Patent Publication Number: US-7720852-B2

Title: Information retrieval engine

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application is a continuation of U.S. patent application Ser. No. 10/167,807 filed Jun. 11, 2002 now U.S. Pat. No. 7,162,482 for “Information Retrieval Engine”, the contents of which are incorporated herein by reference. U.S. patent application Ser. No. 10/167,807, is a continuation-in-part of U.S. patent application Ser. No. 09/848,982 for “Text Equivalencing Engine,” filed May 3, 2001, now U.S. Pat. No. 7,251,665, the contents of which are incorporated herein by reference. U.S. patent application Ser. No. 09/848,982 for “Text Equivalencing Engine” claims priority from provisional U.S. Patent Application Ser. No. 60/201,622, for “Recommendation Engine,” filed May 3, 2000, the contents of which are incorporated herein by reference. 

   BACKGROUND 
   A. Technical Field 
   The present invention is related to information retrieval, and more particularly, to retrieving information about a signal such as a music track or song, and creating a database of signals and the information retrieved about the signals. 
   B. Background of the Invention 
   In many contexts retrieving information about a sequence of unknown items is desirable. One such context is music. Some electronic copies of a piece of music include not only the music itself but also a tag (also known as meta-data). The tag includes information about the music such as the artist, the album name, and the track name. The tag can include any other information associated with the music entered by a listener. 
   However, some electronic copies of songs do not include the tag information. In particular, copies of songs downloaded from Napster or other sites where music can be downloaded only rarely include complete tag information. Additionally, as systems for recognizing music are fielded, it is likely that those providing copyrighted music without license will make efforts to evade any filtering system put into place. This has happened already with systems based on file name filtering. Thus, it is reasonable to expect slight variations of songs to be encountered in which the variations do not change the file substantially for the listener, but might make naïve comparison methods unreliable. Examples of such modifications include slight trimming of the beginning of a song, changes in overall volume, band-limiting and changes in equalization. 
   One way to replace missing tag information is to compare the waveforms of an unknown song (i.e. one that does include tag information) to those of known songs (i.e. songs for which tag information is known). When two waveforms match, tags can be transferred to unknown song. However, this method is inefficient in time and space because direct wave-form comparison is computationally expensive. Also, music files available over the Internet are usually compressed using a compression algorithm such as MP3. Compression makes direct comparison of audio waveforms impossible since there will, by nature, be significant differences between the original signal and the signal recovered from a lossily compressed version. These differences will not generally be audible, but they can be substantial in magnitude, especially when phase information is discarded by the compression algorithm. 
   An alternative method of determining tag information is to perform feature comparison rather than waveform comparison. Feature comparison is the comparison of analog features associated with the waveform such as correlating spectra or comparing extracted rhythm signatures. Feature comparison is also slow because it involves comparison of analog features that occur over time. Thus, comparison using these analog features requires exhaustive comparison and time-alignment. Moreover, many analog features are not invariant under common transformations such as translation in time, volume scaling and small changes in equalization. 
   What is needed is a system and method of determining tag information for an unknown music track that avoids the above-described limitations and disadvantages. What is further needed is a system and method that leads to accurate retrieval of tag information for a compressed, trimmed piece of music of limited bandwidth. What is further needed is a system and method that is efficient in time and memory space. Currently available systems require an exhaustive search and expensive comparisons to each exemplar. Both the exhaustive search and the expensive comparison operations should be avoided if possible. 
   SUMMARY OF THE INVENTION 
   The present invention provides an information retrieval engine capable of retrieving information about an unknown signal. In one embodiment, the unknown signal is an unknown audio signal such as a song and the information is tag information that should be associated with the unknown audio signal. The tag information can be retrieved even for unknown songs that have been trimmed or compressed. The present invention can be used to retrieve the tag information accurately and efficiently. Thus, the present invention overcomes the problem of linear searching and difficulties due to compression and trimming. 
   The information is retrieved by converting analog signals into sequences of symbols that represent the original analog signal. The conversion to symbolic representation is designed to be robust with respect to commonly occurring transformations such as small time shifts and volume changes. The symbolic representation of each signal is further transformed by considering the symbols representing the signal as analogically equivalent to letters in normal text and then grouping the symbols that represent a signal into the analogical equivalent of words and then grouping the words into documents. 
