Patent Grant
9236060
This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2013-004734, filed on Jan. 15, 2013, the entire contents of which are incorporated herein by reference.
The embodiments discussed herein are related to a noise suppression device, a noise suppression method and to a storage medium storing a noise suppression program.
Noise suppression is conventionally performed, for example, in a vehicle mounted car navigation system, a hands-free phone, or a telephone conference system, to suppress noise contained in a speech signal that has mixed-in noise other than a target voice (for example a person's speech). A technique employing a microphone array including plural microphones is known as such noise suppression technology.
In such conventional noise suppression technology using a microphone array, a method has been disclosed in which a phase difference computed from respective input signals to each of the microphones in the microphone array is employed to derive a value representing the likelihood of a sound source being in a specific direction. In this method, based on the derived value, sound signals from sound sources other than the sound source in the specific direction are suppressed. A method has also been described that utilizes an amplitude ratio between input signals of each of the microphones to suppress sound other than from a target direction.
For example, a technique has been proposed that respectively divides waveforms acquired at two points into plural frequency bands, derives time differences and amplitude ratios for each band, and eliminates waveforms that do not match an arbitrarily determined time difference and amplitude ratio. In such a technique, after waveform processing and laying out each of the bands alongside each other, it is possible to selectively extract only the sound of a source at an arbitrary position (direction) by adding together the outputs of each of the bands. Moreover, in this technique, when selectively extracting sound from a sound source that has a difference in distance from two microphones, the phase difference or amplitude ratio are aligned with each other by performing signal delay or amplitude amplification, and then waveforms whose phase difference or amplitude ratio do not match are removed.
There has also been a proposal for a technique in which phase differences are detected between microphones by employing a target sound source direction estimated from the sound received from two or more microphones, and then using the detected phase differences to update a central phase difference value. In such a technique, a noise suppression filter generated using the updated central value is employed to suppress noise received by the microphones, and then sound is output.
There has also been a proposal for a technique in which audible signals received from two sensors placed in various different places are converted, spectral signals arise, a spectral signal is delayed, and many intermediate signals are supplied. Each of the intermediate signals corresponds to different spatial positions with respect to the two sensors, and the locations of noise sources and a desired emitting source, together with the spectral content of the desired signal, are determined from the intermediate signals corresponding to the location of the noise source.
According to an aspect of the embodiments, a noise suppression device includes: a phase difference utilization range computation section that, based on an inter-microphone distance between plural microphones contained in a microphone array and on a sampling frequency, computes, as a phase difference utilization range, a frequency band in which phase rotation of phase difference does not occur for each frequency between respective input sound signals containing a target voice and noise that are input from each of the plural microphones; an amplitude condition computation section that, based on an amplitude ratio or an amplitude difference for each frequency between the input sound signals, computes amplitude conditions to determine whether or not the input sound signals are the target voice or the noise based on the inter-microphone distance and a position of a sound source of the target voice; a phase difference derived suppression coefficient computation section that, over the phase difference utilization range computed by the phase difference utilization range computation section, computes, for each frequency, a phase difference derived suppression coefficient based on a phase difference; an amplitude ratio derived suppression coefficient computation section that computes, for each frequency, an amplitude ratio derived suppression coefficient based on the amplitude ratio or the amplitude difference, and based on the amplitude conditions computed by the amplitude condition computation section; and a suppression section that suppresses noise contained in the input sound signals based on a suppression coefficient determined using the phase difference derived suppression coefficient and the amplitude ratio derived suppression coefficient.
The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention.
Detailed explanation follows regarding an example of an exemplary embodiment of technology disclosed herein, with reference to the drawings.
The microphones 11a and 11b collect peripheral sound, convert the collected sound into an analogue signal and output the analogue signal. The signal output from the microphone 11a is input sound signal 1 and the signal output from the microphone 11b is input sound signal 2. Noise other than the target voice (a voice from a target source, such as for example the voice of a person talking) is mixed into the input sound signal 1 and the input sound signal 2. The input sound signals 1 and 2 output from the microphone array 11 are input to the noise suppression device 10. In the noise suppression device 10 an output sound signal is generated, in which noise contained in the input sound signals 1 and 2 that were input has been suppressed, and then output.
As illustrated in
Based on the inter-microphone distance and the sampling frequency, the phase difference utilization range computation section 12 computes a frequency band in which the phase difference is utilizable to compute suppression coefficients to suppress noise contained in the input sound signal 1 and the input sound signal 2.
