Noise-reduced speech apparatus and noise-reduced speech method

Abstract: A speech transmission apparatus comprises a removing means (10) for removing specific components from an input signal (101, 102) and a level adjusting means (11) for amplifying the resultant voice signal (95) after the removal to obtain a constant-level output. The specific component removing means may be constituted by a high-pass filter (10). The level adjusting means may be constituted by a rectifying circuit (20), an integrating circuit (21) and an automatic signal amplifier (24) adjusted on the basis of the output of the integrating circuit (21). Alternatively the level adjusting means (11) may be constituted by an A/D converter (30), a voice power calculating portion (31), a voice power comparator (32) and a digital volume adjuster (33).

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a speech apparatus in which wire or wireless transmitter side noise is reduced so that voice easy to hear is transmitted. 2. Description of the Related Art Fig. 8 is a configuration diagram showing a conventional example of speech apparatus. In Fig. 8, the reference numeral 91 designates a transmission signal microphone; 92, a transmitter side output terminal; 93, a voice detector for detecting transmission voice; and 94, an attenuator for attenuating the transmission signal on the basis of the output of the voice detector. The operation of the apparatus having the aforementioned configuration will be described below. Not only near-end speaker's voice 101 is inputted to the transmission signal microphone 91 but also near-end ambient noise 102 is also inputted to the microphone. The voice detector 93 detects the near-end speaker's voice 101 inputted to the transmission signal microphone 91. When there is no voice 101, that is, when there is only ambient noise 102, the attenuator 94 for attenuating the transmission signal is operated so that the transmission signal is attenuated. When there is voice 101, contrariwise, the voice detector makes the attenuator 94 inoperative so that the transmission voice is not attenuated. The conventional speech apparatus configured as above has the following problem. The level of near-end speaker's voice varies correspondingly to near-end ambient noise. This is because a voice amplifying operation is carried out in order to improve the S/N of the near- end speaker's transmission signal so that the voice level increases as the near-end ambient noise increases and that the voice level decreases as the near-end ambient noise decreases. Accordingly, the reception voice level fluctuates relative to the far-end speaker. There arises a problem that the voice is hard to hear. US-A-4 747 143 discloses an arrangement for a speech enhancement processor which maintains the processed speech at a constant level regardless of large changes in the associated noise level. The composite speech and noise signal is applied to a first AGC circuit and then to a speech enhancement system which removes noise from the signal. The extracted noise power estimates are subtracted from the constant amplitude signal to provide a gain control signal value to which the gain of a second variable gain amplifier is inversely proportional. US-A-discloses a stationary interference cancellor which utilizes a programmable digital signal processor with cancelling software to provide the required combinations of time delay and filtering. The size of the delay is reduced and an FIR shaping filter and automatic gain control at the output is added. The present invention is designed to solve the aforementioned problem, and it is an object of the invention to provide a speech apparatus in which near-end side noise is reduced to reduce the fluctuation of the level of a transmission signal so that the transmission signal easy to hear on the receiver side is transmitted even under noise. The invention is set out in claims 1 and 7, respectively. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects and features of the present invention will be more apparent from the following description taken in conjunction with the accompanying drawings. Figs. 1A and 1B are configuration block diagrams of a noise reduced speech apparatus and a voice level adjustment portion in Embodiment 1 of the present invention; Fig. 2 is a configuration block diagram of a digital voice level adjustment portion; Fig. 3 is a configuration block diagram of a noise reduced speech apparatus in Embodiment 2 of the present invention; Figs. 4A and 4B are a configuration block diagram of a noise reduced speech apparatus in Embodiment 3 of the present invention and a diagram showing the directivity characteristic of the differential microphone; Fig. 5 is a configuration block diagram of a noise reduced speech apparatus in Embodiment 4 of the present invention; Fig. 6 is a configuration block diagram of a noise reduced speech apparatus in Embodiment 5 of the present invention; Fig. 7 is a configuration block diagram of a noise reduced speech apparatus in Embodiment 6 of the present invention; and Fig. 8 is a configuration block diagram of a conventional speech apparatus. DETAILED DESCRIPTION OF THE DETAILED DESCRIPTION Now, a description will be given in more detail of preferred embodiments of the present invention with reference to the accompanying drawings. (Embodiment 1) A speech apparatus according to Embodiment 1 of the present invention will be described with reference to the drawings. Fig. 1A is a diagram showing the configuration of the speech apparatus in this embodiment. In Fig. 