   A quantization technique is applied to obtain clusters in the space of spectral representations of the signal and then proximity of a signal to a cluster is taken as indicating the occurrence of the symbol corresponding to that cluster. By suitable design of the feature extraction done prior to clustering, it is possible to cluster such that the clusters formed from an uncompressed signal are not significantly different from the clusters formed from the same signal after it has been compressed. A technique called indexing is used to retrieve the tag information based on the words included in the unknown signal. This indexing operation is essentially similar to that performed in conventional text retrieval engines and thus can make use of an extensive technology in that area. 
   Indexing involves building a data structure that relates individual words made up of a sequence of symbols to the known signals that contain those words. The tags for each known signal is also stored in association with the known signal. Each known signal may be broken into multiple documents in which case the index relates words back to the documents in which they occur and then relates documents back to the signals in which they occur. To create an index, each known signal is first converted into symbolic representation, sequences of symbols are grouped into words and then sequences of words are converted into documents which may actually overlap. The index from words to documents and from documents to signals is then updated to represent the occurrence of each word and each document. 
   In order to search for an unknown signal using an index, the unknown signal is first converted into a sequence of symbols, words and documents in exactly the same fashion as was done with the known signals. The index is then used to find all documents that contain any of the words in the unknown signal. Scores are accumulated for all documents that are part of a known signal and that share any word with the unknown signal. There are many methods that are known in the text retrieval literature for scoring documents. The signal corresponding to the documents with the highest score is then considered to be the same signal as the unknown signal. In one embodiment, the tag information for that known signal is retrieved and associated with the unknown signal. The known and unknown signals need not represent audio signals, but merely need to be signals for which a useful quantization operation exists that is invariant with respect to the transformations that might be expected to be encountered. 
   As can be seen from the above description, the present invention may be applied to many different domains, and is not limited to the application of retrieval of information corresponding to audio signals. In addition, information retrieval according to the techniques of the present invention is not limited to the retrieval of tag information for music songs. Many techniques of the present invention may be applied to retrieval of information in other domains. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a block diagram of a functional architecture for one embodiment of the present invention. 
       FIG. 2  is a block diagram of a conceptual architecture for one embodiment of the present invention. 
       FIG. 3  is a block diagram of a functional architecture for information retrieval. 
       FIG. 4  is a flow diagram of a method of information retrieval. 
       FIG. 5  is a block diagram of a functional architecture for quantization. 
       FIG. 6  is a diagram of a signal broken into documents. 
       FIG. 7  is a flow diagram of a method of indexing. 
       FIG. 8  is a flow diagram of a method of grouping letters to form words. 
       FIG. 9  is a flow diagram of a method of information retrieval using an index. 
       FIG. 10  is a flow diagram of a method of scoring documents and queries using the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The following description of preferred embodiments of the present invention is presented in the context of an information retrieval engine for music tracks, such as may be implemented in an Internet-based jukebox or personalized radio station. One skilled in the art will recognize that the present invention may be implemented in many other domains and environments, both within the context of musical information retrieval, and in other contexts. Accordingly, the following description, while intended to be illustrative of a particular implementation, is not intended to limit the scope of the present invention or its applicability to other domains and environments. Rather, the scope of the present invention is limited and defined solely by the claims. 
   Referring now to  FIG. 1 , there is shown a conceptual architecture of one embodiment of the present invention. In the architecture of  FIG. 1 , the invention is implemented in connection with a web-based “jukebox”  103 , or personalized radio station, which accepts a user&#39;s selections of music tracks. The user is able to search for particular tracks and/or artists, and to control the playback of selected tracks. The user is also able to download tracks from other places on the Internet. Some of those tracks may not include tag information. Thus, the information retrieval engine of the present invention is employed to retrieve the unknown tag information. In one embodiment of the present invention, it is desirable to be able to retrieve the tag information from unknown music tracks so that an Internet radio station can be personalized or so that a server can make appropriate recommendations for music to purchase or to download. 
   In one embodiment, the system monitors the user&#39;s behavior with regard to searching, listening, and playback control in order to refine recommendations. Advertising, offers, and other information may be selected and presented to the user based on observations of user behavior and analysis as to which material may be of interest to the user. Thus, in this embodiment, it is useful to retrieve tag information for any unknown music tracks. 
