View source for Synchronous context-free grammar

'''Synchronous context-free grammars''' ('''SynCFG''' or '''SCFG'''; not to be confused with [[Stochastic context-free grammar|stochastic CFGs]]) are a type of [[formal grammar]] designed for use in [[transfer-based machine translation]]. Rules in these grammars apply to two languages at the same time, capturing grammatical structures that are each other's translations.

The theory of SynCFGs borrows from [[syntax-directed transduction]] and [[syntax-based machine translation]], modeling the reordering of clauses that occurs when translating a sentence by correspondences between phrase-structure rules in the source and target languages. Performance of SCFG-based MT systems has been found comparable with, or even better than, state-of-the-art [[phrase-based machine translation]] systems.<ref name="Chiang">{{cite journal
|last1  = Chiang
|first1 = David
|year   = 2007
|title  = Hierarchical phrase-based translation
|journal= [[Computational Linguistics (journal)|Computational Linguistics]]
|volume = 33
|number = 2
|pages  = 201–228
|doi = 10.1162/coli.2007.33.2.201
|s2cid = 3505719
|doi-access = free
}}</ref>
Several algorithms exist to perform translation using SynCFGs.<ref>{{cite conference
|last1  = Venugopal
|first1 = Ashish
|last2  = Zollmann
|first2 = Andreas
|last3  = Vogel
|first3 = Stephan
|year   = 2007
|title  = An efficient two-pass approach to Synchronous-CFG driven statistical MT
|book-title=Proc. NAACL HLT
|pages  = 500–507
|url  = https://www.aclweb.org/anthology/N07-1063
}}</ref>

==Formalism==
Rules in a SynCFG are superficially similar to CFG rules, except that they specify the structure of two phrases at the same time; one in the source language (the language being translated) and one in the target language. Numeric indices indicate correspondences between non-terminals in both constituent trees. Chiang<ref name="Chiang"/> gives the Chinese/English example:

: {{math|''X'' →}} (yu {{math|''X''<sub>1</sub>}} you {{math|''X''<sub>2</sub>}}, have {{math|''X''<sub>2</sub>}} with {{math|''X''<sub>1</sub>}})

This rule indicates that an {{mvar|X}} phrase can be formed in Chinese with the structure "yu {{math|''X''<sub>1</sub>}} you {{math|''X''<sub>2</sub>}}", where {{math|''X''<sub>1</sub>}} and {{math|''X''<sub>2</sub>}} are variables standing in for subphrases; and that the corresponding structure in English is "have {{math|''X''<sub>2</sub>}} with {{math|''X''<sub>1</sub>}}" where {{math|''X''<sub>1</sub>}} and {{math|''X''<sub>2</sub>}} are independently translated to English.

==Software==
* [http://cdec-decoder.org cdec], MT decoding package that supports SynCFGs
* [http://joshua-decoder.org Joshua], a machine translation decoding system written in Java

==References==
<references/>

[[Category:Formal languages]]
[[Category:Machine translation]]
[[Category:Natural language parsing]]
[[Category:Statistical natural language processing]]