Patent GB-1418958-A (GB)
Title: Speech recognition system
IPC: G10L15/10
Filed: 1973-05-14
Abstract: 1418958 Speech recognition systems INTERNATIONAL BUSINESS MACHINES CORP 14 May 1973 [26 May 1972] 22756/73 Heading G4R A speech recognition system includes means for splitting the speech signal into a number of different frequency bands and for detecting the occurrence of peaks of energy concentration in each band. The output of the detecting means is sampled to provide a sequence of signal groups each indicative of the instantaneous value of each of the frequency components. Each group is identified as a component of a word, and a sequence of such components is recognized as a word. A voice-controlled clock unit is employed.  The speech waveform from a microphone 1, Fig. 1, passes to an analyser 2 (Fig. 2, not shown) comprising a number, e.g. 8, of bandpass filters which together cover the speech band. Each filter output is rectified and filtered to provide a signal representing the energy distri - bution in the corresponding frequency band. The analyser feeds the eight outputs to a feature selector 3 (Fig. 3, not shown), where each output is compared with a respective threshold signal which is varied in dependence upon all the outputs of analyser 2. As a result only square wave standard amplitude pulses representing peaks of energy concentration having high enough amplitude are passed to shift register unit 4. The pulses on the eight outputs pass via unit 4 (Fig. 5, not shown), which comprises eight shift registers and is under the control of an irregular clock 12 (Fig. 4, not shown) controlled by the voice signals. It is designed to sample sibilants at a slow rate and vowels at a high rate. Silences within a word cause the clock to stop until the word resumes. Each register stores a sequence of bits which may be displayed at 5. The registers provide a sequence of 8-bit parallel bit words, called uniphones, in true and inverse form to a recognition unit comprising an adaptive memory 6 (Fig. 6, not shown). Each bit of the uniphone indicates whether the energy of the corresponding frequency band is above or below the respective threshold at the sampling instant. The memory is set up during a training phase in which a speaker speaks selected words to cause uniphones pertinent to his voice to be stored. In a subsequent recognition phase the degree of match between each stored uniphone and the uniphones in the word to be recognized is determined. If a uniphone in the word lies within a certain Hamming distance, e.g. 1, of one or more stored uniphones it is identified as that or those uniphones. Signals corresponding to the identified uniphones are passed to a shift register unit 7, comprsing one register for each type of uniphone. Thus the registers store indications of the presence or absence of each uniphone in the order of their occurrence. For each word to be recognized certain of the stages of certain of the registers are coupled, via a plugboard 9 or a computer simulation of a plugboard, to word recognition units. The coupling takes account of the order of the uniphones in the word, and of the uniphones lying within the prescribed Hamming distance. The inputs to each word recognition unit in unit 10 (Fig. 8, not shown) are summed, and the word is identified by an output from that unit receiving the highest sum, provided it is above a certain threshold. The word may be identified even if a uniphone is missing from the input to the word recognition units. The signal representing the identified word is converted to a binary signal which is stored in a multiple shift register store 11 (Fig. 9, not shown). Successively uttered words are stored in store 9 in this form. The speaker may erase a word from the store by saying "mistake". and may clear it by saying "reset".