(19) United States
`(12) Patent Application Publication (10) Pub. No.: US 2003/0031328A1
`(43) Pub. Date:
`Feb. 13, 2003
`Elko et al.
`
`US 2003OO31328A1
`
`(54)
`
`SECOND-ORDER ADAPTIVE
`DIFFERENTIAL MICROPHONE ARRAY
`
`(57)
`
`ABSTRACT
`
`Inventors: Gary W. Elko, Summit, NJ (US);
`Heinz Teutsch, Nurnberg (DE)
`Correspondence Address:
`MENDELSOHN AND ASSOCATES PC
`1515 MARKET STREET
`SUTE 715
`PHILADELPHIA, PA 19102 (US)
`
`Appl. No.:
`
`09/999.298
`
`Filed:
`
`Oct. 30, 2001
`Related U.S. Application Data
`Provisional application No. 60/306.271, filed on Jul.
`18, 2001.
`
`Publication Classification
`
`Int. Cl. .................................................. H04R 3/00
`U.S. Cl. ................................................................ 381/92
`
`(76)
`
`(21)
`(22)
`
`(60)
`
`(51)
`(52)
`
`
`
`A Second-order adaptive differential microphone array
`(ADMA) has two first-order elements (e.g., 802 and 804 of
`FIG. 8), each configured to convert a received audio signal
`into an electrical signal. The ADMA also has (i) two delay
`nodes (e.g., 806 and 808) configured to delay the electrical
`Signals from the first-order elements and (ii) two Subtraction
`nodes (e.g., 810 and 812) configured to generate forward
`facing and backward-facing cardioid signals based on dif
`ferences between the electrical Signals and the delayed
`electrical signals. The ADMA also has (i) an amplifier (e.g.,
`814) configured to amplify the backward-facing cardioid
`Signal by again parameter; (ii) a third Subtraction node (e.g.,
`816) configured to generate a difference signal based on a
`difference between the forward-facing cardioid signal and
`the amplified backward-facing cardioid signal; and (iii) a
`lowpass filter (e.g., 818) configured to filter the difference
`Signal from the third Subtraction node to generate the output
`Signal for the Second-order ADMA. The gain parameter for
`the amplifier can be adaptively adjusted to move a null in the
`back half plane of the ADMA to track a moving noise
`Source. In a Subband implementation, a different gain param
`eter can be adaptively adjusted to move a different null in the
`back half plane to track a different moving noise Source for
`each different frequency Subband.
`
`ld2.
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`Patent Application Publication Feb. 13, 2003 Sheet 1 of 12
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`US 2003/0031328A1
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`Patent Application Publication Feb. 13, 2003 Sheet 3 of 12
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`Forward facing Cardioid
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`Patent Application Publication Feb. 13, 2003 Sheet 5 of 12
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`Patent Application Publication Feb. 13, 2003 Sheet 6 of 12
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`Patent Application Publication Feb. 13, 2003 Sheet 10 of 12 US 2003/0031328A1
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`Patent Application Publication Feb. 13, 2003 Sheet 12 of 12 US 2003/0031328A1
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`

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`US 2003/0031328A1
`
`Feb. 13, 2003
`
`SECOND-ORDER ADAPTIVE DIFFERENTIAL
`MICROPHONE ARRAY
`
`CROSS-REFERENCE TO RELATED
`APPLICATIONS
`0001. This application claims the benefit of the filing date
`of U.S. provisional application No. 60/306.271, filed on Jul.
`18, 2001 as attorney docket no. Elko 18-1.
`
`BACKGROUND OF THE INVENTION
`0002) 1. Field of the Invention
`0003. The present invention relates to microphone arrays
`that employ directionality characteristics to differentiate
`between Sources of noise and desired Sound Sources.
