Signal Processing Apparatus and Signal Processing Method

1. A signal processing apparatus comprising: 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; a ratio calculator that calculates a ratio of the subtracted value to the added value; a gain calculator that calculates a gain based on a value of the ratio and information indicating a relationship between the gain and the value of the ratio which varies according to a distance between the microphones; a determination unit that determines noise being included in the input signals when a difference between the added value and the subtracted value is less than a threshold value; and a noise suppression unit that suppresses noise of the input signals based on the gain calculated by the gain calculator.

2. The signal processing apparatus according to claim 1 , wherein the determination unit determines that noise is included in the input signals when a value of the ratio calculated by the ratio calculator is greater than or equal to a threshold value.

3. The signal processing apparatus according to claim 1 , further comprising: a delay detector that acquires input signals from the plurality of microphones, changes a delay amount of each of the input signals, and detects delay amounts by which a correlation coefficient of the input signals is a maximum; and a delay unit that delays each of the input signals based on the delay amounts detected by the delay detector, wherein the adder calculates the added value based on the input signals delayed by the delay unit, and the subtracter calculates the subtracted value based on the input signals delayed by the delay unit.

4. The signal processing apparatus according to claim 1 , further comprising a frequency converter that acquires input signals from the plurality of microphones and converts each of the input signals into a signal on a frequency axis, wherein the adder calculates the added value based on each signal on the frequency axis, and the subtracter calculates the subtracted value based on each signal on the frequency axis.

5. The signal processing apparatus according to claim 1 , wherein the ratio calculator calculates a ratio of a value obtained by squaring the subtracted value to a value obtained by squaring the added value.

6. The signal processing apparatus according to claim 1 , wherein the ratio calculator calculates a ratio of an absolute value of the subtracted value to an absolute value of the added value.

7. The signal processing apparatus according to claim 1 , wherein the adder calculates a first added value obtained by adding together input signals acquired from a first microphone and a second microphone and a second added value obtained by adding together input signals acquired from a third microphone and a fourth microphone, the subtracter calculates a first subtracted value obtained by subtracting an input signal acquired from the second microphone from an input signal acquired from the first microphone and a second subtracted value obtained by subtracting an input signal acquired from the fourth microphone from an input signal acquired from the third microphone, and the gain calculator calculates a gain based on the first added value, the second added value, the first subtracted value, and the second subtracted value.

8. The signal processing apparatus according to claim 1 , wherein the gain calculator calculates the gain based on a formula g(t)=1−v(t), g(t) indicating the gain, v(t) indicating ratio when a distance between the microphones is 4.2 cm, and calculates the gain based on a decision table which sets a change rate of the gain different from the change rate of the gain calculated by the formula when the distance between the microphones is smaller than 4.2 cm.

9. A signal processing method comprising: acquiring a plurality of input signals from a plurality of microphones and calculating an added value obtained by adding the input signals together; acquiring a plurality of input signals from the plurality of microphones and calculating a subtracted value obtained by subtracting one input signal from the other input signal; calculating a ratio of the subtracted value to the added value; calculating a gain based on a value of the ratio and information indicating a relationship between the gain and the value of the ratio which varies according to a distance between the microphones; determining noise being included in the input signals when a difference between the added value and the subtracted value is less than a threshold value; and suppressing noise of the input signals based on the gain calculated at the calculating the gain.

10. A non-transitory computer-readable recording medium having stored therein a program for causing a computer to execute a signal processing procedure comprising: acquiring a plurality of input signals from a plurality of microphones and calculating an added value obtained by adding the input signals together; acquiring a plurality of input signals from the plurality of microphones and calculating a subtracted value obtained by subtracting one input signal from the other input signal; calculating a ratio of the subtracted value to the added value; calculating a gain based on a value of the ratio and information indicating a relationship between the gain and the value of the ratio which varies according to a distance between the microphones; determining noise being included in the input signals when a difference between the added value and the subtracted value is less than a threshold value; and suppressing noise of the input signals based on the gain calculated at the calculating the gain.