(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2004/0185804 A1
`(43) Pub. Date:
`Sep. 23, 2004
`Kanamori et al.
`
`US 20040185804A1
`
`(54)
`
`MICROPHONE DEVICE AND AUDIO
`PLAYER
`
`(76)
`
`Inventors: Takeo Kanamori, Hirakata (JP);
`Takashi Kawamura, Settsu (JP);
`Tomomi Matsuoka, Ibaraki (JP)
`Correspondence Address:
`WENDEROTH, LIND & PONACK, L.L.P.
`2033 K STREET N. W.
`SUTE 800
`WASHINGTON, DC 20006-1021 (US)
`Appl. No.:
`10/714,857
`
`(21)
`(22)
`(30)
`Nov. 18, 2002
`
`Filed:
`
`Nov. 18, 2003
`Foreign Application Priority Data
`
`(JP)...................................... 2002-333390
`
`Publication Classification
`
`(51) Int. Cl." ....................................................... H04B 1/04
`(52) U.S. Cl. .......................................................... 455/1142
`(57)
`ABSTRACT
`A signal generating Section generates a main signal and a
`noise reference Signal. A determining Section determines
`whether a level ratio is larger than a predetermined value. An
`adaptive filter Section generates a signal indicative of a
`Signal component of a target Sound included in the noise
`reference signal generated by the Signal generating Section,
`and learns a filter coefficient only when the determining
`Section determines that the level ratio is larger than the
`predetermined value. A Subtracting Section Subtracts the
`Signal generated by the adaptive filter Section from the noise
`reference Signal. A noise Suppressing Section Suppresses a
`Signal component of noise included in the main signal by
`using the main signal and the noise reference Signal after
`Subtraction by the Subtracting Section.
`
`
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`REFLECTION CORRECTING
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`
`TIME-WAREANT
`COEFFICIENT FILTER
`SECTION
`NOISE SUPPRESSION
`FILTER COEFFICIENT
`CALCULATING SECTION
`
`40
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`Patent Application Publication Sep. 23, 2004 Sheet 22 of 23
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`Patent Application Publication Sep. 23, 2004 Sheet 23 of 23
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`US 2004/0185804 A1
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`US 2004/0185804 A1
`
`Sep. 23, 2004
`
`MICROPHONE DEVICE AND AUDIO PLAYER
`
`BACKGROUND OF THE INVENTION
`0001) 1. Field of the Invention
`0002 The present invention relates to microphone
`devices and audio playerS and, more Specifically, to a
`microphone device and an audio player which detects a
`desired Sound coming from a specific direction with noise
`being Suppressed.
`0.003
`2. Description of the Background Art
`0004. The configurations of conventional microphone
`devices are described with reference to FIGS. 24 through
`26. FIG. 24 is an illustration showing the configuration of a
`conventional microphone device of Example 1. In FIG. 24,
`the conventional microphone device includes a first micro
`phone unit 1010, a second microphone unit 1020, a signal
`adding section 1030, a first signal subtracting section 1031,
`a signal amplifying Section 1050, an adaptive filter Section
`1060, and a second signal Subtracting section 1062. Each of
`the microphone units 1010 and 1020 is placed so as to be
`oriented to the front (left in FIG. 24). The signal adding
`section 1030 adds a signal output from the first microphone
`unit 1010 and a signal output from the second microphone
`unit 1020. The first signal subtracting section 1031 subtracts
`the signal output from the second microphone unit 1020
`from the signal output from the first microphone unit 1010.
`The signal amplifying section 1050 multiplies a signal
`output from the signal adding section 1030 by 1/2. The
`adaptive filter section 1060 is supplied with a signal output
`from the first Signal Subtracting Section 1031, and outputs a
`Signal obtained through filtering performed by an adaptive
`filter included therein. The Second Signal Subtracting Section
`1062 subtracts a signal output from the adaptive filter
`Section 1060 from a signal output from the Signal amplifying
`section 1050. An output from the second signal subtracting
`section 1062 is an output from the microphone device. The
`adaptive filtersection 1060 learns a filter coefficient from the
`Signal output from the Second Signal Subtracting Section
`1062 and the Signal output from the first Signal Subtracting
`Section 1031.
