9. A system, comprising: a buffer to store samples representing an input audio signal, a speech feature extractor having an input for samples from the buffer, a speech vocabulary storing data representing a predetermined set of speech elements, and a controller comprising a decision circuit to generate a wake up signal in response to a match between speech features output by the extractor and the stored speech elements, wherein the controller is configured to output a wake up signal indicating the match and to output samples received subsequent to the samples stored in the buffer corresponding to the match.
9. A system, comprising: a buffer to store samples representing an input audio signal, a speech feature extractor having an input for samples from the buffer, a speech vocabulary storing data representing a predetermined set of speech elements, and a controller comprising a decision circuit to generate a wake up signal in response to a match between speech features output by the extractor and the stored speech elements, wherein the controller is configured to output a wake up signal indicating the match and to output samples received subsequent to the samples stored in the buffer corresponding to the match. 10. The system of claim 9 , further comprising a speech recognition system, responsive to the wake up signal, to transition from a low power mode to a high power mode and perform speech recognition on samples representing the input audio signal.