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.
16. The system of claim 9 , further comprising an analog-to-digital converter (ADC) to generate the samples, the ADC operative in at least: a first power mode wherein the ADC generates samples at a first dynamic range, and a second power mode wherein the ADC generates samples at a second dynamic range, higher than the first dynamic range.