1A, the reference numeral 10 designates a high-pass filter for attenuating low-frequency components of an input signal to a transmission signal microphone 91; and 11, a level adjusting circuit for adjusting the level of a transmission signal. The reference numeral 92 designates a transmitter side output terminal through which a transmission signal is transmitted to a far-end side. Further, the reference numeral 200 designates a receiver device for receiving the transmission signal from the transmitter side speech apparatus having the aforementioned configuration; 201, a receiver for converting the reception signal received by the receiver device 201 into voice; and 202, a far-end side (receiver side) input terminal. The operation of the apparatus having the aforementioned configuration will be described below. Near-end speaker's voice 101 with ambient noise 102 which enters into the voice 101, is converted into a transmission signal by a transmission signal microphone 91, and then inputted to the high-pass filter 10. Because ambient noise is generally concentrated into a low frequency region, the high-pass filter 10 cuts components equivalent to ambient noise by attenuating the low frequency region of the transmission signal which is the output of the transmission signal microphone 91. After improvement in the S/N of the transmission signal, the high-pass filter 10 supplies the signal as a voice signal 95 to the level adjusting circuit 11. Here, the high-pass filter 10 has cut-off frequency characteristic, for example, of 1 KHz. The level adjusting circuit 11 adjusts the level of a signal to a constant level after ambient noise is removed from the signal by the aforementioned filter. A specific example of the configuration of the level adjusting circuit 11 will be described with reference to Fig. 1B. In Fig. 1B, the reference numeral 20 designates a rectifying circuit for converting the input signal from an AC signal into a DC signal; 21, an integrating circuit for integrating the DC signal with respect to a certain time (for example, several milliseconds) to convert the DC signal into another DC voltage; 22, a reference voice voltage for giving a reference voltage corresponding to a moderate transmission voice signal; 23, a comparator for comparing the DC voltage level of the voice with the reference voltage and outputting a signal corresponding to the difference therebetween; and 24, a volume adjusting circuit for adjusting the level of the transmission signal on the basis of the signal given from the comparator. The reference numeral 11a designates an analog-type voice level adjusting portion constituted by the aforementioned parts. This is one example of the detailed configuration of the level adjusting circuit 11 depicted in Fig. 1A. The operation of the voice level adjusting portion 11a will be described below. A speaker's voice signal is inputted to both the voice adjusting circuit 24 and the rectifying circuit 20. The signal inputted to the rectifying circuit 20 is converted from an AC signal into a DC signal. The DC signal is inputted to the integrating circuit 21. The integrating circuit 21 integrates the intensively fluctuating DC signal with respect to a certain time (for example, several milliseconds) to thereby supply a stabilized DC voltage signal to the comparator 23. The comparator 23 compares the aforementioned DC voltage signal with the reference voice voltage 22 (which is, for example, set to be 70 % of the maximum voice level, as a reference voltage level), and outputs a signal proportional to the difference between the two signals as an adjustment signal for the level adjusting circuit 24. When the level of the DC voltage signal is higher than the reference voltage, the comparator 23 outputs a signal (for example, DC 1 V) so that the gain of the level adjusting circuit 24 is reduced. When the level of the DC voltage signal is contrariwise lower than the reference voltage, the comparator 23 outputs a signal (for example, DC 3 V) so that the gain of the level adjusting circuit 24 is increased. When the level of the DC voltage signal is equal to the reference voltage, the comparator 23 outputs a signal (for example, DC 2 V) so that the gain of the level adjusting circuit 24 becomes 0 dB. The level adjusting circuit 24 adjusts the level of the transmission signal on the basis of the signal given from the comparator 23 and outputs the adjusted signal. When, for example, the level of the output signal of the comparator 23 is 2 V, the level adjusting circuit 24 adjusts the level of the transmission signal to obtain the gain of 0 dB and outputs the adjusted signal, as described above. When, for example, the level of the output signal of the comparator 23 is 1 V, the level adjusting circuit 24 adjusts the level of the transmission signal to obtain the gain of -6 dB and outputs the adjusted signal, as'described above. When, for example, the level of the output signal of the comparator 23 is 3 V, the level adjusting circuit 24 adjusts the level of the transmission signal to obtain the gain of +6 dB and outputs the adjusted signal, as described above. As described above, the voice level adjusting portion 11a compares the level of the fluctuating transmission signal with the reference transmission signal level, and transmits a transmission signal having the level kept constant (reference transmission signal level). Another specific example of the configuration of the level adjusting circuit 11 will be described below with reference to Fig. 2. This is the case where transmission voice level adjustment