Explanation next follows regarding a relationship between inter-microphone distance and sampling frequency, and the phase difference between the input sound signal 1 and the input sound signal 2 (the difference in phase spectra for the same frequency). In the present exemplary embodiment, as illustrated in
As illustrated in
In the phase difference utilization range computation section 12, a frequency band is computed based on the inter-microphone distance d and the sampling frequency Fs such that phase rotation in the phase difference between the input sound signal 1 and the input sound signal 2 does not arise. Then the computed frequency band is set as a phase difference utilization range for determining by utilizing phase difference whether or not there is a target voice or noise present.
More specifically, the phase difference utilization range computation section 12 uses the inter-microphone distance d, the sampling frequency Fs and the speed of sound c to computed an upper limit frequency Fmax of the phase difference utilization range according to the following Equations (1) and (2).
Fmax=Fs/2 when d≦c/Fs (1)
Fmax=c/(d*2) when d>c/Fs (2)
The phase difference utilization range computation section 12 sets a frequency band of the computed Fmax or lower as the phase difference utilization range.
The amplitude condition computation section 14 computes amplitude conditions based on the inter-microphone distance d and the position of the target voice for use when determining whether or not the input sound signal is a target voice or noise based on the amplitude ratio (or amplitude difference) between the amplitude of the input sound signal 1 and the amplitude of the input sound signal 2.
Explanation follows regarding a relationship between the inter-microphone distance and the position of the target voice, and the amplitude ratio between the input sound signal 1 and the input sound signal 2 (the ratio of amplitude spectra at the same frequency).
As illustrated in
Based on the inter-microphone distance d and the sound source position, the amplitude condition computation section 14 accordingly computes the amplitude conditions for determining whether or not the input sound signal is the target voice or noise based on the amplitude ratio of the input sound signal 1 and the input sound signal 2. A range of amplitude ratios expressed by an upper limit and a lower limit to the amplitude ratio capable of determining whether or not the input sound signal is the target voice is then computed as the amplitude conditions.
More specifically, as illustrated in
R={ds/(ds+d×cos θ)}(0≦θ≦180) (3)
When the sound source of the target voice to be left remaining without suppression is present from θmin to θmax then the amplitude ratio R is a value between Rmin and Rmax as expressed by Equation (4) and Equation (5).
Rmin=ds/(ds+d×cos θmin) (4)
Rmax=ds/(ds+d×cos θmax) (5)
The amplitude condition computation section 14 sets as the amplitude condition to determine that the input sound signal is the target voice the condition that the amplitude ratio R of the input sound signal 1 and the input sound signal 2 is contained in the range Rmin to Rmax expressed by the computed Rmin and Rmax.
The sound input sections 16a, 16b input the input sound signals 1 and 2 output from the microphone array 11 to the noise suppression device 10.
The sound receiver 18 respectively converts the input sound signals 1 and 2 that are analogue signals input by the sound input sections 16a, 16b to digital signals at the sampling frequency Fs.
The time-frequency converter 20 respectively converts the input sound signals 1 and 2 that are time domain signals that have been converted to digital signals by the sound receiver 18, into frequency domain signals for each frame, using for example Fourier transformation. Note that the duration of 1 frame may be set at several tens of msec.
The phase difference computation section 22 computes phase spectra respectively for the two input sound signals that have been converted to frequency domain signals by the time-frequency converter 20, in the phase difference utilization range computed by the phase difference utilization range computation section 12 (a frequency band of frequency Fmax or lower). The phase difference computation section 22 then computes as phase differences the difference between the phase spectra at the same frequencies.
The amplitude ratio computation section 24 computes the respective amplitude spectra of the two input sound signals that have been converted into frequency domain signals by the time-frequency converter 20. The amplitude ratio computation section 24 then computes the amplitude ratio Rf as expressed by the following Equation (6), wherein IN1f is the amplitude spectrum of the input sound signal 1 at a given frequency f and IN2f is the amplitude spectrum of the input sound signal 2 at the given frequency f.
