Patent Publication Number: US-2007112839-A1

Title: Method and system for expansion of structured keyword vocabulary

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
      This application claims priority to, and incorporates by reference in its entirety, U.S. Provisional Application No. 60/688,055 filed on Jun. 7, 2005. 
    
    
     TECHNICAL FIELD  
      The present invention relates to classifying media content using a structured vocabulary.  
     BACKGROUND  
      The amount of information available in the World Wide Web and other document corpora continues to expand at a significant pace. Users with a connection to the Internet or other networks are able to search for and identify a large volume of content that would previously have very been difficult or even impossible to find. Unfortunately, the ease of locating certain content varies significantly by the format of the content that is sought. While users seeking textual content on the World Wide Web have a number of good search services such as google.com to identify results responsive to a search query, the performance of tools to search for other types of content have lagged far behind. In particular, many types of media, including but not limited to, audio, video and images, are difficult to classify and locate for retrieval in an automated fashion (an instance of any type of media will hereinafter be referred to as a media content unit).  
      Media content units are particularly challenging to accurately search because there are few reliable algorithms that can programmatically analyze the content in a media content unit and store the media content unit in a manner that allows it to be efficiently located. For example, a human user looking at a picture of a sailboat will very quickly be able to determine not only that the image is a sailboat, but also extract other pieces of information about the image such as the number of people on the sailboat, the color of the sailboat sails, and the condition of the surrounding seas. While image evaluation systems have been developed that use various techniques to describe and classify images, detecting the colors, shapes, and textures of an image to derive the subject matter of the image is an incredibly challenging and complex problem. As a result, automated image evaluation systems have fallen short of the performance necessary to become commercially successful. The same limitations and problems extend to other media content, such as audio or video, as well.  
      Because of the performance limitations of automated media content recognition systems, some systems have relied upon a file name or on metadata associated with a media content unit to predict the contents of the media content unit. Unfortunately, however, using the file name or metadata provided by a large body of users may not provide optimum results since different users often use vastly different naming conventions to describe the same content. For example, an image of a sailboat under sail may be characterized by one user as “hard tack to port” whereas another user may characterize it as “enjoying a gusty day on the seas.” Search engines that rely upon vocabulary selected by a large body of users to characterize media may therefore return inherently unreliable results.  
      A different approach is taken by U.S. Pat. No. 6,735,583 to Bjarnestam et al. entitled “Method and System for Classifying and Locating Media Content,” and assigned to the applicants of the present case. Bjarnestam et al. disclose a structured vocabulary system that may be used in a media classification and search system to provide a better index to media content. Such a system solves many of the problems identified above, but does so with a manually intensive process. While the system disclosed in Bjarnestam et al. provides superior performance to other existing technologies, it would be beneficial to further automate the classification of media content units in order to make available for searching a greater number of media content units. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a block diagram illustrating components of a facility for classifying and searching for media content.  
       FIG. 2  is a block diagram illustrating a facility for analyzing metadata associated with media content units and storing media content units so that they are accessible via a keyword index or keyword navigation.  
       FIG. 3  is a flow diagram illustrating processing of metadata associated with a media content unit in order to associate the media content unit with keywords from a structured vocabulary.  
       FIG. 4  is a flow diagram illustrating the operation of a segmentation tool for processing metadata associated with a media content unit.  
       FIG. 5  is a flow diagram illustrating the operation of a mapping tool for processing descriptive phrases identified from metadata.  
       FIG. 6  is a block diagram illustrating records in a media content unit/keyword database. 
