Continuous speech recognition unit

Abstract: Continuous speech recognition unit recognizes continuous speech associated with the standard patterns of the given units of recognition. Said unit comprises a standard pattern memory means (1) for storing observation likelihoods and transition probabilities as said standard pattern, a similarity computing means (2) for computing similarity between speech patterns inputted to the continuous speech recognition unit and the standard pattern in the interval between respective points in time, a cumulative value computing means for computing, as a cumulative value, the product of the sum of the products of cumulative values of said similarities obtained for all or part of matching passes on said speech pattern of up to a certain point in time and the transition probability, and the observation likelihood obtained for said speech pattern, a cumulative value memory means (23) for storing said cumulative values, a matching pass memory means (24) for storing the matching pass which maximizes the cumulative value at the current point in time, and a result processor means (28) for defining result of recognition which corresponds to the most probable state from said stored cumulative value and matching pass.

Description: The present invention relates to the improvement of continuous speech recognition for recognizing continuous speech composed of continuously uttered words. Conventionally, as speech recognition method, known is a speech recognition method by "Hidden Markov Model" (hereinafter referred as HMM) as described in "Stephen E. Levinson, Structural Methods in Automatic Speech Recognition, Proceedings of the IEEE, Vol. 73, No. 11, November 1985" (hereinafter referred as "literature 1") page 1633. In this method, first the generation phase of speech patterns is modelled as the state transition model by the Markov process. This state transition model is HMM. Speech recognition of the observed speech patterns is performed by determining observation probabilities from this HMM. Let us consider the case where words are recognized by using this process. In this case, first, HMM is formed for each word to be recognized. This method for forming HMM is fully described in the above-mentioned "literature 1", page 1633. When speech pattern is inputted to the speech recognition unit, the observation probability for each HMM is computed, and the result of recognition is obtained as a word for the HMM which gives the highest observation probability. This observation probability can be also considered as the similarity between speech pattern and each HMM, in which HMM is equivalent to the standard pattern. The observation probabilities for HMM can be obtained by the forward algorithm or the Baum algorithm as described in the above-mentioned "reference 1", page 1634. Further the HMM allows continuous speech patterns composed of continuously uttered words to be recognized. Hereinunder, as an example of continuous speech recognition, the case where units of recognition are words is explained. However, any recognition such as vocal sound can be similarly treated. The continuous speech recognition in the case where units of recognition are words can be achieved by means of the Viterbi algorithm as described in the above-mentioned "reference 1", page 1635. The Viterbi algorithm used for continuous speech recognition is an approximation method in which the observation probabilities of words can be obtained from the product of probabilities on a matching pass which is defined as a trace associated with the correspondence between points in time of two patterns. Therefore, the Viterbi algorithm has a disadvantage that the recognizing rate thereof is generally lower compared with the forward algorithm in which the observation probabilities for words can be obtained from probabilities on all possible matching passes. On the other hand, in the forward algorithm, the matching pass giving the maximum probability cannot be uniquely determined. Therefore, the forward algorithm has a disadvantage that the result of recognition cannot be obtained unless computations are performed for all combinations of word sequences in a round robin, in performing continuous speech recognition. Document IEEE TRANSACTIONS ON PATTERN ANALYSIS AND MACHINE INTELLIGENCE, vol. 5, no. 2, March 1983, New-York, pages 179-190; BAHL ET AL.: "A MAXIMUM LIKEYHOOD APPROACH TO CONTINUOUS SPEECH RECOGNITION" also discloses a recognition method using the Viterbi algorithm. SUMMARY OF INVENTION An object of the present invention is to provide a continuous speech recognition unit wherein the efficient continuous speech recognition can be implemented in small amount of computation so that the matching pass giving the maximum probability can be obtained in a word based on the forward algorithm with the high recognition performance. The object of the present invention is achieved by continuous speech recognition unit as set out in Claim 1. In the continuous speech recognition unit according to the present invention, the cumulative value computing means is a computer which computes the product of the sum of the cumulative values of the similarities obtained for matching passes on the speech pattern of up to a certain point in time or the product of the sum of the cumulative values of the similarities obtained for matching passes stored in the matching pass memory wherein positions of start points of units of recognition obtained from the matching passes are coincident