Modification of annotated bilingual segment pairs in syntax-based machine translation

Description

GOVERNMENT INTERESTS

The research and development described in this application were partially supported by the Defense Advanced Research Projects Agency (DARPA), Contract No. HR0011-06-C-0022 and by the Advanced Technology Program (ATP) at the National Institute of Standards and Technology (NIST), Project No. 00-00-6945. The U.S. government may have certain rights in the claimed inventions.

BACKGROUND

1. Field of the Invention

The invention disclosed herein is generally related to machine translation and more specifically to modification of annotated bilingual segment pairs in syntax-based machine translation.

2. Description of the Related Art

To translate written documents from a source language, such as Arabic, to a target language, such as English, machine translation performed by a computer may be used. One technique, statistical machine translation, used to perform machine translation includes generating a translation model comprising translation rules derived from phrases in the source language matched with phrases in the target language These paired phrases include annotated bilingual segment pairs. The annotated bilingual segment pair may be a sentence, a fragment, or a phrase.

In a string-to-tree annotated bilingual segment pair, the target phrase may be represented as a tree having branches separating syntactic structures in the target phrase. The nodes of the tree are typically labeled based on the syntactic structure of the branch. Syntactic structures include noun phrases, verb phrases, adverb phrases, or the like. The annotated bilingual segment pair may further include alignments between the words in the source language and words in the target language.

FIG. 1 is a diagram of a prior art process 100 for deriving translation rules from an annotated bilingual segment pair. The process 100 comprises, in a single iteration, receiving the annotated bilingual segment pair 102 and training a translation engine based on the annotated bilingual segment pair 102 to generate composed rules 104. The composed rules 104 may be used by the translation engine to translate a document from the source language to the target language.

The annotated bilingual segment pair 102 is a tree-to-string annotated bilingual segment pair and comprises one or more parent nodes that are each associated with at least two children. The children may, in turn, be parent nodes for other children. Each node is labeled with a syntactic structure identifier such as noun phrase (NP), verb phrase (VP), adverb phrase (ADVP), or the like. Each endpoint comprises a word in a target language, designated in FIG. 1 by the letter “a.” In the annotated bilingual segment pair 102, words in a target phrase designated by the letter “e” are each aligned via a dotted line to one or more words in the target phrase.

The annotations on a bilingual segment pair are generated automatically by a machine and may include inaccurate or imprecise labels, structures, and/or alignments. In machine translation, millions of the annotated bilingual segment pairs may be used and it may be impractical to correct each of the annotated bilingual segment pairs manually. Further, poor annotated bilingual segment pairs may result in translations that are not comprehensible, nonsensical, or awkward.

SUMMARY

Systems and methods for correcting an annotated bilingual segment pair are provided. In a method according to one embodiment, an annotated bilingual segment pair is received. The annotated bilingual segment pair is processed to generate a plurality of trees based on a tree or set of alignments in the annotated bilingual segment pair. From the plurality of trees, rule sequences are derived. The rule sequences are then processed using an expectation-maximization algorithm to select one of the rule sequences that is most likely to result in an accurate and fluent translation of the source phrase. A second annotated bilingual segment pair based on the selected rule sequence is generated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram of a prior art process for deriving translation rules from an annotated bilingual segment pair;

FIG. 2 depicts an environment in which various embodiments may be practiced;

FIG. 3 is a block diagram of a modification engine according to various embodiments;

FIG. 4 is a flowchart of an exemplary process for modifying annotated bilingual segment pairs according to various embodiments;

FIG. 5 is a diagram of an example of re-labeling an annotated bilingual segment pair according to various embodiments;

FIG. 6 is a diagram of an example of re-structuring an annotated bilingual segment pair according to various embodiments;

FIG. 7 is a diagram of an exemplary forest generated from an annotated bilingual segment pair according to various embodiments;

FIG. 8 is a diagram of an example of re-aligning an annotated bilingual segment pair according to various embodiments; and

FIG. 9 is a diagram of a process for deriving translation rules from a received annotated bilingual segment pair.

DETAILED DESCRIPTION

In machine translation, annotated bilingual segment pairs are used to generate translation rules. The translation rules can then be used to translate documents from a source language to a target language. The translation rules are generated from the annotated bilingual segment pairs using a training process. Translation rules may comprise composed rules and/or minimal rules, as is known in the art. Systems and methods for modifying an annotated bilingual segment pair are presented.