    
    
     DETAILED DESCRIPTION  
      A hardware and software facility is disclosed for analyzing metadata associated with media content units and classifying the media content units using keywords in a structured vocabulary. A media content unit is a media resource that can be electronically stored and that contains images or sound, either separately (e.g. photos, slideshows, silent films or audio recordings), combined (e.g., videos or animation), or in conjunction with other content (e.g., presentations with text, multimedia presentations). The analysis of the metadata and the classification of the associated media content units using the structured vocabulary is preferably done in an automated fashion. The metadata associated with each media content unit is segmented into a series of descriptive phrases, where a descriptive phrase is one or more words, numbers, characters, or other symbols. The facility matches the descriptive phrases to keywords in a structured vocabulary, and associates the identified keywords with the media content units. Descriptive phrases that were not found in the structured vocabulary are tracked as candidate phrases for later addition to the structured vocabulary. A keyword index to the media content units may be constructed. The index is used to identify specific media content units that are responsive to search queries in a reliable and accurate fashion. Alternatively, the keywords may be used in a browse hierarchy to allow users to navigate to desired media content units. The keywords may also be displayed in conjunction with media content units to further characterize the media content units.  
      The facility includes a segmentation tool that applies a rule set to extract certain descriptive phrases from the metadata. Pre-processing may be performed on the metadata to remove words, numbers, characters, and other symbols that are not useful in the classification process. Segmentation of the remaining metadata into descriptive phrases is performed in an automated fashion without human intervention, thereby enabling the efficient classification of a significant number of media content units. In an embodiment of the facility, the segmentation tool relies on an application-specific dictionary to aid in the segmentation process. To improve segmentation efficiency, the application-specific dictionary is a subset of all of the keywords contained in the structured vocabulary. Accurate segmentation significantly increases the number of media content units classified by the facility, and ultimately results in a more accurate matching of media content units to associated keywords. Post-processing of the descriptive phrases may be performed to weight the descriptive phrases according to the relative importance of the descriptive phrase to the media content unit.  
      The facility enables descriptive phrases that are tracked as candidate phrases to be added as keywords to the structured vocabulary. The determination of whether to add a descriptive phrase to the structured vocabulary may be based in part on the type of descriptive phrase. The determination of whether to add a descriptive phrase to the structured vocabulary may also be based in part on the frequency with which the descriptive phrase occurs in the metadata. The descriptive phrase may be automatically added as a keyword to the structured vocabulary, or may be added by an operator.  
      Various embodiments of the invention will now be described. The following description provides specific details for a thorough understanding and enabling description of these embodiments. One skilled in the art will understand, however, that the invention may be practiced without many of these details. Additionally, some well-known structures or functions may not be shown or described in detail, so as to avoid unnecessarily obscuring the relevant description of the various embodiments. The terminology used in the description presented below is intended to be interpreted in its broadest reasonable manner, even though it is being used in conjunction with a detailed description of certain specific embodiments of the invention.  
       FIG. 1  is a block diagram illustrating the components of a media classification and search system  100 . The classification and search system  100  is capable of automatically classifying media content units using keywords from a structured vocabulary and enabling access to the media content units via keyword search or keyword navigation. The media classification and search system  100  is connected via a communication medium, for example the Internet  110 , a proprietary network, or other communication connection, to a number of remote computing systems  120 . Metadata associated with media content units is received by the facility via the communication medium and is processed by a classification service  130 . The received metadata may be structured, unstructured, or a combination of the two. Structured metadata is metadata that has a known identity or format, for example, a portion of the metadata that describes the author of the media content unit. Unstructured metadata is metadata that contains information of an unknown identity or unknown format. Classification service  130  associates one or more keywords contained in a structured vocabulary with each media content unit based on some or all of the metadata. The structured vocabulary may be hierarchical or non-hierarchical. The structured keyword vocabulary is stored in a database  140 , and the mapping of media content units to the keywords in the vocabulary database is stored in a database  150 . The underlying media content unit or a pointer to the location where the media content unit is stored (such as a network path) may be stored in a media content unit database  160 . Once media content units are associated with keywords in the structured vocabulary, the facility may generate a reverse index  170 . The index  170  allows media content units to be identified that are responsive to search terms contained in search queries. Those skilled in the art will appreciate that many different types of indices may be generated, depending on performance and other considerations. Vocabulary database  140 , media content unit/keyword database  150 , media content unit database  160 , and index  170  are all identified as part of a general data storage area  175 . Those skilled in the art will appreciate that the actual implementation of the data storage area  175  may take a variety of forms, and the term “database” is used in the generic sense to refer to any data structure that allows data to be stored and accessed, such as tables, linked lists, arrays, etc.  
