Abstract:
A system and method for detecting the presence of a particular source of audible content using an automatic speech recognizer component which is trained to recognize specific non-speech sounds. Training of the automatic speech recognizer component is accomplished by providing an appropriate model to be recognized and by tuning the automatic speech recognizer to the sounds created by the desired stimulus. The automatic speech recognizer can additionally be provided with processing means for distinguishing the desired stimulus from an irrelevant or unwanted stimulus which may have acoustic signatures with a high degree of similarity to that of the desired stimulus. The invention can be implemented in a portable detector to allow detection of a specific acoustic source at any location.

Description:
FIELD OF THE INVENTION 
   The invention relates generally to acoustics and more specifically to the use of trained speech recognition mean for remote detection and characterization of sounds. 
   BACKGROUND OF THE INVENTION 
   For automatic detection systems that trigger responses such as sounding audible alarms, switching traffic signals, operating lights, or generating other detectable responses, prior art systems have generally relied upon the presence of motion detection, or mechanical activation. For example, mechanical switches coupled to train rails have been used to activate railroad crossing gates and signals. Similarly, counter strips are used to count the number of vehicles approaching a red traffic light and to change the light to green when a threshold number have crossed the counter strip. Mechanical systems suffer from the drawback that the sensors cannot distinguish among the various physical entities which come into contact with them. Therefore, when attempting to detect the presence of a particular physical entity (e.g., an approaching train rather than an approaching handcar), a mechanical sensor would not be able to determine if that particular entity was present. 
   Motion sensors (e.g., infrared detectors) generally react when triggered by visual or heat-sensory stimuli. Like the mechanical detectors, however, motion sensors are not able to distinguish between a target stimulus and other unwanted stimuli. Motion sensors which activate lighting for property perimeters, for example, will therefore often be triggered by moths, small animals, blowing leaves, etc. 
   Detection systems have additionally been proposed which react to audible stimuli. The sound detection systems generally respond to sound at or above a threshold decibel level. However, the latter type of system is also highly likely to react to false stimuli in the same decibel range as that of the expected stimulus. 
   It is desirable, therefore, to have detection equipment which can both detect and characterize sounds. In addition, it may also be desirable to have the detection equipment be portable in order to provide security wherever needed. 
   It is, therefore, an objective of the present invention to provide an improved system and method for detection of audible stimuli. 
   It is another objective of the present invention to provide a sound detection system which is not readily activated in the presence of false stimuli. 
   Yet another objective of the present invention is to train a sound detection system to respond to specific acoustic signatures. 
   Still another objective of the present invention is to effectively characterize the sources of acoustic stimuli in order to activate appropriate responses thereto. 
   SUMMARY OF THE INVENTION 
   The foregoing and other objectives are realized by the present inventive detector comprising an automatic speech recognizer component which is trained to recognize specific non-speech sounds. Training of the automatic speech recognizer component is accomplished by providing an appropriate model to be recognized and by tuning the automatic speech recognizer to the sounds created by the desired stimulus. The automatic speech recognizer can additionally be provided with processing means for distinguishing the desired stimulus from an irrelevant or unwanted stimulus which may have acoustic signatures with a high degree of similarity to that of the desired stimulus. The invention can be implemented in a portable detector to allow detection of a specific acoustic source at any location. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention will now be described in greater detail with specific reference to the appended drawings wherein; 
       FIG. 1  is a schematic illustration of a detection system in accordance with the present invention; and 
       FIG. 2  provides a representative process flow for implementing the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention makes use of an automatic speech recognizer (hereinafter, “ASR”) component  112  in a detector  100  as shown in  FIG. 1 . For purposes of the description, a running example is introduced whereby the detector is provided to detect an approaching train, for the express purpose of alerting track workers that a train is approaching their work location. Clearly, it will be understood that the teachings are extensible to implementation in detectors for a variety of other uses. As depicted in  FIG. 1 , the inventive detector  100  includes an acoustic input device  110 , an ASR  112 , an output signal generator  118  and mounting hardware  120 . The mounting hardware  120  is optional, but is shown for the example implementation wherein the detector may be mounted on a rail. 
