as) United States
`a2) Patent Application Publication 0) Pub. No.: US 2003/0031328 Al
`
` Elkoet al. (43) Pub. Date: Feb. 13, 2003
`
`
`US 20030031328A1
`
`(54) SECOND-ORDER ADAPTIVE
`DIFFERENTIAL MICROPHONE ARRAY
`
`(57)
`
`ABSTRACT
`
`76
`(76)
`
`:
`Inventors: Gary W. Elko, Summit, NJ (US);
`Heinz Teutsch, Nurnberg (DE)
`Correspondence Address:
`teeRSeNeieeSOCIATES PC
`SUITE 715
`PHILADELPHIA,PA 19102 (US)
`
`(21) Appl. No.:
`PPh
`NO-
`4.
`22)
`Filed:
`File
`(22)
`
`09/999,298
`>
`Oct. 30, 2001
`©
`,
`Related U.S. Application Data
`PP
`(60) Provisional application No. 60/306,271, filed on Jul.
`18, 2001.
`
`Publication Classification
`
`CSL) Tt C0 eecccccceeecccssssssssnnsecceesnnnnnnseecsesnnnees HO4R 3/00
`(52) US. C0.
`eeeeccecssesseesseessssnccaseesessnesseenseensceneeses 381/92
`
`A second-order adaptive differential microphone array
`(ADMA)hastwofirst-order elements (e.g., 802 and 804 of
`FIG.8), each configured to convert a received audio signal
`into anelectrical signal. The ADMAalsohas(i) two delay
`nodes(e.g., 806 and 808) configured to delay the electrical
`.
`Pe
`.
`signals from thefirst-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 ADMAalso has(i) an amplifier (e.g.,
`814) configured to amplify the backward-facing cardioid
`signal by a gain 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 movea null in the
`back half plane of the ADMAto track a moving noise
`source. In a subband implementation,a different gain param-
`eter can be adaptively adjusted to movea different null in the
`back half plane to track a different moving noise source for
`each different frequency subband.
`
` IOD2
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`Patent Application Publication
`
`Feb. 13, 2003 Sheet 1 of 12
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`Feb. 13, 2003 Sheet 3 of 12
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`US 2003/0031328 Al
`
`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.
`
`SUMMARYOF THE INVENTION
`
`[0006] Embodiments of the present invention are directed
`to adaptive differential microphone arrays (ADMAs)thatare
`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.
`
`invention is a
`the present
`In one embodiment,
`[0007]
`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 secondfirst-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 thefirst electrical
`signal from thefirst 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 secondelectrical signal from
`the secondfirst-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 secondelectrical 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)
`
`14
`
`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.
`
`In another embodiment, the present invention is an
`[0008]
`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 secondfirst-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 ofa first-order
`fullband ADMAbased on an adaptive back-to-back cardioid
`system;
`
`[0012] FIG. 3 showsthe directivity pattern of the first-
`order ADMAof FIG.2;
`
`[0013] FIG. 4 shows directivity patterns that can be
`obtained by the first-order ADMAfor 8,
`, values of 90°,
`120°, 150°, and 180°;
`
`[0014] FIG. 5 shows a schematic diagram of a second-
`order fullband ADMA;
`
`[0015] FIG. 6 showsthe 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 ADMAformed from two dipole
`elements for @,, values of 90°, 120°, 150°, and 180°;
`
`[0017] FIG. 8 shows a schematic diagram of a subband
`two-element ADMA;
`
`14
`
`