`0004 2. Description of the Related Art
`0005 The presence of background noise accompanying
`all kinds of acoustic Signal transmission is a ubiquitous
`problem. Speech Signals especially Suffer from incident
`background noise, which can make conversations in adverse
`acoustic environments virtually impossible without applying
`appropriately designed electroacoustic transducers and
`Sophisticated Signal processing. The utilization of conven
`tional directional microphones with fixed directivity is a
`limited Solution to this problem, because the undesired noise
`is often not fixed to a certain angle.
`
`SUMMARY OF THE INVENTION
`0006 Embodiments of the present invention are directed
`to adaptive differential microphone arrays (ADMAS) that are
`able to adaptively track and attenuate possibly moving noise
`Sources that are located in the back half plane of the array.
`This noise attenuation is achieved by adaptively placing a
`null into the noise Source's direction of arrival. Such
`embodiments take advantage of the adaptive noise cancel
`lation capabilities of differential microphone arrays in com
`bination with digital Signal processing. Whenever undesired
`noise Sources are spatially non-stationary, conventional
`directional microphone technology has its limits in terms of
`interference Suppression. Adaptive differential microphone
`arrays (ADMAS) with their null-steering capabilities prom
`ise better performance.
`0007. In one embodiment, the present invention is a
`Second-order adaptive differential microphone array
`(ADMA), comprising (a) a first first-order element (e.g., 802
`of FIG. 8) configured to convert a received audio signal into
`a first electrical signal; (b) a Second first-order element (e.g.,
`804 of FIG. 8) configured to convert the received audio
`Signal into a second electrical signal; (c) a first delay node
`(e.g., 806 of FIG. 8) configured to delay the first electrical
`Signal from the first first-order element to generate a delayed
`first electrical signal; (d) a second delay node (e.g., 808 of
`FIG. 8) configured to delay the second electrical signal from
`the Second first-order element to generate a delayed Second
`electrical Signal; (e) a first Subtraction node (e.g., 810 of
`FIG. 8) configured to generate a forward-facing cardioid
`Signal based on a difference between the first electrical
`Signal and the delayed second electrical Signal, (f) a second
`subtraction node (e.g.,812 of FIG. 8) configured to generate
`a backward-facing cardioid signal based on a difference
`between the Second electrical Signal and the delayed first
`electrical signal; (g) an amplifier (e.g., 814 of FIG. 8)
`
`configured to amplify the backward-facing cardioid Signal
`by a gain parameter to generate an amplified backward
`facing cardioid signal; and (h) a third Subtraction node (e.g.,
`816 of FIG. 8) configured to generate a difference signal
`based on a difference between the forward-facing cardioid
`Signal and the amplified backward-facing cardioid signal.
`0008. In another embodiment, the present invention is an
`apparatus for processing Signals generated by a microphone
`array (ADMA) having (i) a first first-order element (e.g., 802
`of FIG. 8) configured to convert a received audio signal into
`a first electrical signal and (ii) a second first-order element
`(e.g., 804 of FIG. 8) configured to convert the received
`audio signal into a Second electrical Signal, the apparatus
`comprising (a) a first delay node (e.g., 806 of FIG. 8)
`configured to delay the first electrical Signal from the first
`first-order element to generate a delayed first electrical
`signal; (b) a second delay node (e.g., 808 of FIG. 8)
`configured to delay the Second electrical Signal from the
`Second first-order element to generate a delayed Second
`electrical Signal; (c) a first Subtraction node (e.g., 810 of
`FIG. 8) configured to generate a forward-facing cardioid
`Signal based on a difference between the first electrical
`Signal and the delayed second electrical signal; (d) a second
`subtraction node (e.g.,812 of FIG. 8) configured to generate
`a backward-facing cardioid signal based on a difference
`between the Second electrical Signal and the delayed first
`electrical signal; (e) an amplifier (e.g., 814 of FIG. 8)
`configured to amplify the backward-facing cardioid Signal
`by a gain parameter to generate an amplified backward
`facing cardioid signal; and (g) a third Subtraction node (e.g.,
`816 of FIG. 8) configured to generate a difference signal
`based on a difference between the forward-facing cardioid
`Signal and the amplified backward-facing cardioid signal.