is achieved by digital signal processing. In Fig. 2, the reference numeral 30 designates an A/D converter for converting an analog voice signal into a digital voice signal; 31, a voice power calculating portion for calculating voice power; 32, a voice power comparator for comparing the calculated voice power with reference voice power and outputting a signal corresponding to the difference between the two power levels; 33, a level adjusting circuit for adjusting the level of the output given from the A/D converter on the basis of the output signal of the voice power comparator, and 34, a D/A converter for converting the digital voice signal into an analog voice signal. The reference numeral 11b designates a digital voice level adjusting portion constituted by the aforementioned parts. A transmitter side output terminal 92 is the same as those shown in Fig. 1A. The operation of the digital level adjusting portion 11b will be described below. After ambient noise is cut by the high-pass filter 10, a speaker's voice signal 95 is inputted to the A/D converter 30. The digital signal obtained by conversion in the A/D converter 30 is inputted both to the voice power calculating portion 31 and to the level adjusting circuit 33. In the voice power calculating portion, the voice power of the input digital signal is calculated, for example, by auto-correlation function calculation which is often used in voice signal processing, or the like. The calculated voice power level is compared with a reference voice power level (which is, for example, set to be 70 % of the maximum voice level, as a reference voice level) stored in the voice power comparator 32 in advance, so that a signal proportional to the difference therebetween is transferred to the level adjusting circuit 33. When, for example, the calculated voice power level is higher than the reference voice power level, a control signal (for example, 5) is supplied to the level adjusting circuit 33 so that the level of the transmission signal is reduced. When, for example, the calculated voice power level is contrariwise lower than the reference voice power level, a control signal (for example, 15) is supplied to the level adjusting circuit 33 so that the level of the transmission signal is increased. When, for example, the calculated voice power level is equal to the reference voice power level, a control signal (for example, 10) is supplied to the level adjusting circuit 33 so that the level of the transmission signal is not changed (the gain is 0 dB). The level adjusting circuit 33 adjusts the level of the digital voice signal on the basis of the signal given from the voice power comparator 32, and outputs the adjusted signal to the D/A converter 34. For example, the level of the transmission signal is adjusted so as to make the gain 0 dB when the output signal of the voice power comparator 32 is 10, to make the gain -6 dB when the output signal of the voice power comparator 32 is 5, and to make the gain +6 dB when the output signal of the voice power comparator 32 is 15, as described above. The D/A converter 34 converts the digital voice signal into an analog voice signal, and supplies the analog voice signal to the transmitter side output terminal 92. As described above, the digital level adjusting portion 11b compares the fluctuating level of the transmission signal based on digital signal processing with the reference transmission signal level, and outputs a transmission signal having the level kept constant (reference transmission signal level). As described above, in the speech apparatus according to this embodiment, ambient noise contained in a transmission signal is removed by the high-pass filter, and the level of transmission voice is kept constant (reference transmission signal level) by the level adjuster in which, specifically, the. voice level adjusting portion 11a or the digital level adjusting portion 11b is used as the level adjusting circuit 11. In this manner, a transmission signal with less fluctuation of the far-end side level can be transmitted with less influence of near-end side ambient noise so that the far-end speaker can hear voice converted by the far-end side receiver device 200 and the receiver 201 with a clear and stable volume. (Embodiment 2) A speech apparatus according to Embodiment 2 of the present invention will be described with reference to the drawing. This embodiment is designed so that ambient noise is collected for cancellation by another microphone as a noise component removing means. Fig. 3 is a diagram showing the configuration of the speech apparatus in Embodiment 2. In Fig. 3, the reference numeral 40 designates a noise microphone for converting near-end side ambient noise 102 into a transmission signal; 41, an invertor for inverting the phase of the noise signal by 180 degrees; 42, an adder for adding two signals; and 11, a level adjusting circuit for adjusting the level of the transmission signal. That is, the reference numeral 10b designates a specific component removing means. The level adjusting circuit 11 corresponds to the voice level adjusting portion 11a in Fig. 1B or the digital level adjusting portion 11b in Fig. 2. A transmission signal microphone 91 and a transmitter side output terminal 92 are the same as those shown in Fig. 1A. The operation of the apparatus having the aforementioned configuration-will be described below. Near-end speaker's voice 101 is converted into a transmission signal by the transmission signal microphone 91, and then the transmission signal is inputted to the adder 42. In