with positions of start points for the matching passes which give the maximum similarities, or the product of the sum of the cumulative values of the similarities obtained for matching passes wherein all or part of recognition results estimated by the result processor means are coincident with the recognition result for the matching passes which give the maximum similarities. Function of continuous speech recognition unit according to the present invention is explained. In the present invention, in continuous word recognition executed with Hidden Markov models described in the above-mentioned literature 1, calculation of probabilities between words is made based on the Viterbi . Namely, it is assumed that the result of recognition about word sequence up to a certain point in time t on speech patterns gives the maximum probability at its point in time. On the other hand, it is assumed that calculation of probabilities in words is made by means of the forward algorithm and the matching pass is obtained. In the present invention, observation probabilities are computed by obtaining forward likelihood which is the cumulative value of the probabilities by means of the forward algorithm. Matching pass is obtained by way of preserving matching pass which gives the maximum forward likelihood at its point in time, by which it becomes possible that observation probabilities are obtained by means of the forward algorithm and matching pass is obtained. Next, this method is explained. Observed speech patterns are expressed in {O t |1<t<T} as time series of characteristic vectors thereof. If probability that speech pattern O t is observed is represented by observation distribution b j (O t ) and transition probability that state changes from i to j is represented by a ij , the forward likelihood α(j,t) at point in time t and in state j can be computed by means of the forward algorithm from the following formula: Further, to obtain the result of recognition for a word, position of the start point of the word is traced back together with the result of recognition at the time of the terminal point thereof, and stored. In frames of respective points in time t, the time before one frame of the start point of the word w giving the best result of recognition can be obtained by using the following back pointer ℓ(j,t). Namely, assuming that in a state of start 1 in HMM of word, ℓ(1,t)=t-1, the forward partial likelihood in a word is computed from the formula (1). Therewith, the position of start point with respect to the matching pass giving the best probability is propagated according to the following formula: wherein represents i giving the maximum of x(i). Time s before one frame of start point of word is preserved in the back pointer ℓ(J,t) obtained at terminal state J of word. Name of word w corresponding to the maximum of forward likelihood α(J,t) and time s before one frame of start point of its word are preserved in the resulting word memory Rw(t)=w and the resulting start point memory Rs(t)=s. When recognition process has been completed up to terminal T of speech pattern inputted to the continuous speech recognition unit, the result of recognition of word sequence can be obtained by following up in order positions of start points of preserved words. Namely, it can be obtained in the following procedures of: (1) settinq t=T; (2) outputting the result of recognition w=Rw(t), and setting t=Rs(t); and (3) terminating the process in case of t<1, and except for this case returning to (2). Outputted results of recognition which are arranged in reverse order corresponds to the result of recognition of word sequence. If contribution of the matching pass giving the maximum probability to forward likelihood is greater, it is possible to obtain results of recognition with high precision in the above mentioned method. However in case of obtaining forward likelihood from the formula (1), there is the possibility that traced-back start point of word differs. When start point of word differs, there is also the possibility that results of recognition of word sequence for the start point differs. In this case, calculation of the formula (1), forward probabilities for various word sequences are unfavorably summed up. Further, in the present invention, the forward probability is computed by adding only forward likelihoods for matching pass wherein a start point is coincident with a start point of the matching pass giving the maximum forward probability at its point in time. Namely, the computation of the forward probability according to the formula (1) is transformed as follows. In the computation, the summation of only forward likelihoods which satisfy ℓ(i,t-1)=ℓ(i max ,t-1) is performed. Therefore increase of processing amount is small. The back pointer ℓ can be computed from the formula (2). Accordingly, there is cancelled a problem that the forward likelihoods for different result of recognition are unfavorably added, since result of recognition is uniquely determined for a start point. However, with regard to matching pass a start point of which differs to some extent, there is the possibility that error is yielded, since forward likelihood for the matching pass is not added. Further in the present invention, the forward likelihood is computed by adding forward likelihoods for matching pass which result of recognition matches the result of recognition of the matching pass giving the maximum probability of up to a certain point in time, including forward