Speech signal processing apparatus for detecting a speech signal from a noisy 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 processor, and more particularly, to a signal processor for analysing noisy input signals in order to derive a noise-suppressed speech signal by separating noise from this signal having characteristic features. Conventionally, there has been suggested an apparatus for discriminating between speech and noise in a noisy speech signal (see, for example, JP-A No. 59-124397/1984, and JP-A No. 59-131997/1984). The discrimination result outputted from the apparatus is used in a speech recognition apparatus for recognizing speech. Furthermore, from a document "SIGNAL PROCESSING. EUROPEAN JOURNAL DEVOTED TO THE METHODS AND APPLICATIONS OF SIGNAL PROCESSING, vol. 15, no. 1, July 1988 pages 43-56, 'Acoustic noise analysis and speech enhancement techniques for mobile radio applications' and in particular paragraphs 4.4. 'Spectral subtraction technique', a spectral subtraction technique is described, and reference is made to an article P. Vary, "Noise suppression by spectral magnitude estimation - Mechanism and theoretical limits", Signal Processing, Vol. 8, No. 4, July 1985, pp.387-400, in which more details are described. A common feature of all speech enhancement techniques based on spectral resolution is the estimation of the signal magnitude during speech activity for a set of frequency bands. Noise suppresion is performed on every band by considering only the magnitude and not the phase of the additive noise. The last mentioned article shows that phase in unimportant in speech enhancement as long as local signal-to-noise ratios are at least about 6 dB. It is an object of the present invention to provide a speech signal processor capable of deriving a noise-suppressed speech signal from a noisy speech signal. This object is being solved by a speech signal processor comprising: Storage means for storing information of predetermined features of a plurality of kinds of predetermined speech signals corresponding to a plurality of speakers, cepstrum analysis means for performing a cepstrum analysis process for the inputted noisy speech signal and outputting a cepstrum of the inputted noisy speech signal, signal judgement means for detecting such a predetermined feature from an inputted cepstrum of the noisy speech signal and outputting a control signal indicating an interval when a speech signal is present, maximum likelihood estimation means for detecting one of the kinds of the predetermined signals of the inputted noisy speech signal by comparing the features of the cepstrum of the input signal in the interval indicated by said control signal with the predetermined features stored in said storage means, and signal output means for outputting the control signal outputted by said signal judgement means and the kind of signal detected by said maximum likelihood estimation as a pair of signals. Preferable embodiments are defined in the dependent claims. These and other features of the present invention will become clear from the following description taken in conjunction with the preferred embodiments thereof with reference to the accompanying drawings throughout which like parts are designated by like reference numerals, and in which: Fig. 1 is a schematic block diagram showing a signal detection apparatus of a first preferred embodiment according to the present invention; Fig. 2 is a schematic block diagram showing a signal detection apparatus of a second preferred embodiment according to the present invention; Fig. 3 is a schematic block diagram showing a noise suppression apparatus of a third preferred embodiment according to the present invention; Figs. 4a and 4b are graphs showing a cepstrum analysis used in the preferred embodiments; Fig. 5 is a graph for showing a noise prediction method used in the preferred embodiments; Figs. 6 and 7 are diagrams showing waveforms for explaining a noise cancellation method used in the preferred embodiments. The preferred embodiments according to the present invention will be described below in detail with reference to the attached drawings. Fig. 1 shows a signal detection apparatus of a first preferred embodiment according to the present invention. In the present preferred embodiment, there is described one example of using a speech signal as a signal to be processed. 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. 4a shows an amplitude spectrum for a short time interval of a speech signal, and Fig. 4b 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. 4b. 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. Namely, as shown in Fig. 2, the signal judgment section 3 may comprise a peak detection section 31 for detecting a peak of the obtained cepstrum, and a speech judgment section 32 for detecting a speech signal interval based on the detected peak information. The pitch shown in Fig. 4b 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 of the present preferred embodiment 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 present preferred embodiment, as 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. 2 shows a signal detection apparatus of a second preferred embodiment according to the present invention. In Fig. 2, the same components as those shown in Fig. 1 are denoted by the same references as those shown in Fig. 1. As shown in Fig. 2, in a manner similar to that of the first preferred embodiment shown in Fig. 1, the signal judgment section 3 comprises the peak detection section 31 and the speech judgment section 32. However, the apparatus further comprises a noise prediction section 8 and a noise power calculation section 9 in addition to the respective sections of the first preferred embodiment. Differences between the first and second preferred embodiments will be described below in detail. The maximum likelihood estimator 4 detects a kind of a speech which is the most like that of an inputted speech signal taking into consideration a predicted noise in the speech in addition to the cepstrum analysis results outputted from the storage unit 2 and the cepstrum analysis result outputted from the cepstrum analysis section 1. Namely, when the speech judgment section 32 detects a time interval during which speech is present, the detection result is outputted from the speech judgment section 32 to the noise prediction section 8. On the other hand, a noisy speech signal is inputted to the noise prediction section 8. Since a portion where there is no speech is a portion where there is only noise (referred to as a noise alone portion hereinafter), the noise prediction section 8 predicts a noise of a speech based on data of the noise alone portion. Namely, the noise prediction section 8, for example, predicts a noise component by respective channels based on the noisy speech signal which are divided into plural m band channels, each band channel having a predetermined band width and respective band channels having bands which are different from each other. Fig. 5 shows one example of a noise prediction method. In Fig. 5, an x-axis is a frequency, a y-axis is a power level of an inputted speech signal, and a z-axis is a time. In a frequency f1, there