Voice signal coding system

Abstract: A voice signal coding system includes a voice signal detector (1, 7) for receiving a mixed signal of voice signal and background noise signal and for detecting the presence and absence of the voice signal contained in the mixed signal. A voice signal period detector (4) is provided for detecting a voice signal period in which the voice signal is present. A coding period control circuit (5) is coupled to the voice signal period detector for producing a coding period control signal during the voice signal period. A coding circuit (6) receives and encodes the mixed signal in response to the coding period control signal. Thus, only the voice signals are coded in the coding circuit.

Description: The present invention relates to a voice signal coding system adapted to encode noise-mixed voice signals. Description of the Related Art For transmitting voice signals to remote places, the voice signals are coded. US-A-4,918,734 defining the closest prior art from which the invention proceeds discloses a speech coding system including apparatus for generating a variable threshold value dependent upon the power of an input speech signal, and a comparator for comparing the power of the input speech signal with the variable threshold value to generate a discriminating signal for discriminating between a period when a speech continues and a period when the speech pauses, to change the coding operation for the input speech signal in accordance with the level of the discriminating signal, thereby forming voiced and unvoiced frames independently of each other. It is further known from "The Journal of the Accustical Society of America", Vol. 41, No. 2, 1967, pages 293 - 309, New York, US; A.M. O: "Cepstrum Pitch Determination" a wanted signal code system comprising band dividing means for dividing a mixed signal into a plurality of frequency ranges and for supplying the divided signals through a plurality of channels, a cepstrum analysis means for cepstrum-analysing the signal in each channel from the band dividing means, and a peak detection means for detecting a cepstrum peak as the cepstrum analysis output of said cepstrum analysis means, whereby a wanted signal is detected as present when a cepstrum peak is detected. Moreover, the technique of the cepstrum analysis on a speech signal including the detection of the cepstrum peak for deciding whether a given interval is a voiced speech interval is known from this document. Furthermore US-A-4,053,712 discloses the detecting of a noise signal period in which the speech signal is absent, calculating the length of the noise signal period and producing a coded noise period data representing the time lengths of the noise signal period, said coded noise period data being inserted in the coded signal. "Electronics and Communications in Japan", Vol. 64-A, No. 9, 1981, pages 21 - 30, H. Nagabuchi et al.: "Quality Improvement of Synthesized Speech in Noisy Speech Analysis-Synthesis Processing" discloses the estimating of a noise signal in the mixed signal and substracting of the estimated noise signal from the mixed signal to cancel the noise signals in the mixed signal. According to the conventional coding methods, the voice signals are coded together with background noise signals. However, in such coding methods, since the data which is really necessary is the voice data, the coding of the background noise signal is of waist. An essential object of the present invention is to provide a voice signal coding system which can solve the foregoing problem involved in conventional systems and is particularly adapted to code only the voice signals, wherein the noise signals may be coded separately, if necessary. In accomplishing these and other objects, according to the present invention, there is provided a wanted signal coding system comprising: a wanted signal detection means for receiving a mixed signal of a wanted signal and a background noise signal and for detecting the presence and absence of said wanted signal contained in said mixed signal; a wanted signal period detecting means for detecting a wanted signal period in which said wanted signal is present; a coding period control means for producing a coding period control signal during the wanted signal period; and a coding means for encoding said mixed signal in response to said coding period control signal, whereby said mixed signal is coded only in the periods during which the wanted signal is present; said wanted signal coding system being characterized by: a noise prediction means for predicting a noise signal in said mixed signal by evaluating noise signals obtained in a predetermined past time; a cancellation means for subtracting the predicted noise signal from said mixed signal to cancel the noise signals in said mixed signal; a noise signal period detecting means for detecting a noise signal period in which said wanted signal is absent; a coding-compression control means for calculating the length of the noise signal period from said coding period control signal and for producing a coded noise period data representing the time length of the noise signal period, said coded noise period data being inserted in the coded