likelihood for matching pass having different position of start point. Since there is high possibility that matching pass having the forward likelihood close to the maximum likelihood yields the same result of recognition in spite of different position of start point thereof, this method is effective. To implement this method, results of recognition of up to a certain point in time are propagated as in back pointer obtained from the formula (2). Namely, result pointer r(j,t) is provided. It is assumed that at a start point for HMM of a word (state 1), r(1,t) = c where c represents coded sequence of words which has been recognized before that point in time. In a word, result pointer is computed as follows: r(j,t) = r(i max ,t-1) The forward likelihood is computed from the following formula. Thereby, since the forward likelihood is obtained by adding forward likelihoods which have same result of recognition, the forward likelihood can be obtained with high precision. Herein, sequence of words recognized before that point in time is kept as a code c at the result pointer r(j,t). Where number of words which has been recognized, code c becomes larger, therefore there is the possibility that processing amount for comparison computation is increased. To deal with this problem, only few recent results of recognition which has been recognized are coded instead of coding all of results of recognition which has been recognized, by which code c can be made to an appropriate volume. According to the present invention it is possible to implement a continuous speech pattern recognition unit in which continuous speech pattern recognition can be performed with high precision in small amount of computation. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a schematic diagram showing a first embodiment according to the present invention, Fig. 2 is a schematic diagram showing a second embodiment according to the present invention, and Fig. 3 is a schematic diagram showing a third embodiment according to the present invention. PREFERRED EMBODIMENT OF THE INVENTION One embodiment of continuous speech recognition unit according to the present invention is explained with reference to the drawings. Fig. 1 is a schematic diagram showing the first embodiment according to the present invention. The continuous speech recognition unit of the first embodiment according to the present invention comprises a standard pattern memory 1, an observation likelihood computer 2, a transition probability computer 5, a forward likelihood computer 6, a maximum detector 7, a matching pass memory 4, a cumulative value memory 3 and a result processor 8. The standard pattern memory 1 is connected to the observation likelihood computer 2 and the transition probability computer 5. The observation likelihood computer is connected to the forward likelihood computer 6. The transition probability computer 5 is connected to the forward likelihood computer 6 and the maximum detector 7. The forward likelihood computer 6 is connected to the cumulative value memory 3, which are connected to the transition probability computer 5 and the result processor 8. The maximum detector 7 is connected to the matching pass memory 4, which is in turn connected to the result processor 8. Standard patterns of word unit are kept, as observation likelihood table b j (O) and transition probability a ij which are parameters of HMM as shown in the expression (1), in the standard memory 1. Speech pattern O t is inputted to the observation likelihood computer 2. At the observation likelihood computer 2, observation likelihood b j (O t ) of the speech pattern Ot is computed by using the observation likelihood table b j (O). The observation likelihood is obtained by vector quantization as described in the above-mentioned literature 1, page 1634. Forward likelihoods α(j,t) are kept in the cumulative value memory 3. For an initial value, α(1,1)=1.0 while for the other j,t,α is 0.0. Further the back pointer ℓ(j,t) representing a position of start point with respect to a matching pass is kept in the matching pass memory 4. For an initial value, value of ℓ(1,t)=t-1 is set at j=1 of the state corresponding to the start point of word. When the observation likelihood b j (O t ) for time t is computed at the observation likelihood computer 2, the transition probability a ij is read out from the standard pattern memory 1, and successively the forward likelihood α(j,t-1) is read out from the cumulative value memory 3. Product α(i,t-1)·a ij is computed at the transition probability computer 5. Said value is inputted from the transition probability computer 5 to the forward likelihood computer 6 while the observation likelihood b j (O t ) is inputted from said value and the observation likelihood b j (O t ), the forward likelihood α(j,t) is computed according to the formula (1). The obtained forward likelihood α(j,t) is kept at the cumulative value memory 3. Therewith, the product α(i,t-1)·a ij is inputted from the transition probability computer 5 to the maximum detector 7, and i max which is i giving the maximum of obtained forward likelihoods at its point in time is computed according to the formula (3) at the maximum detector 7. The back pointer ℓ(j,t) is computed from the i max read out from the maximum detector 7 according to the formula (2) at the matching pass memory 4 and kept therein. When the above-mentioned process has been completed for time t, the forward likelihoods α(J,t) and the back pointer ℓ(J,t) for a final