Rf=IN2f/IN1f (6)
The phase difference derived suppression coefficient computation section 26 computes the phase difference derived suppression coefficient in the phase difference utilization range computed by the phase difference utilization range computation section 12. The phase difference derived suppression coefficient computation section 26 uses the phase difference computed by the phase difference computation section 22 to identify a probability value representing the probability that the sound source that should remain unsuppressed is present in the sound source direction, namely the probability that the input sound signal is the target voice. The phase difference derived suppression coefficient computation section 26 then computes the phase difference derived suppression coefficient based on the probability value.
For example, explanation follows regarding an example of a computation method of a phase difference derived suppression coefficient α, wherein α is a phase difference derived suppression coefficient.
αf=1.0 when f>Fmax
αf=1.0 when f≦Fmax, and the phase difference is within the diagonally shaded range
αf=αmin when f≦Fmax, and the phase difference is outside the diagonally shaded range
Note that αmin is a value such that 0<αmin1, and when a suppression amount of −3 dB is desired, αmin is about 0.7, and when a suppression amount of −6 dB is desired αmin is about 0.5. When the phase difference is outside of the diagonally shaded range, the phase difference derived suppression coefficient α is computed so as to gradually change from 1.0 to αmin as the phase difference moves away from the diagonally shaded range.
The amplitude ratio derived suppression coefficient computation section 28 determines whether or not the input sound signal is the target voice or noise based on the amplitude conditions computed by the amplitude condition computation section 14, and computes the amplitude ratio derived suppression coefficient.
For example explanation follows regarding an example of a computation method of an amplitude ratio derived suppression coefficient β wherein β is the amplitude ratio derived suppression coefficient. When the amplitude conditions computed by the amplitude condition computation section 14 have an amplitude ratio Rf contained in the range Rmin to Rmax as described above, the amplitude ratio derived suppression coefficient β is computed as shown in the following when determining the target voice.
βf=1.0 when Rmin≦Rf≦Rmax
βf=βmin when Rf<Rmin, or Rf>Rmax
Note that βmin is a value such that 0<βmin<1, and when a suppression amount of −3 dB is desired, βmin is about 0.7, and when a suppression amount of −6 dB is desired βmin is about 0.5. For the amplitude ratio derived suppression coefficient β, similarly to for the phase difference derived suppression coefficient α, when the amplitude ratio Rf is outside the amplitude conditions range, then the amplitude ratio derived suppression coefficient β is computed so as to gradually change from 1.0 to βmin, as shown below as the amplitude ratio moves away from the amplitude condition range.
βf=1.0 when Rmin≦Rf≦Rmax
βf=10(1.0−βmin)Rf−10Rmin(1.0−βmin)+1.0 when Rmin−0.1≦Rf≦Rmin
βf=−10(1.0−βmin)Rf+10Rmax(1.0−βmin)+1.0 when Rmax≦Af≦Rmax+0.1
βf=βmin when Rf<Rmin−0.1,Rf>Rmax+0.1
The suppression coefficient computation section 30 computes a suppression coefficient for each frequency to suppress noise from the input sound signal, based on the phase difference derived suppression coefficient computed by the phase difference derived suppression coefficient computation section 26 and based on the amplitude ratio derived suppression coefficient computed by the amplitude ratio derived suppression coefficient computation section 28.
For example, explanation follows regarding an example of a method for computing a suppression coefficient γ based on the phase difference derived suppression coefficient α and the amplitude ratio derived suppression coefficient β. A suppression coefficient γf at frequency f may be computed as illustrated below by multiplying phase difference derived suppression coefficient αf by amplitude ratio derived suppression coefficient βf.
γf=αf×βf
There however no limitation to the above example, and suppression coefficient γ may be computed by the average or weighted sum of α and β.
Moreover, as another method of computing suppression coefficient γ, the larger degree of suppression out of the phase difference derived suppression coefficient α and the amplitude ratio derived suppression coefficient β may be computed as the suppression coefficient γ. Since the degree of suppression is larger the smaller the values of α and β, the suppression coefficient γf at frequency f may be computed according to the following:
γf=αf when αf<βf
γf=βf when αf>βf
The suppression signal generation section 32 generates a suppression signal in which noise has been suppressed by multiplying the amplitude spectrum of the frequencies corresponding to the input sound signal by the suppression coefficient for each frequency computed by the suppression coefficient computation section 30.
The frequency-time converter 34 converts the suppression signal that is a frequency domain signal generated by the suppression signal generation section 32 into an output sound signal that is a time domain signal by employing, for example, an inverse Fourier transform, and outputs the output sound signal.
