1. An accelerator for calculating distances for a speech recognition circuit, the accelerator comprising: a calculating circuit for calculating distances indicating the similarity between a feature vector and a plurality of predetermined acoustic states of an acoustic model, wherein the feature vector comprises a plurality of extracted and/or derived quantities from an audio signal during a defined audio time frame; first and second storage circuits, each for storing calculated distances for at least one said audio time frame, and for making said stored distances available for use by another part of the speech recognition circuit; and a control circuit for controlling read and write access to the first and second storage circuits, said control circuit being configured to allow writing to one said storage circuit while the other said storage circuit is available for reading, to allow first calculated distances for one audio time frame to be written to one said storage circuit while second calculated distances for an earlier audio time frame are made available for reading from the other said storage circuit.