Abstract:
A system and method of selective sampling for sound signal classification is disclosed. The method of the present invention discloses the elements of: receiving a sound signal; specifying meta-data to be extracted from the sound signal; dividing the sound signal into a set of frames; applying a fitness function to the frames to create a set of fitness data; selecting a frame from the set of frames, if the frame&#39;s corresponding fitness datum within the set of fitness data exceeds a predetermined threshold value; extracting the meta-data from the selected frames; and classifying the sound signal based on the meta-data extracted from the selected frames. The system of the present invention discloses means for implementing the method.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates generally to systems and methods for sound signal classification, and more particularly to selective sampling techniques for sound signal classification.  
         [0003]     2. Discussion of Background Art  
         [0004]     Interactive Voice Response (IVR) systems are an increasingly important tool for providing information and services in a more cost efficient manner. IVR systems are typically hosted by a server, which includes an array of Digital Signal Processors (DSPs), and enable speakers to interact with corporate databases and services over a telephone using a combination of voice utterances and telephone button presses. IVR systems are particularly cost effective when a large number of speakers require data or services that are very similar in nature and thus can be handled in an automated manner. A speaker using an IVR system may or may not eventually be connected to a live operator, depending upon the complexity of the speaker&#39;s request.  
         [0005]     Due to the significant cost savings often realized with IVR systems, there is a growing demand for such systems to provide more functionality and a richer speaker experience. Toward those ends, IVR systems responsive to a speaker&#39;s age range, gender, language, accent, dialect, identity, and so on are desirable. Such functionality often is possible when a speaker&#39;s vocal utterance (a.k.a. speech or sound signal) is first digitized and then analyzed, so that a set of meta-data (e.g. the speaker&#39;s age range, and so on) can be extracted from the utterance, without requiring the speaker to provide such information directly to the IVR system.  
         [0006]     While such meta-data extraction has a potential to improve speech recognition of the speaker and enable some novel IVR applications directed to a speaker&#39;s particular characteristics, current techniques for meta-data extraction are very computationally intensive and have further burdened IVR system servers and support hardware to the point of creating speed bottlenecks even during normal use.  
         [0007]     What is needed is a system and method for sound signal classification that overcomes the problems of the prior art.  
       SUMMARY OF THE INVENTION  
       [0008]     The present invention is a system and method for sound signal classification. The method of the present invention includes the elements of: receiving a sound signal; specifying meta-data to be extracted from the sound signal; dividing the sound signal into a set of frames; applying a fitness function to the frames to create a set of fitness data; selecting a frame from the set of frames, if the frame&#39;s corresponding fitness datum within the set of fitness data exceeds a predetermined threshold value; extracting the meta-data from the selected frames; and classifying the sound signal based on the meta-data extracted from the selected frames. The system of the present invention includes means for implementing the method.  
         [0009]     These and other aspects of the invention will be recognized by those skilled in the art upon review of the detailed description, drawings, and claims set forth below.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a dataflow diagram of one embodiment of a system for sound signal classification;  
         [0011]      FIG. 2  is one example of a data structure for maintaining a set of fitness data; and  
         [0012]      FIGS. 3A and 3B  are a flowchart of one embodiment of a method for sound signal classification.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0013]     The present invention discusses a selective frame sampling technique for extracting, from a speaker&#39;s voice utterance/speech/sound signal, meta-data such as age range, gender, language, accent, dialect, and identity. The invention not only increases the speed at which an Interactive Voice Response (IVR) system can extract meta-data from (i.e. classify) a speaker&#39;s sound signal, but also the accuracy of the extracted meta-data.  
         [0014]      FIG. 1  is a dataflow diagram of one embodiment of a system  100  for sound signal classification.  FIG. 2  is one example of a data structure  200  for maintaining a set of fitness data.  FIG. 3A and 3B  are a flowchart of one embodiment of a method  300  for sound signal classification.  FIGS. 1, 2 ,  3 A, and  3 B are now discussed together.  