The noise suppression device 10 may for example be implemented by a computer 40 as illustrated in
The storage section 46 may be implemented for example by a Hard Disk Drive (HDD) or a flash memory. The storage section 46 serving as a storage medium is stored with a noise suppression program 50 for making the computer 40 function as the noise suppression device 10. The CPU 42 reads the noise suppression program 50 from the storage section 46, expands the noise suppression program 50 in the memory 44 and sequentially executes the processes of the noise suppression program 50.
The noise suppression program 50 includes a phase difference utilization range computation process 52, an amplitude condition computation process 54, a sound input process 56, a sound receiving process 58, a time-frequency converting process 60, a phase difference computation process 62 and an amplitude ratio computation process 64. The noise suppression device 50 includes a phase difference derived suppression coefficient computation process 66, an amplitude ratio derived suppression coefficient computation process 68, a suppression coefficient computation process 70, a suppression signal generation process 72 and a frequency-time converting process 74.
The CPU 42 operates as the phase difference utilization range computation section 12 illustrated in
Note that the noise suppression device 10 may be implemented by for example a semiconductor integrated circuit, or more specifically by an Application Specific Integrated Circuit (ASIC) and a Digital Signal Processor (DSP).
Explanation next follows regarding operation of the noise suppression device 10 according to the first exemplary embodiment. When the input sound signal 1 and the input sound signal 2 are output from the microphone array 11, the CPU 42 expands the noise suppression program 50 stored in the storage section 46 into the memory 44 and executes the noise suppression processing illustrated in
At step 100 of the noise suppression processing illustrated in
At the next step 102, the phase difference utilization range computation section 12 employs the inter-microphone distance d, the sampling frequency Fs and the speed of sound c received at step 100, and computes the Fmax according to Equation (1) and Equation (2). The phase difference utilization range computation section 12 then sets a frequency band of computed Fmax or lower as the phase difference utilization range.
At the next step 104, the amplitude condition computation section 14 uses the inter-microphone distance d, the sound source direction θ, and the distance ds from the sound source to the microphone 11a that were received at step 100, and computes the Rmin as expressed by Equation (4) and the Rmax as expressed by Equation (5). The amplitude condition computation section 14 then sets amplitude conditions to determine whether or not the input sound signal is the target voice when the amplitude ratio R between the input sound signal 1 and the input sound signal 2 is contained within the range Rmin to Rmax expressed by the computed Rmin and Rmax.
At the next step 106, the sound input sections 16a, 16b input the noise suppression device 10 with the input sound signal 1 and the input sound signal 2 that have been output from the microphone array 11. The sound receiver 18 then respectively converts the input sound signal 1 and the input sound signal 2 that are analogue signals input by the sound input sections 16a, 16b into digital signals at sampling frequency Fs.
At the next step 108, the time-frequency converter 20 respectively converts the input sound signal 1 and the input sound signal 2 that are time domain signals converted into digital signals at step 106 into frequency domain signals for each frame.
At the next step 110, the phase difference computation section 22 computes phase spectra in the phase difference utilization range computed at step 102 (the frequency band of frequency Fmax or lower) for each of the two input sound signals that were converted into frequency domain signals at step 108. The phase difference computation section 22 then computes as the phase difference the difference between the phase spectra at the same frequencies.
At the next step 112, the phase difference derived suppression coefficient computation section 26 computes the phase difference derived suppression coefficient αf based on the probability that the input sound signal is the target voice for each of the frequencies f in the phase difference utilization range computed at step 102.
At the next step 114, the amplitude ratio computation section 24 computes the amplitude spectra of each of the two input sound signals that were converted into frequency domain signals at step 108. Then the amplitude ratio computation section 24 computes the amplitude ratio Rf as expressed by Equation (6), wherein the amplitude spectrum of the input sound signal 1 at frequency f is IN1f and the amplitude spectrum of the input sound signal 2 is IN2f.
At the next step 116, the amplitude ratio derived suppression coefficient computation section 28 determines whether or not the input sound signal is the target voice or noise and computes the amplitude ratio derived suppression coefficient of for each of the frequencies f based on the amplitude conditions computed at step 104. Specifically, the amplitude ratio derived suppression coefficient computation section 28 computes an amplitude ratio derived suppression coefficient βf according to whether or not the amplitude ratio Rf computed at step 114 lies within the range Rmin to Rmax computed at step 104.