      When a search is to be performed to locate a particular media content unit or category of media content units, a search service  180  receives the search query or search request and applies the search terms contained in the query against index  170 . The search query may include both a text query and other information that further defines the parameters of the search. The index  170  is used to identify media content units in the database  160  that are responsive to the search query. Those skilled in the art will appreciate that standard search techniques may be used to pre-process the search query, as well as to post-process and prioritize the resulting search results that are responsive to the query. Those skilled in the art will also appreciate that some or all of the keywords may be used in a browse hierarchy to allow users to navigate to desired media content units. In some applications, the keywords may also displayed in conjunction with media content units to further characterize the media content units. The use of a structured keyword vocabulary greatly increases the speed and likelihood that users submitting search queries or browsing will be able to identify one or more media content units that are responsive to their search. Once the user identifies one or more media content units, the displayed keywords may provide an improved context in which the user may utilize the media content units.  
       FIG. 2  is a block diagram illustrating the various components of the classification service  130 , which receives metadata associated with media content units, analyzes the metadata, and associates keywords from the structured vocabulary with the media content units based on all or part of the metadata. In some embodiments, the classification service receives as input a media content unit  210 . Associated with each media content unit is metadata  220  that provides information about the media content unit. The metadata may be provided in a variety of different formats, and may contain information such as the originating artist, the title, a general description of the contents, the date, and the size and format of the content unit. In general, each media content unit will typically have a file name and a file extension indicating the type of content (e.g., picture.bmp). In many cases, substantially more metadata may be associated with the media content unit. For example, in some embodiments a user submitting a media content unit to the facility may be required to complete a form and input certain required metadata (such as a caption and detailed description) for the submission to be accepted. The format and the amount of metadata may therefore vary significantly from media content unit to media content unit. In the example depicted in  FIG. 2 , the media content unit  210  is an image of Lance Armstrong at the Tour de France. Metadata  220  associated with the image provides additional details about the contents of the media content unit, specifically “Getty Images:Paris: Lance Armstrong crosses the finish to win his seventh Tour de France. This is Lance&#39;s last year in the peloton.” While an image of Lance Armstrong will be used as an example in the discussion below, it will be appreciated that the image and metadata format is merely representative of the type of media content unit and metadata that may processed by the facility. In an alternate embodiment, the facility may only receive metadata that is currently, or in the future will be, associated with media content units. Such alternate embodiment will be described in additional detail below.  
      The media content unit  210  and associated metadata  220  is received by a flow manager  230  which manages the processing of the metadata through a variety of steps to be described in greater detail below. Flow manager  230  is connected to a segmentation tool  240 , a mapping tool  250 , a vocabulary management tool  260 , and a translation tool  270 . The segmentation tool  240  parses the metadata  220  associated with the media content unit, and separates the metadata into one or more descriptive phrases for further processing. Descriptive phrases may be comprised of one or more words, numbers, characters, or other symbols that are compared to keywords in the structured vocabulary in order to classify the media content unit. Further details about the segmentation tool are described with respect to  FIG. 4 .  
      The mapping tool  250  receives as input the descriptive phrases identified by the segmentation tool. The mapping tool  250  is connected to a database  140  that contains the structured keyword vocabulary. For each descriptive phrase associated with a media content unit, the mapping tool performs a lookup in the structured keyword vocabulary and attempts to match the descriptive phrase with a keyword contained in the vocabulary. Information about the structured keyword vocabulary, including how it is constructed and modified, may be found in U.S. Pat. No. 6,735,583 entitled “Method and System for Classifying and Locating Media Content,” which is herein incorporated by this reference in its entirety. Keywords that are identified from the vocabulary database as matching the descriptive phrases are stored in association with the media content units as is described in additional detail below. Descriptive phrases that are not mapped to keywords in the vocabulary database  140  are stored as a list of candidate phrases that may be added to the structured vocabulary in the future. Further details about the mapping tool are described with respect to  FIG. 5 .  