   The acoustic input device,  110 , receives acoustic input and provides same to the ASR  112 . The acoustic input device  110  can be realized as a microphone or as any other mechanical-to-electrical transducer, such as a piezoelectric element. The acoustic input device may include an adjustable pre-amplifier circuit (not shown) for adjusting the amplitude sensitivity of the acoustic input device which permits the device to be used in a plurality of different environments. In addition to this amplitude sensitivity adjustment (i.e., for adjusting the sensitivity of the acoustic input device to a range of input volumes), frequency adjustments may be provided at the input stage  110  in the form of one or more acoustic filters (not shown), for filtering out certain ambient sounds which are expected to be encountered in the detection environment. By filtering out irrelevant ambient sounds, the acoustic input device can reduce the required processing by the ASR and can, thereby, provide a more responsive detector. It is additionally contemplated that the acoustic input device could dynamically tune the filters and the pre-amplifier gain in situ to improve the filtering out of specific unanticipated ambient sounds for a particular detection location (e.g., in a railroad tunnel). 
   It is particularly advantageous to utilize an acoustic input device that digitizes the acoustic input prior to providing same to the ASR. While analog signal processing can perform the signal analysis needed for the present invention, it is generally recognized that digital signal processing can result in faster and more accurate results. However, should the acoustic input device  110  not be equipped with analog-to-digital (A-D) signal conversion means, the ASR can process the analog input or can, itself, digitize the signal if appropriately equipped with its own A-D converter. It is also to be noted that the acoustic input device&#39;s gain and frequency control functionality could be incorporated into the ASR. 
   With specific reference to  FIG. 1 , the ASR  112  is coupled to the acoustic input device  110  and includes at least an input component  113  for receiving the acoustic signal from the acoustic input device, a processing component  114  for processing the input acoustic signal and for generating a signal at output  115 , and output  115  for providing output to the output signal generator  118  (discussed in greater detail below). The processing component  114  of ASR  112  of the present inventive detector  100  is trained to classify sounds and/or sound sources using one or more of a plurality of features of the acoustic signal input, such as amplitude, frequency, pitch, onset and offset of sound, frequency transitions and audio segmentation. The ASR is tuned to a specific signal feature or to a plurality of specific signal features which represent the acoustic signature of the desired stimulus (i.e., in the present running example, the signature of the approaching train to be detected). The training information, including one or a plurality of acoustic signal signature features, can be stored in a storage location (not shown) accessible by the processing component. 
   The processing component of the ASR analyzes the acoustic input signal in accordance with the parameters specified by its tuning. For example, the first signal analysis step may be to determine the frequency of the input signal and to compare that frequency to one or a plurality of “target” frequencies for desired stimuli. If the input signal has a frequency which is at the target frequency or in the target range of frequencies, the processing component may next look at another signal feature, such as the duration of the signal, in order to determine if the input is from the desired stimulus or is simply a signal having the same frequency as the desired stimulus. For example, a low frequency signal may be received; but, based on its amplitude and signal duration, the processing component may determine that it is from a distant truck rather than an approaching train. Clearly it is not necessary for the ASR to affirmatively identify a signal source, but rather, simply to determine that the input signal is or is not from the desired (a.k.a., target) source. Should definitive identification of the signal source be desired, however, the ASR processing component could be provided with a database (not shown) of acoustic signal signatures and sources in order to match the processed input to a database entry. An alternative signal processing progression could be to first analyze the signal amplitude followed by a frequency analysis. In that way, for example, a signal from a plane flying overhead could be distinguished from a signal generated by an approaching train. As above, it will probably not be necessary to identify the signal source, but simply to classify it as “from a target source” or “not from a target source”. The details of a basic acoustic input detector and the training/tuning of such a detector can be found in the reference entitled “A Maximum Likelihood Approach to Continuous Speech Recognition”, by L. R. Bahl, F. Jelnek and R. L. Mercer, IEEE Transactions on Pattern Analysis and Machine Intelligence, Vol. PAMI-5, pp. 179–190 (March 1983). 
