Signal processing apparatus and signal processing method

A signal processing apparatus includes an adder that acquires a plurality of input signals from a plurality of microphones and calculates an added value obtained by adding the input signals together, a subtracter that acquires a plurality of input signals from the plurality of microphones and calculates a subtracted value obtained by subtracting one input signal from the other input signal, and a determination unit that determines whether noise is included in the input signals based on the added value and the subtracted value.

CROSS-REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2011-274742, filed on Dec. 15, 2011, the entire contents of which are incorporated herein by reference.

FIELD

The embodiments discussed herein are directed to a signal processing apparatus and the like.

BACKGROUND

When wind hits a microphone, a diaphragm included in the microphone vibrates largely, so that noise is included in an input signal. Here, if noise is included in the input signal, when speech recognition is performed or a hands-free phone call is made, the accuracy of the speech recognition and the quality of the phone call degrade. Therefore, in conventional speech recognition and a hands-free phone call, a conventional apparatus which suppresses irregular noise generated when wind hits the microphone is used.

FIG. 10is a diagram illustrating an example of the conventional apparatus. As illustrated inFIG. 10, a conventional apparatus10is connected to microphones10aand10b. The microphones10aand10bare spaced apart from each other by a predetermined distance. The conventional apparatus10includes a correlation analyzer11, a noise analyzer12, and a noise reduction unit13.

The microphone10ais a microphone which converts collected sound into an input signal Lin(t) and outputs the input signal Lin(t) to the conventional apparatus10. The microphone10bis a microphone which converts collected sound into an input signal Rin(t) and outputs the input signal Rin(t) to the conventional apparatus10. Here, t corresponds to a sampling number of the input signal.

The correlation analyzer11calculates a correlation value r(t) between the input signal Lin(t) and the input signal Rin(t) by using the formula (1). In the formula (1), the correlation analyzer11sequentially changes the value of i from 1 to n and calculates a correlation value r(t). For example, the value of n is 128. The correlation analyzer11outputs the correlation value r(t) to the noise analyzer12.
r(t)=ΣLin(t−i)Rin(t−i)/((ΣLin(t−i)2)1/2(ΣRin(t−i)2)1/2)  (1)

When the value of the correlation value r(t) is small, the noise analyzer12determines that there is noise of wind and outputs control information to turn down volume to the noise reduction unit13.

The noise reduction unit13adjusts the volumes of the input signals Lin(t) and Rin(t) and outputs the input signals Lin(t) and Rin(t) to an external apparatus. For example, when the noise reduction unit13receives the control information to turn down volume, the noise reduction unit13turns down the volumes of the input signals Lin(t) and Rin(t) and outputs the input signals Lin(t) and Rin(t) to the external apparatus.

However, in the conventional technique described above, there is a problem that noise included in a target input signal such as voice is not sufficiently suppressed.

For example, in the conventional technique, when wind blowing against the microphone is strong, the noise of the input signal can be suppressed. However, when wind blowing against the microphone is weak, the difference of correlation coefficient between a section including voice and a section including wind hitting noise is small, so that the noise is not accurately suppressed.

SUMMARY

According to an aspect of an embodiment, a signal processing apparatus includes: an adder that acquires a plurality of input signals from a plurality of microphones and calculates an added value obtained by adding the input signals together; a subtracter that acquires a plurality of input signals from the plurality of microphones and calculates a subtracted value obtained by subtracting one input signal from the other input signal; and a determination unit that determines whether noise is included in the input signals based on the added value and the subtracted value.

DESCRIPTION OF EMBODIMENTS

Preferred embodiments of the present invention will be explained with reference to accompanying drawings. The present invention is not limited by the embodiments.

[a] First Embodiment

FIG. 1is a diagram illustrating a configuration of a signal processing apparatus according to a first embodiment. As illustrated inFIG. 1, a signal processing apparatus100is connected to microphones10aand10b. The microphones10aand10bare spaced apart from each other by a predetermined distance.

The signal processing apparatus100includes an adder110, a subtracter120, and power calculators130aand130b. The signal processing apparatus100also includes a ratio calculator140, a determination unit150, a gain calculator160, and a noise suppression unit170.

The microphone10ais a microphone which converts collected sound into an input signal Lin(t) and outputs the input signal Lin(t) to the signal processing apparatus100. The microphone10bis a microphone which converts collected sound into an input signal Rin(t) and outputs the input signal Rin(t) to the signal processing apparatus100. Here, t corresponds to a sampling number of the input signal.

