A speech recognition system for an automotive vehicle derive a spoken instruction start signal and a spoken instruction end signal when a smoothed spoken instruction signal exceeds or drops below a predetermined threshold level representing the intensity of the background noise for more than first and second predetermined time periods, respectively. Noise is determined by converting the output of a microphone transducing the spoken instruction into a single polarity signal that is smoothed with a long time constant. The single polarity variation is also smoothed with a shorter time constant. The signals with the long and short constants are applied to a comparator that derives a bi-level output signal. In response to transitions in first and second directions of the bi-level signal lasting for first and second durations, the start and end signals are respectively derived.