      The vocabulary management tool  260  is a tool used to manage the composition and structure of the vocabulary database. It is primarily a tool used by operators to maintain the integrity and value of the structured vocabulary database. One function of the vocabulary management tool is to receive descriptive phrases that were not contained in the structured keyword vocabulary. Descriptive phrases are added to the vocabulary database if the descriptive phrase represents a valuable concept that is missing in the vocabulary database. Descriptive phrases are not added if the phrase is perceived to add little or no value to the structured vocabulary.  
      The translation tool  270  may be used to translate descriptive phrases, and particularly descriptive phrases that are candidate phrases, into other languages that are supported by the facility. The translation tool may be beneficial when a candidate phrase is not contained in the structured keyword vocabulary in the received language. By translating the candidate phrase into each of the supported languages, the candidate term may be searched or browsed in different languages without manual translation. The translation tool also be beneficial when a candidate phrase is not contained in the structured keyword vocabulary in the received language, but it is contained in the structured keyword vocabulary in a different language. By translating the candidate phrase into a language where the translated form already exists as a keyword in the structured vocabulary, the facility may leverage all pre-existing relationships that the keyword may have or information about the keyword in order to better characterize the candidate phrase.  
      The flow manager  230  is also connected to the media content unit database  160 . When a media content unit  210  and associated metadata  220  is received by the flow manager, the media content unit  210  or a pointer to the media content unit  210  is stored in the media content unit database  160  for subsequent access. The flow manager ensures the orderly classification of media content units by the facility, making calls to the various tools and receiving results from each of the tools when processing is complete.  
       FIG. 3  is a flow diagram presenting additional detail about the classification process  300  implemented by the facility. At a block  310 , the facility receives a media content unit and associated metadata. Media content units and metadata may be received singly or in groups from a variety of internal and external sources, including third-party submissions via a Web form, third-party submissions via Email, third-party submissions via an FTP transfer, and other methods. At a block  320 , the metadata is processed by the segmentation tool  240  to identify descriptive phrases contained in the metadata. The method of processing the metadata is described in greater detail in  FIG. 4 .  
       FIG. 4  is a flow diagram of a segmentation process  400  used to automatically segment metadata into descriptive phrases. At a block  410 , the facility optionally pre-processes the metadata to remove information that has been experimentally proven to be of less value in classifying a media content unit. The information that is removed will vary depending on the type of media content unit and the intended use of the classification and search system. For example, in an embodiment of the system that processes photographic images submitted by photographers, it was found that performing one or more of the following pre-processing steps to the metadata improved the classification results: (i) removing numbers; (ii) removing parentheses and the contents of any parenthetical; (iii) removing any attributions of source; (iv) removing all sentences other than the first sentence in the metadata; (v) removing some or all non-grammatical symbols; and (vi) removing all structured metadata and leaving only unstructured metadata. Note that “remove” as used in this context may mean either the physical deletion of metadata or merely the tagging of the identified metadata so that it is not further processed. The pre-processing rules may be generated manually or in an automated fashion.  
      At a block  420 , the facility extracts descriptive phrases from the metadata that remains after the pre-processing step (if performed) or from all of the metadata (if no pre-processing). Descriptive phrases are any words, numbers, characters, or other symbols that are useful to classify the media content unit in a manner that allows the media content unit to be easily identified though search or navigation. Those skilled in the art will recognize that there are many different technologies that may be used to extract descriptive phrases from text, including statistical processing and linguistic processing (including morphological, lexical, syntactic, and semantic processing). In an embodiment of the facility, an application that is used to extract descriptive phrases is Linguistic DNA software provided by Nstein Technologies of Montreal, Canada (nstein.com). The Linguistic DNA software relies on a proprietary set of linguistic rules and a proprietary dictionary to extract content from documents. The Linguistic DNA dictionary may also be supplemented with an application-specific dictionary of terms. When applied to the example metadata  220  depicted in  FIG. 2 , the facility may return the descriptive phrases “Lance Armstrong,” “finish line,” “win,” and “Tour de France.” Each of these phrases is helpful in classifying the associated media content unit  210 . Other components of the metadata that are less helpful to classifying the media content unit, such as “seventh,” “last year,” and “peloton,” have been ignored by operation of the pre-processing and extraction processing step.  