The adder110calculates an added value p(t) by adding the input signal Lin(t) and the input signal Rin(t) together. The added value p(t) is represented by the formula (2). The adder110outputs the added value p(t) to the power calculator130a.
p(t)=Lin(t)+Rin(t)  (2)

The subtracter120calculates an subtracted value m(t) by subtracting the input signal Rin(t) from the input signal Lin(t). The subtracted value m(t) is represented by the formula (3). The subtracter120outputs the subtracted value m(t) to the power calculator130b.
m(t)=Lin(t)−Rin(t)  (3)

The power calculator130acalculates a power Ppow(t) by summing up squared values of the added value p(t) of different sampling numbers. The power Ppow(t) is represented by the formula (4). The power calculator130asequentially changes the value of j from 1 to n and calculates the power Ppow(t). The value of n is a natural number. For example, the value of n is 128. The power calculator130aoutputs the power Ppow(t) to the ratio calculator140.
Ppow(t)=Σp(t−j)2(4)

The power calculator130bcalculates a power Mpow(t) by summing up squared values of the subtracted value m(t) of different sampling numbers. The power Mpow(t) is represented by the formula (5). The power calculator130bsequentially changes the value of j from 1 to n and calculates the power Mpow(t). The value of n is a natural number. For example, the value of n is 128. The power calculator130boutputs the power Mpow(t) to the ratio calculator140.
Mpow(t)=Σm(t−j)2(5)

The ratio calculator140calculates a ratio v(t) of the power Ppow(t) and the power Mpow(t). For example, the ratio calculator140calculates the ratio v(t) by the formula (6). The ratio calculator140outputs the ratio v(t) to the determination unit150and the gain calculator160. Here, v(t) may be a square root of the ratio of the power Ppow(t) and the power Mpow(t).
v(t)=Mpow(t)/Ppow(t)  (6)

The determination unit150determines whether noise is included in the input signals Lin(t) and Rin(t) based on the ratio v(t). When the value of the ratio v(t) is greater than or equal to a predetermined threshold value, the determination unit150determines that noise is included in the input signals Lin(t) and Rin(t). The determination unit150outputs a determination result to the gain calculator160.

The gain calculator160calculates a gain based on the ratio v(t). The gain calculator160first calculates g′(t) by the formula (7) and thereafter calculates a gain g(t) by the formula (8).
g′(t)=1−v(t)  (7)
g(t)=max(g′(t),α)  (8)

In the formula (8), α is a lower limit value greater than or equal to 0 and smaller than or equal to 1. The gain calculator160calculates g′(t) and thereafter calculates the larger value of g′(t) and α as the gain g(t). The gain calculator160outputs the gain g(t) to the noise suppression unit170.

The gain calculator160may calculate the gain g(t) when noise is included in the input signals Lin(t) and Rin(t) based on the determination result of the determination unit150. On the other hand, when noise is not included in the input signals Lin(t) and Rin(t), the gain calculator160may set 1 to the gain g(t) and output the gain g(t) to the noise suppression unit170.

The noise suppression unit170suppresses noise by multiplying the input signals Lin(t) and Rin(t) by the gain g(t). For example, the noise suppression unit170generates output signals Lout(t) and Rout(t) from the input signals Lin(t) and Rin(t) and outputs the generated signals to an external apparatus. For example, the output signals Lout(t) and Rout(t) are represented by the formula (9) and the formula (10).
Lout(t)=g(t)Lin(t)  (9)
Rout(t)=g(t)Rin(t)  (10)

Next, a processing procedure of the signal processing apparatus100according to the first embodiment will be described.FIG. 2is a flowchart illustrating a processing procedure of the signal processing apparatus according to the first embodiment. For example, the process illustrated inFIG. 2is performed when input signals are received from the microphones10aand10b.

As illustrated inFIG. 2, the signal processing apparatus100calculates the added value p(t) obtained by adding together the input signals Lin(t) and Rin(t) of the microphones10aand10b(step S101). The signal processing apparatus100calculates the subtracted value m(t) obtained by subtracting the input signal Rin(t) of one microphone10bfrom the input signal Lin(t) of the other microphone10a(step S102).

The signal processing apparatus100calculates the power Ppow(t) based on the added value p(t) (step S103). The signal processing apparatus100calculates the power Mpow(t) based on the subtracted value m(t) (step S104).