state are read out from the cumulative value memory 3 and the matching pass memory 4, respectively, and the best result of recognition is defined according to the method described in the above-mentioned literature 1 at the result processor 8 and kept therein. When the above-mentioned process has been completed, the result of recognition of word sequence R can be obtained by following up positions of start points of words which are kept in the above-mentioned method. Then a second embodiment of the present invention is explained. Fig. 2 is a schematic diagram showing the second embodiment according to the present invention. The continuous speech recognition unit of the second embodiment according to the present invention comprises a standard pattern memory 21, an observation likelihood computer 22, a transition probability computer 25, a forward likelihood computer 26, a maximum detector 27, a back pointer comparator 29, a matching pass memory 24, a cumulative value memory 23 and a result processor 28. The standard pattern memory 21 is connected to the observation likelihood computer 22 and the transition probability computer 25. The observation likelihood computer 22 is connected to the forward likelihood computer 26. The transition probability computer 25 is connected to the forward likelihood computer 26 and the maximum detector 27. The maximum detector 27 is connected to the matching pass memory 24, which are connected to the back pointer comparator 29 and the result processor 28. The forward likelihood computer 26 is connected to the cumulative value memory 23, which are connected to the transition probability computer 25 and the result processor 28. As in the first embodiment, the forward likelihood α(j,t) is kept in the cumulative value memory 23. For the initial value, α(1,1)=1.0 while for the other j,t,α is 0.0. Further the back pointer ℓ(j,t) representing position of start point with respect to a matching pass is kept in the matching pass memory 24. As an initial value, value of ℓ(1,t)=t-1 is set at j=1 of the state corresponding to the start point of word. When the observation likelihood b j (O t ) for time t is computed at the observation likelihood computer 22, the transition probability a ij is read out from the standard pattern memory 21 and successively the forward likelihood α(j,t-1) is read out from the cumulative value memory 23. Product α(i,t-1)·a ij is computed at the transition probability computer 25. Said value is inputted from the transition probability computer 25 to the maximum detector 27, at which i max which is i giving the maximum of obtained forward likelihood at its point in time is computed according to the formula (3). The back pointer ℓ(j,t) is computed from the i max read out from the maximum detector 27 according to the formula (2) at the matching pass memory 24, and kept therein. ℓ(i,t-1) for state i successively read out from the matching pass memory 24 is compared with ℓ(i max ,t-1) giving the maximum at the back pointer comparator 29, and state ic in which the former matches the latter is outputted from the back pointer comparator 29 to the forward likelihood computer 26, at which the forward likelihood α(j,t) is obtained as product of sum of products α(i,t-1).a ij accumulated with regard to the state ic and the observation likelihood b j (O t ) according to the formula (4), at the forward likelihood computer 26. The obtained forward likelihood α(j,t) is outputted from the forward likelihood computer 26 to the cumulative value memory 23 and kept therein. As in the first embodiment, when the above-mentioned process for time t has been completed, the forward likelihood α(J,t) and the back pointer ℓ(J,t) for a final state are read out from the cumulative value memory 23 and the matching pass memory 24, respectively, and the best result of recognition is defined at the result processor 28 and kept therein. When the above-mentioned process has been completed, the result of recognition of word sequence R can be obtained by following up positions of start points of words, which are kept in the above-mentioned method. Then, a third embodiment of the present invention is explained. Fig. 3 is a schematic diagram showing the third embodiment according to the present invention. The continuous speech recognition unit of the third embodiment according to the present invention comprises a standard pattern memory 31, an observation likelihood computer 32, a transition probability computer 35, a forward likelihood computer 36, a maximum detector 37, a result comparator 40, a cumulative value memory 33, a matching pass memory 34, a result memory 39, and a result processor 38. The standard pattern memory 31 is connected to the observation likelihood computer 32 and the transition probability computer 35. The transition probability computer 35 is connected to the forward likelihood computer 36 and the maximum detector 37. The observation likelihood computer 32 is connected to the forward likelihood computer 36. The maximum detector 37 is connected to the matching pass memory 34 and the result memory 39. The matching pass memory 34 is connected to the result processor 38. The result memory 39 is connected to the result comparator 40, which is connected to the forward likelihood computer 36. The forward likelihood computer 36 is connected to the cumulative value memory 33, which are conneted to the transition probability computer 35 and. the result processor 38. As in the first embodiment, the forward likelihood