The modification of the annotated bilingual segment pair may comprise re-labeling the syntactic structures in the annotated bilingual segment pair, re-structuring the nodes in the annotated bilingual segment pair, and/or re-aligning words in a source phrase to words in a target phrase. To modify the annotated bilingual segment pair, the annotated bilingual segment pair is processed to generate a plurality of trees. Each tree represents a possible modification of the annotated bilingual segment pair. Any number of trees may be generated. A set of rule sequences used to explain each tree is derived. From the derived rule sequences, one of the derived rule sequences is selected using an expectation-maximization algorithm. The expectation-maximization algorithm calculates a probability that a derived rule sequence is correct for any given translation and then compares the probability to the other probabilities associated with the other derived rule sequences. Based on the comparison, the derived rule sequence having the highest probability of generating the correct translation is selected.

Using the selected rule sequence, a new annotated bilingual segment pair is generated for the source phrase and the target phrase. The new annotated bilingual segment pair is based on the rule sequence having the highest probability, as discussed herein. The new annotated bilingual segment pair may be used to generate a translation rule and/or train a translation engine as part of a larger set of annotated bilingual segment pairs.

FIG. 2 depicts an environment 200 in which various embodiments may be practiced. The environment 200 may comprise a bilingual engine, or bilingual data engine 202, a modification engine 204, and a training engine 206 configured to communicate over a network 208. The network 208 may comprise a public or private network such as a local area network (LAN), a wide area network (WAN), or the Internet. The bilingual data engine 202, the modification engine 204, and the training engine 206 may be implemented in hardware, software, or any combination thereof as is known to those skilled in the art.

The bilingual data engine 202 is configured to receive and store bilingual data. In some embodiments, the bilingual data engine 202 is configured to receive pairs of source phrases and target phrases that are translations of one another. The bilingual data engine 202 may process the bilingual data to generate annotated bilingual segment pairs or other data structures that can be used to translate documents including, but not limited to, translation memories, context databases, dictionaries, or the like. According to various embodiments, the annotated bilingual segment pairs may be tree-to-string, tree-to-tree, and/or string-to-tree. The bilingual data engine 202 may communicate the bilingual data to the training engine 206.

The modification engine 204 may be configured to receive the bilingual data and process the bilingual data to generate a modified annotated bilingual segment pair that can be used by the training engine 206 to generate translation rules. The modification engine 204 is configured to re-label, re-structure, and/or re-align a previously generated annotated bilingual segment pair. The modification engine 204 may receive the annotated bilingual segment pair from the bilingual data engine 202, modify the annotated bilingual segment pair, and output a modified annotated bilingual segment pair to the bilingual engine 202 and/or the training engine 206. The modification engine 204 is discussed in greater detail herein in connection with, at least, FIG. 3.

The training engine 206 is configured to receive the modified annotated bilingual segment pair, compose translation rules, and output the translation rules. The translation rules may be composed according to systems and methods known to those skilled in the art and may comprise composed rules and/or minimal rules. The translation rules, according to some embodiments, may be generated using systems and methods similar to those used by the modification engine 204 to generate derived rules.

FIG. 3 is a block diagram of a modification engine 204 according to various embodiments. The modification engine 204 comprises a processing module 302, a derivation module 304, a training module 306, and a distillation module 308. The modules may be implemented as hardware, software, and/or firmware as known to those skilled in the art.

The processing module 302 is configured to receive an annotated bilingual segment pair and process the annotated bilingual segment pair. As a result of the processing, a plurality of trees is generated. In some embodiments, the plurality of trees is generated as a target forest as further described herein, at least, in connection with FIG. 4. The trees in the plurality of trees each represent an alternative annotated bilingual segment pair or alignment rule sequence that can be used to express a relationship between the words in the bilingual segment pair. As part of the processing, the processing module 302 may re-label, re-structure, and/or re-align the annotated bilingual segment pair. Re-labeling the annotated bilingual segment pair includes changing the labels at one or more of the nodes in the tree. Re-structuring includes adding parent nodes to the tree. In some embodiments, the tree may be binarized. Re-aligning the annotated bilingual segment pair includes generating translation rules based on the initial alignments in the annotated bilingual segment pair.

