An acoustic processing unit for a speech recognition system is characterized in that the parameters are extracted from the speech data, including correlation functions, the zero crossing number of the original waveforms, the zero crossing number of the differential waveforms, and the average level of the waveforms. A suitable threshold is selected from a plurality of thresholds preliminarily stored, depending on the inputted speech volume level. The inputted speech volume level of the speaker is detected so that feedback of the detected volume level is obtained. The selected threshold set is then compared with each of the parameters to thereby make the phonemic classification. Because the plurality of threshold sets are thus automatically selected depending on the inputted speech volume level, flexible phonemic classification can be obtained to exactly detect the speech sections.
Phonemic classification in speech recognition system having accelerated response time