Abstract:
Provided are an automatic speech translation system and a method for obtaining accurate translation performance with a simple structure. Because input and output sentences are written in different languages, automatic speech translation requires techniques for processing different languages. Repetition of text processing like morpheme analysis or sentence parsing in conventional automatic speech translation can complicate the overall translation process. Meanwhile, although input and output sentences are written in different languages, they have to have the same meaning and a corresponding sentence form and words. Accordingly, the corresponding words and sentence forms of the two languages can be expressed with a simple structure and utilized in the automatic speech translation process, thereby maintaining consistency during the process and avoiding unnecessary process repetition, which reduces errors and improves performance.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to and the benefit of Korean Patent Application No. 2006-0108979, filed Nov. 6, 2006, the disclosure of which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to an automatic speech translation system, and more particularly, to an automatic speech translation method and system based on a corresponding sentence template. 
     2. Discussion of Related Art 
     Generally, three steps are needed for automatic speech translation: a speech recognition step of converting speech in one language into a written sentence, a language translation step of translating the sentence into another language, and a speech synthesis step of producing and outputting speech corresponding to the translated sentence. Here, each step commonly uses text sentences as input/output. 
     However, these three steps in the automatic speech translation process have been developed to be performed independently. This causes that the same kinds of processes are repeated in each step. Consequently, not only the efficiency of overall translation process is poor, but also the quality of overall automatic speech translation tends to be poorer due to the error accumulation of repeated processes. 
     That is, even though morpheme analysis data is used in the speech recognition step, the speech recognition result is output simply as text which is morpheme-analyzed or parsed again in the language translation step. Also, the sentence output from the language translation step is transferred to the speech synthesis step in text form where it is morpheme-analyzed yet again in order to extract syntax data, prosody data, and so on. 
     During these steps, processing errors may be accumulated, thereby degrading automatic speech translation performance. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to an automatic speech translation method and system based on a corresponding sentence template in which each step generates output as a sentence template rather than as text and transfer the output to a next step, thereby avoiding functionally unnecessary and repetitive processes and maintaining and utilizing data obtained from a previous process. 
     One aspect of the present invention provides an automatic speech translation system based on a corresponding sentence template, the system including: a speech recognizer searching a sentence template network in response to an input speech signal and outputting the best matching sentence template along with the corresponding words for the classes included in the sentence template; a language translator receiving the sentence template and the words which are generated by the speech recognizer, and outputting a translated sentence template and translated words using a corresponding sentence template dictionary and a corresponding word dictionary, respectively; and a speech synthesizer searching for prosody data corresponding to the translated sentence template in a sentence template prosody data dictionary and synthesizing speech based on the search result. 
     Another aspect of the present invention provides an automatic translation method based on a corresponding sentence template, the method including the steps of: (a) extracting speech features used for speech recognition from an input speech signal, and outputting an sentence template most closely corresponding to the input speech signal; (b) searching for an output sentence template corresponding to the input sentence template and translated words, with respect to words included in the input sentence template; (c) extracting prosody data with respect to the output sentence template; and (d) synthesizing a speech signal based on the prosody data. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of the present invention will become more apparent to those of ordinary skill in the art by describing in detail exemplary embodiments thereof with reference to the attached drawings in which: 
         FIG. 1  is a block diagram of an automatic speech translation system based on a corresponding sentence template according to the present invention; 
         FIGS. 2A to 2C  illustrate a sentence template network of a speech recognizer according to the present invention; 
         FIGS. 3A and 3B  illustrate sentence templates output from the speech recognizer and attached word lists according to the present invention; 
         FIG. 4  illustrates a language translation process according to the present invention; and 
         FIG. 5  illustrates a sentence template prosody data expression used in a speech synthesizer according to the present invention. 
     
    
    
     DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS 
     The foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of exemplary embodiments of the invention, as illustrated in the accompanying drawings. While, the present invention may be applied to automatic speech translation between any languages, automatic speech translation from Korean to English will be described herein for convenience. 