The signal processing apparatus100calculates the ratio v(t) (step S105) and determines whether noise is included (step S106). The signal processing apparatus100calculates the gain g(t) based on the ratio v(t) (step S107). The signal processing apparatus100outputs the output signals Lout(t) and Rout(t) whose noise is suppressed based on the gain g(t) (step S108).

Next, effects of the signal processing apparatus100according to the first embodiment will be described. In the signal processing apparatus100, the determination unit150determines whether noise is included in the input signals Lin(t) and Rin(t) based on the added value p(t) calculated by the adder110and the subtracted value m(t) calculated by the subtracter120. Therefore, it is possible to accurately determine whether noise is included, so that noise included in the input signals can be sufficiently suppressed by using the determination result.

Generally, the lower the frequency, the stronger the correlation between the input signals. This is because the lower the frequency, the smaller the variation of waveforms in the time axis direction, so that the waveforms have similar shapes. An extreme example is a direct current. Therefore, in order to strengthen the correlation, the adder110sums up the input signals. On the other hand, in order to suppress the correlation, the subtracter120subtracts one input signal from the other input signal.

Therefore, for example, in a case where the input signals of the microphone have a correlation, such as cases of voice and noise in a driving car, a relationship of the formula (11) is established.
“power of(Lin(t)+Rin(t))”>>“power of(Lin(t)−Rin(t))”  (11)

On the other hand, in a case where the correlation between the input signals of the microphone is weak, such as case of wind hitting noise, a relationship of the formula (12) is established.
“power of(Lin(t)+Rin(t))”≈“power of(Lin(t)−Rin(t))”  (12)

Therefore, in the signal processing apparatus100according to the first embodiment, noise can be accurately detected by using the ratio of the “power of (Lin(t)−Rin(t))” based on the “power of (Lin(t)+Rin(t))”. Also, an appropriate gain can be calculated by using the ratio of the “power of (Lin(t)-Rin(t))” based on the “power of (Lin(t)+Rin(t))”, and a noise component included in the input signals can be suppressed by using the gain.

FIG. 3is a diagram for explaining an effect of the present invention compared with a conventional apparatus. Signals20A inFIG. 3indicates a relationship between the sampling number and the gain g(t) of the conventional apparatus. Signals20B inFIG. 3indicates a relationship between the sampling number and the gain g(t) of the signal processing apparatus100. Sections21in the signals20A and20B are sections including voice. When comparing the signals20A and20B, in the signals20B, the gain g(t) in sections including only wind hitting noise is reduced while the gain g(t) in the sections21is substantially maintained at 1. Therefore, in the present invention, it is possible to turn down volume of a part of input signals including noise without turning down volume of whole input signals in voice sections.

By the way, in the signal processing apparatus100according to the first embodiment, the determination unit150determines whether noise is included based on the value of the ratio v(t). However, it is not limited to this. The determination unit150may obtain a difference between the Ppow(t) and the Mpow(t), and when the difference value is smaller than a predetermined threshold value, the determination unit150may determine that noise is included in the input signals.

The signal processing apparatus100may be configured by integrating the determination unit150and the gain calculator160together.

[b] Second Embodiment

Next, a signal processing apparatus according to a second embodiment will be described.FIG. 4is a diagram illustrating a configuration of the signal processing apparatus according to the second embodiment. As illustrated inFIG. 4, a signal processing apparatus200is connected to the microphones10aand10b. The microphones10aand10bare spaced apart from each other by a predetermined distance. The description about the microphones10aand10bis the same as that in the first embodiment.

The signal processing apparatus200includes a delay detector201and delay devices202aand202b. The signal processing apparatus200also includes an adder210, a subtracter220, and power calculators230aand230b. The signal processing apparatus200also includes a ratio calculator240, a determination unit250, a gain calculator260, and a noise suppression unit270.

The delay detector201calculates a correlation coefficient c(d) between the input signal Lin(t) and the input signal Rin(t) by the formula (13). The delay detector201changes a delay amount d and detects a delay amount dmax by which the value of correlation coefficient c(d) is the maximum.
c(d)=ΣLin(t)Rin(t−d)/((ΣLin(t)2)1/2(ΣRin(t−d)2)1/2)  (13)

After detecting the dmax, the delay detector201calculates a delay amount dL applied to the input signal from the microphone10aand a delay amount dR applied to the input signal from the microphone10bin order to synchronize the input signals inputted from the microphones10aand10b. A calculation example of the delay amounts dL and dR will be described below.

