Codec translator modules provide one or more implementations of the ast_translator interface. A translator has source and destination format attributes. It also provides a callback that will be used to convert a chunk of media from the source to the destination format. It knows nothing about the concept of a phone call. It only knows how to convert media from one format to another.
For more detailed information about the translator API, see include/asterisk/translate.h and main/translate.c. Implementations of the translator abstraction can be found in the codecs directory.