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`0009. Other aspects, features, and advantages of the
`present invention will become more fully apparent from the
`following detailed description, the appended claims, and the
`accompanying drawings in which:
`0010 FIG. 1 shows a schematic representation of a
`first-order adaptive differential microphone array (ADMA)
`receiving an audio Signal from a signal Source at a distance
`where farfield conditions are applicable;
`0011 FIG. 2 shows a schematic diagram of a first-order
`fullband ADMA based on an adaptive back-to-back cardioid
`System;
`0012 FIG. 3 shows the directivity pattern of the first
`order ADMA of FIG. 2;
`0013 FIG. 4 shows directivity patterns that can be
`obtained by the first-order ADMA for 0 , values of 90,
`120°, 150°, and 180°:
`0014 FIG. 5 shows a schematic diagram of a second
`order fullband ADMA;
`0015 FIG. 6 shows the directivity pattern of a second
`order back-to-back cardioid System;
`0016 FIG. 7 shows the directivity patterns that can be
`obtained by a second-order ADMA formed from two dipole
`elements for 0 values of 90°, 120°, 150, and 180°:
`0017 FIG. 8 shows a schematic diagram of a Subband
`two-element ADMA;
`
`Page 14 of 19
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`

`

`US 2003/0031328A1
`
`Feb. 13, 2003
`
`0018 FIGS. 9A and 9B depict the fullband ADMA
`directivity patterns for first-order and Second-order arrayS,
`respectively; and
`0019 FIGS. 10 and 11 show measured directivity of
`first- and Second-order Subband implementations of the
`ADMA of FIG. 8, respectively, for four simultaneously
`playing Sinusoids.
`
`DETAILED DESCRIPTION
`0020 First-Order Fullband ADMA
`0021
`FIG. 1 shows a schematic representation of a
`first-order adaptive differential microphone array (ADMA)
`100 receiving audio signal s(t) from audio source 102 at a
`distance where farfield conditions are applicable. When
`farfield conditions apply, the audio signal arriving at ADMA
`100 can be treated as a plane wave. ADMA 100 comprises
`two zeroth-order microphones 104 and 106 separated by a
`distance d. Electrical signals generated by microphone 106
`are delayed by inter-element delay T at delay node 108
`before being Subtracted from the electrical Signals generated
`by microphone 104 at Subtraction node 110 to generate the
`ADMA output y(t). The magnitude of the frequency and
`angular dependent response H (f, 0) of first-order ADMA
`100 for a signal point source at a distance where farfield
`conditions are applicable can be written according to Equa
`tion (1) as follows:
`
`H (f, 6) = '" 1
`(2it fT+k-d)
`-2s2. It rid
`
`(1)
`
`0022 where Y (f, 0) is the spectrum of the ADMA
`output signal y(t), S(f) is the spectrum of the Signal Source,
`k is the Sound vector, k=k=2 f/c is the wavenumber, c is
`the Speed of Sound, and d is the displacement vector between
`microphones 104 and 106. As indicated by the term Y (f, 0)
`, the ADMA output signal is dependent on the angle 0
`between the displacement vector d and the Sound vector k as
`well as on the frequency f of the audio signal s(t).
