Speech signal processing apparatus for detecting a speech signal

Abstract: There is disclosed a signal detection apparatus for detecting a noise-suppressed speech signal. In the signal detection apparatus, a band division process including a Fourier transformation is performed for an inputted speech signal, thereby outputting spectrum signals of plural channels. A cepstrum analysis process is performed for the spectrum signals, and a peak of the obtained cepstrum is detected in response to the cepstrum analysis result. Thereafter, a speech signal interval of the inputted noisy speech signal is detected in response to the detected peak, and a noise is predicted in the speech signal in response to the detected speech signal interval. Then, the predicted noise is canceled in the spectrum signals thereby outputting noise-suppressed spectrum signals. Finally, the noise-suppressed spectrum signals are combined and are inverse Fourier-transformed, thereby outputting a noise-suppressed speech signal.

Description: The present invention relates to a speech signal processing apparatus, and more particularly, to a signal detection apparatus for detecting a noise-suppressed speech signal from a noisy speech signal and detecting a kind of the detected speech signal, a signal processing apparatus for discriminating between speeches of plural speakers to be provided in the previous step of a speech recognition apparatus, and a speech recognition apparatus for recognizing a speech spoken by a speaker. Conventionally, there has been suggested an apparatus for discriminating between a speech and a noise in response to a noisy speech signal (See, for example, the Japanese patent laid open publication (JP-A) No. 59-124397/1984, and the Japanese patent laid open publication (JP-A) No. 59-131997/1984). The discrimination result outputted from the apparatus is used in an apparatus for recognizing a speech such as a speech recognition apparatus. However, the conventional apparatuses for discriminating between a noise and a speech can not detect a kind of the speech, namely, can not detect who speaks. In a general speech recognition apparatus, it is desired that a kind of the speech is previously detected. Further, conventionally, in a conventional speech recognition apparatus, a word spoken by a speaker is often recognized by inputting a speech signal of a particular speaker thereto. Namely, in order to heighten a recognition rate upon pattern-matching of speech features, there is considered that an information who now speaks is inputted to the speech recognition apparatus. Alternatively, in a speech recognition apparatus provided in a system requiring for keeping a secret, it is desired that only a particular speech is accepted. Namely, it is desired to judge who speaks and to use information of the judgment result at the previous step of the speech recognition apparatus. Further, recently, there has been developed a system which can be used only if a particular word is spoken in a place requiring for keeping a secret such as a laboratory. In this system, by utilizing a speech recognition technique, when a key word is spoken toward this system using key words such as "one", "three", "five", "seven", this system is enabled. In the system of this type requiring for keeping a secret, it is necessary not only to use the above-mentioned key word but also to limit the speaker to a particular one. Namely, it is desired that the system is constituted so that the system is enabled only when a predetermined particular one speaks a key word toward the system in order to further keep a secret. Document "Journal of the Acoustical Society of America" vol. 47, no. 2, February 1970, N.X., U.S., pages 634-648, Schafer et al, : "System for automatic formant analysis of voiced speech" discloses a system which can be used for speaker recognition. The object of the present invention is therefore to provide a signal detection apparatus capable of detecting a noise-suppressed speech signal from a noisy speech signal. Another object of the present invention is to provide a signal detection apparatus capable of detecting a speech and a kind of the speech in response to a speech signal. A further object of the present invention is to provide a signal processing apparatus capable of discriminating between speeches of plural speakers and outputting respective discriminated speech signals. A still further object of the present invention is to provide a speech recognition apparatus capable of recognizing only a speech of a predetermined particular speaker and outputting a speech signal thereof. In accordance with the invention, a signal processing apparatus is provided, comprising band division means for performing a band division process including a Fourier transformation for an inputted speech signal and outputting spectrum signals of plural channels; cepstrum analysis means for performing a cepstrum analysis process for the spectrum signals of plural channels outputted from said band division means and outputting a cepstrum analysis result; formant analysis means for performing a formant analysis process for the cepstrum analysis result outputted from said cepstrum analysis means and outputting a formant analysis result; speech detection means for detecting a feature of a speech of the inputted speech signal in response to the formant analysis result outputted from said formant analysis means and the cepstrum analysis result outputted from said cepstrum analysis means; speech judgment control means for storing predetermined features of speeches of plural speakers and sequentially outputting the stored features; speech judgment means for detecting which speaker among said plural speakers the feature detected