1. A system for calculating the look ahead probabilities at the nodes in a language model look ahead tree, wherein the words of the vocabulary of the language are located at the leaves of the tree, said apparatus comprising a processor adapted to: to assign a language model probability to each of the words of the vocabulary using a first low order language model; to calculate the language look ahead probabilities for all nodes in said tree using said first low order language model; to determine if the language model probability of one or more words of said vocabulary can be calculated using a higher order language model and updating said words with the higher order language model; and to update the look ahead probability at only the nodes which are affected by the words where the language model has been updated.