When the value of the dmax is greater than or equal to 0, the delay detector201sets the value of dL to 0 and sets the value of dR to the value of the dmax. Then, the delay detector201outputs the dL to the delay device202aand outputs the dR to the delay device202b.

On the other hand, when the value of the dmax is smaller than 0, the delay detector201sets the value of dL to the absolute value of the dmax and sets the value of dR to 0. Then, the delay detector201outputs the dL to the delay device202aand outputs the dR to the delay device202b.

The delay device202aperforms a delay process of the input signal Lin(t) from the microphone10a. The delay device202aoutputs an input signal Lin(t−dL) obtained by performing a delay process on the input signal Lin(t) to the adder210and the subtracter220.

The delay device202bperforms a delay process of the input signal Rin(t) from the microphone10b. The delay device202boutputs an input signal Rin(t−dR) obtained by performing a delay process on the input signal Rin(t) to the adder210and the subtracter220.

The adder210calculates an added value p(t) by adding together the input signals Lin(t−dL) and Rin(t−dR) whose delay amounts are adjusted. The adder210outputs the added value p(t) to the power calculator230a.

The subtracter220calculates an subtracted value m(t) by subtracting the input signal Rin(t−dR) from the input signal Lin(t−dL). The subtracter220outputs the subtracted value m(t) to the power calculator230b.

The descriptions about the power calculators230aand230b, the ratio calculator240, the determination unit250, the gain calculator260, and the noise suppression unit270are the same as those of the power calculators130aand130b, the ratio calculator140, the determination unit150, the gain calculator160, and the noise suppression unit170illustrated inFIG. 1.

Next, a processing procedure of the signal processing apparatus200according to the second embodiment will be described.FIG. 5is a flowchart illustrating the processing procedure of the signal processing apparatus according to the second embodiment. For example, the process illustrated inFIG. 5is performed when input signals are received from the microphones10aand10b.

The signal processing apparatus200detects the maximum value of the correlation coefficient c(d) between the input signal Lin(t) and the input signal Rin(t) and calculates the delay amounts dL and dR for synchronization (step S201). The signal processing apparatus200calculates the p(t) by performing synchronous addition (step S202) and calculates the m(t) by performing synchronous subtraction (step S203).

The signal processing apparatus200calculates the power Ppow(t) based on the added value p(t) (step S204). The signal processing apparatus200calculates the power Mpow(t) based on the subtracted value m(t) (step S205).

The signal processing apparatus200calculates the ratio v(t) (step S206) and determines whether noise is included (step S207). The signal processing apparatus200calculates the gain g(t) based on the ratio v(t) (step S208). The signal processing apparatus200outputs the output signals Lout(t) and Rout(t) whose noise is suppressed based on the gain g(t) (step S209).

Next, effects of the signal processing apparatus200according to the second embodiment will be described. In the signal processing apparatus200, the delay detector201calculates the delay amounts dL and dR and the delay devices202aand202bsynchronize the input signals. In the signal processing apparatus200, the determination unit250determines whether noise is included in the input signals Lin(t) and Rin(t) based on the added value p(t) calculated by the adder210and the subtracted value m(t) calculated by the subtracter220. Therefore, even if the timings when voice of a user is collected by the microphones10aand10bare different from each other, the input signals can be synchronized by adjusting the delay amounts. Thus, it is possible to accurately determine whether noise is included in a direction of a user whose voice is desired to be recorded, so that noise included in the input signals can be suppressed by using the determination result.

Next, a signal processing apparatus according to the third embodiment will be described.FIG. 6is a diagram illustrating a configuration of the signal processing apparatus according to the third embodiment. As illustrated inFIG. 6, a signal processing apparatus300is connected to the microphones10aand10b. The microphones10aand10bare spaced apart from each other by a predetermined distance. The description about the microphones10aand10bis the same as that in the first embodiment.

The signal processing apparatus300includes FFTs (Fast Fourier Transforms)301aand301b, an adder310, a subtracter320, and power calculators330aand330b. The signal processing apparatus300also includes a ratio calculator340, a determination unit350, a gain calculator360, a noise suppression unit370, and IFFTs (Inverse FFTs)371aand371b.