   Stream delivery system  150  interacts with jukebox  103  to specify a sequence of audio files to deliver to jukebox  103 . System  150  also delivers the audio files. Stream delivery system  150  also communicates with real-time subscription authorization module  157 , which includes real-time server  154  and database server  156  that keeps track of which user accounts are active and enforces global business rules about which accounts can listen to the radio at a given time. Within stream delivery system  150 , there are a number of distinct software entities. These include sequence server  153 , authorization server  152 , and content server  151 . Sequence server  153  selects which files to play according to estimated listener preferences as well as pre-determined station formats. Authorization server  152  keeps a record of the files that are selected by sequence server  153  and is consulted by content server  151  when files are requested. If jukebox  103  requests files without the necessary security information or in an unexpected order, authorization server  152  flags this anomaly and content server  151  declines to provide the data. 
   Content database  102  contains descriptions of music tracks, and in one embodiment contains digitized representations of the music tracks themselves. Content database  102  is stored using conventional database storage means, and in one embodiment includes several fields providing descriptive information regarding music tracks, such as title, album, artist, type of music, track length, year, record label, and the like. For tracks without tag information, this descriptive information is retrieved using the information retrieval engine  160 . 
   Referring now to  FIG. 2 , there is shown a block diagram of a functional architecture for one embodiment of the present invention. Content index  210  provides a concise index of content stored in database  202 , and is generated by conventional database generation means, to enable more efficient searching and updating of database  202 . 
   Index and search module  204  facilitates functionality for accepting user input and searching database  202  for particular music tracks. In one embodiment, the user enters input by accessing web site  206 , which provides an interactive user interface for accessing the functions of the present invention. Web site  206  provides the main point of contact with users. A user interacts with web site  206  over a network, using a conventional web browser  205  (such as Microsoft Internet Explorer), running on a client computer. Module  204  accesses database  202  and index  210  in response to user queries. Results are returned to the user via web site  206 . In one embodiment, index and search module  204  also dynamically updates content index  210  in order to provide improved efficiency for future searches. Such indexing techniques are well known in the art. 
   In addition, web site  206  offers the capability for suggesting tracks and artists that may interest the user, based on personal criteria  211 , profiles  212 , of track-level discovered relationships based on observed user listening behavior determined by log analysis and information retrieval  213  of play logs  214 , as described in more detail below. Log analysis and information retrieval  213  also retrieves tag information for unknown songs using the information retrieval engine of the present invention. 
   Play log  214  is a database that monitors and stores information describing user behavior. Specifically, the user&#39;s interaction with jukebox  103 , including track selection, repeats, aborts and skips, and the like, are recorded and stored in play log  214 . Log analysis and information retrieval module  213  analyzes play log  214  in order to generate a profile of the user, which is stored in profile database  212 . Log analysis and information retrieval module  213  also retrieves tag information in accordance with the present invention. The retrieved tag information is stored in play log  214  for unidentified tracks. Log analysis and information retrieval module  213  periodically updates the play log  214  as the information about unknown music tracks is retrieved. In one embodiment, the tag information is retrieved by the information retrieval engine module  213  prior to log analysis in the log analysis module  213 . 
   Now referring to  FIG. 3 , there is shown an overview block diagram according to the present invention. In  FIG. 3 , an unknown signal  301  is identified. The unknown signal can be an entire song, a trimmed portion of an entire song, a compressed version of an entire song, a compressed version of a trimmed portion of an entire song, or any other version of a song. The music  301  may be in any recognizable format, for example, MP3, MP2, M3U, ASX, ASF, PLS, WMA, WM, WMX, WAV, AVI, QT, CDA, MPG, MPEG, MPV2, MP2V, or MPA. 
   Waveform converter  305  converts signal  301  to a waveform  308 . The waveform  308  is an alternate representation of the signal  301  input into the waveform converter  305 . The waveform  308  is input into a spectral feature detector  310 . The spectral feature detector detects spectral features  312  of the waveform  308 . 
   The spectral feature detector first converts the signal from audio to Mel Spectral. Mel is a nonlinear frequency scale that matches human frequency discriminatory capabilities. The spectral feature detector then performs another transformation from Mel Spectral to Mel Cepstral as described in “Mel-Frequency Cepstral Analysis,” by Unjung Nam available on the Internet at http://ccrma-www.stanford.edu/˜unjung/mylec/mfcc.html. In one embodiment, Mel Cepstral is represented in terms of multiple coefficients  312 . A first coefficient  312  encodes amplitude, a second coefficient  312  encodes fundamental frequency, and a subsequent coefficients  312  encodes successively refined aspects of overall spectral shape. The coefficients  312  are then quantized, by quantizer  315 , using any one of several quantization techniques. The quantization techniques are described below in reference to  FIG. 5 . 