`0023 For small element spacing and short inter-element
`delay (kd.<<IC and T-72?, Equation (1) can be approximated
`according to Equation (2) as follows:
`
`0024. As can be seen, the right side of Equation (2)
`consists of a monopole term and a dipole term (cos0). Note
`that the amplitude response of the first-order differential
`array rises linearly with frequency. This frequency depen
`dence can be corrected for by applying a first-order lowpass
`filter at the array output. The directivity response can then be
`expressed by Equation (3) as follows:
`
`(3)
`
`0.025 Since the location of the source 102 is not typically
`known, an implementation of a first-order ADMA based on
`
`Equation (3) would need to involve the ability to generate
`any time delay T between the two microphones. AS Such,
`this approach is not Suitable for a real-time System. One way
`to avoid having to generate the delay T directly in order to
`obtain the desired directivity response is to utilize an adap
`tive back-to-back cardioid System
`0026 FIG. 2 shows a schematic diagram of a first-order
`fullband ADMA 200 based on an adaptive back-to-back
`cardioid system. In ADMA 200, signals from both micro
`phones 202 and 204 are delayed by a time delay T at delay
`nodes 206 and 208, respectively. The delayed signal from
`microphone 204 is subtracted from the undelayed signal
`from microphone 202 at forward subtraction node 210 to
`form the forward-facing cardioid signal C(t). Similarly, the
`delayed signal from microphone 202 is subtracted from the
`undelayed signal from microphone 204 at backward Sub
`traction node 212 to form the backward-facing cardioid
`Signal c(t), which is amplified by gain? at amplifier 214.
`The signal y(t) is generated at Subtraction node 216 based on
`the difference between the forward and amplified backward
`signals. The signal y(t) is then lowpass filtered at filter 218
`to generate the ADMA output signal y(t).
`0027 FIG. 3 shows the directivity pattern of the first
`order back-to-back cardioid system of ADMA200. ADMA
`200 can be used to adaptively adjust the response of the
`backward facing cardioid in order to track a possibly moving
`noise Source located in the back half plane. By choosing
`T=d/c, the back-to-back cardioid can be formed directly by
`appropriately Subtracting the delayed microphone signals.
`0028. The transfer function H(f, 0) of first-order ADMA
`200 can be written according to Equation (4) as follows:
`
`You (f, 6)
`H (f, 8) =
`S(f)
`
`2je
`
`(sink
`
`d(1 + cost
`d(1 - cost
`( on- sink (
`!).
`
`(4)
`
`0029 where Y(f, 0) is the spectrum of the ADMA
`output signal y(t).
`0030 The single independent null angle 0 of first-order
`ADMA200, which, for the present discussion, is assumed to
`be placed into the back half plane of the array
`(90's 0s 180), can be found by setting Equation (4) to
`Zero and Solving for 0=0, which yields Equation (5) as
`follows:
`
`8 =
`2
`f3- 1
`arccosiaret f3+ 1
`
`kid
`2 ),
`
`(5)
`
`0031 which for small spacing and short delay can be
`approximated according to Equation (6) as follows:
`
`6 & arccos.f3-
`
`1
`
`6
`(6)
`
`Page 15 of 19
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`

`US 2003/0031328A1
`
`Feb. 13, 2003
`
`0032 where 0s Bs1 under the constraint (900s 180°).
`FIG. 4 shows the directivity patterns that can be obtained by
`first-order ADMA200 for 0 values of 90°, 120°, 150°, and
`180°.
`0033. In a time-varying environment, an adaptive algo
`rithm is preferably used in order to update the gain param
`eter B. In one implementation, a normalized least-mean
`square (NLMS) adaptive algorithm may be utilized, which
`is computationally inexpensive, easy to implement, and
`offerS reasonably fast tracking capabilities. One possible
`real-valued time-domain one-tap NLMS algorithm can be
`written according to Equation 2 (7a) and (7b) as follows:
`y(i)=CE(i)-f(i)CE(i),
`(7a)
`0034)
`
`fi+1) = f(i)+ files (i)}(i),
`
`(7b)
`
`0035) where c(i) and c(i) are the values for the forward
`and backward-facing cardioid Signals at time instance i, u is
`an adaptation constant where 0<u<2, and C. is a Small
`constant where C.20.
`0.036
`Further information on first-order adaptive differ
`ential microphone arrays is provided in U.S. Pat. No.
`5,473,701 (Cezanne et al.), the teachings of which are
`incorporated herein by reference.
