Abstract:
In some speech recognition applications, not only the language of the utterance is not known in advance, but also a single utterance may contain words in more than one language. At the same time, it is impractical to build speech recognizers for all expected combinations of languages. Moreover, business needs may require a new combination of languages to be supported in short period of time. The invention addresses this issue by a novel way of combining and controlling the components of the single-language speech recognizers to produce multilingual speech recognition functionality capable of recognizing multilingual utterances at a modest increase of computational complexity.

Description:
FIELD OF THE INVENTION 
   The invention relates to systems and methods of automatic speech recognition. More specifically, the invention is a method of multilingual speech recognition capable of recognizing utterances, which may be by themselves multilingual. The method utilizes existing components of single-language speech recognizers by combining and controlling them in such a way that multilingual speech recognition results. 
   DESCRIPTION OF PRIOR ART 
   There is a growing demand for speech recognition applications supporting several languages simultaneously. Not only does the language of an utterance to be recognized is not known in advance, but also the language may be changed during the utterance. The following are some examples of voice-activated applications where this may be the case. 
   1. Dictation/Transcription: Consider English phrases like “He&#39;s lately been ‘hundemuede’” or “As the French say, C&#39;est la vie.” 
   2. Name lookup/Name dialing: Consider commands like “Display Richard the Third,” “Display Richard Strauss,” “Display Richard the Rocket.” In each case, “Richard” is likely to be pronounced differently per English, German and French tradition respectively, yet there is no way of predicting the pronunciation based on the spelling. Moreover, it is not uncommon for a person to have the first and the last names coming from different language backgrounds, so even the full name in a contact database may by itself be a multilingual phrase. 
   3. Device navigation: Consider, e.g., a command to start an application by filename, where the command itself is spoken in a native language (e.g., Chinese) and the filename in the language of the vendor or of the operating system (e.g., English). This is an example of an application where the languages (but perhaps not the accents) are completely independent. 
   Prior art addresses multilingual recognition problems in one of the following ways: 
   1. Requiring the application to pre-specify the language before the recognition can occur; 
   2. Determining the language based on a small vocabulary of, essentially, wake-up, or activation, commands, and selecting this language for the main utterance; 
   3. Running a plurality of single-language recognizers in parallel and declaring the best overall hypothesis the recognition result; 
   4. Creating a complete multilingual recognizer based on a “Holy Grail” model, encompassing all languages, for example, all universal phonemes. 
   Method (1) does not detect the language automatically and does not support multilingual utterances. 
   Method (3), either, cannot handle utterances inside which the language change occurs. Besides, it is computationally inefficient: its execution time is the sum of execution times of individual recognizers. 
   Method (2) cannot support multilingual utterances or a language switch between the activation command and the main utterance. 
   Strictly speaking, method (4), (e.g., of which U.S. Pat. No. 5,799,278 to Cobbett et al., U.S. Pat. No. 6,212,500 to Kohler) has the correct approach. However, from a product standpoint, the method in general suffers from two related major drawbacks: 
   First, the models of words in the vocabulary (such as phoneme-based acoustic models and language models, if any) must cover all supported languages. Those models necessarily become rather large. When adding support for a new language, the models must be rebuilt to reflect training data for the new language. 
   Second, the approach doesn&#39;t scale well. Business needs may call in short order for a product supporting an arbitrary subset of generally supported languages. Dropping some languages may be mandatory in order to reduce the memory footprint and execution time of the combined recognizer and perhaps to improve the recognition quality. However, this requires re-training the models. Besides, the number of such custom-tailored projects grows combinatorially with the number of generally supported languages and is very difficult to maintain. 
   SUMMARY OF INVENTION 
   The invention relates to systems and methods of automatic speech recognition. More specifically, the invention is a method of multilingual speech recognition capable of recognizing utterances, which may be by themselves multilingual. The method utilizes existing components of single-language speech recognizer engines by combining and controlling them in such a way that multilingual speech recognition results. 