   The quantizer  315  outputs a symbol that represents an approximation of the signal  301  characteristics for a short period of time. These symbols are referred to here as letters. The sequence of letters  320  reflects the time evolution of the signal as it changes to be more or less like the clusters centroids. Because they are ordered and taken from a reasonably small vocabulary, these symbols can be analogized to letters in the textual domain. Therefore, in this application the terms symbols, clusters, and letters will be used interchangeably. The letters  320  are grouped into groups of one or more letters  320  by reference to a dictionary of letter groups that is created by examination of the frequencies of all letter groups in a large training set of signal inputs. These groups of letters are termed words, again following an analogy to the textual domain where the letters may be actual letters and the words actual words. The organization module  330  uses an algorithm described below with reference to  FIG. 8  to determine a set of words or dictionary of letter groups  335 . 
   The words  335  are then used to access an index. An index records all of the documents that contain particular words. By examining all index entries for the words in the words  335 , all of the documents that have one or more words in common with the words  335  can be found. The known signals used to create the index are temporally separated into sections called documents. Each document is quantized and organized into words which are then indexed so that there is an entry for each unique word in any known signal. The index can thus be searched for words in the unknown signal  301 . Every indexed document containing a word in common with the unknown signal  301  is scored in the score and weight module  340 . Every word in the unknown signal  301  is weighted in the score and weight module  340 . Scoring and weighting algorithms are described in detail below in reference to  FIG. 10 . Scores and weights  345  are output from the score and weight module. The score  345  is used to select a known signal and the tag information  355  associated with that known signal is then ascribed to the the unknown signal  301 . 
   The information retrieval engine according to the present invention can determine the tag information for unknown music tracks that have been compressed or otherwise distorted in a fashion that leaves them psycho-acoustically similar to the original. By using quantization techniques and an index of quantized known tag information, tag information can be determined quickly and efficiently. The invention divides signals into documents, so that the known signal is broken down into smaller sections, documents, allowing the information retrieval engine to retrieve information for trimmed signals. One property of the quantization used in this embodiment is that the quantization of a compressed signal and the quantization of the same signal prior to compression are similar because the comparison operation that underlies the quantization process is designed to place psycho-acoustically similar signals near each other. Thus, a quantization technique allows the information retrieval engine to retrieve information for compressed signals. 
   Now referring to  FIG. 4 , there is shown a flow chart of the method of determining tag information. In one embodiment, a signal is converted to a waveform and the spectral features determined as described above, before the steps of  FIG. 4  are performed. The spectral features are quantized  405  by quantizer  315  to obtain letters  405 . The letters are grouped to form words  410  by organization module  330 . Documents in an existing index are searched for the unknown words  415 . An existing index is a database including known signals and their corresponding tags. The known signals are analyzed to recognize spectral features and their corresponding quantized representation exactly as is the unknown signal. The letters for the known signals are grouped into words and words are stored in the index with a pointer back to the original known signal. The unknown words are weighted  418  by score and weight module  340 . Indexed documents are scored  418  by score and weight module  340 . The scoring is based on the number of words in the unknown signal appearing in each segment of the known signals according to an algorithm described in detail below in reference to  FIG. 10 . The information for the known signal achieving the highest score is associated with the unknown signal. The information for the known signal achieving the highest score is output  420  by information retrieval engine  350 . 
   Now referring to  FIG. 5 , there is shown a block diagram of some examples of clustering techniques. Clustering is a process of associating groups of instances that are similar. Since the process of clustering groups instances that are similar and since similarity is defined in this embodiment by extracting spectral features that reflect human auditory perception, the clustering process will tend to group together signals that sound similar to a human listener. This property provides robustness to the system relative to distortions and compression algorithms that are psychoacoustically transparent. This property thus allows information retrieval when the unknown signal is compressed. By noting the successive clusters that are close to successive parts of a signal, the signal can be translated to a sequence of discrete symbols, referred to here as letters. There are several different clustering techniques. In one embodiment, any clustering technique can be used such that the desired result of obtaining letters is achieved. Clustering techniques include, for example: “k-means”, incremental clustering, EM, and Gaussian mixtures. 
   Sets of coefficients representing spectral features are called input instances  501  and are clustered in one embodiment using the k as follows. First, a cluster centroid is chosen  505 . Then, each cluster is built around the cluster centroids  510 . Finally, the clusters are refined in some fashion. In one embodiment, the cluster centroids  510  are initially chosen at random. In one embodiment, the cluster centroids  510  are chosen by selecting an input instance as the initial value for each cluster centroid. Every instance is assigned to a cluster  515 . In one embodiment, the instances  501  are assigned to the cluster  520  to which they are most proximal. 