      In an embodiment of the system, the facility provides a portion of the keywords from the keyword vocabulary database  140  as an application-specific dictionary to supplement the Linguistic DNA dictionary. The application-specific dictionary provided to the segmentation tool is less than the entire vocabulary that is contained in the vocabulary database  140 . The structured vocabulary will typically contain additional vocabulary that is helpful for searching or editorial purposes, but which doesn&#39;t serve a beneficial purpose when classifying media content units. A representative example of a keyword that is helpful for searching or editorial purpose is the term “between” when used in conjunction with photographic images. When used in the context of search, “between” is useful to specify the location of one or more objects in the photographic image. When “between” is found in metadata, however, it rarely is used in a manner to connote the relative position of objects in the image. As a result, in most cases the association of the keyword “between” to an image is one that is best done manually by an editor. For that reason, although the keyword “between” will appear in the vocabulary database  140 , it will not be included in the reduced vocabulary set that is used to automatically extract descriptive phrases from metadata.  
      At a block  430 , optional post-processing is performed on the one or more descriptive phrases that were identified in block  420 . Post-processing may involve formatting, spelling correction, or other manipulations of the descriptive phrase list. Post-processing may also involve assigning weights to the identified descriptive phrases. The assigned weights are values that represent the relative importance of a descriptive phrase as applied to the associated media content unit. Using the example described above, the image may be primarily directed to the concept identified by the descriptive phrase “Lance Armstrong,” with the other identified descriptive phrases being applicable to the image to a lesser extent (e.g., “finish line”). In such cases, the descriptive phrase or phrases associated with the primary concept of the media content unit would receive higher weights, while the descriptive phrase or phrases associated with other concepts would receive a lower weight. Weights may be determined in a variety of ways, including but not limited to: (i) assigning a weight depending on the location of the descriptive phrase in the metadata (e.g., descriptive phrases located at the beginning of the metadata would receive a higher weight than descriptive phrases located that the end of metadata); (ii) assigning a weight based on a subjective human analysis of the media content unit; (iii) assigning a weight based on whether the descriptive phrase is contained in structured or unstructured metadata; and (iv) assigning a weight based on a linguistic analysis of the descriptive phrases. Those skilled in the art will appreciate that other methods are equally applicable for assigning weights to the descriptive phrases. At a block  440 , the facility outputs the list of identified descriptive phrases. The phrases may be returned in an object list or other form to allow for subsequent manipulation.  
      Returning to  FIG. 3 , after processing the metadata to identify descriptive phrases, at a block  330  the mapping tool  250  searches the structured vocabulary database  140  to match the descriptive phrases with keywords in the vocabulary. As is described in U.S. Pat. No. 6,735,583, a structured vocabulary is used to provide a taxonomy of descriptive terms to classify and retrieve various types of media content. A benefit of using a common keyword vocabulary is that users are more easily able to identify media content units by browsing or using search queries. The method of matching the descriptive phrases to keywords is described in greater detail in  FIG. 5 .  
       FIG. 5  is a flow diagram of a mapping process  500  used to automatically match descriptive phrases with keywords from the structured vocabulary. At a block  505 , the facility selects a descriptive phrase from the descriptive phrases previously identified from the metadata. At a block  510 , the facility queries the structured vocabulary database  140  to determine if the descriptive phrase matches a keyword contained in the structured vocabulary. Those skilled in the art will appreciate that a match may be an exact match or a close match (e.g., to account for variations in spelling, uses of wildcards, diacritics in words). Moreover, since both the descriptive phrase and the keywords in the structured vocabulary may each comprise one or more terms, all of the terms in the descriptive phrase and the keyword must match or closely match for there to be a determination that the descriptive phrase is the same as the keyword. At decision block  515 , the facility determines whether the descriptive phase is contained in the structured vocabulary. If the descriptive phrase is not in the vocabulary, the facility continues to blocks  520 - 535 . If the descriptive phrase is in the vocabulary, the facility continues to blocks  540 - 555 .  