       FIG. 1  is a block diagram of an automatic speech translation system based on a corresponding sentence template according to the present invention. 
     As illustrated in  FIG. 1 , the system includes a speech recognizer  100 , a speech translator  200  and a speech synthesizer  300 . 
     First, the speech recognizer  100  includes a feature extracting part  110  extracting speech features used for speech recognition from an input speech signal, an acoustic model  120  containing the acoustic model of phonemes, and a pronunciation dictionary  130  representing the pronunciation of each word by a series of phonemes, a sentence template network  140  representing the whole sentence template space which corresponds to all the candidate sentences to be recognized, and a sentence searching part  150  determining a sentence most closely matching the speech signal by searching the sentence template network  140  using the acoustic model  120  and the pronunciation dictionary  130  on the basis of the speech features extracted from the feature extracting part  110  and outputting the sentence template and its word components most closely corresponding to the input sound signal as the recognition result. 
     The sentence template network  140  is a group of sentence templates in which each sentence to be recognized is marked as a network of words. A sentence template is designed to be divided into a set of sentence elements so that each sentence element can have its corresponding counterpart between two given languages. The sentence element can be shared among sentence templates in order for the sentence template network to be more compact. 
     Also, a sentence element may contain one or more word classes which represent a set of words having the same characteristic. Using the word class, the sentence element becomes more expressive, that is, can accommodate more sentence instances. 
       FIGS. 2A to 2C  illustrate a sentence template network of a speech recognizer according to the present invention, and  FIG. 2A  illustrates a sentence template network in which each sentence template is represented as a path from “Start” node to ‘End’ node. For example, the sentence template for the sentence, “                                         ? (How long does it take from Incheon to New York by plane?)”, is illustrated in  FIG. 2A . For readability, English expressions are followed with parentheses for Korean expressions from now on.
     Referring to  FIG. 2A , the illustrated sentence template is composed of four sentence elements: ‘&lt;name of place&gt;            (from)’, ‘&lt;name of place&gt;         (to)’, ‘&lt;transportation&gt;         (by)’, and                    ? (How long does it take?)’. The first three sentence elements are shown to include the classes, ‘&lt;name of place&gt;’ and ‘&lt;transportation&gt;’, in addition to words or phrases.
       FIGS. 2B and 2C  illustrate a class network of the classes, ‘&lt;name of place&gt;’ and ‘&lt;transportation&gt;’, respectively. The class network of ‘&lt;name of place&gt;’ is shown to include            (Seoul)’,          (Incheon)’,          (New York)’, and          (Chicago)’.
     Accordingly, as listed in the table of  FIG. 3A , the speech recognizer  100  outputs the recognition result as a sentence template formed of at least one sentence element, and words corresponding to the class included in the sentence template. 
     In particular, referring to  FIG. 3B , in order to mark a part omitted from a basic sentence template, sentence elements capable of being omitted are serially numbered in advance and output with tags showing whether they are contained in the recognition result of the speech recognizer  100  to be translated. For example, in response to the input “                               ? (How long does it take to New York by plane?)”, the element ‘from &lt;name of place 1&gt;’ is omitted, and a tag ‘X’ is attached to show that the sentence element is not included.
     As such, the speech recognizer  100  according to the present invention outputs a specific sentence template and class-matching words included therein in response to the input speech signal. 
     The language translator  200  includes a corresponding sentence template dictionary  210  consisting of corresponding sentence template pairs of different languages, a corresponding sentence template selecting part  220  searching for an output sentence template corresponding to an input sentence template using the corresponding sentence template dictionary  210 , a corresponding word dictionary  230  including corresponding word pairs of different languages, and a corresponding word selecting part  240  searching for corresponding words with respect to the words included in the input sentence template using the corresponding word dictionary  230 . 