   In one embodiment, the clustering groups are exclusive such that any instance belongs to only one group. In another embodiment, the clustering groups are overlapping such that one instance could belong to two groups. 
   In one embodiment, “k-means” clustering is used. In “k-means” clustering, the number of clusters sought is specified, k. Then points are chosen at random for cluster centroids. Each instance is assigned its closest cluster based on distance. The mean of all instances in each cluster is calculated. For each cluster, the mean of instances assigned to that cluster is considered to be the new cluster centroid and the process is repeated iteratively. Iteration can continue until the instances are assigned to the same clusters in consecutive iterations or until the error between consecutive iterations is sufficiently small, or until some other threshold is reached. One skilled in the art will note that there are many variants of clustering that could be used interchangeably with k-means clustering. Such variants as Gaussian mixtures, Expectation-Maximization, hierarchical clustering and so on are well known in the art. 
   Now referring to  FIG. 6 , there is shown a diagram of the segments of signals. Prior to building an index of known signals and their associated information and words, the signals, such as signal  301  shown in  FIG. 6 , are separated into sections, such as sections  605  and  610  of signal  301 , called documents. The sections can be of any length that provides accurate results for tag identification. In one embodiment of the present invention, the documents are  30  seconds long. Each document contains  30  seconds of the song. 
   In one embodiment of the present invention, the documents are overlapping. Any amount of overlap, or no overlap, can be used. In one embodiment of the present invention, the document overlap is 20 seconds. In an alternative embodiment, the document overlap is 15 seconds. 
   Known signals are broken into documents to provide smaller sections of the signal to group into words. The documents also aid in signal identification if a signal has been trimmed. If an unknown signal is only a 30-second segment out of a 4-minute signal, the unknown signal may not be identified if the entire known signal is scored. However, by breaking down the known signals into documents, the unknown signal is more likely to be identified. 
   It is also possible that the unknown signal is 30 seconds long, however it is not the first 30 seconds of the signal or the second 30 seconds, etc. It could be, for example, the 30 seconds that occurs from 7 seconds into the signal until 37 seconds into the signal. In this case, if there were no overlap at all between the documents, the unknown signal may also be hard to identify because it does not correspond in time to one of the known documents. However, if the known documents overlapped by either 5 or 10 seconds, the unknown signal would be easily identifiable because it would correspond closely in time to a known signal document. 
   If an unknown signal is longer than the predetermined length of a known document, the unknown song is itself separated into documents. In one embodiment, the documents of the unknown signal are the same length as the documents of the known signals. Also, the overlap of the documents of the unknown song are usually the same as the documents of the known songs, though this is not necessary to practice the present invention. 
   Now referring to  FIG. 7 , there is shown a flow chart of a method of building an index of known signals. The index of known songs is used to retrieve information about an unknown signal. To build an index, a signal for which the information is available, a known signal, is input into the index. In one embodiment, a database is used to store the known signal and its corresponding tag information  703 . The known signal is sectioned into documents  705 . The documents are further described above in reference to  FIG. 6 . In one embodiment, a database is used to keep track of the documents associated with the known signal  708 . 
   The signal for each document is converted into its corresponding waveform and the spectral features of the waveform are detected as described above in reference to  FIG. 3 . The coefficients are then quantized  710  using one of the above described techniques. When the coefficients have been quantized  710 , clusters of data are output, representing the original audio signal. These clusters are referred to as letters. 
   The letters are grouped together to form words  715 . A word is a group of one or more letters. Words are formed based on the frequency of the groups of letters. If a letter or group of letters appears within certain threshold frequency limits, then the letter or group of letters are organized to form words. The formation of words is described in further detail below with reference to  FIG. 8 . 
   In one embodiment, the database also associates the documents and their corresponding words  720 . This method is repeated for as many known signals as is desirable to enter into the index. In one embodiment of the present invention, an index can be built that contains every signal currently known. In an alternative embodiment, an index can be built that in some way classifies the signals so that it does not contain every known signal, but instead contains a subset of every known signal. The classification could be based on any criteria as long as it is a plausible design. 
   Once an index has been built it can be used to identify an unknown signal. The unknown signal can also be broken into documents, quantized, and grouped into words. In one embodiment of the present invention, the words in the unknown documents can be compared to the words in the known documents in order to find a match and identify an unknown piece of music by its tag information. 