this occasion, the transmission signal contains a near-end speaker's voice signal 101, and a signal of near-end side ambient noise 102. On the other hand, the near-end side ambient noise 102 is converted into a noise signal by the noise microphone 40, and then the noise signal is inputted to the invertor 41. The invertor 41 inverts the phase of the output signal of the noise microphone 40 by 180 degrees, and supplies the inverted noise signal to the adder 42. The adder 42 adds the output signal of the transmission signal microphone 91 to the output signal of the invertor 41. In this occasion, noise outputted from the transmission signal microphone and noise outputted from the invertor cancel out each other because the phases thereof are inverted just by 180 degrees from each other. Accordingly, a transmission voice signal having the S/N improved is outputted as an S/N-improved voice signal 95 to the level adjusting circuit 11. The level adjusting circuit 11 detects the average of transmission signal levels in a certain time, for example, of the order of tens of milliseconds, and supplies the average of the signal having the level kept constant (reference transmission signal level) to the transmitter side output terminal 92 in the same manner as the voice level adjusting portion 11a in Fig. 1B or the digital level adjusting portion 11b in Fig. 2. As described above, in the speech apparatus according to this embodiment, ambient noise contained in a transmission signal is removed by cancellation by using two microphones and an adder, and then the transmission signal level is kept constant (reference transmission signal level). Accordingly, a transmission signal without fluctuation of the level can be transmitted to the far-end side without influence of near-end side ambient noise so that the far-end speaker can hear voice with a clear and stable volume. (Embodiment 3) A speech apparatus according to Embodiment 3 of the present invention will be described with reference to the drawing. This embodiment is designed so that noise is collected for cancellation by a differential microphone as a noise component removing means. Fig. 4A is a diagram showing the configuration of the speech apparatus in this embodiment. In Fig. 4A, the reference numeral 50 designates a transmission signal differential microphone. The reference numeral 11 designates a level adjusting circuit for adjusting the level of a transmission signal. The level adjusting circuit 11 corresponds to the voice level adjusting portion 11a in Fig. 1B or the digital level adjusing portion 11b in Fig. 2. A transmitter side output terminal 92 is the same as that shown in Fig. 1A. The operation of the apparatus having the aforementioned configuration will be described below. A near-end speaker's voice signal 101 is converted into a transmission signal by the transmission signal differential microphone 50. In this occasion, both the near-end speaker's voice signal 101 and a signal of near-end side ambient noise 102 are inputted to the transmission signal differential microphone. Noise is removed by the function of the differential microphone, so that an S/N-improved transmission signal is inputted to the level adjusting circuit 11. Specifically, a close-talking microphone unit (EM-124) made by Primo Co., Ltd. is an example of the differential microphone 50. The directivity of this microphone is eccentric in the front and the rear as shown in Fig. 4B, so that ambient noise inputted to the front and rear of the microphone simultaneously is canceled out. The level adjusting circuit 11 keeps the level of the transmission signal constant (reference transmission signal level) and outputs the signal to the transmitter side output terminal 92. As described above, in the speech apparatus according to this embodiment, ambient noise is removed by using the differential microphone, and then the transmission signal level is kept constant by the level adjusting circuit. Accordingly, a transmission signal with less fluctuation of the far-end side level can be transmitted without influence of near-end side ambient noise so that the far-end speaker can hear voice with a clear and stable volume. (Embodiment 4) A speech apparatus according to Embodiment 4 of the present invention will be described with reference to the drawing. In this embodiment, a digital arithmetic operation portion is used so that the noise component removing means serves also as a level adjuster. Fig. 5 is a diagram showing the configuration of the speech apparatus in this embodiment. In Fig. 5, the reference numeral 60 designates a noise canceler portion for removing ambient noise inputted to the transmission signal microphone 91 by digital signal processing. The same A/D converter 30, the same voice power calculating portion 31, the same voice power comparator 32, the same level adjusting circuit 33 and the same D/A converter 34 as shown in Fig. 2 are used. Accordingly, the reference numeral 11c designates a portion equivalent to the digital level adjusting portion 11b in Fig. 2. The operation of the apparatus having the configuration in Fig. 5 will be described below. Near-end speaker's voice 101 is converted into a transmission signal by the transmission signal microphone 91 and then the transmission signal is inputted to the A/D converter 30. The digital signal obtained by conversion in the A/D converter 30 is inputted to the noise canceler portion 60. The noise canceler portion 60 removes ambient noise selectively from the transmission signal and outputs the signal both to the voice power calculating