   A new component, the multilingual dispatcher (MLD) envelops language-independent components and invokes language-specific components to perform language-dependent processing. Moreover, language-specific components themselves are not affected when a language is added to or deleted from the project; a language support works like a replaceable plug-in. Such a structure enables scalable deployment of any subset of supported languages in short order. 
   The key elements of the invention are: 
   a heuristic way to make numeric scores of hypotheses (such as Viterbi scores) comparable even if produced by different language-specific recognizers; 
   a heuristic way of propagation of (seeding) a hypothesis from a hypothesis in a different language. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  shows a logical representation of a recognition vocabulary. The MLD does not know the format in which the pronunciations (as models of the word) are stored. In  FIG. 2  on, boxes with bold borders represent existing, usually language-specific, components. 
       FIGS. 2 and 3  illustrate how the MLD accesses language-specific components: 
       FIG. 2  shows a flowchart of the vocabulary builder adding a new word. Depending on the application, this procedure can be performed in advance or on the fly as needed. 
       FIG. 3  shows a flowchart of the state update procedure for the new slice of speech. 
       FIG. 4  shows a flowchart of seeding a pronunciation by an exit state of another pronunciation. There, L(P) denotes the language corresponding to the pronunciation P. 
   

   DETAILED DESCRIPTION 
   The invention combines and controls existing common components of language-specific speech recognition engines that implement vocabulary-based recognition whereby each word is equipped with one or more pronunciations and each pronunciation&#39;s runtime state (hypothesis) is represented during recognition as required by the language, but so that it contains an exit state comprising the current score of the pronunciation and history information sufficient to retrieve the best (lowest-score) utterance ending in this pronunciation. 
   NOTE: The term “pronunciation” is used here without any particular connotation to mean the model of a written word appropriate for a given language-specific recognizer. 
   Those versed in the art will immediately see that a popular Hidden Markov Model scheme falls in this class. In HMM, a pronunciation&#39;s state is a directed graph of states of phonetic elements of the pronunciation, and the terminal states of the graph are the exit states of the pronunciation. Also, in HMM, the score has the semantics of (scaled) log-likelihood. 
   Also, it is outside of scope of the proposed invention to specify how exactly the language-specific pronunciations are generated. However, those versed in the art know that it can be, for example, a pre-built or updateable dictionary, or some sort of pronunciation guessing engine, or a combination thereof. 
   According to the invention, recognition of an utterance is performed on a combined vocabulary, where each word is equipped with at least one pronunciation for each language in which the word is allowed. Each pronunciation is marked with (or otherwise knows) the language it is in. 
   The invention comprises the multilingual dispatcher (MLD) controlling a set of existing language-specific components: 
   Pronunciation generator, which, given a spelling, produces a set of pronunciations in a format sufficient to produce word models with distinct initial and exit states; 
   Pronunciation state update mechanism which is capable of pruning the unlikely states using an externally supplied threshold and producing the next pruning threshold; 
   Feature vector converter which transforms current slice of speech into the feature vector in the language-specific format; 
   Pronunciation seeding component, which effectively computes, in a computing system, the entry state of a pronunciation&#39;s model by iterating over the exit states supplied to the seeding component (such as taking the exit state with the minimum score adjusted by an appropriate language model, if any). The computing system can include at least a processor, memory, text and audio input devices and a text input device. 
   The MLD builds a vocabulary in a form semantically equivalent to containing, for each word, the following information: 
   Spelling; 
   A pair &lt;Language Id, pronunciation&gt; for all languages supported and perhaps with more than one pair for a given language. The format of a pronunciation is known to the appropriate language-specific component but is unknown to the MLD. 
   This is illustrated in  FIG. 1 . Those versed in the art know that in some cases substantial computational benefits may be achieved by storing pronunciations in compacted data structures, such as lexical trees. This is not essential for the MLD as long as the information shown in  FIG. 1  can be recovered. 
   Depending on the application, the recognition vocabulary can be built off-line, before the product is shipped (e.g., closed command-and-control applications) or online on as-needed basis (e.g., when synchronizing with a contact database in a PDA). 