   Once the ASR has identified the signal as being from a target source, the processing component  114  will generate an output at  115  which is provided to the output signal generator  118 . The output signal generator  118  can be a component which itself emits the desired output signal (e.g., audible alarm) or can be a component which is coupled to another signal emitting component or components. If the output signal generator  118  is found in a stand-alone detector, it is advantageous that the output signal generator  118  itself emit the desired signal by, for example, sounding an audible alarm, switching on warning lights, or a combination of the foregoing. Alternative embodiments could have the output signal generator coupled to a display for generating a visual indicator, coupled to a modem for communicating the event detection to a control location, etc. 
   It is to be noted that, if the ASR is adapted to definitively identify the source of the acoustic input, it may be desirable to trigger different responses at the output signal generator, depending upon the detected source. In the example of a detector for guarding property, therefore, if the detected source is a truck approaching the gates at high speed, the detector will “pull out all stops” (e.g., lights and alarms) to warn guards; while, if the detected source is a car passing by the gates, the detector may simply activate a signal light indicating that a proximate acoustic signal source has been detected but does not present an imminent threat. 
     FIG. 2  provides a representative process flow for implementation of the present invention. The process flow is for a stationary detector, but would easily be augmented to include placement or mounting of a mobile detector and dynamic filter tuning, as needed, for the specific detection environment. As shown in  FIG. 2 , a detector is first “trained” at  210 . By training the detector, what is meant is tuning the ASR of the detector to the specific target sounds to be detected. The training of the ASR can be done prior to or after inclusion of the ASR in a detector system. In addition, the training of the ASR may be dynamically adjusted (i.e., its comparison tables updated), so that one detector can be trained innumerable times to perform detection of different stimuli, assuming that no different signal processing is required for the alternative training. 
   Once the ASR has been trained, the next step in the  FIG. 2  process flow is detection of the sound at step  212 , wherein the acoustic input device  110  picks up the input and provides it to the ASR  112 . Once the ASR  112  has received the input, it performs step  214  by categorizing the sound. If the sound is one for which a response is necessary, the system then triggers the response at step  216 . As noted above, the ASR of the detector may simply be adapted to determine whether the input acoustic signal is from a target source or is not from a target source; in which case, the categorizing step simply comprises a “yes” or “no” determination. If, however, the ASR has been programmed to more completely analyze the acoustic signal, the categorizing step may include consulting its database to specifically identify the source of the sound, followed by triggering a specific response thereto. 
   Since the inventive detector,  100  of  FIG. 1 , is trainable, its accuracy for detecting target stimuli and for rejecting input from non-target stimuli can be improved by allowing for and incorporating a feedback control step  217  into the system. If this feedback mechanism is present, the acoustic signature database can be dynamically updated in situ by allowing for a human operator to correct mis-categorized stimulus signals. Thus, the system will learn to distinguish the target stimulus, even when a closely related non-target stimulus is present in its environment. 
   Given the fact that many objects emit sounds with an amplitude such that they can be detected and classified well before they are within the usable range for current infrared or ultrasonic proximity or motion detectors, the present invention allows one to react earlier to the triggering event than was possible with prior art systems. This aspect of the invention is quite desirable in time-critical applications, such as burglar alarms. In addition, in the case of the running example, it is possible to take advantage of the increased speed of sound within a rail to identify the stimulus early enough to provide a warning to clear the tracks where necessary, and to disregard other stimuli where there is no danger, such as a car crossing the tracks. It is noteworthy that such a system can be a portable and self-contained product, and thus can be used by railroad repairmen while they are working on tracks far from a station, terminal, or gated crossing where there are typically no train detectors. For the rail-mounted detector embodiment, transducers affixed directly to the rails can provide signals from faraway trains, as sounds dissipate less along the rails than atmospherically. Additionally, signal intensity analysis can be used to estimate vehicular distance, obviating the need for a series of detectors spaced at various distances from the work area. 
   While the invention has been described with reference to several preferred embodiments, it will be understood by one having skill in the relevant art that modifications can be made without departing from the spirit and scope of the invention as set forth in the appended claims.