`0037) Second-Order Fullband ADMA
`0.038
`FIG. 5 shows a schematic diagram of a second
`order fullband ADMA 500 comprising two first-order
`ADMAS 502 and 504, each of which is an instance of
`first-order ADMA 100 of FIG. 1 having an inter-element
`delay T. After delaying the signal from first-order array 504
`by an additional time delay T at delay node 506, the
`difference between the two first-order Signals is generated at
`subtraction node 508 to generate the output signal y(t) of
`ADMA 500.
`0.039 When farfield conditions apply, the magnitude of
`the frequency and angular dependent response H(f, 0) of
`second-order ADMA 500 is given by Equation (8) as fol
`lows:
`
`Y(f,0). If T + (d. cos()/c)
`S(f) -Isin, longer
`
`0040 where Y(f, 0) is the spectrum of the ADMA
`output signal y(t). For the special case of Small spacing and
`delay, i.e., kd, kd-Ju and T, T-72?, Equation (8) may
`be written as Equation (9) as follows:
`
`array 500 consists of a monopole term, a dipole term (cos 0),
`and an additional quadrapole term (cos’0). Also, a quadratic
`rise as a function of frequency can be observed. This
`frequency dependence can be equalized by applying a
`Second-order lowpass filter. The directivity response can
`then be expressed by Equation (10) as follows:
`
`2
`T
`D: (0) = (T. d. 1." (1– T
`
`T
`
`cost).
`
`(10)
`
`0042 which is a direct result of the pattern multiplication
`theorem in electroacoustics.
`0043. One design goal of a second-order differential
`farfield array, such as ADMA 500 of FIG. 5, may be to use
`the array in a host-based environment without the need for
`any Special purpose hardware, e.g., additional external DSP
`interface boards. Therefore, two dipole elements may be
`utilized in order to form the Second-order array instead of
`four omnidirectional elements. As a consequence, T=0
`which means that one null angle is fixed to 0=90. In this
`case, although two independent nulls can be formed by the
`Second-order differential array, only one can be made adap
`tive if two dipole elements are used instead of four omni
`directional transducers. The implementation of Such a Sec
`ond-order ADMA may be based on first-order cardioid
`ADMA200 of FIG. 2, where d=d, T=T, B=|B, and d is
`the acoustical dipole length of the dipole transducer. Addi
`tionally, the lowpass filter is chosen to be a Second-order
`lowpass filter. FIG. 6 shows the directivity pattern of Such
`a Second-order back-to-back cardioid System. Those skilled
`in the art will understand that a second-order ADMA can
`also be implemented with three omnidirectional elements.
`0044) The transfer function H(f, 0) of a second-order
`ADMA formed of two dipole elements can be written
`according to Equation (11) as follows:
`
`- 'out? () -al-ju?t
`Hit?, 0) = c
`= -4e".
`sin(d) (sink d2 (1 o - B sink d2 (1 oil)
`
`(11)
`
`0045 with null angles given by Equations (12a) and
`(12b) as follows:
`0-90,
`0046)
`
`(12a)
`
`A3 - 1
`622 & arccos f3 + 1
`
`(12b)
`
`H2(f, 0) s (27tf)
`
`I
`
`(T + (dcosé) fc).
`
`(9)
`
`0041 Analogous to the case of first-order differential
`array 200 of FIG. 2, the amplitude response of second-order
`
`0047 where 0s Bs1 under the constraint 90’s B23
`180. FIG. 7 shows the directivity patterns that can be
`obtained by a second-order ADMA formed from two dipole
`elements for 0 values of 90°, 120°, 150, and 180°.
`0048. As shown in Elko, G. W., “Superdirectional Micro
`phone Arrays, Acoustic Signal Processing for Telecommu
`
`Page 16 of 19
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`

`

`US 2003/0031328A1
`
`Feb. 13, 2003
`
`nication, J. Benesty and S. L. Gay (eds.), pp. 181-236,
`Kluwer Academic Publishers, 2000, a second-order differ
`ential array is typically Superior to a first-order differential
`array in terms of directivity index, front-to-back ratio, and
`beamwidth.