   Now referring to  FIG. 8 , there is shown a method of forming a word from letters. Words are formed from a series of letters in a given document. In one embodiment, there are two thresholds that together define whether a word is considered “frequently appearing” in each document. The threshold values are chosen such that the words yield an accurate, fast, and memory-efficient result of identifying an unknown signal. The first threshold is a minimum number of appearances of a word in a document. The first threshold is referred to as t 1 . In one embodiment, the second threshold is a maximum number of appearances of a word in a document. The second threshold is referred to as t 2 . A word is considered to be “frequently appearing” if its frequency lies between the thresholds. In an alternate embodiment, only one of the two thresholds is used. 
   When words are formed, initially, each letter is assumed to be a word  805 . Then, in one embodiment of the present invention, the system looks for pairs of frequently appearing letters  810 . These are also considered words. In one embodiment, the system then looks for frequently appearing 3-grams  815  (a three letter word). In one embodiment, the system then looks for frequently appearing 4-grams  815 . Finally, the system looks for frequently appearing n-grams  815 , where n is any positive integer value greater than 2. In the event of overlap between a 2-gram and any n-gram, the system favors the longer word  825 . The result is a series of the longest frequently appearing n-grams. 
   The following is a hypothetical example of word formation for an arbitrary string of letters from the alphabet. 
   agecqrtlmsdsplcqragetlprageslrpsas 
   Assume for this hypothetical example that the lower threshold t 1  is 2 and the upper threshold t 2  is 4. Each letter is assumed to be a word  805 . The letter pair “ag” appears 3 times, making it a frequently appearing word  810 , according to the threshold values. The 3-gram, “age,” appears 3 times making it a frequently appearing word  815 . There is no letter that could be added to “age” to form a frequently appearing 4-gram. The words “agec,” “aget,” and “ages” each only appear once. Therefore no more letters should be added and the 3-gram “age” is the longest n-gram that can be formed to meet the frequently appearing word criteria. The same process could be performed on the second letter in the string, “g.” However, since “ge” overlaps with “age”, the “ge” word would be eliminated when favoring longer words  820 . 
   The same process could be performed to form the word “cqr” as to form the word “age.” After the system finished forming words on this string the following words would be formed: “age” appearing 2 times, “cqr” appearing 2 times, “tl” appearing 2 times, “s” appearing 4 times, “p” appearing 3 times, “l” appearing 2 times, and “r” appearing 2 times. 
   Now referring to  FIG. 9 , there is shown a flow diagram of a method for information retrieval. In one embodiment of the present invention, the information retrieved is tag information identifying a music track. In one embodiment of the present invention, the information retrieval engine  350  receives an unidentified signal, a query  905 . The query comprises an unknown signal. The information retrieval engine  350  uses a previously built index to identify the unknown signal. The information engine determines whether or not the query is longer than the document length used when the index was built  905 . If the query is longer than the length of the documents used when the index was built  906 , then the query is divided into documents  908  as described above with reference to  FIG. 6 . 
   Each document of the query, or the entire query if it was not divided into documents, is quantized  910 . Any technique of quantization can be used. Several techniques of quantization are described above with reference to  FIG. 5 . Words are formed out of the quantized letters for each document in the query  915 . Each word in the query is weighted  918 . The weights are described further below with reference to  FIG. 10 . For each word in each document of the query, the known documents containing those words are located  920 . The known documents containing those query words are scored  925 . The scoring is also described further below in reference to  FIG. 10 . The documents with the highest scores are considered the same as the query documents or the unknown signal. The information retrieval engine consults the database to identify a signal corresponding to the documents with the highest score, and outputs that information  930 . The tag information for the signal with the highest score is output as the tag information for the query or unknown signal. 
   In one embodiment, the present invention employs a binomial log likelihood ratio analysis for weighting and scoring words in the query and known documents in the index. The log likelihood ratio is a measure of how well a null hypothesis fits the observed data. If the null hypothesis is the assumed independence of occurrence of a word in the query and a word in a known document, for example, the log likelihood ratio measures the likelihood that such independence is a valid assumption. It follows, then, that the log likelihood ratio is a useful indicator of the relationship between the occurrences of the two words, if any. 
   The log likelihood ratio is based on a likelihood ratio. A likelihood ratio is the ratio of the maximum likelihood of the observed data for all models where the null hypothesis holds to the maximum likelihood of the observed data for all models where the null hypothesis may or may not hold. The log likelihood ratio is the logarithm of the likelihood ratio. 
   For the present invention, the log likelihood ratio is employed to determine whether a given word is more likely to appear in a first known document than in a second known document. Based on this measure, subsets of documents are defined so as to identify those documents most likely to contain the query words. 
   In one embodiment, the log likelihood ratio is applied in the present invention to determine whether a particular track occurs more frequently than expected in the selections of a subset of users. Variables are defined as follows: 
   N=the total number of users; 
   k(w)=the number of words in each document; 
   i=document 
   j=word 
   The following equations are applied: 
   