The derivation module 304 is configured to derive a derivation forest from the target forest. The target forest comprises the plurality of trees represented as a single large tree that can be decoded to derive rule sequences. Each tree in the derivation forest comprises a set of rule sequences derived from a tree in the target forest. The derived rule sequences may be generated according to an extraction algorithm adapted to receive a target forest instead of a tree as is apparent to those skilled in the art and as discussed herein in connection with, at least, FIG. 4.

The training module 306 is configured to select one of the derived rule sequences in the derivation forest based on a probability, such as a probability that the derived rule sequence may likely result in a more accurate translation than the other rule sequences in the derivation forest. The selection may be made based on an expectation-maximization algorithm.

The distillation module 308 is configured to distill a modified annotated bilingual segment pair from the selected rule sequence. The modified annotated bilingual segment pair may be utilized to produce a translation rule used to translate documents. The translation rules that are derived from the modified annotated bilingual segment pairs are thus more likely, when combined with other translation rules from other annotated bilingual segment pairs, to result in more accurate or fluent translations in the machine translation system. Although the modification engine 204 is illustrated as having the processing module 302, the derivation module 304, the training module 306, and the distillation module 308, fewer or more modules may comprise the modification engine 204 and still fall within the scope of various embodiments.

FIG. 4 is a flowchart of an exemplary process 400 for modifying annotated bilingual segment pairs according to various embodiments. The process 400 may be performed by the modification engine 204, according to some embodiments. For the sake of illustration, the process 400 includes sub-processes for re-labeling, re-structuring, and re-aligning the annotated bilingual segment pair. It is understood that various embodiments may comprise performing any combination comprising one or more of these sub-processes. According to some embodiments, the actual combination and order may be previously selected by a user. Further, the process 400 may be performed multiple times using a different sub-process or combination of sub-processes each time. The process 400 begins when an annotated bilingual segment pair is received in an exemplary step 402.

In an exemplary optional step 404, if the annotated bilingual segment pair is re-labeled, a forest comprising re-labeled trees is generated. Re-labeling comprises combining two or more types of syntactic categories into one category and/or dividing a syntactic category into two or more syntactic categories. According to exemplary embodiments, step 404 may be performed multiple times to generate a plurality of the re-labeled trees. Each of the re-labeled trees is associated with a separate set of syntactic category combinations and/or divisions. In some embodiments, the target tree may be re-labeled according to a technique as discussed herein in connection with, at least, FIG. 5. The plurality of re-labeled trees may be generated as a target forest as described herein in connection with, at least, FIG. 7 and as will be apparent to those skilled in the art.

In an exemplary optional step 406, a forest comprising re-structured trees may be generated from the target tree. Re-structuring may be performed by generating new parent nodes to the target tree. In some embodiments, only one new parent node will be formed at a time. In some embodiments, the target tree may be re-structured according to a binarization technique as discussed herein in connection with, at least, FIG. 6.

In the step 406, to generate a forest, a parallel binarization technique may be used. The forest comprises additive forest nodes and multiplicative forest nodes. A multiplicative node corresponds to a tree node in an unbinarized tree. The multiplicative node may then generate two or more additive nodes corresponding to the nodes in the unbinarized tree. The additive nodes may further comprise a leaf in the binarized tree and/or a multiplicative node. A forest generated using a parallel binarization technique is further discussed herein in connection with, at least, FIG. 7.

In an exemplary optional step 408, to re-align the annotated bilingual segment pair, multi-level tree-to-string translation rules are extracted based on the alignments in the received annotated bilingual segment pair. Extracting the translation rules is discussed further herein in connection with, at least, FIG. 8.

In an exemplary step 410, a derivation forest of rule sequences is built. The derivation forest may be built according to various techniques based on the combination of re-labeling, restructuring, and/or re-aligning techniques that are performed. A derivation forest comprises a plurality of rule sequences represented as trees that correspond to the target forest and/or the extracted translation rules.

If re-labeling and/or re-structuring are performed, the rule sequences may be extracted from the target forest. A forest-based extraction algorithm is configured to receive a target forest, a source string, and an alignment and output a derivation forest comprising translation rules.

In exemplary embodiments, the forest-based extraction algorithm is configured to act on two conditions. In a first condition, if an additive target forest node is reached, the multiplicative target forest nodes that are children of the additive node are processed to recursively extract rules according to a second condition to generate multiplicative derivation forest nodes. The new multiplicative derivative forest nodes are children of the additive derivative forest nodes.