      If the descriptive phrase is not in the vocabulary, at a decision block  520  the facility checks to see if the descriptive phrase is on a list of candidate phrases that are to be considered for addition to the structured vocabulary. In order to allow the structured vocabulary to expand over time to include new concepts, the facility maintains a list of all descriptive phrases that were identified in media content unit metadata, but which aren&#39;t currently in the structured vocabulary. The list of descriptive phrases is composed of the descriptive phase, a candidate ID number (an identifier that uniquely identifies the descriptive phrase), and a count of the number of times that the descriptive phrase has been found in metadata from all media content units processed by the facility. Items in the list of descriptive phrases are referred to as “candidate phrases” since they are maintained on the list until promoted to the structured vocabulary or deleted from the list for consideration. If the facility finds the descriptive phrase on the list of candidate phrases at decision block  520 , processing proceeds to block  535  where the count associated with the candidate phrase is incremented by one to indicate another occurrence of the phrase. At a block  530 , the candidate ID number is associated with the media content unit from which the descriptive phrase was derived. Assigning a candidate ID number to the media content unit allows the descriptive phrase to be associated with the media content unit in the event that the descriptive phrase is promoted to be in the keyword vocabulary in the future. Assigning a candidate ID number to the media content unit also allows the media content to be located when a user searches on the particular descriptive phrase, even if that descriptive phrase hasn&#39;t yet been added to the structured vocabulary. If the facility finds that the descriptive phrase is not on the list of candidate phrases, at a block  525  the descriptive phrase is added to the list of candidate phrases and a new candidate ID number is assigned to the descriptive phrase. The facility then proceeds to block  530  where the candidate ID number is associated with the media content unit from which the descriptive phrase was derived. Following block  530 , the facility proceeds to decision block  560  where a test is made to determine if any additional descriptive phrases remain to be processed by the facility. If additional descriptive phrases remain, processing continues at a block  505 . Otherwise, the mapping process  500  is complete and processing returns to control of the flow manager  230 .  
      In an embodiment of the facility, immediately or on a periodic basis the list of candidate phrases is reviewed and a decision is made on whether to remove a descriptive phrase from the list or add it as a new keyword in the structured vocabulary. The determination of whether to add the descriptive phrase is aided by the type of descriptive phrase. For example, descriptive phrases that correspond to names of famous people (e.g., celebrities, politicians, athletes), names of locations (e.g., city names, venue names, geographic feature names), and nouns (e.g., bicycle, tree) may be more likely to be added to the structured vocabulary than descriptive phrases that correspond to adjectives or descriptive phrases that correspond to similes. The determination of whether to add the descriptive phrase to the structured vocabulary is also aided by the count that is maintained of the number of occurrences of the descriptive phrase in all metadata that has been analyzed. A high count for a particular phrase may indicate that the phrase is either very topical or has been adopted by the public for general use. In such a case, the descriptive phase may be more readily added to the structured vocabulary. A low count for a particular phrase may indicate that the phrase is rarely used. In such a case, the descriptive phrase may be removed from the list of candidate phrases and added to an exclusion list. The exclusion list contains those descriptive phrases that are specifically excluded from being added to the structured vocabulary in the future. The review of the list of candidate phrases may be done on a periodic basis by an operator, with the review occurring more frequently if one or more of the candidate phrases have high counts. For example, an alarm may be associated with a count crossing a threshold, thereby triggering a review to occur. Alternatively, the review and promotion may take place automatically, such as a process whereby descriptive phrases are automatically promoted to the keyword vocabulary when the count of a particular descriptive phrase reaches a threshold number that suggests widespread adoption of the descriptive phrase.  