`
`0049 Subband ADMA
`0050 FIG. 8 shows a schematic diagram of a Subband
`two-element ADMA 800 comprising two elements 802 and
`804. When elements 802 and 804 are omnidirectional ele
`ments, ADMA 800 is a first-order system; when elements
`802 and 804 are dipole elements, ADMA 800 is a second
`order system. ADMA 800 is analogous to fullband ADMA
`200 of FIG. 2, except that one additional degree of freedom
`is obtained for ADMA 800 by performing the adaptive
`algorithm independently in different frequency Subbands. In
`particular, delay nodes 806 and 808 of Subband ADMA 800
`are analogous to delay nodes 206 and 208 of fullband
`ADMA200; subtraction nodes 810, 812, and 816 of ADMA
`800 are analogous to subtraction nodes 210, 212, and 216 of
`ADMA 200; amplifier 814 of ADMA 800 is analogous to
`amplifier 214 of ADMA 200; and lowpass filter 818 of
`ADMA 800 is analogous to lowpass filter 218 of ADMA
`200, except that, for ADMA 800, the processing is indepen
`dent for different frequency Subbands.
`0051) To provide Subband processing, analysis filter
`banks 820 and 822 divide the electrical signals from ele
`ments 802 and 804, respectively, into two or more Subbands
`l, and amplifier814 can apply a different gain f(1,i) to each
`different Subband 1 in the backward-facing cardioid Signal
`c(li). In addition, Synthesis filter bank 824 combines the
`different Subband Signals y(1,i) generated at Summation node
`816 into a single fullband signal y(t), which is then lowpass
`filtered by filter 818 to generate the output signal y(t) of
`ADMA 800.
`0.052 The gain parameter B(li), where 1 denotes the
`Subband bin and i is the discrete time instance, is preferably
`updated by an adaptive algorithm that minimizes the output
`power of the array. This update therefore effectively adjusts
`the response of the backward-facing cardioid c(1,i) and can
`be written according to Equations (13a) and (13b) as fol
`lows;
`
`0053)
`
`, , Flyl ice (li)
`,
`,
`, ,
`,
`f3(l, i + 1) = f3(l, i) + ca?t, ill? ai) + a
`
`0054 where
`
`f3(l, i) = {
`
`B(l, i), 0 < B(l, i) s 1
`0,
`B(l, i) < 0
`1,
`B(l, i) > 1
`
`(13b)
`
`(14)
`
`0.055 and u is the update parameter and C. is a positive
`COnStant.
`
`0056 By using this algorithm, multiple spatially distinct
`noise Sources with non-overlapping spectra located in the
`back half plane of the ADMA can be tracked and attenuated
`Simultaneously.
`Implementation and Measurements
`0057)
`0058 PC-based real-time implementations running under
`the MicrosoftTM WindowsTM operating system were realized
`using a Standard Soundcard as the analog-to-digital con
`verter. For these implementations, the demonstrator's analog
`front-end comprised two omnidirectional elements of the
`type Panasonic WM-54B as well as two dipole elements of
`the type Panasonic WM-55D 103 and a microphone pream
`plifier offering 40-dB gain comprise the analog front-end.
`The implementations of the first-order ADMAS of FIGS. 2
`and 8 utilized the two omnidirectional elements and the
`preamplifier, while the implementation of the Second-order
`ADMA of FIG. 5 utilized the two dipole elements and the
`preamplifier.
`0059) The signals for the forward-facing cardioids c(t)
`and the backward-facing cardioids c(t) of the first-order
`ADMAs of FIGS. 2 and 8 were obtained by choosing the
`spacing d between the omnidirectional microphones Such
`that there is one sample delay between the corresponding
`delayed and undelayed microphone Signals. Similarly, the
`Signals for the forward- and backward-facing cardioids of
`the second-order ADMA of FIG. 5 were obtained by choos
`ing the Spacing d between the dipole microphones Such that
`there is one Sample delay between the corresponding
`delayed and undelayed microphone signals. Thus, for
`example, for a sampling frequency f. of 22050 Hz, the
`microphone spacing d=d=1.54 cm. For the Panasonic
`dipole elements, the acoustical dipole length d was found to
`be 0.8 cm.