`0005 The operation of the conventional microphone
`device of Example 1 is described below. In order to detect
`a sound coming from the front, the microphone units 1010
`and 1020 each output approximately the same Signal. In
`order to detect a Sound coming from other directions, the
`microphone units 1010 and 1020 output signals that are
`different in phase. The output signals from the microphone
`units 1010 and 1020 are then added together by the signal
`adding section 1030. The resultant signal obtained through
`addition is then normalized in level by the Signal amplifying
`section 1050. That is, the amplitude of the signal is amplified
`by 1/2. With this, a main Signal having components of the
`Sound coming from the front can be obtained. Also, with the
`output from the first signal subtracting section 1031, it is
`possible to achieve a directivity characteristic Such that the
`main axis of directivity is oriented to a direction of 90
`degrees with respect to the front and the front direction is a
`direction of a minimum Sensitivity in the directivity (that is,
`the sensitivity of directivity is minimum in the front direc
`tion). That is, the Signal output from the first signal Sub
`tracting Section 1031 serves as a noise reference Signal
`which does not include the components of the Sound coming
`
`from the front. The adaptive filter section 1060 uses the main
`signal output from the signal amplifying section 1050 and
`the noise reference Signal output from the first signal Sub
`tracting section 1031 to achieve adaptive directivity. That is,
`the direction of a minimum sensitivity in the directivity is
`uniquely determined to be oriented to a noise Sound coming
`from a direction other than the front direction.
`0006 FIG.25 is an illustration showing the configuration
`of a conventional microphone device of Example 2. In FIG.
`25, the conventional microphone device includes a first
`microphone unit 1010, a second microphone unit 1020, a
`first adaptive filter section 1040, a first signal delaying
`section 1041, a first signal subtracting section 1042, a
`Second adaptive filter Section 1060, a Second Signal delaying
`section 1061, and a second signal Subtracting section 1062.
`0007. The first adaptive filter section 1040 is supplied
`with an output signal from the second microphone unit 1020
`and then outputs the filtering results obtained by an adaptive
`filter included therein. The first signal delaying section 1041
`delays a signal output from the first microphone unit 1010.
`The first Signal Subtracting Section 1042 Subtracts a signal
`output from the first adaptive filter section 1040 from a
`Signal output from the first Signal delaying Section 1041. The
`first adaptive filter section 1040 learns a filter coefficient
`from a signal output from the first signal Subtracting Section
`1042 and a Signal output from the Second microphone unit
`1020. The second signal delaying section 1061 delays the
`Signal output from the first signal delaying Section 1041. The
`second adaptive filter section 1060 is supplied with a signal
`output from the first signal Subtracting Section 1042, and
`then outputs the filtering results obtained by an adaptive
`filter included therein. The Second Signal Subtracting Section
`1062 subtracts a signal output from the second adaptive filter
`section 1060 from a signal output from the second signal
`delaying section 1061. The subtraction result is an output
`from the microphone device. The second adaptive filter
`section 1060 learns a filter coefficient from a signal output
`from the Second Signal Subtracting Section 1062 and a signal
`output from the first signal subtracting section 1042.
`0008. The operation of the conventional microphone
`device of Example 2 is described below. The first adaptive
`filter section 1040, the first signal delaying section 1041, and
`the first Signal Subtracting Section 1042 performs a canceling
`operation on Sound waves coming to the microphone units
`1010 and 1020. That is, the signal output from the first signal
`Subtracting Section 1042 Serves as a noise Signal for the
`second adaptive filter section 1060. That is, the signal output
`from the first signal Subtracting Section 1042 is a signal
`Serving a purpose Similar to that of the Signal output from the
`first subtracting section 1031 in FIG. 24. However, the
`conventional microphone device of Example 2 is different
`from that of Example 1 in the following point. That is, the
`directivity is fixed in Example 1, whilst the directivity can
`be changed by using the adaptive filters in Example 2.