At the next step 118, the suppression coefficient computation section 30 computes suppression coefficient γf each of the frequencies f, based on the phase difference derived suppression coefficient αf computed at step 112 and the amplitude ratio derived suppression coefficient βf computed at step 116.
Then at step 120, the suppression signal generation section 32 generates a suppression signal in which noise has been suppressed for each of the frequencies by multiplying the amplitude spectra of the frequency corresponding to the input sound signal by the suppression coefficient γf at each of the frequencies f computed at step 118.
At the next step 122, the frequency-time converter 34 converts the suppression signal that is the frequency domain signal generated at step 122 into an output sound signal that is a time domain signal, and outputs the output sound signal at step 124.
At the next step 126, determination is made as to whether or not the sound input sections 16a, 16b have input following input sound signals. Processing proceeds to step 128 when input sound signals have been input, and determination is made as to whether or not any of the setting values of the phase difference utilization range computation section 12 and the amplitude condition computation section 14 have changed. Processing returns to step 106 when none of the setting values have changed, and the processing of steps 106 to 126 is repeated. However, when for example there are plural types of the sampling frequency prepared, such that the sampling frequency automatically switches over according to the output destination of a voice, then determination is made that one of the setting values has changed in cases such as when switching of the sampling frequency has been detected. In such cases, processing returns to step 100, and the changed setting value is received, and then the processing of steps 100 to 126 are repeated.
The noise suppression processing is ended when it is determined at step 126 that no following input sound signals have been input.
As explained above, according to the noise suppression device 10 of the first exemplary embodiment, a frequency band in which phase rotation does not occur is computed based on the inter-microphone distance and the sampling frequency, and a phase difference derived suppression coefficient is computed by utilizing the phase difference in this frequency band. Amplitude conditions are also computed based on the inter-microphone distance and the sound source position when determining whether or not the input sound signal is the target voice or noise by amplitude ratio, and an amplitude ratio derived suppression coefficient is computed according to the inter-microphone distance and the sound source position. Then, using a suppression coefficient computed from the phase difference derived suppression coefficient and the amplitude ratio derived suppression coefficient, the noise contained in the input sound signal is suppressed. Thus even in cases where phase rotation occurs due to the inter-microphone distance, it is possible to perform suppression in a frequency band where phase rotation does not occur by utilizing phase difference to achieve a higher suppression precision than were an amplitude ratio to be employed. Moreover, even when an amplitude ratio is utilized, more appropriate suppression is enabled to be performed by amplitude conditions according to the inter-microphone distance and the sound source position. This accordingly enables noise suppression to be performed with an appropriate suppression amount and low audio distortion even in cases in which there are limitations to the placement positions of a microphone array.
Note that in the amplitude ratio derived suppression coefficient computation section 28, as for example expressed by the following, in the phase difference utilization range (the frequency band of the upper limit frequency Fmax or lower), the range in which no suppression is performed may be made wider than the frequency band greater than Fmax.
Rmin=0.7, and Rmax=1.4 when f>Fmax
Rmin=0.6, and Rmax=1.5 when f≦Fmax
This thereby enables excessive suppression to be avoided in a phase difference utilization range in which suppression is performed utilizing phase difference.
Moreover, configuration may be made such that other than the above formulae, in the suppression coefficient computation section 30 over the phase difference utilization range the phase difference derived suppression coefficient α is employed as the suppression coefficient γ irrespective of the value of the amplitude ratio derived suppression coefficient β. Moreover, when computing the suppression coefficient γ from the phase difference derived suppression coefficient α and the amplitude ratio derived suppression coefficient β, weighting may be performed to give a greater weighting to the phase difference derived suppression coefficient α.
The noise suppression device 210 includes a phase difference utilization range computation section 12, an amplitude condition computation section 14, sound input sections 16a, 16b, a sound receiver 18, a time-frequency converter 20, a phase difference computation section 22 and an amplitude ratio computation section 24. The noise suppression device 210 includes a phase difference derived suppression coefficient computation section 226, an amplitude ratio derived suppression coefficient computation section 228, a suppression coefficient computation section 230, a suppression signal generation section 32, a frequency-time converter 34, a stationary noise estimation section 36, and a stationary noise derived suppression coefficient computation section 38. Note that the phase difference computation section 22 and the phase difference derived suppression coefficient computation section 226 are an example of a phase difference derived suppression coefficient computation section of technology disclosed herein. The amplitude ratio computation section 24 and the amplitude ratio derived suppression coefficient computation section 228 are an example of an amplitude ratio derived suppression coefficient computation section of technology disclosed herein. The suppression coefficient computation section 230 and the suppression signal generation section 32 are an example of a suppression section of technology disclosed herein. The stationary noise estimation section 36 and the stationary noise derived suppression coefficient computation section 38 are an example of a stationary noise derived suppression coefficient computation section of technology disclosed herein.