      Returning to decision block  515 , if the descriptive phrase is in the structured vocabulary, the facility proceeds to a decision block  540 . At decision block  540  the facility checks to see if the descriptive phrase is ambiguous. A descriptive phrase is ambiguous if there are multiple possible keyword concepts that map to the same descriptive phrase. For example, the phrase “Henry Ford” gives no indication of which Henry Ford is the appropriate one to associate with the media content unit, as there are numerous Henry Fords (e.g., the founder of the Ford Motor Company, the illustrator, the early New Orleans jazz string base player, or the British ecological geneticist, among others). Ambiguous keywords may be marked in the structured vocabulary as ambiguous, or may be identified in the structured vocabulary as being ambiguous by the fact that two or more keywords in the structured vocabulary match or closely match the descriptive phrase. If a match or close match exists between the descriptive phrase and a keyword that is considered ambiguous, at a block  545  the descriptive phrase is queued for disambiguation by an operator or by another process. The disambiguation process resolves the ambiguity of the descriptive phrase by selecting the appropriate keyword that should be associated with the media content unit. Until the descriptive phrase has been disambiguated, the descriptive phrase is not searchable by the facility using keywords from the structured vocabulary. After queuing for disambiguation, the facility continues to decision block  560  where a loop is implemented to process remaining descriptive phrases.  
      Returning to decision block  540 , if the descriptive phrase is not ambiguous, processing continues at a block  550 . At block  550 , the facility assigns a keyword ID number to the media content unit. The keyword ID number is a number that uniquely identifies a keyword in the structured vocabulary. If optional post-processing had been performed on the list of descriptive phrases to weight the descriptive phrases, the weighting assigned to the descriptive phrase may also be assigned to the keyword as well. After assigning a keyword ID, the facility continues to decision block  560  where a loop is implemented to process any remaining descriptive phrases.  
      At decision block  560 , a test is made by the facility to determine if any additional descriptive phrases remain to be processed. If phrases remain, processing continues at block  505 . Otherwise, the mapping process  500  is complete and processing returns to control of the flow manager  230 . At this point in the processing, each media content unit may be associated with one or more keywords in the structured vocabulary database, with one or more candidate phrases, or may have no associations based on the descriptive phrases that were identified in the media content unit&#39;s metadata.  
      Returning to  FIG. 3 , at a block  340  the facility stores a mapping of the media content unit with keywords from the structured vocabulary and with candidate phrases. A representative mapping table of media content units is depicted in  FIG. 6 . At a block  350 , the facility stores the media content unit or a pointer to a location of the media content unit in the database  160  so that the media content unit can be subsequently accessed.  
       FIG. 6  is a block diagram of a representative table  600  that correlates each media content unit with keywords associated with that media content unit. Table  600  also correlates each media content unit with any candidate phrases. Each record in the table contains a media content unit ID field  610 , one or more keyword fields  620 , and one or more candidate fields  640 . The media content ID field  610  contains an identification number that uniquely identifies a media content unit stored in the database  160 . The one or more keyword fields  620  contain one or more keywords that have been determined to be associated with the media content unit by an analysis of the media content unit&#39;s metadata, and the one or more candidate fields  640  contain one or more candidate phrases that have been determined to be associated with the media content unit by an analysis of the media content unit&#39;s metadata. If the analysis of the metadata associated with a media content unit failed to identify any keywords or candidate phrases in the metadata, the keyword fields and candidate fields would be empty. Using the example media content unit shown in  FIG. 2 , a record  650  has been created in table  600 . In record  650 , the media content unit has been identified by a unique identifier “0167298,” and has been associated with the keywords “Lance Armstrong,” “finish line,” “win,” and “Tour de France.” As a second example, record  660  identifies the keywords “mountain,” “K2,” “climbing,” and “summit” for a media content unit identified by the unique identifier “7295863.” Moreover, a candidate phrase “Ed Viesturs” has also been associated with the media content unit. Correlating the candidate phrase with the media content unit allows the media content unit to be identified when a search for “Viesturs” is performed, even though the descriptive phrase “Ed Viesturs” may not have been introduced into the structured vocabulary yet. It will be appreciated that while the keywords and candidate phrases are represented in table  600  in textual form for purposes of clarity, an implementation of the table would instead contain reference numbers or other pointers that uniquely identify the respective keyword or candidate phrase vocabulary database  140 . It will also be appreciated that while  FIG. 6  depicts a table whose contents and organization is designed to make it more comprehensible by a human reader, those skilled in the art will appreciate that the actual data structure used by the facility to store this information may differ from the table shown, and that they, for example, may be organized in a different manner (e.g., may be implemented using a number of tables rather than a single table), may contain more or less information than shown, may be compressed and/or encrypted, and may be optimized in a variety of ways.  