`0009 FIG. 26 is an illustration showing the configuration
`of a conventional microphone device of Example 3. The
`conventional microphone device illustrated in FIG. 26
`includes a first unidirectional microphone unit 1011, a
`second unidirectional microphone unit 1012, a first FFT
`section 1070, a second FFT section 1080, a two-input-type
`spectrum subtraction section 1090, and a voice recognition
`Section 2000.
`
`Page 25 of 47
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`Sep. 23, 2004
`
`0010. In FIG. 26, the first unidirectional microphone unit
`1011 is placed so that the main axis of its directivity is
`oriented to the front. The Second unidirectional microphone
`unit 1012 is placed so that the main axis of its directivity is
`oriented to the back. The first FFT section 1070 is supplied
`with a Signal output from the first unidirectional microphone
`unit 1011 to find a frequency spectrum. The second FFT
`section 1080 is supplied with a signal output from the second
`unidirectional microphone unit 1012 to find a frequency
`Spectrum. The two-input-type spectrum Subtraction Section
`1090 is supplied with signals output from both of the FFT
`sections 1070 and 1080 to subtract, in a power spectrum
`region, the Signal Spectrum derived by the Second FFT
`section 1080 from the signal spectrum derived by the first
`FFT section 1070, thereby outputting a spectrum of a target
`signal. The voice recognition section 2000 is supplied with
`the Spectrum of the target Signal output from the two-input
`type spectrum subtraction section 1090 for voice recogni
`tion.
`0.011 The operation of the conventional microphone
`device of Example 3 is described below. In Example 3, the
`first unidirectional microphone unit 1011 has a directivity
`characteristic of collecting a desired Sound (target Sound)
`from the front. The Second unidirectional microphone unit
`1012 has a directivity characteristic of mainly collecting
`noise. Therefore, a main Signal m1 is obtained from the first
`unidirectional microphone unit 1011, while a noise reference
`Signal m2 is obtained from the Second unidirectional micro
`phone unit 1012. Then, a spectrum of the main Signal m1 is
`found by the first FFT section 1070, while a spectrum of the
`noise reference signal m2 is found by the second FFT
`section 1080. The power spectrum of the noise reference
`Signal is Subtracted from the power spectrum of the main
`Signal by the two-input-type spectrum Subtraction Section
`1090. With this, the power spectrum of the signal compo
`nents are estimated. Note that, in a one-input-type spectrum
`Subtraction Scheme, a noise Spectrum is estimated, assuming
`that noise is Stationary during a time Section in which the
`target Sound has not yet arrive. Therefore, in the one-input
`type spectrum Subtraction Scheme, only Suppression of
`Stationary noise is possible. On the other hand, according to
`the configuration of the microphone device of Example 3
`adopting a two-input-type spectrum Subtraction Scheme, the
`Spectrum of the noise reference Signal can always be
`obtained by the second unidirectional microphone unit 1012.
`Therefore, Suppression of non-Stationary noise is possible.
`AS Such, according to the microphone device of Example 3,
`the ratio of Voice recognition at the Voice recognition Section
`2000 at a later stage can be improved by Suppressing
`Stationary noise and non-Stationary noise. Note that,
`although the device illustrated in FIG. 26 is dedicated for
`Voice recognition, the device can be used as a microphone
`device by performing IFFT at the last stage to convert the
`Spectrum to a time signal and then to a waveform Signal with
`frame overlap.
`0012. In the microphone device of Example 1, a large
`noise Suppressing effect can be achieved under an environ
`ment where noise is coming from a certain direction. How
`ever, the microphone device of Example 1 does not handle
`noise coming from a plurality of directions. Therefore, under
`the actual noisy environment where noise Sources Simulta
`neously exist in various directions, the microphone device of
`
`Example 1 can merely achieve a noise Suppressing effect
`equivalent to that obtained by conventional unidirectional
`microphone devices.
`0013 In the microphone device of Example 2, the noise
`reference signal is obtained by using the first adaptive filter.
