Patent GB-981153-A (GB)
Title: Improved phonetic typewriter system
IPC: G10L25/00
Filed: 1961-06-29
Abstract: 981,153. Recognising spoken words. NIPPON TELEGRAPH & TELEPHONE PUBLIC CORPORATION. June 29, 1961 [March 20, 1961], No. 23623/61. Heading G4R. In a phonetic typewriter control system the input sound signal is applied to a phoneme classifying and distinctive feature extracting means, analogue-to-digital converting means in which the sound is converted into a pattern representing the time variation of certain parameters, analysing means and "stability" and "distance" detectors which give indications of the immobility and change of the parameter pattern. There are also means for detecting pressures, transitions from one phoneme to another and vowels and a sampling control based on the sound itself rather than upon time interval. Speech signals from a source 1, are applied to units 2, 3 and 4 of which the first is adapted to classify phonemes and extract distinctive features, the second is the analogue-to-digital converter and the third is an analysing part. Signals from unit 3 are applied to units 5 and 6 which detect "stability"and "distance" as explained below. The output of unit 6 passes to unit 7 which detects plosives. The outputs of both units 5 and 6 are applied to units 8 and 9 for determining transitions and vowels respectively. The unit 10 receiving the output of both of the previous units generates sampling pulses for units 4 and 11 the latter being for discriminating phonemes. Unit 12 is an output device. Unit 2 is shown in Fig. 9 and consists of blocks 901-911. L-P and H-P filters in block 901 extract the fundamental frequency component and the formants and noise components. Circuit 902 detects that speech is present giving a signal (termed Q) when the speech envelope exceeds a certain level, and circuit 903 responds in the same way (signal X) when the high frequency energy exceeds a certain level. Circuit 904 responds (signal Y) when the output of the low frequency filter exceeds a certain value. Circuit 905 compares the outputs of the two filters and gives an output (Z) when the high pass filter has the greater output. These four signals in combination are received by "section" detector 909 which divides the soundwith sections as follows: vowel section (X.Y. Z). unvoiced consonant section (X.Y. Z), voiced constant section(#X.Y.#Z.)and nasal section (X.Y.#Z). The corresponding outputs are applied to block 910 which rejects or retains the section signals according as to whether they persist for more or less than a certain time (say 50 milliseconds). Circuit 906 detects a sudden rise of the high frequency signals which indicates a stop constant. The speech signal is also passed through filters 912-916 forming part of unit 4 and units 917-921 are zero-crossing circuits as described below. Circuit 907 counts the number of zero-crossings detected by circuit 919 during theconsonant sampling interval and the circuit 908 counts the number of waves with a width of less than 150 microseconds. This information is used to reveal the presence of a fricative consonant. The consonant sampling signal is derived from circuit 911. The outputs of all the circuits of block 2 are passed to block 9. In blocks 3 and 4 the method of zero-crossing analysis is used. In this method the speech wave is converted into a series of square pulses of different widths by amplifying and clipping. Pulses are counted for the duration of each pulse and the counter produces a signal on a channel lead according to the count. Over a certain period there appear a number of pulses in each channel, according to the distribution of pulse frequencies in the input signal. Counters in each channel give the sums of the pulses in the corresponding channels, during the sample period. Unit 3 consists of circuits for the first and second formants, having frequency filters 601, 602 and zero crossing analysis circuits 603, 605 and 604, 606. Circuits 607, 608 are threshold circuits and circuits 609, 610 are threshold level generators. For every period, each channel, five for the first frequency band and nine for the second, therefore produces a pulse or not according as to whether there have been more than a certain number of pulses of corresponding width. These signals are entered into parallel shift registers 611 as shown in Fig. 2. Units 5 and 6, Fig. 1 receive the signals from the store 611 and extract "stability" and "distance" features. Stability is a measure of the number of periods in which a signal is present in any particular channel. It is detected by looking at successive groups of say 6 periods and the presence of a signal in each of them, or in a certain number of them is taken to indicate stability in that particular channel. In Fig.2 stability is indicated by rows of successive dots, e.g. in the 4th channel of Fig. 2 from t1-t6. Stability is continuously detected by logical circuits. It may imply that the format is in the corresponding channel. "Distance" is the number of channels which change f om "1" to "0" or vice versa between successive periods. For example between t1 and t2 there are changes in F1 channels 1, 3 and 4 and F2 channels 1, 2, 3, and 7 making 7 in all. The outputs of these two units pass to units 8 and 9. In unit 8 a segmentation signal is generated to distinguish a certain section of a sound corresponding to a phoneme from a section corresponding to another phoneme. It is done by detecting when stability changes from one channel to another in either or both frequency bands. The transition signals are applied to unit 10 which generates sampling period signals for the analysing circuit 4. The unit 10 also receives signals from unit 9 indicating that a change of vowel has been detected. The analysing circuit 4 is shown in Fig. 9. It has two vowel frequency channels given by filters 912 and 913. Groups of channel counters 922, 923 are controlled by the sampling period signal and in each of the two frequency band channels one of the counters 922, 923 will produce a highest output. The highest is found by the peak detectors 927,927 and the vowel is determinedby circuit 930. There are also three consonant frequency filters 914-916 designed for unvoiced consonant, voiced consonant and nasal sounds and corresponding zero-crossing circuits 919-921. The counters 924-926 apply outputs to threshold detectors 929 so that binary signals are produced indicating the presence or absence of these sounds. The outputs of all circuits are successively stored, e.g. on a magnetic drum or tape, core memory or flip-flops. The signals stored represent a syllable and a diode logical circuit determines the syllable. In reading out a voltage is generated representing the number of pulses in each channel and converted to binary form by a transistor threshold device. Specification 981,154 is referred to.