by said speech detection means corresponds to by comparing the feature detected by said speech detection means with the stored features outputted from said speech judgment control means and outputting a detection result; switch control means for outputting a switch control signal according to the detection result outputted from said speech judgment means; and switch means for outputting speech signals discriminating them by respective speakers according to the switch control signal outputted from said switch control means in response to the inputted speech signal. Fig. 1 is a schematic block diagram showing a signal detection apparatus useful for describing the present invention; Figs. 2a and 2b are graphs showing a cepstrum analysis used in the present invention; Fig. 3 is a schematic block diagram showing a signal processing apparatus useful for describing the present invention; and Fig. 4 is a schematic block diagram showing a signal processing apparatus of a preferred embodiment according to the present invention. Referring to Fig. 1, the signal detection apparatus comprises a cepstrum analysis section 1, a storage unit 2, a signal judgment section 3, a maximum likelihood estimator 4 and a signal output section 5. The cepstrum analysis section 1 performs a cepstrum analysis for an inputted speech signal which is previously Fourier-transformed by a band division section. A cepstrum is obtained by calculating a logarithm of an amplitude spectrum for a short time interval of a waveform of the inputted speech signal and inverse-Fourier-transforming the calculated logarithm. Fig. 2a shows an amplitude spectrum for a short time interval of a speech signal, and Fig. 2b shows a cepstrum of the speech signal. The storage unit 2 is provided for previously storing predetermined feature information of known kinds of plural speech signals, and is constituted by a RAM or ROM. Namely, for example, the storage unit 2 stores information of a relationship between a cepstrum analysis result of a speech signal of a speaker A and the speaker A. The cepstrum analysis result includes at least a presence position of a peak or a pitch of the obtained cepstrum and formant information, as shown in Fig. 2b Similarly, the cepstrum analysis result of speeches of the other speakers B, C and so on is previously stored in the storage unit 2. The presence positions of peaks or pitches and the formant information of respective speakers are different from each other every speaker. It is to be noted that, as feature information of a speech, there may be used any speech feature such as an amplitude and a phase of a speech and so on in addition to the above-mentioned cepstrum analysis result. The signal judgment section 3 discriminates between a speech and a noise of a noisy speech signal by utilizing the cepstrum obtained by the cepstrum analysis section 1. As a method for detecting a speech signal interval by utilizing the cepstrum of the speak signal, there has been known to those skilled in the art, for example, a method for discriminating between a speech signal and a noise by detecting a peak or a pitch of the cepstrum. The pitch shown in Fig. 2b represents a peak of the speech signal, and a time interval when the peak exits in the cepstrum is a speech signal interval. The peak is detected by, for example, presetting a predetermined threshold value for a cepstrum and comparing respective power levels of the cepstrum with the preset threshold value. At a time when a speech signal interval is detected by the signal judgment section 3, a control signal is outputted from the signal judgment section 3 to the storage unit 2, and then, feature information previously stored in the storage unit 2 is inputted to the maximum likelihood estimator 4. On the other hand, the feature information of the cepstrum analysis result of the speech is inputted from the cepstrum analysis section 1 to the maximum likelihood estimator 4. The maximum likelihood estimator 4 compares the inputted cepstrum analysis result with the feature information outputted from the storage unit 2, sequentially, switching the feature information in an order of the feature information of the speakers A, B, C, D and so on, searching the feature information which is the most like inputted cepstrum analysis result, and detecting which speaker corresponds to the inputted speech. As a method for detecting the degree of the similarity therebetween, there can be used a pattern matching method, a method for detecting a degree of the similarity between amplitudes thereof, and a method for detecting a degree of the similarity between phases thereof, which are well known to those skilled in the art. The signal output section 5 outputs a pair of the detection results in response to the detection result outputted from the signal judgment section 3 and the detection result outputted from the maximum likelihood estimator 4. Namely, the signal output section 5 outputs not only presence information of the speech or information of the speak signal interval but also information of the speaker of the speech signal. An operation of the signal detection apparatus will be described below. First of all, cepstrum analysis results of speeches of respective speakers are previously stored in the storage unit 2. Thereafter, a noisy speech signal is inputted to the cepstrum analysis section 1, and then, the cepstrum analysis section 1 performs the cepstrum analysis for the inputted noisy speech signal as described above. It is to be noted that the cepstrum analysis of speeches of respective speakers may be performed by the cepstrum analysis section 1 as indicated by a line 6 (Fig. 1). The signal judgment section 3 detects a speech signal interval of the inputted