The FFT301apositions a Hanning window with 50% overlap on the input signal Lin(t) acquired from the microphone10aand converts the input signal Lin(t) into a signal on the frequency axis by fast Fourier transform. The input signal Lin(t) converted into a signal on the frequency axis is represented as LIN(i, f). Here, i is a number of an analysis frame on which FFT is performed and f indicates the frequency. The FFT301aoutputs the LIN(i, f) to the adder310, the subtracter320, and the noise suppression unit370.

The FFT301bpositions a Hanning window with 50% overlap on the input signal Rin(t) acquired from the microphone10band converts the input signal Rin(t) into a signal on the frequency axis by fast Fourier transform. The input signal Rin(t) converted into a signal on the frequency axis is represented as RIN(i, f). The FFT301boutputs the RIN(i, f) to the adder310, the subtracter320, and the noise suppression unit370. The FFTs301aand301bare an example of a frequency converter.

The adder310is a processing unit which calculates an added value p(i, f) by adding the input signal LIN(i, f) and the input signal RIN(i, f) together. The added value p(i, f) is represented by the formula (14). The adder310outputs the added value p(i, f) to the power calculator330a.
p(i, f)=LIN(i, f)+RIN(i, f)  (14)

The subtracter320calculates a subtracted value m(i, f) by subtracting the input signal RIN(i, f) from the input signal LIN(i, f). The subtracted value m(i, f) is represented by the formula (15). The subtracter320outputs the subtracted value m(i, f) to the power calculator330b.
m(i, f)=LIN(i, f)−RIN(i, f)  (15)

The power calculator330acalculates a power Ppow(i, f) by summing up squared values of the added value p(i, f) of different frame numbers. The power Ppow(i, f) is represented by the formula (16). The power calculator330asequentially changes the value of j from 1 to n and calculates the power Ppow(i, f). The value of n is a natural number. For example, the value of n is 128. The power calculator330aoutputs the power Ppow(i, f) to the ratio calculator340.
Ppow(i, f)=Σp(i−j, f)  (16)

The power calculator330bcalculates a power Mpow(i, f) by summing up squared values of the subtracted value m(i, f) of different frame numbers. The power Mpow(i, f) is represented by the formula (17). The power calculator330bsequentially changes the value of j from 1 to n and calculates the power Mpow(i, f). The value of n is a natural number. For example, the value of n is 128. The power calculator330boutputs the power Mpow(i, f) to the ratio calculator340.
Mpow(i, f)=Σm(i−j, f)  (17)

The ratio calculator340calculates a ratio v(i, f) of the power Ppow(i, f) and the power Mpow(i, f). For example, the ratio calculator340calculates the ratio v(i, f) by the formula (18). The ratio calculator340outputs the ratio v(i, f) to the determination unit350and the gain calculator360.
v(i, f)=Mpow(i, f)/Ppow(i, f)  (18)

The determination unit350determines whether noise is included in the input signals LIN(i, f) and RIN(i, f) based on the ratio v(i, f). When the value of the ratio v(i, f) is greater than or equal to a predetermined threshold value, the determination unit350determines that noise is included in the input signals LIN(i, f) and RIN(i, f). The determination unit350outputs a determination result to the gain calculator360.

The gain calculator360calculates a gain based on the ratio v(i, f). The gain calculator360first calculates g′(i, f) by the formula (19) and thereafter calculates a gain g(i, f) by the formula (20).
g′(i, f)=1−v(i, f)  (19)
g(i, f)=max(g′(i, f),α)  (20)

In the formula (20), α is a lower limit value greater than or equal to 0 and smaller than or equal to 1. The gain calculator360calculates g′(i, f) and thereafter calculates the larger value of g′(i, f) and α as the gain g(i, f). The gain calculator360outputs the gain g(i, f) to the noise suppression unit370.

The gain calculator360may calculate the gain g(i, f) when noise is included in the input signals LIN(i, f) and RIN(i, f) based on the determination result of the determination unit350. On the other hand, when noise is not included in the input signals LIN(i, f) and RIN(i, f), the gain calculator360may set 1 to the gain g(i, f) and output the gain g(i, f) to the noise suppression unit370.