      Returning to  FIG. 3 , after the mapping of the media content unit to one or more keywords in the keyword vocabulary is performed at block  350 , classification is complete for that particular media content unit. The classification process  300  may be repeated by the facility as required for each received media content unit.  
      After a media content unit has been classified by the classification service  130 , the index  170  may be updated to allow the media content unit to be quickly identified. Additional information about the indexing and searching of media content units may be found in U.S. Pat. No. 6,735,583. If the optional weighting of keywords has been performed by the facility, enhanced search functionality may be provided as the weighting can be used to order or refine the search results. Those media content units having responsive keywords with greater weighting may be promoted in the search results, and those media content units having responsive keywords with lesser weighting may be demoted in the search results The keywords may also be used in a browse hierarchy to allow users to easily navigate to desired media content units, or may be displayed next to media content units to better characterize the media content units. As previously discussed, using the structured vocabulary greatly increases the likelihood of a user finding a media content unit or group of media content units in which they are interested. The automated segmentation and mapping that is managed by the flow manager  230  also ensures that a large volume of media content units can be quickly classified. As a result, search facility and navigation performance will be further improved by making the media content units capable of being located on a timely basis.  
      While various embodiments are described in terms of the environment described above, those skilled in the art will appreciate that various modifications may be made to the facility without impacting the overall functionality. For example, while it is contemplated that the media content unit and the metadata are stored as separate data structures, in certain formats the media content unit and the metadata may be merged into a single data structure or may be split between multiple data structures. In such an event, the facility may first separate the received media content unit and metadata into two portions that may then be manipulated in accordance with the processes described herein.  
      Moreover, in an alternate embodiment, when the metadata is first received by the facility it may not already be associated with media content units. For example, the received metadata may describe a type of media content unit that will be received or identified in the future. The delay in having media content units to associate with keywords does not impact processing of the metadata. The metadata is processed as described herein, with keywords identified from descriptive phrases segmented from the metadata. The keywords identified from the metadata are then stored and assigned to one or more media content units as they are identified or received in the future. In this fashion, a “bucket” of media contents units may be defined and keywords for the bucket identified in advance. As media content units are received into the bucket, they are automatically correlated with the keywords. Similarly, keywords for a single media content unit may be defined in advance and the keywords assigned to the media content unit when it is identified or received in the future.  
      It will also be appreciated that the facility may be implemented in a variety of environments including a single, monolithic computer system, a distributed system, as well as various other combinations of computer systems or similar devices connected in a variety of ways. The facility may utilize third-party services and data to implement all or portions of the aforementioned functionality. Moreover, as used herein, the term “database” should be interpreted to include any data structure that allows data to be stored and accessed, such as tables, linked lists, arrays, etc. Those skilled in the art will also appreciate that the steps shown in  FIGS. 3, 4 , and  5  may be altered in a variety of ways. For example, the order of the steps may be rearranged, substeps may be performed in parallel, steps may be omitted, or other steps may be included.  
      From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the spirit and scope of the invention. Accordingly, the invention is not limited except as by the appended claims.