`0060 FIGS. 9A and 9B depict the fullband ADMA
`directivity patterns for first-order and Second-order arrayS,
`respectively. These measurements were performed by plac
`ing a broadbandjammer (noise Source) at approximately 90
`with respect to the array's axis (i.e., 0 for the first-order
`array and 0 for the Second-order array) utilizing a standard
`directivity measurement technique. It can be seen that deep
`nulls covering wide frequency ranges are formed in the
`direction of the jammer.
`0061 FIGS. 10 and 11 show measured directivity of
`first- and second-order Subband implementations of ADMA
`800 of FIG. 8, respectively, for four simultaneously playing
`sinusoids. For the first-order Subband implementation, four
`loudspeakers Simultaneously played sinusoidal Signals while
`positioned in the back half plane of the arrays at 0 values
`of approximately 90°, 120°, 150, and 180°. For the second
`order Subband implementation, four loudspeakers Simulta
`neously played sinusoidal signals while positioned in the
`back half plane of the arrays at 0 values of approximately
`110, 120, 150, and 180. As can be seen, these measure
`ments are in close agreement with the Simulated patterns
`shown in FIGS. 4 and 7.
`0062. In order to combat the noise amplification proper
`ties inherent in differential arrays, the demonstrator included
`a noise reduction method as presented in Diethorn, E. J., “A
`Subband Noise-Reduction Method for Enhancing Speech in
`Telephony & Teleconferencing, IEEE Workshop On Appli
`cations of Signal Processing to Audio and Acoustics,
`Mohonk, USA, 1997, the teachings of which are incorpo
`rated herein by reference.
`
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`Feb. 13, 2003
`
`0063 Conclusions
`0064.
`First- and second-order ADMAS which are able to
`adaptively track and attenuate a possibly moving noise
`Source located in the back half plane of the arrays have been
`presented. It has been shown that, by performing the calcu
`lations in Subbands, even multiple Spatially distinct noise
`Sources with non-overlapping spectra can be tracked and
`attenuated Simultaneously. The real-time implementation
`presents the dynamic performance of the ADMAS in real
`acoustic environments and shows the practicability of using
`these arrays as acoustic front-ends for a variety of applica
`tions including telephony, automatic Speech recognition, and
`teleconferencing.
`0065. The present invention may be implemented as
`circuit-based processes, including possible implementation
`on a single integrated circuit. AS would be apparent to one
`skilled in the art, various functions of circuit elements may
`also be implemented as processing Steps in a Software
`program. Such Software may be employed in, for example,
`a digital Signal processor, micro-controller, or general-pur
`pose computer.
`0.066 The present invention can be embodied in the form
`of methods and apparatuses for practicing those methods.
`The present invention can also be embodied in the form of
`program code embodied in tangible media, Such as floppy
`diskettes, CD-ROMs, hard drives, or any other machine
`readable Storage medium, wherein, when the program code
`is loaded into and executed by a machine, Such as a
`computer, the machine becomes an apparatus for practicing
`the invention. The present invention can also be embodied in
`the form of program code, for example, whether Stored in a
`Storage medium, loaded into and/or executed by a machine,
`or transmitted over Some transmission medium or carrier,
`Such as over electrical wiring or cabling, through fiber
`optics, or via electromagnetic radiation, wherein, when the
`program code is loaded into and executed by a machine,
`Such as a computer, the machine becomes an apparatus for
`practicing the invention. When implemented on a general
`purpose processor, the program code Segments combine
`with the processor to provide a unique device that operates
`analogously to specific logic circuits.