portion 31 and to the level adjusting circuit 33 after improvement of S/N. Here, the noise canceler portion 60 is realized by digital signal processing. For example, the noise canceler 60 is carried out by algorithm or the like disclosed in the section 5.2.1.2 "Noise Canceler" in the first volume of Japanese Digital Car Telephone System Standard Specification RCR STD-27D. In the voice power calculating portion 31, the voice power of the input digital signal is calculated, for example, by auto-correlation function calculation or the like which is often used in voice signal processing. The calculated voice power level is compared with a reference voice power level stored in the voice power comparator 32 in advance, so that a signal proportional to the difference between these two levels is transferred to the level adjusting circuit 33. When, for example, the calculated voice power level is higher than the reference voice power level, a control signal is supplied to the level adjusting circuit 33 so that the level of the transmission signal is reduced. When the calculated voice power level is lower than the reference voice power level, contrariwise, a control signal is supplied to the level adjusting circuit 33 so that the level of the transmission signal is increased. When the calculated voice power level is equal to the reference voice power level, a control signal is supplied to the level adjusting circuit 33 so that the level of the transmission signal is not changed. As described above in Embodiment 1, the level adjusting circuit 33 adjusts the level of the digital voice signal on the basis of the signal given from the voice power comparator 32 and outputs the adjusted signal to the D/A converter 34. The D/A converter 34 converts the digital voice signal into an analog voice signal and outputs the analog voice signal to the transmitter side output terminal 92. In practical use, the noise cancellation, voice power calculation, voice power comparison and level adjustment are executed by a general-use processor and programs installed in advance in a memory. As described above, in the speech apparatus according to this embodiment, the S/N of the transmission signal is improved by the noise canceler using digital signal processing so that the transmission signal level is kept constant (reference transmission signal level) by the level adjusting circuit. Accordingly, a transmission signal with less fluctuation of the level can be transmitted to the far-end side without influence of near-end side ambient noise so that the far-end speaker can hear voice with a clear and stable volume. (Embodiment 5) A speech apparatus according to Embodiment 5 of the present invention will be described with reference to the drawing. This embodiment is designed so that the voice signal is made as natural as possible but the S/N is improved to suppress the fluctuation of the voice level when noise is large. Fig. 6 is a diagram showing the configuration of the speech apparatus in this embodiment. In Fig. 6, the reference numeral 12 designates a noise removing portion which is, for example, equivalent to the high-pass filter 10 in Fig. 1A. The reference numeral 71 designates a rectifying circuit for converting an AC signal into a DC signal; 72, an integrating circuit for measuring the level of ambient noise from the transmission signal; 73, a reference noise level; 74, a comparator; 75, a path switching circuit for switching the path of the transmission signal; and 11, a level adjusting circuit for adjusting the level of the transmission signal. These parts constitute a noise level detecting portion 70. Incidentally, the level adjusting circuit 11 corresponds to the voice level adjusting portion 11a in Fig. 1B or the digital level adjusting portion 11b in Fig. 2. A transmission signal microphone 91 and a transmitter side output terminal 92 are the same as those shown in Fig. 1A. The operation of the apparatus having the aforementioned configuration will be described below. Near-end speaker's voice 101 and ambient noise 102 are converted into a transmission-signal by the transmission signal microphone 91 and then the transmission signal is inputted to the noise removing portion 12 and to the rectifying circuit 71 in the noise level detecting portion 70. The noise removing portion 12 removes only a noise signal from the transmission signal. Specifically, this portion may be, for example, the high-pass filter 10 in Fig. 1A or the noise canceler portion 60 in Fig. 5. After the removal of noise by the portion 12, the voice signal is supplied to the path switching circuit 75. On the other hand, the rectifying circuit 71 converts the transmission signal from an AC signal into a DC signal and supplies the DC signal to the integrating circuit 72. The integrating circuit 72 detects only the level of noise which has a gentle leading edge, for example, of 1 to 2 seconds and a sharp trailing edge, for example, of the order of tens of milliseconds and which varies slowly with the passage of time. The comparator 74 compares the output of the integrating circuit 72, that is, the measured noise level, with a reference noise level 73. When the comparison results that the measured noise level is higher than the reference noise level, the comparator 74, for example, outputs a signal of "high" ("1"). When the comparison contrariwise results that the noise level is lower than the reference noise level, the comparator 74, for example, outputs a signal of "low" ("0"). When the output of the comparator 74 is "high" ("1"), the path switching circuit 