Patent US 3,940,565

A time domain speech recognition system is disclosed wherein a speech signal is infinitely clipped in order to derive its zero crossover pattern. A pitch pulse detector generates standardized marker pulses in synchronism with the glottal pressure pulses occurring during voiced sounds. Using these marker pulses as trigger signals, a sampling gate samples the infinitely clipped speech signal in synchronism with the glottal pulses. In the absence of a voiced signal, sampling is performed at a pseudo random rate. The zero crossing samples obtained are normalized with respect to voice pitch and are classified as belonging to a particular set of speech sounds called phonemes in accordance with a number of parameters including number and length of the zero crossover intervals, the length of the pitch pulse interval and the relative number of changes in the duration of the crossover intervals in a pitch pulse interval.