In the second condition, if a multiplicative derivative forest node is reached, rules may be extracted. In some embodiments, the rules may be extracted according to the techniques disclosed in Galley et al. “Scalable Inference and Training of Context Rich Syntactic Models.” Proceedings of the 44^th Annual Meeting of the Association for Computational Linguistics (ACL) 2006. After the rules are extracted, the process returns to condition 1 to form a derivation forest based on the nodes of the newly-extracted rules to generate additive derivation forest nodes.

If re-aligning is performed, the rule sequences may be extracted from the initial alignments in exemplary step 408. Thus, in exemplary step 410, the derivation forest may be constructed independent of the initial alignments. In exemplary embodiments, derivation forests may be built from a forced-decoding algorithm. For example, a standard CKY-style decoder used in machine translation may be configured to limit its search to the training pair. An exemplary CKY-style decoder is described in Galley et al. “Scalable Inference and Training of Context Rich Syntactic Models.” Proceedings of the 44^th Annual Meeting of the Association for Computational Linguistics (ACL) 2006.

In an exemplary step 412, one or more rule sequences in the derivation forest are selected. In some embodiments, all of the rule sequences may be selected. The rule sequences may be selected using an expectation-maximization (EM) algorithm. The EM algorithm selects rule sequences so as to maximize the probability of the entire training corpus comprising many annotated bilingual segment pairs. Thus, the EM algorithm may prefer to assign probabilities such that one rule or rule sequence is used many times rather than several different rules for the same situation.

If the annotated bilingual segment pair is re-labeled and/or re-aligned, the EM algorithm may be used to generate a set of probabilities based on which the rile sequence can be selected. A selected rule sequence, or a selected binarization, of a tree may be calculated according to the formula:

β * = arg ⁢ ⁢ max β ⁢ p ⁡ ( τ β , f , a | θ * )
where β* is the binarization that results in the highest likelihood of the re-labeled or restructured training data comprising (τ_β,f,a)-tuples. Other formulas or algorithms apparent to those skilled in the art may be used. In the tuples, τ_β represents a generated target tree, f represent a word or phrase in the source string, and a represents the alignment associated with f. Selected parameters or rule probabilities, θ*, are obtained such that:

θ * = arg ⁢ ⁢ max θ ⁢ p ⁡ ( τ , f , a ) θ * = arg ⁢ ⁢ max θ ⁢ p ⁡ ( β , τ , f , a ) θ * = arg ⁢ ⁢ max θ ⁢ p ⁡ ( τ β , f , a )
where (β,τ)=τ_β if bar notation is used to label the new intermediate nodes added using binarization. To store the binarizations, a packed forest may be used. For model estimation, an inside-outside algorithm may be used. The probability p(τ_β,f,a) of a (τ_β,f,a)-tuple can be calculated by aggregating the rule probabilities p(r) in each derivation, ω, in the set of all derivations, Ω, using the equation:

p ⁡ ( τ β , f , a ) = 1  Ω  ⁢ ∑ ω ∈ Ω ⁢ ∏ r ∈ ω ⁢ ⁢ p ⁡ ( r )
In some embodiments, the probability p(τ_β,f,a) may be decomposed using minimal rules during running of the EM algorithm.

In exemplary embodiments in which the annotated bilingual segment pair is re-aligned, EM algorithms described in U.S. nonprovisional patent application Ser. No. 11/082,216 filed Mar. 15, 2005 and entitled “Training Tree Transducers for Probabilistic Operations” may be used.

In exemplary step 414, the modified annotated bilingual segment pair may be distilled from the one or more selected rule sequences. The selected rule sequences may comprise Viterbi derivations and/or Viterbi alignments. From the rule sequences a modified annotated bilingual segment pair is generated as is known to those skilled in the art. The modified annotated bilingual segment pair may then be used to train an MT system.

FIG. 5 is a diagram of an example of re-labeling an annotated bilingual segment pair according to various embodiments. Each node in the tree in the annotated bilingual segment pair is labeled with a syntactic category. The syntactic category may be, for example, sentence (S), noun phrase (NP), noun phrase-complement (NP-C), verb phrase (VP), adverb phrase (ADVP), or the like. Re-labeling comprises combining two or more types of syntactic categories into one category and/or dividing a syntactic category into two or more syntactic categories.