         [0015]     In step  302 , an IVR system  102  receives a vocal utterance/speech/sound signal  104  from a speaker. The sound signal  104  will most commonly include of human utterances, such as words, phrases, and sentences. However, the sound signal  104  may also include sounds made from a recording, an animal, an inanimate object, and a computer synthesizer. In step  304  the IVR system  102  sends a meta-data request to a classifier selection module  106 . The meta-data request specifies what classes of meta-data shall be extracted from the sound signal  104  for a speaker who authored the sound signal  104 . The meta-data classes include: age range, gender, language, accent, dialect, identity, and so on. Those skilled in the art, however, will recognize that different types of meta-data may be extracted from the non-human speech/sound signals listed above.  
         [0016]     In step  306 , the IVR system  102  passes the sound signal  104  to a sound signal digitization module  108 . The digitization module  108  includes one or more Digital Signal Processors for converting analog sound signals into a digitized form and performing additional processing on the sound signal  104  if necessary. The additional processing may include sound signal noise reduction, echo cancellation, speech detection, and so on. In step  308 , the digitization module  108  passes a digitized version of the sound signal  104  back to the IVR system  102  for further processing or storage, depending upon how the IVR system  102  is designed. In step  310 , the digitization module  108  passes a digitized version of the sound signal  104  on to a sound signal framing module  110 . In step  312 , the framing module  110  divides the sound signal  104  into time frames of a predetermined length. Preferably the time frames are of equal length.  
         [0017]     In step  314 , the classifier selection module  106  selects one or more fitness functions, from a fitness function database  112 , corresponding to the meta-data to be extracted from the sound signal  104 . A fitness function is herein defined as a mathematical calculation to be performed on one or more sound signal frames.  
         [0018]     While a different fitness function may be used for each class of meta-data to be resolved, preferably a single fitness function that calculates each frame&#39;s overall sound signal strength is used. The sound signal strength of a frame is herein defined in the alternative to be: the frame&#39;s total signal power, an average of peak amplitudes within the frame, a total energy within the frame, a frame volume equal to a logarithmic value of the sound signal&#39;s amplitude, and so on, depending upon a particular implementation of the present invention.  
         [0019]     In step  316 , the classifier selection module  106  passes the selected fitness functions to a frame selection module  114 . In step  318 , the frame selection module  114  applies each of the selected fitness functions individually to each frame received from the sound signal framing module  110 , thereby creating the set of fitness data  200 . In step  320 , the frame selection module  114  stores the fitness data in a fitness data database  116 .  
         [0020]      FIG. 2  shows one example of the data structure  200  for maintaining the set of fitness data. In the example data structure  200 , the sound signal  104  has been divided into ten separate frames, labeled by “frame number.” A set of fitness data is calculated for each of the meta-data classes (e.g. age range, gender, accent, etc.). An exemplary set of fitness data for meta-data class # 1  is shown, and will be used to illustrate the method steps that follow.  
         [0021]     Generally, the frame selection module  114  preferably identifies a sub-set of the sound signal frames from which the selected meta-data may be accurately extracted. The preferred method selects those sound signal frames that have a greatest relative signal strength for further meta-data extraction.  
         [0022]     Specifically, in step  322 , the frame selection module  114  identifies a greatest fitness datum within a meta-data class (e.g. Frame  5  having a value of 12.0 in the example). In step  324 , the frame selection module  114  accesses a predetermined margin (e.g. a margin of 2 in the example) for that meta-data class from the classifier selection module  106 . This “margin” effectively sets a sampling rate for the meta-data class. In step  326 , the frame selection module  114  calculates a fitness datum threshold equal to the greatest fitness datum minus the margin (e.g. 12.0 minus 2=10.0 in the example).  