     
       
         
           
             
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   Referring now to  FIG. 10 , there is shown a flow diagram of a method of weighting words and scoring documents according to the present invention. The method illustrated in  FIG. 10  is shown in terms of matching words in an information retrieval system. One skilled in the art will recognize that the method may be adapted and applied to many other domains and techniques. 
   A total number of documents N is determined 1002. The system determines a document frequency (the number of times document occurred, or 
                 DF   j     =     ∑     (       k   ij     &gt;   0     )         )     ⁢   1004.         
The results are weighted according to a product of up to three components: l=the number of times a word appears in the query document(s); g=the number of times a word appears in the known documents; and n=a normalizing factor based on how many words there are, in total 1005.
 
   The first weighting factor, l, is a local weighting factor. It represents the frequency of the word within the query document. It may be represented and defined according to the following alternatives: 
   l T =k ij =Number of words in the query; or 
   l L =log k ij  (or log(k ij +1) ); or 
   l X =1 (a constant, used if this weighting factor is not to be considered). 
   l may be adjusted. 
   The second weighting factor, g, represents the frequency of the words within all the known documents. It may be represented and defined according to the following alternatives: 
   
     
       
         
           
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                   j 
                 
                 + 
                 1 
               
             
           
         
       
     
       
       
         
           (inverse document frequency, i.e., the log of the total number of documents divided by the document frequency); or 
           g x =1 (a constant, used if this weighting factor is not to be considered). 
         