signal produced by said coding means; a noise extraction means for extracting the noise signal during said noise signal period; a noise joining means for performing a switching operation to connect the extracted noise signal with the predicted noise signal to produce a continuing noise signal; and a noise signal coding means for encoding said continuing noise signal. Further advantageous embodiments are defined in the subclaims. BRIEF DESCRIPTION OF THE DRAWINGS These and other objects and features for the present invention will become apparent from the following description taken in conjunction with the preferred embodiment thereof with reference to the accompanying drawings, in which: Fig. 1 is a block diagram of a voice signal coding system according to a preferred embodiment of the present invention; Fig. 2 is a graph showing an operation of the present invention; Figs. 3a and 3b are graphs for explaining the cepstrum analysis used in the present invention; Fig. 4 is a graph for explaining a noise prediction method used in the present invention; and Figs. 5a, 5b, 5c, 5d and 5e are graphs for explaining a canceling method used in the present invention. DETAILED DESCRIPTION OF THE INVENTION Before the description of the present invention proceeds, it is to be noted that like parts are designated by like reference numerals throughout the accompanying drawings. Referring to Fig. 1, a block diagram of a voice signal coding system according to a preferred embodiment of the present invention is shown. In Fig. 1, a band dividing circuit 1 is provided for A/D conversion and for dividing the A/D converted input voice signal accompanying noise signal (noise mixed voice input signal) into a plurality of, such as m, frequency ranges by way of Fourier transformation at a predetermined sampling cycle. The divided signals are transmitted through m-channel parallel lines. The noise signal is present continuously as in the white noise signal, and the voice signal appears intermittently. Instead of the voice signal, any other data signal may be used. A voice signal detection circuit 7 receives the noise mixed voice input signal and detects the voice signal portion within the background noise signal and produces a signal indicative of absence\presence of the voice signal. For example, the voice signal detection circuit 7 includes a cepstrum analyzing circuit (not shown) which detects the portion wherein the voice signal is present by the cepstrum analysis, and a peak detection circuit (not shown) for detecting the peak of the cepstrum obtained by the cepstrum analysis circuit. Figs. 3a and 3b show spectrum analysis and cepstrum analysis to obtain the peak (i.e., pitch). In the above arrangement, it is also possible to provide average calculation circuit (not shown) to calculate the average of the cepstrum obtained by the cepstrum analysis circuit, and a voice discrimination circuit (not shown) to discriminate voice portions using the peak of the cepstrum fed by the peak detection circuit and the average value of the cepstrum fed by the average calculation circuit. This arrangement allows discrimination between vowels and consonants, making it possible to accurately discriminate the voice portions. More specifically, when there is a signal input from the peak detection circuit indicating that a peak has been detected, a vowel portion of the voice signal is detected. For discrimination of consonants, on the other hand, when a cepstrum average value fed from the average calculation circuit is greater than a predetermined specified value, or when the increment of the cepstrum average (differential coefficient) is greater than a predetermined specified value, it is informed that a consonant portion of the voice signal is detected. Then the resulting output is either a vowel/consonant representing signal, or one that represents a voice interval including vowels and consonants. The voice detection circuit 7 is not limited to one in this embodiment, and may be substituted by another method. A voice period detector 4 serves to discriminate a voice period, for example, the start time and end time of a voice signal depending on voice signal portion from the voice detection circuit 7. A coding period control circuit 5 serves to produce a control signal for encoding a voice period. A coding circuit 6 encodes a voice signal depending on the control signal from the coding period control circuit 5. The coding circuit 6 is selected depending on the circuit that is connected in the following stage. For example, the coding circuit may be of a type that includes the method of linear conversion using an analog-to-digital converter or the µ-law coding that involves logarithmic compression. The operation of the above described embodiment of the present invention is explained in connection with Fig. 3. In Fig. 2, row (a), a noise-mixed voice signal is shown, in which the high-level portions (such as t 1 -t 2 , t 3 -t 4 ) are the voice portions, and the low-level portions (such as t 0 -t 1 , t 2 -t 3 , t 4 -t 5 ) are the noise portions. The band dividing circuit 1 receives the noise-mixed voice signal (row (a)). The cepstrum analysis circuit of the voice signal detection circuit 