         [0023]     In step  328 , the frame selection module  114  stores a copy of each digitized sound signal frame that has a signal strength equal to or greater than the fitness data threshold (e.g. Frames  4  and  5  in the example) in a sampled frames database  118 . In step  330 , the frame selection module  114  stores a copy of each digitized sound signal frame that has a signal strength less than the fitness data threshold (e.g. Frames  1 - 3  and  6 - 10  in the example) in a discarded frames database  120 . Alternatively, the frame selection module  114  could just delete these discarded frames. Typically a sound signal&#39;s middle frames have a higher Signal-to-Noise Ratio (SNR) (i.e. signal strength) when compared with the sound signal&#39;s leading and trailing frames, and thus most often become the sampled frames stored in the sampled frames database  118 . As a quick second example, if the margin was set to 4, then the threshold would be 12.0 minus 4=8, and Frames  1 ,  3 ,  4 , and  5  would have been stored in the sampled frames database  118 .  
         [0024]     In step  332 , a classifier module  122  classifies each frame stored in the sampled frames database  118  according to the selected meta-data criteria. In one embodiment of the present invention, the classifier  122  uses a Multi-Layer Perceptron (MLP) neural network trained to recognize the meta-data class patterns.  
         [0025]     If the sound signal  104  is a speech signal, the MLP neural network will typically have at least three layers: an input layer with 12 nodes, corresponding to the 12 Mel-Cepstral components of a speech signal; a hidden layer with 20 nodes; and an output layer with a number of nodes corresponding to each class within the meta-data class (e.g. 2 nodes, “male” and “female,” if the meta-data class is “gender”). Back propagation (BP) is used to train the neural network. After being trained on a ground-truth set of about 200,000 frames, the classifier  122  can achieve a meta-data class recognition rate of about 70% for a gender meta-data class at the frame level.  
         [0026]     Next, in step  334 , after having classified each of the sound signals&#39;  104  sampled frames individually, the classifier module  122  classifies the entire sound signal  104  according to the selected meta-data classes and stores the result in a sound signal meta-data database  124 .  
         [0027]     One way to classify the entire sound signal  104  is by voting. Voting classifies the sound signal  104  based on which meta-data class is supported by a greatest number of the sampled frames.  
         [0028]     However, a preferred method for classifying the entire sound signal  104  adds together each of the sampled frame&#39;s confidence scores, which were generated by the neural network. That meta-data class with a highest overall total confidence score is chosen as the final class for the entire sound signal  104 . The confidence score approach results in a lower classification “error rate,” and is even more effective as the “selective sampling” rate is decreased.  
         [0029]     Another approach classifies the entire sound signal  104  as that class having a statistically longest run-length. The run length of a class is equal to a longest number of continuous sampled frames having been assigned a same meta-data class.  
         [0030]     Then, in step  336 , the sound signal meta-data  124  for the sound signal  104  is provided to the IVR system  102 . IVR systems can benefit from such meta-data in a variety of ways, including: improved customer service; added IVR system functionality; and improved statistical record keeping.  
         [0031]     Empirical tests comparing the present invention&#39;s selective sampling to even sampling were run on about 1,200 speech files in a “Test” directory of TIDIGITS corpus. There were about 250,000 frames in total. “Even sampling” is herein defined as when the sampled frames from a sound signal are equally spaced with respect to each other, independent of their signal strength. For example, if the “sampling rate” is 1:3, then “even sampling” will select Frames  1 , 4 , 7 , and  10  in  FIG. 2 , while “selective sampling” will select Frames  1 ,  3 ,  4 , and  5 .  
         [0032]     It was found that regardless of the “sampling rate”, the present invention&#39;s “selective sampling” achieved a lower sound signal meta-data  124  error rate when compared to “even sampling.” It was also found that there was general “sweet sampling rate spot” for “selective sampling” between 1:2 and 1:3, for gender meta-data classes. Sampling rates greater than 1:2 tended to include too many low quality sound signal frames, while sampling rates lower than 1:3 tended to discard too many high quality sound signal frames. Those frames discarded by selective sampling also tend to have a lower confidence score than the sampled sound signal frames. Also, since not all of the sound signal&#39;s frames are analyzed by the classifier module  122 , the speed with which the sound signal meta-data  124  is calculated is also increased.  
         [0033]     While one or more embodiments of the present invention have been described, those skilled in the art will recognize that various modifications may be made. Variations upon and modifications to these embodiments are provided by the present invention, which is limited only by the following claims.