`Here, in order to stably operate the first adaptive filter under
`the actual environment, it is required to cause the first
`adaptive filter to learn a filter coefficient only when the voice
`from the talker is Sufficiently larger than the Surrounding
`noise. Therefore, the microphone device of Example 2
`cannot achieve a noise Suppression effect until filter con
`Vergence has been completed. Moreover, under the noisy
`environment, filter convergence is difficult. Further, as with
`Example 1, the microphone device of Example 2 cannot
`handle a plurality of noise Sources. Still further, Since the
`microphone device of Example 2 was devised with the aim
`of Suppressing wind noise, which has no correlation between
`unit Signals, the direction of the target Sound cannot be
`restricted. In other words, the largest one of the Sounds that
`has arrived at the microphone device is regarded as the target
`Sound. Therefore, it is impossible to performing a process of
`collecting Sounds with a Sound in a specific direction being
`enhanced.
`0014.
`In the microphone device of Example 3, the main
`Signal and the noise reference Signal are converted into
`Spectrums. Then, noise is Suppressed based on the power
`Spectrums by using a spectrum Subtraction Scheme. With
`this, even if noise Sources exist in a plurality of directions,
`their noise can be simultaneously Suppressed. In the micro
`phone device of Example 3, however, inclusion of even a
`Slightest amount of components of the target Sound in the
`noise reference Sound will significantly deteriorate the Sound
`quality of the processed Sound or, at worse, may cancel the
`target Sound itself. Moreover, in the actual Sound field, a
`reflected wave may be diffracted to enter the microphone
`device even if the direction of a minimum sensitivity in the
`directivity of the unidirectional microphone unit are oriented
`to the direction of the target Sound. Further, in normal
`microphone units, the amount of attenuation in the direction
`of a minimum sensitivity in the directivity is not infinite but
`on the order of 10 to 15 db. Therefore, the direct wave of the
`target Sound may not be completely eliminated and may be
`included in the noise reference Signal. Still further, in the
`Spectrum Subtraction Scheme, a process delay will occur due
`to a frame processing. Therefore, the microphone device
`using the Spectrum Subtraction Scheme is not Suitable for
`Simultaneous calls or loudspeakers.
`0015 Moreover, the above conventional microphone
`devices focus on Suppressing additive noise, which is dif
`ferent from the target Sound. The above conventional micro
`phone devices cannot SuppreSS multiplicative noise, which
`arrives after being reflected on a Surface of reflection, Such
`as a wall, a desk, or a floor. Therefore, the frequency
`characteristic of the target Sound may be distorted due to, for
`example, the influence of reflection in a Sound field where
`the microphone device is actually used. For this reason,
`particularly for the purpose of Voice recognition, a mismatch
`in recognition may occur, leading to erroneous recognition.
`
`SUMMARY OF THE INVENTION
`0016. Therefore, an object of the present invention is to
`provide a microphone device capable of Stably operating
`
`Page 26 of 47
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`Sep. 23, 2004
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`even under noise from a plurality of noise Sources in the
`actual use environment and also achieving a high S/N ratio.
`0.017. Another object of the present invention is to pro
`vide a microphone device which Suppresses multiplicative
`noise caused by, for example, a reflective wave of a target
`Sound or other factors and additive noise caused by accu
`mulation of noise.
`0.018
`Still another of the present invention is to generate
`a main Signal and noise reference Signal used in a noise
`Suppressing process with a simple Scheme.
`0019. In order to attain the objects mentioned above, the
`present invention adopts the following Structures. That is, a
`first aspect of the present invention is directed to a micro
`phone device which detects a target Sound coming from a
`direction of the target Sound. The microphone device
`includes a signal generating Section, a determining Section,
`an adaptive filter Section, a Subtracting Section, and a noise
`Suppressing Section. The signal generating Section generates
`a main Signal indicative of a result obtained through detec
`tion with a Sensitivity in the direction of the target Sound and
`a noise reference Signal indicative of a result obtained
`through detection with a Sensitivity higher in another direc
`tion than in the direction of the target Sound. The determin
`ing Section determines whether a level ratio indicative of a
`ratio of a level of the main Signal to the noise reference
`Signal generated by the Signal generating Section is larger
`than a predetermined value. The adaptive filter Section
`generates a signal indicative of a signal component of the
`target Sound included in the noise reference Signal generated
`by the Signal generating Section by performing, by an
`adaptive filter included in the adaptive filter Section, a
`filtering process on the main Signal generated by the Signal
`generating Section, and learns a filter coefficient only when
`the determining Section determines that the level ratio is
`larger than the predetermined value. The Subtracting Section
`Subtracts the Signal generated by the adaptive filter Section
`from the noise reference Signal generated by the Signal
`generating Section. The noise Suppressing Section Suppresses
`a signal component of noise included in the main signal by
`using the main Signal and the noise reference Signal after
`Subtraction by the Subtracting Section.