       
     
  
   g may be adjusted in a similar manner as is l. 
   The third weighting factor, n, represents a normalizing factor, which serves to reduce the bias for scoring long documents higher than short ones. Using a normalizing factor, a short relevant document should score at least as well as a longer document with general relevance. n may be represented and defined according to the following alternatives: 
   
     
       
         
           
             
               n 
               c 
             
             = 
             
               1 
               
                 
                   
                     ∑ 
                     j 
                   
                   ⁢ 
                   
                     
                       
                         ( 
                         
                           l 
                           j 
                         
                         ) 
                       
                       2 
                     
                     ⁢ 
                     
                       
                         ( 
                         
                           g 
                           ij 
                         
                         ) 
                       
                       2 
                     
                   
                 
               
             
           
           ; 
           or 
         
       
     
   
   n X =1 (a constant, used if this weighting factor is not to be considered). 
   By employing the above-described combination of three weighting factors in generating weights for words and scores for documents, the present invention avoids the problems of overstating frequently appearing words and overstating coincidental co-occurrence. If a word is frequently occurring, the second weighting factor will tend to diminish its overpowering effect. In addition, the effect of coincidental co-occurrence is lessened by the normalization factor. 
   In one embodiment, the system of the present invention generates scores as follows. For each track of interest, a large N-dimensional vector is determined. For each known document, a large N-dimensional vector is determined. The techniques of assigning meaning to such vectors and training the vector set to represent similarities among vectors are well known in the art, as described for example in Caid et al., U.S. Pat. No. 5,619,709, for “System and method of context vector generation and retrieval.” 
   A score can be generated for one or more documents, and the highest-scoring document is treated as if it is the same as the query document. A score for a document is determined by taking the dot product of the query vector and the document vector. In one embodiment of the present invention, the above-described weighting factors are applied to the vector terms in order to improve the results of the scoring process. 
   The words are weighted using the general formula l ij g j n i , where l, g, n are defined above 1005. The known documents are scored using the general formula l ij g j n i , where l, g, n are defined above 1005. The specific options can be chosen based on the desired results. In one embodiment of the present invention, it is desired to preserve diversity, to give rarity a bonus, and to normalize. In that embodiment, the weighting options L, I, and C may be chosen. Weighting option L acts to preserve diversity, option I acts to give variety a bonus, and C acts to normalize the results. Thus, in that embodiment the weighting would be equal to l L g I n C . In other embodiments, different weighting options can be used. In one embodiment, the present invention uses the same weighting for the query words as it does for scoring the known documents. In an alternative embodiment, different weighting options can be chosen for the query words and for scoring the known documents. 
   Once the query has been weighted and the known documents have been scored and the dot product of two vectors determined, the document or documents with the highest score are determined. The highest scoring documents are then looked up in the database to determine the corresponding signal. The corresponding signal is considered to be the query signal. Thus, the present invention has identified the unknown signal using the information retrieval engine. The information for the unknown signal is the same as the information for the corresponding signal in the index. In one embodiment, the present invention associates the information with the unknown signal. In one embodiment, the present invention outputs the information to a user of the information engine  1006 . 
   From the above description, it will be apparent that the invention disclosed herein provides a novel and advantageous system and method for information retrieval. The foregoing discussion discloses and describes merely exemplary methods and embodiments of the present invention. As will be understood by those familiar with the art, the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. For example, the invention may be applied to other domains and environments, and may be employed in connection with additional applications where information retrieval is desirable. Accordingly, the disclosure of the present invention is intended to illustrative, but not limiting, of the scope of the invention, which is set forth in the following claims.