     Thus, the language translator omits complicated steps performed in a conventional language transition system (for example, morpheme analysis, phrase analysis and sentence parsing) and obtains a corresponding sentence template by means of a simple dictionary search, which reduces processing time while achieving more accurate translation performance. 
     Here, in building the corresponding sentence template dictionary  210 , the inclusion range of the input sentence may be expanded by adding sentence template pairs of the two languages. 
     Such a language translator  200  searches the corresponding sentence template dictionary  210  for a target language sentence template corresponding to the given sentence template from the speech recognizer  100 , and extracts translated words corresponding to the class included in the sentence template from the corresponding word dictionary  230 , thereby outputting the translation result. 
     For example, in response to the input speech signal “                                         ? (How long does it take from Incheon to New York by plane?)”, a sentence template such as Table  410  and class matching words such as Table  420  in  FIG. 4  are input to the language translator.
     Then, the corresponding sentence template selecting part  220  chooses Table  450  of an English sentence template corresponding to Table  410  of a Korean sentence template using the corresponding sentence template dictionary  210 . And the corresponding word selecting part  240  chooses for the translated words ‘Incheon’, ‘New York’ and ‘plane’ corresponding to the words            (Incheon),          (New York) and          (plane) included in respective sentence elements using the corresponding word dictionary  230 , as listed in Table  440 . Subsequently, Table  450  of a sentence template and a set of words  440  corresponding to a class included therein is output as a translation result.
     The language translator  200  according to the present invention has a simple structure and clearly processes a corresponding sentence template and a translated word. This improves translation accuracy. Accordingly, to translate various sentences, the corresponding sentence template dictionary  210  has to be built in advance to include corresponding sentence template pairs for expected input sentences. 
     Meanwhile, the speech synthesizer  300  includes a sentence template prosody data dictionary  310  including prosody data corresponding to the sentence template, a sentence template prosody data extracting part  320  extracting prosody data with respect to a translated sentence template input to the speech synthesizer  300 , a synthetic speech database  330  which contains a set of speech segments or feature parameters for synthesis units, and a speech signal synthesizing part  340  which generates the synthesized speech signal based on the speech data stored in the synthetic speech database  330 . 
     The speech synthesizer  300  needs various prosody data, such as spacing between words of a sentence and stress applied to each word, in order to generate natural synthetic speech. This data is dependant on the sentence template. Thus, the sentence template prosody data dictionary  310  containing appropriate prosody data for each sentence template is built in advance, and the prosody data corresponding to the sentence template transferred from the language translator  200  is extracted from this dictionary  310 , and then, a speech signal is generated by a speech signal synthesizing part  340  using this prosody data, based on a synthetic sound piece connection method or a method of generating a speech signal from parameters, and finally a synthesized speech is output. 
     An example of expressing sentence template prosody data as stress and length of pause between words is illustrated in  FIG. 5  with respect to the sentence described above. This example may be designed according to specific application or may follow any standard like a Speech Synthesis Markup Language (SSML) standard provided in the World Wide Web Consortium (W3C). 
     Consequently, since the speech synthesizer  300  may easily obtain appropriate prosody data for an input sentence template by searching the sentence template prosody data dictionary  310 , avoiding the load of the conventional method of extracting prosody data by analyzing a sentence again. 
     As described above, a method of processing a sentence template instead of text in each step of automatic speech translation according to the present invention results in a simple structure, simple operation and high-quality automatic speech translation performance. In particular, the inventive method avoids needless repetition of the same processes and the performance degradation such repetition can cause. 
     Moreover, according to the present invention, necessary data such as corresponding sentence templates and prosody data is built in advance by off-line processes, which can greatly reduce workload in real-time operation. 
     As a result, an automatic speech translation system according to the present invention can realize high-quality automatic speech translation with structural and operational simplicity, and may be applied to various automatic speech translation products and services. 
     While the invention has been shown and described with reference to certain exemplary embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.