The stationary noise estimation section 36 estimates the level of stationary noise for each of the frequencies based on input sound signals that have been converted by the time-frequency converter 20 into frequency domain signals. Conventional technology may be employed as the method of estimating the level of stationary noise, such as for example the technology described in JP-A No. 2011-186384.
The stationary noise derived suppression coefficient computation section 38 computes the stationary noise derived suppression coefficient based on the level of stationary noise estimated by the stationary noise estimation section 36. Explanation follows regarding an example of a method for computing a stationary noise derived suppression coefficient ε wherein ε is, for example, the stationary noise derived suppression coefficient. When sound from a sound source other than the stationary noise does not occur, the ratio of the input sound signal level and the stationary noise level is a value close to 1.0. However, when sound from a sound source other than the stationary noise is emitted, the ratio of the input sound signal level and the stationary noise level deviates from 1.0.
When the input sound signal level/stationary noise level is a value close to 1.0 (for example 1.1) the stationary noise derived suppression coefficient computation section 38 computes the stationary noise derived suppression coefficient ε as for example shown below as a stationary noise derived suppression range.
ε=εmin when input sound signal level/stationary noise level<1.1
ε=1.0 when input sound signal level/stationary noise level≧1.1.
Note that εmin is a value such that 0<εmin<1, and for example, when a suppression amount of −3 dB is desired, εmin is about 0.7, and when a suppression amount of −6 dB is desired εmin is about 0.5. Similarly to with the phase difference derived suppression coefficient α and the amplitude ratio derived suppression coefficient β, when the input sound signal level/stationary noise level is outside the suppression range, the stationary noise derived suppression coefficient ε is computed so as to gradually change from 1.0 to εmin on progression away from the suppression range.
The phase difference derived suppression coefficient computation section 226 computes a phase difference derived suppression coefficient outside of the stationary noise derived suppression range. The method of computing the phase difference derived suppression coefficient is similar to that of the phase difference derived suppression coefficient computation section 26 of the first exemplary embodiment.
The amplitude ratio derived suppression coefficient computation section 228 computes an amplitude ratio derived suppression coefficient outside of the stationary noise derived suppression range. The method of computing the amplitude ratio derived suppression coefficient is similar to that of the amplitude ratio derived suppression coefficient computation section 28 of the first exemplary embodiment.
Note that there are cases in the above example in which the stationary noise derived suppression coefficient ε is 1.0 outside of the stationary noise derived suppression range. Moreover, when ε holds values from εmin to 1.0, configuration may be made such that cases in which ε is a specific threshold value εthr or greater, namely cases in which the degree of suppression derived from stationary noise is a specific value or lower, are treated as being outside the stationary noise derived suppression range.
The suppression coefficient computation section 230 computes a suppression coefficient for each frequency to suppress the noise included in the input sound signal based on the stationary noise derived suppression coefficient, the phase difference derived suppression coefficient, and the amplitude ratio derived suppression coefficient. Explanation follows regarding an example of a computation method of a suppression coefficient γ.
When the stationary noise derived suppression coefficient ε is made 1.0 outside of the stationary noise derived suppression range, the suppression coefficient γ may be computed outside the stationary noise derived suppression range as set out below using the phase difference derived suppression coefficient α and the amplitude ratio derived suppression coefficient β.
γ=ε when ε≠1.0
γ=α×β, or γ=the smallest of α or β when ε=1.0
As another computation method, configuration may be made such that the suppression coefficient γ outside of the stationary noise derived suppression range is computed using the α and the β as set out below when the stationary noise derived suppression coefficient ε is the specific threshold value εthr or greater, as cases outside of the stationary noise suppression range.