`0067. The use of figure reference labels in the claims is
`intended to identify one or more possible embodiments of
`the claimed Subject matter in order to facilitate the interpre
`tation of the claims. Such labeling is not to be construed as
`necessarily limiting the Scope of those claims to the embodi
`ments shown in the corresponding figures.
`0068. It will be further understood that various changes in
`the details, materials, and arrangements of the parts which
`have been described and illustrated in order to explain the
`nature of this invention may be made by those skilled in the
`art without departing from the Scope of the invention as
`expressed in the following claims.
`What is claimed is:
`1. A Second-order adaptive differential microphone array
`(ADMA), comprising:
`(a) a first first-order element (e.g., 802 of FIG. 8) con
`figured to convert a received audio Signal into a first
`electrical Signal;
`(b) a second first-order element (e.g., 804 of FIG. 8)
`configured to convert the received audio signal into a
`Second electrical Signal;
`
`(c) a first delay node (e.g., 806 of FIG. 8) configured to
`delay the first electrical signal from the first first-order
`element to generate a delayed first electrical Signal;
`(d) a second delay node (e.g., 808 of FIG. 8) configured
`to delay the Second electrical Signal from the Second
`first-order element to generate a delayed Second elec
`trical Signal;
`(e) a first subtraction node (e.g.,810 of FIG.8) configured
`to generate a forward-facing cardioid Signal based on a
`difference between the first electrical Signal and the
`delayed Second electrical Signal;
`(f) a second subtraction node (e.g., 812 of FIG. 8)
`configured to generate a backward-facing cardioid Sig
`nal based on a difference between the Second electrical
`Signal and the delayed first electrical Signal;
`(g) an amplifier (e.g., 814 of FIG. 8) configured to
`amplify the backward-facing cardioid Signal by a is
`gain parameter to generate an amplified backward
`facing cardioid Signal; and
`(h) a third subtraction node (e.g., 816 of FIG. 8) config
`ured to generate a difference Signal based on a differ
`ence between the forward-facing cardioid signal and
`the amplified backward-facing cardioid Signal.
`2. The invention of claim 1, further comprising a lowpass
`filter (e.g., 818 of FIG. 8) configured to filter the difference
`Signal from the third Subtraction node to generate an output
`signal for the second-order ADMA.
`3. The invention of claim 1, wherein the first and second
`first-order elements are two dipole elements.
`4. The invention of claim 1, wherein each of the first and
`Second first-order elements is a first-order differential micro
`phone array (e.g., 100 of FIG. 1).
`5. The invention of claim 4, wherein each first-order
`differential microphone array comprises:
`(1) a first omnidirectional element (e.g., 104 of FIG. 1)
`configured to convert the received audio signal into an
`electrical signal;
`(2) a second omnidirectional element (e.g., 106 of FIG. 1)
`configured to convert the received audio signal into an
`electrical signal;
`(3) a delay node (e.g., 108 of FIG. 1) configured to delay
`the electrical Signal from the Second omnidirectional
`element to generate a delayed electrical Signal; and
`(4) a first subtraction node (e.g., 110 of FIG. 1) configured
`to generate the corresponding electrical Signal for the
`first-order element based on a difference between the
`electrical Signal from the first omnidirectional element
`and the delayed electrical Signal from the delay node.
`6. The invention of claim 1, wherein the gain parameter
`for the amplifier is configured to be adaptively adjusted to
`move a null located in a back half plane of the Second-order
`ADMA to track a moving noise Source.
`7. The invention of claim 6, wherein the gain parameter
`is configured to be adaptively adjusted to minimize output
`power from the second-order ADMA.
`8. The invention of claim 1, further comprising:
`(i) a first analysis filter bank (e.g., 820 of FIG. 8)
`configured to divide the first electrical signal from the
`
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`Feb. 13, 2003
`
`first first-order element into two or more Subband electrical
`Signals corresponding to two or more different frequency
`Subbands;
`(j) a second analysis filter bank (e.g., 822 of FIG. 8)
`configured to divide the Second electrical signal from
`the Second first-order element