α(j,t) is kept in the cumulative value memory 33. For an initial value, α(1,1)=1.0 while for the other j,t,α is 0.0. Further the back pointer ℓ(j,t) representing position of start point with respect to a matching pass is kept in the matching pass memory 34. As an initial value, value of ℓ(1,t)=t-1 is set at j=1 of the state corresponding to the start point of word. Further, the result pointer r(j,t) is kept in the result memory 39. As an initial value, coded result of recognition c is set. When the observation likelihood b j (O t ) for time t is computed at the observation likelihood computer 32, the transition probability is read out from the standard pattern memory 31 and successively the forward likelihood α(j,t-1) is read out from the cumulative value memory 33. Product α(i,t-1 ).a ij is computed at the transition probability computer 35. Said value is inputted from the transition probability computer 35 to the forward likelihood computer 36 and the maximum detector 37. imax which is i producing the maximum is computed according to the formula (3), at the maximum detector 37. The back pointer ℓ(j,t) is computed from the i max read out from the maximum detector 37 according to the formula (2) at the matching pass memory 34 and kept therein. Similarly, the result pointer r(j,t) is computed according to the formula (7) at the result memory 39 and kept therein. r(i,t-1) for state i successively read out from the result memory 39 is compared with the result pointer r(i max ,t-1) giving the maximum read out therefrom at the result comparator 40, and state ic in which the former matches the latter is outputted from the result comparator 40 to the forward likelihood computer 36, at which the forward likelihood α(j,t) is obtained as product of sum of products α(i,t-1)·a ij summed up with regard to the state ic and the observation likelihood b j (O t ) according to the formula (8) at the forward likelihood computer 36. The obtained forward likelihood α(j,t) is outputted from the forward likelihood computer 36 to the cumulative value memory 33 and kept therein. As in the first embodiment, when the above mentioned process for time t has been completed, the forward likelihood α(J,t) and the back pointer ℓ(J,t) for a final state are read out from the cumulative value memory 23 and the matching pass memory 24, respectively, and the best result of recognition is defined at the result processor 38 and kept therein. At the same time, the best result of recognition is outputted as result c from the result processor 38 to the result memory 39. It is kept as initial value of the result pointer r(j,t) as shown in the formula at the result memory 39. When the above-mentioned process has been completed, the result of recognition of word sequence R can be obtained by following up positions of start points of words, which are kept in the above-mentioned method.

Claims: Continuous speech recognition unit for recognizing continuous speech associated with the standard patterns of the given units of recognition comprising: a standard pattern memory means (1, 21, 31) for storing observation distributions and transition probabilities as said standard pattern; a similarity computing means (2, 22, 32) for computing similarity between speech patterns inputted to the continuous speech recognition unit and the standard pattern in the correspondence between respective points in time; a cumulative value computing means (6, 26, 36) for computing, as a cumulative value, the sum of products of respective cumulative values and similarities obtained for all or part of matching passes on said speech pattern of up to a certain point in time; a cumulative value memory means (3, 23, 33) for storing said cumulative values; a matching pass memory means (4, 24, 34) for storing the matching pass which maximizes the cumulative value; and a result processor means (8, 28, 38) for defining result of recognition which corresponds to the most probable state from said stored cumulative value and matching pass. Continuous speech recognition unit as claimed in claim 1, wherein said cumulative value computing means (6, 26, 36) computes, as said cumulative value, the sum of the products of respective cumulative values and similarities obtained for matching passes on said speech pattern of up to a certain point in time. Continuous speech recognition unit as claimed in claim 1, wherein said cumulative value computing means (6, 26,36) computes, as said cumulative value, the sum of the products of respective cumulative values and similarities obtained for matching passes stored in the matching pass memory wherein start positions of units of recognition obtained from said matching passes are coincident with positions of start positions for the matching passes which give the maximum similarities. Continuous speech recognition unit as claimed in claim 1, wherein said cumulative value computing means (6, 26, 36) computes, as said cumulative value, the sum of the products of respective cumulative values and similarities obtained for matching passes wherein all or part of recognition results estimated by the result processor means are coincident with the recognition result for the matching passes which give the maximum similarities. Continuous speech recognition unit as claimed in any of claims 1 to 4, wherein said similarity is forward likelihood.

Inventors: YOSHIDA KAZUNAGA, YOSHIDA, KAZUNAGA
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1989
Main classification: G10L5
IPC codes: G10L11, G10L15