain output signal  25  may be sampled in 0.5 second increments  104  for processing by a fast fourier frequency transform algorithm  106  whose output  108  may be converted to polar coordinate 110 for further processing. The polar coordinate output signal representation may be scanned to filter out background noise  112  and the resulting scanned signal may be stored in the memory of the digital signal processor as a plot of sample data points in the form of a histogram  114 . When 2.0 seconds  116  of sample data have been processed the 2.0 seconds buffer of data may be scanned on a first-in-last-out basis to identify the siren sound signal characteristics to compare to stored memory siren sound characteristics. 
   A siren wail or yelp sound may be characterized as a frequency modulated signal with a fixed amplitude. The siren emits sound with frequencies that are continuously contiguous. The digital signal processor  50  firmware may scan for contiguous blocks of 156 Hz frequency  118 . Most sirens may emit frequencies between 800 Hz and 1800 Hz with contiguous blocks of frequencies between 900 Hz to 1056 Hz with a consistent amplitude being an example for which to search. The actual data captured for a siren sound may be much greater than the minimum 156 Hz selected based on the resolution of the sampling selected. One or more of the alarm indicators may be activated  120 . 
   The distance from the siren source may be estimated  122 . The signal characteristics of a siren wail and yelp are different because of the difference in cycle periods with a wail having a longer period than a yelp signal. Different comparison tables may be stored in the digital signal processor memory for comparison of a yelp or a wail signal to estimate distance to the siren sound source. The presence of a wail or yelp signal may be determined  124  and the histogram scanned for largest magnitude  125 . Adjacent frequency magnitude, within approximately plus or minus 50 Hz, may be examined  126  to confirm contiguous signal  128 . If the sample is contiguous, average the maximum magnitude with adjacent magnitudes  130  and compare to the stored magnitude/distance tables  132 . Based on the distance to the siren source, the warning indicator repetition rate  134  may be controlled. 
   A battery monitoring  60  and charging circuit  61  with batteries  62  may be used to provide electric power to the unit. With an adapter, for example, a connector for an automobile power outlet socket, the emergency vehicle alert system  10  may be used in land vehicles. With battery  62  only power, the system  10  may be carried by a person or used in other situations where external power may not be available. The implementation of a low power initial detection stage  12  and power save mode signal identification stage  14  may allow for longer operation when battery  62  only operation is used. 
   While the invention has been particularly shown and described with respect to the illustrated embodiments thereof, it will be understood by those skilled in the art that the foregoing and other changes in form and details may be made therein without departing from the spirit and scope of the invention.