is data p1, p2, ..., pi as a time is elapsed, and then, data pj presenting after data pi on line of data p1, p2, ..., pi is predicted. For example, an average value of noise data p1 to pi is calculated, and then, the calculated average value is set at a predicted value pj. Alternatively, when a speech signal continuously exits, data pj is multiplied by an attenuation factor, and then, the product of the data pj and the attenuation factor is set at a predicted value. The noise power calculation section 9 calculates a power level of the predicted noise such as an average value of an amplitude thereof. When the calculated power level of the predicted noise is larger than a predetermined threshold value, even though the cepstrum analysis result stored in the storage unit 2 is slightly different from the cepstrum analysis result outputted from the cepstrum analysis section 1, the maximum likelihood estimator 4 judges that they are substantially similar to each other, and the level of the similarity judgment standard is lowered. On the other hand, when the power level of the predicted noise is smaller than the predetermined threshold value, the maximum likelihood estimator 4 judges that there is substantially no noise, and then, the level of the similarity judgment standard is heightened. Fig. 3 shows an apparatus including a noise suppression apparatus of a third preferred embodiment according to the present invention. In Fig. 3, the same components as those shown in Figs. 1 and 2 are denoted by the same references as those shown in Figs. 1 and 2. The noise suppression apparatus of the third preferred embodiment is characterized in outputting a noise-suppressed speech signal utilizing a prediction value of a predicted noise, and further comprises a band division section 10, a cancellation section 11 and a band combining section 12 in addition to the second preferred embodiment. Referring to Fig. 3, a noisy speech signal is inputted to the band division section 10, and then, in response thereto, the band division section 10 converts the inputted analogue noisy speech signal into digital noisy speech signal, Fourier-transforms the digital noisy speech signal into a spectrum signal, and then, divides the transformed spectrum signal into spectrum signals of plural m channels. The spectrum signals of plural m channels outputted from the band division section 10 are inputted to the above-mentioned cepstrum analysis section 1 and the noise prediction section 8. The noisy spectrum signals of respective frequency channels outputted from the band division section 10 is inputted to the cancellation section 11, and the cancellation section 11 rejects or cancels a noise of a prediction value predicted by the noise prediction section 8 as follows. Fig. 6 shows one example of a general cancellation method on a time axis. As shown in Fig. 6, a noise waveform (b) predicted by the noise prediction section 8 is subtracted from a noisy speech signal (a), thereby extracting only a speech signal (c). In the present preferred embodiment, there is used a cancellation method on a frequency axis shown in Fig. 7. Referring to Fig. 7, a noisy speech signal (a) is Fourier-transformed into a spectrum signal (b), and then, a spectrum (c) of the predicted noise is subtracted from the converted spectrum signal (b) so as to obtain the subtracted spectrum result (d). Further, the subtracted spectrum result (d) is inverse-Fourier-transformed into a speech signal (e) including no noise or a noise-suppressed speech signal (e). The band combining section 12 combines the spectrum signals of plural m channels outputted from the cancellation section 11, and inverse-Fourier-transforms the combined spectrum signals into a noise-suppressed speech signal. As described above, in the preferred embodiment, a noise component is canceled from the noisy speech signal by the cancellation section 11, and a speech signal is obtained by the band combining section 12. Therefore, a noise-suppressed speech signal can be obtained together with information of a speaker of the speech signal. It is to be noted that, in the preferred embodiment, as features to be stored in the storage unit 2, there is selected peak information of the cepstrum analysis results of respective speakers.

Claims: A speech signal processor comprising: storage means (2) for storing information of predetermined features of a plurality of kinds of predetermined speech signals corresponding to a plurality of speakers; cepstrum analysis means (1) for performing a cepstrum analysis process for the inputted noisy speech signal and outputting a cepstrum of the inputted noisy speech signal; signal judgement means (3) for detecting such a predetermined feature from an inputted cepstrum of the noisy speech signal and outputting a control signal indicating an interval when a speech signal is present; maximum likelihood estimation means (4) for detecting one of the kinds of the predetermined signals of the inputted noisy speech signal by comparing the features of the cepstrum of the input signal in the interval indicated by said control signal with the predetermined features stored in said storage means, and signal output means (5) for outputting the control signal outputted by said signal judgement means (3) and the kind of signal detected by said maximum likelihood estimation as a pair of signals. The speech signal processor as claimed in claim 1, further comprising noise prediction means (8) for predicting noise of the signal in response to the inputted noisy signal; wherein said maximum likelihood estimation means (4) detects the kind of the speech signal in response to the noise predicted by said noise prediction means (8). A speech signal processor as claimed in claim 1, further comprising: band division means (10) 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 (1) for performing a cepstrum analysis process for the spectrum signals of plural channels outputted from said band division means (10) and outputting a cepstrum analysis result; said judgement means (3) consisting of peak detection means (31) for detecting a peak of a cepstrum in response to the cepstrum analysis result outputted from said cepstrum analysis means, and speech judgement means (32) for detecting a speech signal interval of the inputted noisy speech signal based on the peak detected by said peak detection means and outputting the detected speech signal interval; noise prediction means (8) for predicting a noise in the speech of the inputted noisy speech signal outside the detected speech signal interval outputted from said speech judgement means (32); cancellation means (11) for cancelling noise from the inputted noisy speech signal by subtracting the spectrum of predicted noise by said noise prediction means (8) from the spectrum signals of plural channels outputted from said band division means (10) and outputting noise-suppressed spectrum signals of plural channels; and band combining means (12) for combining the noise-suppressed spectrum signals of plural channels, inverse Fourier-transforming the combined spectrum signals, and outputting a transformed 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: G10L3
IPC codes: G10L11, G10L15, G10L21