`0020 Note that “a main signal indicative of a result
`obtained through detection with a Sensitivity in the direction
`of the target Sound” means that the main signal can be not
`only a signal output from a microphone unit, but also a
`Signal obtained by performing a predetermined process on a
`Signal by the microphone unit. That is, the main signal can
`be not only a signal output from a microphone unit whose
`main axis of directivity is oriented to the direction of the
`target Sound, but also a signal obtained by performing a
`predetermined proceSS on a Signal output from any micro
`phone unit (that is, a non-directional microphone unit or a
`directional microphone unit whose main axis of directivity
`is oriented to a predetermined direction). Similarly, "a noise
`reference signal indicative of a result obtained through
`detection with a Sensitivity higher in another direction than
`in the direction of the target Sound” means that the noise
`reference Signal can be not only a signal output from a
`microphone unit, but also a signal obtained by performing a
`predetermined proceSS on a Signal output from any micro
`phone unit.
`0021. A second aspect of the present invention is directed
`to a microphone device which detects a target Sound coming
`
`from a direction of the target Sound. The microphone device
`includes a signal generating Section, a determining Section,
`an adaptive filter Section, a Subtracting Section, a reflection
`information calculating Section, and a reflection correcting
`Section. The Signal generating Section generates a main
`Signal indicative of a result obtained through detection with
`Sensitivity in the direction of the target Sound and a noise
`reference signal indicative of a result obtained through
`detection with a Sensitivity higher in another direction than
`in the direction of the target Sound. The determining Section
`determines whether a level ratio indicative of a ratio of a
`level of the main Signal to the noise reference Signal gen
`erated by the Signal generating Section is larger than a
`predetermined value. The adaptive filter Section generates a
`Signal indicative of a Signal component of the target Sound
`included in the noise reference signal generated by the Signal
`generating Section by performing, by an adaptive filter
`included therein, a filtering process on the main Signal
`generated by the Signal generating Section, and learns a filter
`coefficient only when the determining Section determines
`that the level ratio is larger than the predetermined value.
`The Subtracting Section Subtracts the Signal generated by the
`adaptive filter Section from the noise reference Signal gen
`erated by the Signal generating Section. The reflection infor
`mation calculating Section calculates information about a
`difference in arrival time between a direct wave of the target
`Sound and a reflected wave of the target Sound. The reflec
`tion correcting Section corrects, based on the information
`calculated by the reflection information calculating Section,
`distortion in a frequency characteristic of the main Signal
`caused by the reflected wave.
`0022.
`In a third aspect, the signal generating Section
`includes a first microphone unit and a Second microphone
`unit. The first microphone unit is placed So that a main axis
`of directivity is oriented to the direction of the target Sound.
`The Second microphone unit is placed So that a minimum
`sensitivity axis of directivity is oriented to the direction of
`the target Sound (a direction of a minimum sensitivity in the
`directivity).
`0023. Also, in a fourth aspect, the microphone device
`further includes a signal delaying Section. The Signal delay
`ing Section is provided between an output end of the noise
`reference Signal in the Signal generating Section and the
`Subtracting Section, and delays the noise reference Signal So
`as to Satisfy conditions of convergence of the adaptive filter
`of the adaptive filter Section.
`0024.
`Furthermore, in a fifth aspect, the predetermined
`value is changeable.