A first syntactic category may be combined with another syntactic category. For example, categories may be combined if the first syntactic category rarely occurs or the syntax of source language renders the category irrelevant. In these embodiments, the label may be changed to an existing label or a new label may be created for the combined category. For example, in FIG. 5, the syntactic category “NP-C” is combined with the category “NP.” The syntactic label “NP” is now used to indicate both the syntactic categories noun phrase and noun phrase-complement.

A syntactic category may be added if a category is broadly defined. For example, the syntactic category “verb phrase” may include, in an English tree, both non-finite verbs such as “to go” and “going,” and finite verbs such as “goes” and “went.” In this instance, the category “finite verbs” (VP-FIN) may be added, as depicted in FIG. 5. In some embodiments, the syntactic category “VP” may be preserved.

FIG. 6 is a diagram of an example of re-structuring an annotated bilingual segment pair according to various embodiments. More specifically, FIG. 6 depicts a diagram of a tree that has been right-binarized. In right-binarization, a new node is formed over the rightmost child and dominates all of the children except the furthest left child. The process is then repeated with respect to the children of the new node until only two children remain under the newest node. In other embodiments, left-binarization may be performed in which a new node is formed over the leftmost child as will be apparent to those skilled in the art. Parallel binarization includes performing both left and right binarization.

FIG. 7 is a diagram of an example of a forest comprising additive (⊕) and multiplicative ({circle around (x)}) nodes. These nodes are added by first recursively parallel binarizing child nodes in the tree to generate additive nodes at each of the child nodes in the generated trees. Next, the nodes are right-binarized by adding an intermediate tree node. The intermediate tree node is recursively binarized to generate an additive binarization forest node. A multiplicative forest node is added as the parent of the intermediate tree node and the child. Then the nodes are left-binarized by adding an intermediate tree node. The intermediate tree node is recursively binarized to generate an additive binarization forest node. A multiplicative forest node is added as the parent of the intermediate tree node and the child. Finally, an additive node is added as the parent of the added multiplicative forest nodes.

FIG. 8 is a diagram of an example of re-aligning an annotated bilingual segment pair according to various embodiments. The annotated bilingual segment pair 902 may comprise a tree-to-string annotated bilingual segment pair, a tree-to-tree annotated bilingual segment pair, or a string-to-tree annotated bilingual segment pair. In the embodiment shown, the annotated bilingual segment pair 902 comprises a string-to-tree annotated bilingual segment pair. The alignments in the annotated bilingual segment pairs may have been generated without regard to syntactic structure and/or the use of a tree and, therefore, may be inaccurate. To extract translation rules from the alignments, an alignment algorithm, such as the algorithm described in Galley et al. “What's in a Translation Rule” HLT-NAACL 2004: Main Proceedings, pages 273-280, may be used. In embodiments where the annotated bilingual segment pair is re-aligned, a target forest may not be generated.

FIG. 9 is a diagram of a process 900 for deriving translation rules from a received annotated bilingual segment pair. In the process 900, an annotated bilingual segment pair 902 is received. The annotated bilingual segment pair 902 may comprise a tree-to-string annotated bilingual segment pair, a tree-to-tree annotated bilingual segment pair, or a string-to-tree annotated bilingual segment pair. In the embodiment shown, the annotated bilingual segment pair 902 comprises a string-to-tree annotated bilingual segment pair. Next, based on the received annotated bilingual segment pair 902, a set of rule sequences 904 are generated. According to some embodiments, the rule sequences may be generated by re-labeling, re-structuring, and/or re-aligning the received annotated bilingual segment pair 902. From the rule sequences 904, a modified annotated bilingual segment pair 906 is generated. The modified annotated bilingual segment pair 906 may then be used to generate translation rules 908 that can be used to train an MT system.

The above-described functions and components can be comprised of instructions that are stored on a storage medium. The instructions can be retrieved and executed by a processor. Some examples of instructions are software, program code, and firmware. Some examples of storage medium are memory devices, tape, disks, integrated circuits, and servers. The instructions are operational when executed by the processor to direct the processor to operate in accord with various embodiments. Those skilled in the art are familiar with instructions, processor(s), and storage medium.

While various embodiments have been described above, it should be understood that they have been presented by way of example only, and not limitation. The scope of the present disclosure is in no way limited to the languages used to describe exemplary embodiments. Thus, the breadth and scope of a preferred embodiment should not be limited by any of the above-described exemplary embodiments.