7 effects cepstrum analysis with respect to the signal from the band dividing circuit 1. The peak detection circuit of the voice signal detection circuit 7 detects the peak of the cepstrum analysis result. The voice period detector 4 discriminates a voice period depending on the result of peak detection. In Fig. 2, row (b), blocks A, B and C represent the voice signal periods during which the coding is executed, and the intervening periods p, q and r are skip periods during which the coding is not executed. Then the coding period control circuit 5 produces a control signal depending on the voice signal period information. The coding circuit 6 encodes only the voice signal periods A, B and C in the example shown in Fig. 2 in accordance with the control signal. As a result, the noise signal periods are compressed, as shown in Fig. 2, row (c), in which the coded voice signals, each accompanying start and end codes, are connected without any interval. Furthermore, a noise prediction circuit 11 and a cancellation circuit 12 are provided so that the noise signal existing in the voice/noise signal is eliminated. The noise prediction circuit 11 includes a noise level detector for detecting the level of the actual noise signal at every sampling cycle but only during the absence of the voice signal, a storing circuit for storing noise levels obtained during predetermined number of sampling cycles before the present sampling cycle, and a noise level predictor for predicting the noise level of the next sampling cycle based on the stored noise signals. The prediction of the noise signal level of the next sampling cycle is carried out by evaluating the stored noise signals, for example by taking an average of the stored noise signals. In this case, the predictor is an averaging circuit. The noise prediction circuit 11 receives the noise mixed voice input signal that has been transformed to Fourier series, as shown in Fig. 4, in which X-axis represents frequency, Y-axis represents noise level and Z-axis represents time. Noise signal data pl-pi during the predetermined past time is collected in the noise prediction circuit 11, and is evaluated, such as taking an average of pl-pi, to predict a noise signal data pj in the next sampling cycle. Preferably, such a noise signal prediction is carried out for each of the m-channels of the divided bands. Thus in the noise prediction circuit 11, during absence of the voice signal as detected by the signal detector 7, the noise signal level of the next sampling cycle is predicted using the stored noise signals. The predicted noise signal level is sent to a cancellation circuit 12. After that, the predicted noise signal is replaced with the actually detected noise signal and is stored in the storing circuit. Thus, during the absence of the voice signal, the storing circuit stores actually detected noise signal at every sampling cycle, and the prediction is effected in predictor by the actually detected noise signal. On the other hand, during presence of the voice signal as detected by signal detector 7, the noise signal level of the next sampling cycle is predicted in the same manner as described above, and is sent to the cancellation circuit 12. After that, since there is no actually detected noise signal at this moment, the predicted noise signal is stored in the storing circuit together with other noise signals obtained previously. Thus, during the presence of the voice signal, the actual noise signals of the past data as stored in the storing circuit are sequentially replaced by the predicted noise signals. The cancellation circuit 12 is provided to cancel the noise signal in the voice signal by subtracting the predicted noise signal from the Fourier transformed noise mixed voice input signal, and is formed, for example, by a subtractor. A combining circuit 13 is provided after the cancellation circuit 12 for combing or synthesizing the m-channel signals to produce a voice signal with the noise signals being canceled not only during the voice signal absent periods, but also during the periods at which the voice signal is present. The combing circuit 13 is formed, for example, by an inverse Fourier transformation circuit and a D/A converter. In Fig. 1, signal sl is a noise mixed voice input signal (Fig. 5a) and signal s2 is a signal obtained by Fourier transforming of the input signal sl (Fig. 5b). Signal s3 is a predicted noise signal (Fig. 5c) and signal s4 is a signal obtained by canceling the noise signal (Fig. 5d). It is to be noted that in Fig. 1, only one signal s2 is shown for the sake of brevity, but there are m signals s2 for m-channels, respectively. Similarly, there are m signals s3 and m signals s4. Signal s5 is a signal obtained by inverse Fourier transforming of the noise canceled signal (Fig. 5e). Accordingly, the noise-mixed voice signal is divided into a plurality of channels by the band dividing circuit 1, and the divided signals are applied to voice detection circuit 7 and also to the noise prediction circuit 11. The voice detection circuit 7 performs cepstrum analysis, as described above, and further detects the peak depending on the cepstrum analysis result. The noise prediction circuit 11 