`0025 Still further, in a sixth aspect, the signal generating
`Section includes a first microphone unit, a Second micro
`phone unit, a delaying Section, an amplifying Section, a first
`Subtracting Section, and a Second Subtracting Section. The
`Second microphone unit has a characteristic identical to a
`characteristic of the first microphone unit. The delaying
`Section outputs a signal output from the first microphone
`unit as being delayed by a predetermined delay amount. The
`amplifying Section amplifies the Signal output from the delay
`Section. The first Subtracting Section Subtracts the Signal
`amplified by the amplifying Section from a Signal output
`from the Second microphone unit to generate the main
`Signal. The Second Subtracting Section Subtracts the Signal
`output from the delaying Section from the Signal output from
`
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`US 2004/0185804 A1
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`Sep. 23, 2004
`
`the Second microphone unit to generate the noise reference
`Signal. The predetermined delay amount is Set So that the
`noise reference Signal includes components of a Sound
`coming from a direction other than the direction of the target
`Sound more than components of the target Sound. The
`amplification factor in the amplifying Section is Set So as to
`cause a difference in a Sensitivity to the target Sound between
`the main Signal and the noise reference Signal.
`0.026
`Still further, in a seventh aspect, the microphone
`device further includes a Setting Section for changing the
`predetermined delay amount used in the delay Section.
`0.027
`Still further, in an eighth aspect, the signal gener
`ating Section includes a first microphone unit, a Second
`microphone unit, and a combining Section. The Second
`microphone unit has a characteristic identical to a charac
`teristic of the first microphone unit. The combining Section
`generates, based on Signals output from the first and Second
`microphone unit, the main Signal with Sensitivity in the
`direction of the target Sound, and generating a noise Signal
`with minimum Sensitivity in the direction of the target
`Sound.
`0028 Still further, in a ninth aspect, the signal generating
`Section includes a first microphone unit, a Second micro
`phone unit, a signal adding Section, and a signal Subtracting
`Section. The Second microphone unit is placed So that a main
`axis of directivity is oriented to a direction which is different
`from a main axis of directivity of the first microphone unit.
`The signal adding section adds a first signal output from the
`first microphone unit and a Second signal output from the
`Second microphone unit to generate the main Signal. The
`Signal Subtracting Section Subtracts a third signal, which is
`either one of the first signal and the Second Signal, from a
`fourth Signal, which is either one of the first signal and the
`Second Signal but other than the third signal, to generate the
`noise reference signal.
`0029 Still further, in a tenth aspect, the signal generating
`Section includes a first microphone unit, a Second micro
`phone unit, a Stereo signal generating Section, an inverse
`combining Section, and a combining Section. The Second
`microphone unit has a characteristic identical to a charac
`teristic of the first microphone unit. The Stereo signal
`generating Section generates, based on the first and Second
`microphone units, a Stereo signal formed by a right channel
`Signal and a left channel Signal. The inverse combining
`Section generates, based on the Stereo signal, Signals output
`from the first and Second microphone units. The combining
`Section generates the main Signal and the noise reference
`Signal based on the Signals generated by the inverse com
`bining Section.
`0030 Still further, in an eleventh aspect, the signal gen
`erating Section includes a first microphone unit, a Second
`microphone unit, a Stereo signal generating Section, a signal
`adding Section, and a signal Subtracting Section. The Second
`microphone unit has a characteristic identical to a charac
`teristic of the first microphone unit. The Stereo signal
`generating Section generates, based on the first and Second
`microphone units, a Stereo signal formed by a right channel
`Signal and a left channel Signal. The Signal adding Section
`adds he right channel Signal and the left channel Signal to
`generate the main Signal. The Signal Subtracting Section
`Subtracts a first Signal, which is either one of the right
`channel Signal and the left channel Signal, from a Second
`
`Signal, which is either one of the right channel Signal and the
`left channel Signal but other than the first Signal, to generate
`the noise reference Signal.
`0031 Still further, in a twelfth aspect, the microphone
`device further includes a reflection information calculating
`Section and a reflection correcting Section. The reflection
`information calculating Section calculates, based on the filter
`coefficient of the adaptive filter Section, information about a
`difference in arrival time between a direct wave of t