75 switches the path to the level adjusting circuit 11 side. When the output of the comparator 74 is "low" ("0"), contrariwise, the path switching circuit 75 switches the path to the transmitter side output terminal 92 side. The level adjusting circuit 11 outputs a transmission signal having the level kept constant (reference transmission signal level) to the transmitter side output terminal 92 in the same manner as the voice level adjusting portion 11a in Fig. 1B or the digital level adjusting portion 11b in Fig. 2. As described above, the speech apparatus according to this embodiment regards the level fluctuation of the transmission signal inputted to the microphone as being large when the level of ambient noise contained in the transmission signal is higher than a reference level, and adjusts the level of the transmission signal to be a constant level. When the level of ambient noise contained in the transmission signal is contrariwise lower than the reference level, the speech apparatus regards the level fluctuation of the transmission signal inputted to the microphone as being small, and transmits the transmission signal with no change. As a result, when the level of ambient noise is small, the far-end speaker can hear the received voice signal with a more natural voice level. (Embodiment 6) A speech apparatus according to Embodiment 6 of the present invention will be described below with reference to the drawing. In this embodiment, the thought of the previous embodiment is realized by another circuit structure. Fig. 7 is a diagram showing the configuration of the speech apparatus in this embodiment. In Fig. 7, the reference numerals 80 and 82 designate variable resistors each having the resistance value changed by voltage; 81 and 83, resistors; and 84, an operation amplifier. The same noise removing portion 12, the same rectifying circuit 71 and the same integrating circuit 72 as shown in Fig. 6 are used. The same rectifying circuit 20, the same integrating circuit 21 , the same reference voice voltage 22 and the same level adjusting circuit 24 as shown in Fig. 1B are used. The operation of the apparatus having the aforementioned configuration will be described below. Voice 101 and ambient noise 102 are converted into a transmission signal by the transmission signal microphone 91 and then the transmission signal is inputted both to the noise removing portion 12 and to the rectifying circuit 71. The noise removing portion 12 removes only the noise signal from the transmission signal. The rectifying circuit 71 converts the transmission signal from an AC signal into a DC signal and supplies the DC signal to the integrating circuit 72. The integrating circuit 72 has a gentle leading edge and a sharp trailing edge, that is, has a certain time constant and detects the level of noise which changes slowly with the passage of time. The noise level output of the integrating circuit 72 is used as a signal for controlling the resistance values of the variable resistors 80 and 82. The rectifying circuit 20 converts the transmission signal after the removal of noise from an AC signal into a DC signal. The integrating circuit 21 integrates the intensively fluctuating DC signal with respect to a certain time constant to thereby output a stabilized DC voltage signal. The variable resistors 80 and 82, the resistors 81 and 83 and the operation amplifier 84 constitute a differential amplifier which amplifies the difference between the level of the DC voice signal as the output of the integrating circuit 21 and the reference voice voltage 22 and outputs a signal as a control signal of the level adjusting circuit 24. As described above in Embodiment 1, the differential amplifier outputs a signal so that the gain of the level adjusting circuit 24 is reduced, for example, when the DC voice signal is higher than the reference voice voltage 22, while the differential amplifier outputs a signal so that the gain of the level adjusting circuit 24 is increased when the level of the DC voltage signal is lower than the reference voltage. When the level of the DC voltage signal is equal to the reference voltage, the differential amplifier outputs a signal so that the gain of the level adjusting circuit 24 is set to 0 dB. The amplification factor of the aforementioned differential amplifier is determined on the basis of the ratio of the resistor 81 to the variable resistor 80 and the ratio of the resistor 83 to the variable resistor 82. That is, the control signal given to the level adjusting circuit 24 can be adjusted by controlling the values of the variable resistors 80 and 82 (correspondingly to the magnitude of ambient noise), so that the level of the transmission signal to be transmitted can be adjusted correspondingly to the level of ambient noise. For example, if characteristic is given so that the resistance values of the variable resistors 80 and 82 are reduced when the level of ambient noise is large, the gain of the differential amplifier is increased so that a control signal having the level accurately (for example, within ±5 %) coincident with the reference voice level which corresponds to the reference voice voltage 22 is outputted to the level adjusting circuit 24. If characteristic is contrariwise given so that the resistance values of the variable resistors 80 and 82 are increased when the level of ambient noise is small, the gain of the differential amplifier is reduced so that a control signal near (for example, within ±20 %) the reference voice level which corresponds to the reference voice voltage 22 is