     FIG. 2  illustrates a process of adding a new word to the vocabulary. (From  FIG. 2  on, boxes with bold borders represent existing, usually language-specific, components.) For the recognition phase, the MLD needs a way to compare exit scores of pronunciation hypotheses in different languages: since single-language recognizers are independent, the scores produced by them may not be directly comparable. 
   To address this issue, the MLD employs a separate score format and, for each language, a function converting the language-specific score to the MLD score. The exact nature of this function depends on the single-language recognition engine involved; in general, it is a best-fit statistically derived function. The basis for the best-fit problem is a heuristic criterion that MLD scores of certain hypotheses on qualified utterances be equal (in least-squares or similar statistical sense). More specifically, the training set should contain noise “utterances” according to the application requirements (such as a mixture of car, airport and babble noise of varied duration). The hypotheses that need to score equally are universally present silence hypotheses in all languages and, if present, noise hypotheses. (To resolve ambiguity, one selected language score may be postulated to be equal to the MLD score.) By the way of illustration, the score-converting function can be as simple as scaling the language-specific score S with a language L specific factor k(L): MLD_score=k(L)*S. From now on, this simple form will be used for the sake of clarity. 
   Speech processing usually involves a language-independent feature extraction (such as computing cepstral coefficients from the slice of speech) and a language-specific computing of the “feature vector” (such as applying an LDA transformation to the language-independent features). The MLD invokes an existing language-independent component to produce the language-independent features F and then, for each active language L, invokes an existing language-specific component to convert F to the language-specific format F(L), as illustrated in the flowchart depicted in  FIG. 3 . 
   Then, as shown on  FIG. 3 , the MLD processes all active pronunciations. For each active pronunciation P (accessed through the recognition vocabulary), its language L=L(P) is retrieved and the language L specific update component is invoked. Parameters that the MLD passes to the language-specific update component are the pronunciation P, the feature vector F(L) corresponding to the language L and the pruning threshold T transformed to the language L score format, T/k(L).). The language-specific update component is responsible for update of active states and also for update and propagation of the language-specific silence model(s). Thus, the initial silence manipulation is encapsulated within the language-specific component and not exposed to the MLD. 
   At the end of the update process, for each language, the result of computations is the same as if only that language&#39;s recognizer component was run. In particular, the new value of the pruning threshold, T(L), is computed. The MLD retrieves T(L) for each language L and computes the new threshold value (in the MLD score format) as the smallest language-specific threshold, T=min{k(L)*T(L)}, which may be relaxed by a tuning parameter as shown on  FIG. 3 . 
   Those versed in the art know that the state (hypotheses) update is a computationally expensive operation, and so it is common practice to prune away (internal) states based on a dynamically varying score threshold as unlikely and to update only remaining (active) states and the states immediately reachable from them. Within the framework of the invention, the over-the-threshold pruning of active states is occurring for all pronunciations (and thus all languages) together, against a common threshold. Typically, this means that the number of active states is significantly smaller than the sum of active states if individual language engines were to run independently. (In other words, unlikely languages will prune away.) This results in reduction of computational complexity. 
   It is a commonly used mechanism of chaining the words in the utterance recognition that an exit state of each pronunciation can “seed” or “propagate to” another pronunciation&#39;s entry state as an active hypothesis with appropriate score. (E.g., in the HMM setting, the initial state(s) of the pronunciation&#39;s graph are seeded to be the current least-score seeding state where the score may be adjusted, say, per language model.) In a single-language environment, this happens according to the (application-dependent) grammar rules and/or language model. The invention extends this mechanism to multilingual utterances. 
   Consider an exit state E of pronunciation P in the language L(P) seeding a pronunciation Q in the language L(Q) (see  FIG. 4 ). If the pronunciation which is being seeded (Q) and the seeding exit state&#39;s pronunciation (P) belong to the same language (i.e., L(P)=L(Q)), that language&#39;s seeding rules and algorithm apply. The MLD merely invokes the language-specific seeding component. 