noisy speech signal utilizing the peak etc. of the cepstrum analysis result outputted from the cepstrum analysis section 1. Then, at a timing when a speech signal interval is detected by the signal judgment section 3, a control signal is outputted from the signal judgment section 3 to the storage unit 2 as indicated by a line 7, and then, the cepstrum analysis result stored in the storage unit 2 is outputted to the maximum likelihood estimator 4. On the other hand, the cepstrum analysis result is inputted from the cepstrum analysis section 1 to the maximum likelihood estimator 4. The maximum likelihood estimator 4 compares the cepstrum analysis result inputted from the cepstrum analysis section 1 with the cepstrum analysis results inputted from the storage unit 2, and determines one of the cepstrum analysis results stored in the storage unit 2 which is the most like cepstrum analysis result outputted from the cepstrum analysis section 1. If there is not any one of the cepstrum analysis results which is like cepstrum analysis result outputted from the cepstrum analysis section 1 with a similarity larger than a threshold value, the maximum likelihood estimator 4 judges that there is no cepstrum analysis result which is like cepstrum analysis result of the inputted speech signal. Finally, the signal output section 5 outputs a pair of the detection result outputted from the maximum likelihood estimator 4 and the detection result outputted from the signal judgment section 3. In the description, a speech signal is used as an example of an input signal, there is used a speech signal, however, the present invention is not limited to this. In stead of the speech signal, there may be used a signal of a sound of an automobile or a signal of a sound of an airplane etc., and then, the other signals are judged as noise. In this case, the cepstrum analysis result is not used, and the other feature information thereof is used. Fig. 3 shows a signal processing apparatus useful for describing the present invention. It comprises a band division section 101, a cepstrum analysis section 102, a peak frequency detection section 103, a peak frequency judgment controller 104, a peak frequency judgment section 105, a switch controller 106 and a switch 107. Referring to Fig. 3, the band division section 101 performs a band division process for speech signals of plural speakers, and for example, includes an analogue to digital converter for converting inputted analogue speech signals into digital speech signals, and a fast Fourier transformer (referred to as an FFT hereinafter) for Fourier-transforming the A/D converted speech signals into spectrum signals. The cepstrum analysis section 102 performs a cepstrum analysis for the spectrum signals outputted from the band division section 101 and outputs data of the cepstrum analysis result to the peak frequency detection section 103. The peak frequency detection section 103 detects a peak or pitch frequency of the cepstrum analysis result in a manner similar to that of the signal judgment section 3 of Fig. 1. The peak frequency judgment controller 104 previously stores and outputs a peak frequency of each of speeches of plural speakers, sequentially. The peak frequencies of the speeches of plural speakers are different from each other. Therefore, the peak frequencies of the speeches of plural speakers are previously measured, and are stored therein in order to output them. The peak frequency judgment section 105 compares the peak frequency detected by the peak frequency detection section 103 with various kinds of peak frequencies outputted from the peak frequency judgment controller 104, and detects which speaker the peak frequency detected by the peak frequency detection section 103 corresponds to. For example, the peak frequency judgment section 105 searches the peak frequency of a speaker which is closest to the detected peak frequency, and outputs the detection information to the switch controller 106. The switch controller 106 outputs a switch control signal to the switch 107 according to the detection information inputted from the peak frequency judgment section 105. On the other hand, the above-mentioned plural speech signals are inputted to the switch 107. The switch 107 outputs the speech signals by discriminating them by respective speakers according to the switch control signal outputted from the switch controller 106. For example, as shown in Fig. 3, speech signals are inputted to the switch 107 in an order of speakers D, A, B and C. These speech signals are outputted to the output terminals A, B, C, D, ..., N by discriminating them by respective speakers. An operation of the signal processing apparatus will be described below in detail. The band division section 101 performs the band division process including the Fourier transformation for plural speech signals. The cepstrum analysis section 102 performs a cepstrum analysis for the spectrum signals of plural channels outputted from the band division section 101, and outputs the cepstrum analysis result to the peak frequency detection section 103. The peak frequency detection section 103 detects the peak frequency based on the cepstrum analysis result. On the other hand, the peak frequency judgment controller 104 outputs the peak frequencies of the speeches of respective speakers, sequentially, which are stored therein. The peak frequency judgment section 105 compares the detected peak frequency with the peak frequency sequentially transferred from the peak frequency judgment controller 104, detects which peak frequency of a speaker the detected peak frequency corresponds to, and outputs the detection information to the switch controller 106. The switch controller 106 outputs a