γ=ε when ε<εthr
γ=α×β, or γ=the smallest of α or β when ε≧εthr
Moreover, configuration may be made such that without partitioning into a stationary noise derived suppression range, and outside the range, the suppression coefficient γ is computed as set out below according to whether or not the input sound signal level is greater than the estimated stationary noise level.
γ=ε when the input sound signal level≦the stationary noise level
γ=smallest of α, β or ε when the input sound signal level>the stationary noise level
The noise suppression device 210 may be implemented by a computer 240 as illustrated in
The storage section 46 may be implemented for example by a Hard Disk Drive (HDD) or a flash memory. The storage section 46 serving as a storage medium is stored with a noise suppression program 250 for making the computer 240 function as the noise suppression device 210. The CPU 42 reads the noise suppression program 250 from the storage section 46, expands the noise suppression program 250 in the memory 44 and sequentially executes the processes of the noise suppression program 250.
The noise suppression program 250 includes, in addition to each of the processes of the noise suppression program 50 according to the first exemplary embodiment, a stationary noise estimation process 76 and a stationary noise derived suppression coefficient computation process 78.
The CPU 42 operates as the stationary noise estimation section 36 illustrated in
Thus the computer 240 executing the noise suppression program 250 functions as the noise suppression device 210.
Note that the noise suppression device 210 may be implemented by for example a semiconductor integrated circuit, or more specifically by an ASIC and a DSP.
Explanation follows regarding operation of the noise suppression device 210 according to the second exemplary embodiment. When the input sound signal 1 and the input sound signal 2 are output from the microphone array 11, the CPU 42 expands the noise suppression program 250 stored in the storage section 46 into the memory 44, and executes the noise suppression processing illustrated in
Through execution of the steps 100 to 108 of the noise suppression processing illustrated in
At the next step 200, the stationary noise estimation section 36 estimates the stationary noise level for each frequency based on the input sound signals that have been converted into frequency domain signals at step 108.
At the next step 202, the stationary noise derived suppression coefficient computation section 38 computes the stationary noise derived suppression coefficient ε based on the ratio of the input sound signal level and the stationary noise level as estimated at step 200.
The stationary noise derived suppression coefficient computation section 38 then determines whether or not the input sound signal is within the stationary noise derived suppression range, based on the stationary noise derived suppression coefficient ε computed at step 202. Processing proceeds to step 206 when inside the stationary noise derived suppression range. Processing proceeds to step 110 when outside the stationary noise derived suppression range, the phase difference derived suppression coefficient α and the amplitude ratio derived suppression coefficient β are computed through steps 110 to 116, and processing proceeds to step 206.
At step 206, the suppression coefficient computation section 230 takes the suppression coefficient γ as the stationary noise derived suppression coefficient ε computed at step 202 when within the stationary noise derived suppression range. The phase difference derived suppression coefficient α and the amplitude ratio derived suppression coefficient β are employed to compute the suppression coefficient γ at each frequency when outside the stationary noise derived suppression range.
In the following steps 120 to 128, similar processing is performed to that of the first exemplary embodiment, an output sound signal is output, and the noise suppression processing is ended.
As explained above, according to the noise suppression device 210 according to the second exemplary embodiment, in addition to the advantageous effects of the first exemplary embodiment, suppression is also enabled for stationary noise which is only slightly affected by noise suppression utilizing phase difference or amplitude ratio.
Note that explanation has been given in each of the exemplary embodiments above of cases in which input values are received for the sound source direction and the distance between the microphones and the sound source, however configuration may be made that utilizes a sound source direction and a distance from the sound source to the microphone estimated based on the phase difference computed at the phase difference computation section 22.
Thus according to the method of the technology disclosed herein, the degrees of freedom is increased for the placement positions for each of the microphones, enabling implementation with a microphone array mounted to various devices such as smart phones that are becoming increasingly thinner, and enabling noise suppression to be executed without audio distortion.
Note that explanation has been given above of a mode in which the noise suppression programs 50 and 250 serving as examples of a noise suppression program of technology disclosed herein are pre-stored (pre-installed) on the storage section 46. However the noise suppression program of technology disclosed herein may be supplied in a format such as stored on a storage medium such as a CD-ROM or DVD-ROM.
An aspect of technology disclosed herein has the advantageous effect or enabling noise suppression to be performed with an appropriate suppression amount and low audio distortion even when there are limitations to the placement positions of the microphone arrays.
All examples and conditional language provided herein are intended for the pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although one or more embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.
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