15. An apparatus, comprising: a memory; and a processor coupled to the memory and configured: to receive input speech to be decoded; to convert a non-back-off language model that assigns a probability to each fixed order n-gram generated with a given vocabulary to a back-off language model that assigns a probability to one or more fixed order n-grams assigned a zero probability by the non-back-off language model by backing off to a given lower order n-gram corresponding to the given fixed order n-gram; and to prune the converted back-off language model; and to utilize the converted back-off language model in a speech decoding process to generate a decoded speech output from the input speech; wherein converting the non-back-off language model comprises, starting with an initial non-back-off language model associated with a lowest order n-gram and hierarchically progressing with one or more higher order non-back-off language models until the fixed order n-gram is reached, converting the initial non-back-off language model and the one or more higher order non-back-off language models into respective back-off language models; wherein the given fixed order n-gram comprises a given word and an associated history; wherein the given lower order n-gram corresponding to the given fixed order n-gram comprises the given word and a truncated version of the associated history.
16. The apparatus of claim 15 , wherein converting the non-back-off language model to the back-off language model utilizes a background language model.