switch control signal according to the detection information outputted from the peak frequency judgment section 105, to the switch 107. According to the switch control signal, the switch 107 makes the input terminal thereof connect to the output terminal A thereof if the inputted speech signal is judged as a speech signal of a speaker A. Further, the switch 107 makes the input terminal thereof connect to the output terminal B thereof if the inputted speech signal is judged as a speech signal of a speaker B. Thus, the speech signals of respective speakers are discriminated by respective speakers, and are outputted to the predetermined output terminals corresponding to the speakers. Fig. 4 shows a signal processing apparatus of a preferred embodiment according to the present invention. In Fig. 4, the same components as those shown in Fig. 3 are denoted by the same references as those shown in Fig. 3. The signal processing apparatus of the preferred embodiment is characterized by, as compared with Fig. 3, comprising a formant analysis section 108, a speech detection section 111, a speech judgment section 113 and a speech judgment controller 112 in stead of the peak frequency detection section 103, the peak frequency judgment controller 104, the peak frequency judgment section 105 of Fig. 3. Since the band division section 101, the cepstrum analysis section 102, the switch controller 106 and the switch 107 are the same as those shown in Fig. 3, the description of these components are omitted therein. The formant analysis section 108 analyzes a formant shown in Fig. 2b of a cepstrum which is obtained by the cepstrum analysis section 102. It is to be noted that the formant corresponds to an envelop of a spectrum of a speech signal. The pitch information etc. of the cepstrum analysis result outputted from the cepstrum analysis section 102 and the formant analysis result outputted from the formant analysis section 108 are inputted to the speech detection section 111, and in response thereto, the speech detection section 111 detects speech features of a speech signal and outputs data of the speech features to the speech judgment section 113. The speech judgment controller 112 outputs speech features of speeches of plural speakers, sequentially. The speech features of plural speakers are different from each other. Therefore, the speech features of plural speakers are previously measured and are stored therein in order to output them, sequentially, to the speech judgment section 113. The speech judgment section 113 compares the speech features detected by the speech detection section 111 with the speech features outputted from the speech judgment controller 112, and detects which speech features of a speaker the detected speech features of the inputted speech signal correspond to. In the speech judgment section 113, for example, there is used a pattern matching method which is well known to those skilled in the art. In the above-mentioned preferred embodiments, the cepstrum analysis section, the maximum likelihood detection section, the noise interval judgment section, the peak frequency detection section, the noise prediction section, the speech detection section, the formant analysis section and the first and second comparators are constituted utilizing a computer by software. However, these sections can be constituted by hardware only used for these sections. In the preferred embodiments, a speech signal may be handled as a noise. Namely, in the present invention, a noise means a signal other than a specified signal. In the above-mentioned preferred embodiments, there is provided plural comb type band-pass filters. However, only one comb type band-pass filter may be provided, and then, may be controlled by a time division multiplex method.

Claims: A signal processing apparatus comprising: band division means (101) for performing a band division process including a Fourier transformation for an inputted speech signal and outputting spectrum signals of plural channels; cepstrum analysis means (102) for performing a cepstrum analysis process for the spectrum signals of plural channels outputted from said band division means (101) and outputting a cepstrum analysis result; formant analysis means (108) for performing a formant analysis process for the cepstrum analysis result outputted from said cepstrum analysis means (102) and outputting a formant analysis result; speech detection means (111) for detecting a feature of a speech of the inputted speech signal in response to the formant analysis result outputted from said formant analysis means (108) and the cepstrum analysis result outputted from said cepstrum analysis means (102); characterised by speech judgment control means (112) for storing predetermined features of speeches of plural speakers and sequentially outputting the stored features; speech judgment means (113) for detecting which speaker among said plural speakers the feature detected by said speech detection means (111) corresponds to by comparing the feature detected by said speech detection means (111) with the stored features outputted from said speech judgment control means (112) and outputting a detection result; switch control means (106) for outputting a switch control signal according to the detection result outputted from said speech judgment means (113); and switch means (107) for outputting speech signals discriminating them by respective speakers according to the switch control signal outputted from said switch control means (106) in response to the inputted speech signal.

Inventors: KANE JOJI, NOHARA AKIRA, KANE, JOJI, NOHARA, AKIRA
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1991
Main classification: G10L21
IPC codes: G10L21, G10L11