outputted to the level adjusting circuit 24. As described above, in the speech apparatus according to this embodiment, the level of ambient noise is monitored so as to be used as a reference signal for adjusting the level of the transmission signal. In this manner, because the fluctuation of the speaker's voice level increases as the level of ambient noise increases, the level adjusting circuit adjusts the level of the transmission signal so that the level of the transmission signal approaches the reference voice signal level. Because the fluctuation of the speaker's voice level contrariwise decreases as the level of ambient noise decreases, the level adjusting circuit adjusts the level of the transmission signal so that the level of the transmission signal roughly approaches the reference voice signal level. As a result, the far-end speaker can hear the voice with a stable and natural volume regardless of the level of near-end side ambient noise. As described above, according to the present invention, there are provided a specific component removing means for removing ambient noise inputted to a microphone, and a level adjusting means for adjusting the voice level of the transmission signal after the removal of ambient noise to a constant level. Accordingly, there arises an effect that clear and stable voice is transmitted to a far-end while the level of near-end side ambient noise is reduced. Furthermore, there are provided a specific component removing means, a noise level detecting means, a changeover switch, and a transmission level adjusting means. Accordingly, there arises an effect that the level adjusting means is bypassed to transmit more natural voice to a far-end when ambient noise is small. Furthermore, there are provided a specific component removing means, a noise level detecting means, and a transmission level adjusting means. Accordingly, there arises an effect that voice having a moderate level can be transmitted to a far-end even in the case where ambient noise is extremely large or extremely small. The foregoing description of a preferred embodiment of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiment was chosen and described in order to explain the principles of the invention and its practical application to enable one skilled 'in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents.

Claims: Speech transmission apparatus, comprising: specific component removing means (10) for removing specific components from an input signal containing voice (101) and ambient noise (102); and level adjusting means for amplifying the resultant voice signal (95) after removal of said specific components to obtain a constant level output, wherein said level adjusting means (11c) comprises an A/D converter (30), a voice power calculating portion (31), a voice power comparator (32) and a digital level adjuster (33) which is adjustable in gain on the basis of a value obtained from the power calculation and comparison, and wherein said voice signal after the level adjustment is transmitted, and wherein the voice power comparator (32) compares the value obtained from the power calculation with a reference voice power level stored in the voice power comparator (32) in advance. An apparatus according to Claim 1, wherein said specific component removing means (10) comprises a high-pass filter. An apparatus according to Claim 1, wherein said specific component removing means (10b) comprises a noise collection microphone (40) and a portion equivalent to a subtractor (42) for subtracting noise components collected by said noise collection microphone (40) from the input signal (101, 102). An apparatus according to Claim 1, wherein a differential microphone (50) with two inputs which converts input signal (101, 102) into an electric signal is provided as said specific component removing means, one input of the microphone being used for voice input (101) and noise collection, the other input being used for noise collection. An apparatus according to Claim 1, 2 or 3, further comprising: a changeover switch (75) for bypassing said level adjusting means (11) provided between said specific component removing means (12) and said level adjusting means (11); and a noise level detecting means (70); and wherein when the noise level detected by said noise level detecting means (70) is small, said changeover switch (75) is adjusted so that said level adjusting means (11) is bypassed. An apparatus according to Claim 1, further comprising a noise level detecting means (71, 72) for detecting the noise level on the basis of which the gain of said level adjusting means (24) is adjusted. Digital method of noise reduction in input signal comprising the steps of: cancelling noise components in said input signal containing voice (101) and ambient noise (102) by means of frequency analysis of the output of an A/D converter (30); calculating the power level of the resultant voice signal (95) after the noise cancellation; comparing the calculated power level of said resultant voice signal (95) with a reference power level stored in advance; and adjusting the level of said resultant voice signal (95) on the basis of the result of the comparison. An apparatus according to claim 1, wherein a digital noise canceller (60) for performing noise cancellation by frequency analysis of the output of said A/D converter (30) is provided as said specific component removing means.

Inventors: HIGUCHI KOJI, HIGUCHI, KOJI
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1996
Main classification: G10L21
IPC codes: G10L15, H04M1, H04B1, G10L21