Patent US 3,946,157

In an apparatus for speech recognition, intended for controlling machines, spoken words are synthesized from two to twelve phoneme classes. Phonemes are recognized by analysis of speech sounds, measuring and comparing energy and time-rate of change of energy in certain frequency bands. Words are recognized by further logic means analyzing phoneme classes. One particular feature determines the plosive class "T" versus the fricative class "S" by measuring energy rise-time.