predicts the noise signal level of voice portions in each channel. The cancellation circuit 12 eliminates the noise signal in each channel using the predicted noise. The combining circuit 13 combines the noiseless voice signal in the plurality of channels. The coding circuit 6 encodes the combined signal only during the presence of the voice signal in accordance with a coding period control signal. The present embodiment further includes circuits 31, 32, 33, and 34, whereby the noise signals are coded separately from the voice signal. The noise period detector 31 detects a noise period depending on the voice information detected by the voice detection circuit 7. The noise cutout circuit 32 cuts noise signal from the above-mentioned divided signal depending on the resulting noise period information to extract only the noise signal. The noise signal joining circuit 33 performs switching operation that connects the extracted noise signal and the predicted noise signal predicted by the noise prediction circuit 11 to produce a continuing noise signal. The noise signal coding circuit 34 is circuit for encoding the continuing noise signal. The present embodiment allows to obtain coded signal of a continuing noise signal separately from the coded voice signals. For instance, if the voice is a singing voice and the noise signal is of orchestral music played as background, then the singing voice and the background orchestral music can be separated from each other. A coding-compression control circuit 40 is further provided after the coding period control circuit 5 for receiving a coding control signal of the voice and producing noise-compression control information. This enables the coding circuit 6 to add the length of the original noise period as information when it compresses the noise periods. In the foregoing embodiment, it is possible to assemble the system by way of hardware or by way of software employing a computer to do the function of various circuits. As apparent from the above description, since the voice coding system according to the present invention is adapted to encode only voice portions out of a noise-mixed voice signal and, in turn, compresses noise portions thereof, it is possible to obviate the useless processing of encoding noise signals. Thus the data transmission rate can be improved. Furthermore, the voice coding system of the present invention can cancel noise signals effectively by predicting the noise signal in the voice signal portions. Still further, according to the present invention it is possible to obtain noise signals in coded form separately from the coded voice signals.

Claims: A wanted signal coding system comprising: a wanted signal detection means (1, 7) for receiving a mixed signal of a wanted signal and a background noise signal and for detecting the presence and absence of said wanted signal contained in said mixed signal; a wanted signal period detecting means (4) for detecting a wanted signal period in which said wanted signal is present; a coding period control means (5) for producing a coding period control signal during the wanted signal period; and a coding means (6) for encoding said mixed signal in response to said coding period control signal, whereby said mixed signal is coded only in the periods during which the wanted signal is present; said wanted signal coding system being characterized by: a noise prediction means (11) for predicting a noise signal in said mixed signal by evaluating noise signals obtained in a predetermined past time; a cancellation means (12) for subtracting the predicted noise signal from said mixed signal to cancel the noise signals in said mixed signal; a noise signal period detecting means (31) for detecting a noise signal period in which said wanted signal is absent; a coding-compression control means (40) for calculating the length of the noise signal period from said coding period control signal and for producing a coded noise period data representing the time lengt of the noise signal period, said coded noise period data being inserted in the coded signal produced by said coding means (6); a noise extraction means (32) for extracting the noise signal during said noise signal period; a noise joining means (33) for performing a switching operation to connect the extracted noise signal with the predicted noise signal to produce a continuing noise signal; and a noise signal coding means (34) for encoding said continuing noise signal. A wanted signal coding system as claimed in claim 1, wherein said wanted signal detection means (1, 7) comprises: a band dividing means (1) for dividing said mixed signal into a plurality of frequency ranges and for supplying said divided signals through a plurality of channels; a cepstrum analysis means for cepstrum-analysing the signal in each channel from said band dividing means (1); and a peak detection means for detecting a cepstrum peak in the cepstrum analysis output of said cepstrum analysis means, whereby a wanted signal is detected as present when a cepstrum peak is detected. A wanted signal coding system as claimed in claim 1, wherein said wanted signal is a voice 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, G10L19