   Otherwise, if the languages L(P) and L(Q) are different, the MLD makes a heuristic adjustment to the score of the seeding exit state E of pronunciation P. In short, the score adjustment involves adding adjustments from the language-specific components, if available, and the MLD&#39;s own adjustments. The detailed description follows. 
   Language-specific recognizers may have each a component to quantify score “price” of switching (a) to and (b) from a different language and in general dependent on the pronunciations and the history involved. The following examples illustrate this notion. 
   Example 1 
   Language-specific recognizers are equipped with class-based language models and the classes themselves are language-independent. In this case, language score penalties for the seeding pronunciation P are the score penalties for going from the class of word of P to the class of word of Q. For the language L(P) of P this is the from-language score (in the L(P) scale and computed with L(P) class transitions); for the language L(Q) of the pronunciation Q this is the to-language score (in the L(Q) scale and computed with L(Q) class transitions). 
   Example 2 
   In all languages, the class model has a class for a generic foreign-language word. Alternatively, a commonly used “generic word” class may be used for a foreign-language word. In these two cases, classes need not be the same across languages. 
   In general, the MLD assumes that a language-specific facility exists for each language, which produces out-of-language score adjustment S_out and into-language score adjustment, S_in, given the pronunciations P, Q and the exit state E. (In applications where such a facility is not available, S_out and S_in are always zeros.) The MLD computes the score adjustment (in the MLD score scale) as a weighted average of S_out and S_in: w 1 *k(L(P))*S_out+w 2 *k(L(Q))*S_in, where the weight coefficients w 1  and w 2  are tuning parameters owned by the MLD. To be consistent with seeding a same-language pronunciation, the sum of the weights should be a 1. 
   In addition, the MLD uses a “meta-language model” that provides a score adjustment S_MLD=S_MLD(L 1 ,L 2 ) for switching between any two languages L 1  and L 2  regardless of the specific pronunciations involved. In analogy with class-based language models, the whole language is thus can be though of as a “meta-class” with score adjustments for transitions between any two languages. 
   This meta-language model is thus a set of parameters owned and tuned by the MLD. Since the number of those parameters is rather large (and thus hard to tune), it is preferred that such parameter be the sum of out-of-language adjustment and into-language adjustment. This would leave two parameters per language to tune, independently of the number of languages. In some applications, a single parameter may suffice. These simplifications are worth mentioning but they are not essential to the invention. 
   Summarizing, the MLD retrieves language adjustments S_out and S_in ( FIG. 4 ) and computes the language adjustment 
   S_a=[w 1 *k(L(P))*S_out+w 2 *k(L(Q))*S_in]+S_MLD(L(P),L(Q)). 
   and taking into account the score scales, the score S of the exit state E of the seeding pronunciation P is replaced with (k(L(P))*S+S_a)/k(L(Q)). The MLD then invokes the seeding component specific to the language L(Q) of the pronunciation Q, which is being seeded (as shown on  FIG. 4 ). 
   NOTE: Some applications employ application-dependent grammar rules prohibiting the seeding of a pronunciation in certain situations. In many cases, the grammar rules apply to the words of the recognition vocabulary rather than to their pronunciations and as such are language-independent. In cases where language-specific grammar rules do differ among languages, the grammar rules for multilingual phrases must be defined in addition to the language-specific grammars. This is an application level issue, and the MLD can handle it by invoking an application-dependent function. It is worth noting a special case where the multilingual grammar is reduced to the rules of the language L(Q) of the pronunciation Q that is being seeded. (An example of it is a language-independent grammar.) In this case, the language-specific grammar-checking component can be a part of the language-specific seeding component and not be exposed to the MLD (though this is computationally less efficient). This is why any grammar checking is omitted in  FIG. 4 . 
   The result of the recognition is the word sequence recovered from the exit state with the least MLD score among all exit states of all pronunciations in all languages at the moment when end of utterance is detected by existing means. 
   Coverage 
   The description above is concerned with a single pass of recognition. There exist multiple-pass speech recognizers. The invention of combining single-language recognizers into a multilingual recognizer applies to any or all passes of the of multiple-pass engines. 
   The invention applies equally to discrete- and continuous-speech recognition engines.