The noise suppression unit370suppresses noise by multiplying the input signals LIN(i, f) and RIN(i, f) by the gain g(i, f). For example, the noise suppression unit370generates output signals LOUT(i, f) and ROUT (i, f) from the input signals LIN(i, f) and RIN(i, f) and outputs the generated signals to the IFFTs371aand371b. For example, the output signals LOUT(i, f) and ROUT (i, f) are represented by the formulas (21) and (22).
LOUT(i, f)=g(i, f)LIN(i, f)  (21)
ROUT(i, f)=g(i, f)RIN(i, f)  (22)

The IFFT371ais a processing unit which generates a signal Lin(t) on the time axis by performing an inverse Fourier transform and 50% overlap addition on the LOUT(i, f). The IFFT371aoutputs the output signal Lin(t) to an external apparatus.

The IFFT371bis a processing unit which generates a signal Rin(t) on the time axis by performing an inverse Fourier transform and 50% overlap addition on the ROUT(i, f). The IFFT371boutputs the output signal Rin(t) to an external apparatus.

Next, a processing procedure of the signal processing apparatus300according to the third embodiment will be described.FIG. 7is a flowchart illustrating the processing procedure of the signal processing apparatus according to the third embodiment. For example, the process illustrated inFIG. 7is performed when input signals are received from the microphones10aand10b.

As illustrated inFIG. 7, the signal processing apparatus300calculates the LIN(i, f) and the RIN(i, f) by performing a Fourier transform on the input signals Lin(t) and Rin(t) of the microphones (step S301). The signal processing apparatus300calculates the added value p(i, f) obtained by adding together the input signals LIN(i, f) and RIN(i, f) of the microphones (step S302).

The signal processing apparatus300calculates the subtracted value m(i, f) obtained by subtracting the input signal RIN(i, f) of one microphone from the input signal LIN(i, f) of the other microphone (step S303). The signal processing apparatus300calculates the power Ppow(i, f) based on the added value p(i, f) (step S304).

The signal processing apparatus300calculates the power Mpow(i, f) based on the subtracted value m(i, f) (step S305). The signal processing apparatus300calculates the ratio v(i, f) (step S306) and determines whether noise is included (step S307).

The signal processing apparatus300calculates the gain g(i, f) based on the ratio v(i, f) (step S308). The signal processing apparatus300outputs the output signals LOUT(i, f) and ROUT(i, f) whose noise is suppressed based on the gain g(i, f) (step S309). The signal processing apparatus300performs an inverse Fourier transform on the output signals LOUT(i, f) and ROUT(i, f) and outputs Lout(t) and Rout(t) (step S310).

Next, effects of the signal processing apparatus300according to the third embodiment will be described. The signal processing apparatus300performs a Fourier transform on the input signals Lin(t) and Rin(t) and determines whether there is noise at each frequency. Then, the signal processing apparatus300calculates the gain g(i, f) at each frequency and multiplies the input signals LIN(i, f) and RIN(i, f) at each frequency by the gain g(i, f). Therefore, noise at each frequency can be suppressed.

The processes of the aforementioned signal processing apparatuses100to300are an example. Hereinafter, another process of the signal processing apparatuses of the present invention will be described as a fourth embodiment. The process according to the fourth embodiment will be described with reference to the configuration diagram inFIG. 1.

Another process of the gain calculator160will be described. Although the gain calculator160calculates the gain g(t) by using the formula (7) and the formula (8), it is not limited to this. The gain calculator160may calculate the gain g(t) by using a decision table indicating a relationship between the gain g(t) and the ratio v(t) for each distance between the microphones10aand10b.

FIG. 8is a diagram for explaining the other process of the gain calculator. For example, the gain calculator160defines that when the distance between the microphones10aand10bis 4.2 cm, the relationship between the gain g(t) and the ratio v(t) is a relationship changing as illustrated by a straight line30A inFIG. 8. The relationship illustrated by the straight line30A corresponds to the relationship of the formula (7).

On the other hand, the gain calculator160defines that when the distance between the microphones10aand10bis smaller than 4.2 cm, the relationship changes as illustrated by a curved line30B inFIG. 8. In other words, the gain calculator160sets the change rate of the gain g(t) larger than that indicated by the straight line30A when the value of the ratio v(t) is about 1 to 0.8. The gain calculator160performs the process as described above, so that it is possible to accurately suppress noise corresponding to the change of characteristics of the input signals caused by the difference of the distance between the microphones.

The gain calculator160may acquire information of the distance between the microphones10aand10bin any manner. A user may input the distance into the signal processing apparatus100by using an input device.

