Patent GB-1508741-A (GB)
Title: Keyword detection in continuous speech using continuous asynchronous correlation
IPC: G10L15/08
Filed: 1976-01-08
Abstract: 1508741 Speech recognition system ROCKWELL INTERNATIONAL CORP 8 Jan 1976 [2 April 1975] 675/76 Heading G4R A speech recognition system for detecting keywords in speech generates digitized samples from continuous speech, produces correlation signals from predetermined sequences of the samples and is responsive to the correlation signals to determine whether the keyword has been detected. A speech input signal is gain controlled using an a.g.c. circuit, is applied to a pre-emphasis circuit and is fed to a spectral analyser 11 which produces N analog speech parameter signals on respective channels, e.g. to indicate the energy in 200 Hz wide contiguous bandpasses. The speech signal may pass through K low pass filters and respective logarithmic amplifiers and the outputs from the filters fed to a feature extractor which supplies N-K further parameter signals (Fig. 2, not shown). The N parameter signals feed an STV (Spectral Time Vector) generator which time division multiplexes and digitizes the parameter signals. A multiplexer samples each of the N parameter signals every 10 milliseconds and supplies the samples serially to an A/D converter which supplies N digital values every 10 msec. Each of the N samples (N may be 16) may be a word six bits wide and the N words form the STV data. Each of the STV data words is fed to a random access memory addressed by a counter which resets when a predetermined value is reached so that the memory stores, e.g. 16 STV words and the seventeenth word entered replaces the first word entered. After each word is entered (during a load state) a run state occurs and data from the random access memory and from a mask memory are read out and a correlation operation is performed. Data is recorded in the mask memory according to the keyword to be indentified to represent subelements of the keyword i.e. phoneme pairs and/or phoneme triplets. On average six masks are required for the detection of a desired keyword. The masks can vary from 1 to 16 STV's in length. The correlation operation determines the closeness between the keyword and the data in the RAM. Each address location of the mask memory contains an eighteen bit word, 6 bits of amplitude data r i , 5 bits of threshold data # i , 5 bits of weighting data w i  and End of Mask and End of Run bits. The STV data in the random access memory is read out in the form of words X i  each of six bits. These X i  are correlated with the eighteen bit words of the masks using a series of mathematical operations as shown in Fig. 12 in which one of the five discriminant functions d is performed by calculating the values m i , t i  and c i  as shown from the known values X i , r i , O i  and W i . A correlation signal d 1  to d m  is obtained for each of the m masks in the memory and the relative amplitude of the correlation signals determines the closeness between the stored mask and the speech. The word or words required generally necessitate the use of more than one mask, e.g. "Los Angeles" requires masks for "Los", "An", "ge" and "les". The words are examined through windows which are time positioned relative to each other and which probably overlap, e.g. "Los" occurs before "An" &c., e.g. The maximum values of d occurring in the windows are summed and compared with a threshold value to determine whether or not a desired keyword is present.