Another process of the power calculator130awill be described. The power calculator130aobtains the power Ppow(t) based on the formula (4). However, instead of the power Ppow(t), an average Pamp(t) of the absolute values may be calculated. For example, the power calculator130aobtains the average of the absolute values by the formula (23).
Pamp(t)=Σ|p(t−j)|  (23)

Similarly, the power calculator130bmay calculate an average Mamp(t) of the absolute values instead of the power Mpow(t). For example, the power calculator130bobtains the average of the absolute values by the formula (24).
Mamp(t)=Σ|m(t−j)|  (24)

It is known that when the average of the absolute values is obtained, the processing load of the computer is smaller than when the power is obtained. Therefore, when it is configured so that the power calculators130aand130bcalculate the average of the absolute values instead of the power, the cost of the signal processing apparatus100can be reduced.

By the way, the signal processing apparatus100may suppress noise by using microphones10cand10dnot illustrated in the drawings in addition to the pair of microphones10aand10b. For example, the signal processing apparatus100calculates the ratio v(t) from the microphones10aand10bby the same process as that in the first embodiment. The v(t) calculated from the microphones10aand10bis defined as v1(t).

Also, the ratio v(t) is calculated from the microphones10cand10dby the same process as that in the first embodiment. The v(t) calculated from the microphones10cand10dis defined as v2(t). When both the v1(t) and v2(t) are greater than a predetermined threshold value, the signal processing apparatus100may adjust the input signals Lin(t) and Rin(t) by the gain g(t).

For example, the signal processing apparatus100may obtain g′(t) by the formula (25) or the formula (26). The processes after calculating the g′(t) is the same as that in the first embodiment. The g′(t) may be obtained by subtracting the average value of the v1(1) and v2(2) from 1.
g′(t)=1−v1(t)  (25)
g′(t)=1−v2(t)  (26)

In this way, the gain g(t) is calculated by using the microphones10ato10d, so that it is possible to accurately suppress noise of the input signals even when there are a plurality of users.

The processing units included in the signal processing apparatuses100to300described in the first to the fourth embodiments correspond to integrated devices such as, for example, ASIC (Application Specific Integrated Circuit) and FPGA (Field Programmable Gate Array). Also, the processing units correspond to electronic circuits such as CPU and MPU (Micro Processing Unit).

Next, an example of a computer that executes a signal processing program which realizes the same function as that of the signal processing apparatuses100to300described in the embodiments will be described.FIG. 9is a diagram illustrating an example of the computer that executes the signal processing program.

As illustrated inFIG. 9, a computer400includes a CPU401that performs various calculation processes, an input device402that receives input of data from a user, and a display403. The computer400also includes a reading device404that reads a program and the like from a storage medium and an interface device405that transmits and receives data to and from another computer through a network. The computer400also includes microphones406aand406b. The computer400also includes a RAM407that temporarily stores various information and a hard disk device408. The devices401to408are connected to a bus409.

The hard disk device408includes, for example, an addition program408a, a subtraction program408b, a power calculation program408c, a ratio calculation program408d, a determination program408e, a gain calculation program408f, and a noise suppression program408g. The CPU401reads the programs408ato408gand develops the programs in the RAM407.

The addition program408afunctions as an addition process407a. The subtraction program408bfunctions as a subtraction process407b. The power calculation program408cfunctions as a power calculation process407c. The ratio calculation program408dfunctions as a ratio calculation process407d. The determination program408efunctions as a determination process407e. The gain calculation program408ffunctions as a gain calculation process407f. The noise suppression program408gfunctions as a noise suppression process407g.

For example, the addition process407acorresponds to the adder110. The subtraction process407bcorresponds to the subtracter120. The power calculation process407ccorresponds to the power calculators130aand130b. The ratio calculation process407dcorresponds to the ratio calculator140. The determination process407ecorresponds to the determination unit150. The gain calculation process407fcorresponds to the gain calculator160. The noise suppression process407gcorresponds to the noise suppression unit170.

The programs408ato408gneed not necessarily be stored in the hard disk device408from the beginning. For example, the programs are stored in a “portable physical medium” such as a flexible disk (FD), a CD-ROM, a DVD disk, a magneto-optical disk, and an IC card which are inserted in the computer400. The computer400may read the programs408ato408gfrom these media and execute the programs. The programs of the signal processing apparatuses200and300described in the second to the fourth embodiments are executed by the computer400in the same manner as the programs of the signal processing apparatus100.

According to the disclosed signal processing apparatus, there is an effect that noise included in the input signal can be suppressed.