`

`US 2003/0031328 Al
`
`Feb. 13, 2003
`
`the fullband ADMA
`[0018] FIGS. 9A and 9B depict
`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
`ADMAof 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
`ADMAoutput 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:
`
`Equation (3) would need to involve the ability to generate
`any time delay T between the two microphones. As such,
`this approachis 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 responseis to utilize an adap-
`tive back-to-back cardioid system
`
`[0026] FIG. 2 shows a schematic diagram ofa 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 B 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 lowpassfiltered at filter 218
`to generate the ADMAoutputsignal y,,,(t).
`
`[0027] FIG. 3 showsthe directivity pattern of the first-
`order back-to-back cardioid system of ADMA 200. ADMA
`200 can be used to adaptively adjust the response of the
`backward facing cardioid in orderto 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 microphonesignals.
`
`
`[0028]
`Thetransfer function H,(f, 8) of first-order ADMA
`_ deine T+ cons
`200 can be written according to Equation (4) as follows:
`
`Lif. 01=|REO |= 1 seer )
`
`
`
`
`Yourf. 9
`[0022] where Y,(f, ) is the spectrum of the ADMA
`Sf)
`output signal y(t), S(f) is the spectrum of the signal source,
`_,da(1 -—cosé)
`ne + cos) _
`= jer(si
`rink
`k is the sound vector, |k|=k=22f/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, 8)
`,
`the ADMAoutput 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 signals(t).
`
`Af, =
`
`(4)
`
`[0029] where Y,,,(f, 9) is the spectrum of the ADMA
`output signal y,,,,(t).
`
`[0023] For small element spacing and short inter-element
`delay (kd<<z and T<<%f, Equation (1) can be approximated
`according to Equation (2) as follows:
`lH, , ®)|~2xf[T+(d cos 0)/c].
`
`(2)
`
`the right side of Equation (2)
`[0024] As can be seen,
`consists of a monopole term and a dipole term (cos@). 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:
`
`[0030] The single independentnull angle 6, of first-order
`ADMA200, which,for the present discussion, is assumed to
`be placed into the back half plane of
`the
`array
`(90°=8, =180°) , can be found by setting Equation (4) to
`zero and solving for 6=6,, which yields Equation (5) as
`follows:
`
`a=
`2
`B-1\
`kad
`,= accvos{are ais|}
`
`(5)
`
`[0031] which for small spacing and short delay can be
`approximated according to Equation (6) as follows:
`
`(3)
`
`6)
`
`1 B
`
`+
`
`@, = arccosf —
`
`Di (0) =
`
`T
`T
`Frat ('- Fea)
`
`cosé.
`
`[0025] Since the location of the source 102 is not typically
`known, an implementation ofa first-order ADMAbased on
`
`15
`
`15
`
`

`

`US 2003/0031328 Al
`
`Feb. 13, 2003
`
`[0032] where 06 =1 under the constraint (90°60, =180°).
`FIG.4 showsthe directivity patterns that can be obtained by
`first-order ADMA200 for 6, values of 90°, 120°, 150°, and
`180°.
`
`In a time-varying environment, an adaptive algo-
`[0033]
`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 NLMSalgorithm can be
`written according to Equation 2 (7a) and (7b) as follows:
`y@Q=er)-BOcRO;
`(7a)
`
`[0034]
`
`a
`H
`ye ag
`AG+l=pO+ TTlesoO
`
`(7b)
`
`[0035] where c,(i) and c,(i) are the valuesfor the forward-
`and backward-facing cardioid signals at time instance1, 4 1s
`an adaptation constant where O<u<2, and a@ is a small
`constant where a>0.
`
`[0036] 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
`
`[0038] 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 twofirst-order signals is generated at
`subtraction node 508 to generate the output signal y(t) of
`ADMA500.
`
`[0039] When farfield conditions apply, the magnitude of
`the frequency and angular dependent response H.(f, 8) of
`second-order ADMA 500 is given by Equation (8) as fol-
`lows:
`
`array 500 consists of a monopole term,a dipole term (cos8),
`and an additional quadrapole term (cos?6). 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,
`T,
`D2(0) = I| Cowan + (1 - Ferayeho)
`vel
`
`(10)
`
`[0042] whichis 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 meansthat one null angle is fixed to 6,,=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=6.,, and d, is
`the acoustical dipole length of the dipole transducer. Addi-
`tionally, the lowpassfilter is chosen to be a second-order
`lowpassfilter. 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, 8) of a second-order
`ADMA formed of two dipole elements can be written
`according to Equation (11) as follows:
`
`
`Hacf 6) = Let) = _gorinste
`Sf)
`
`sin((sine dy(1 veel) ~ pysink dy(1 —
`
`a)
`
` 2
`[0045] with null angles given by Equations (12a) and
`8)
`YF, 8)
`w Af[size cos
`
`(12b) as follows:
`[ef O1=|So
`vel
`654=90°,
`(12a)
`
`is the spectrum of the ADMA
`[0040] where Y.(f, 8)
`output signaly,(t). For the special case of small spacing and
`delay, ie., kd,, kd.<<z and T,, T.<<%f, Equation (8) may
`be written as Equation (9) as follows:
`
`[0046]
`
`622 = arccos
`
`
`fo -1
`Bot?
`
`(12b)
`
`
`
`An(f, 8)
`
`
`
`:
`= (nf) | [Ty + (decos0) fe].
`vel
`
`9)
`
`[0047] where 0£6,=1 under the constraint 90°=8,,.23
`180°. FIG. 7 shows the directivity patterns that can be
`obtained by a second-order ADMAformed from two dipole
`elements for @,, values of 90°, 120°, 150°, and 180°.
`
`[0041] Analogous to the case of first-order differential
`array 200 of FIG.2, the amplitude response of second-order
`
`[0048] As shownin Elko, G. W., “Superdirectional Micro-
`phone Arrays,’Acoustic Signal Processing for Telecommu-
`
`16
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`US 2003/0031328 Al
`
`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 ADMA800
`are analogous to delay nodes 206 and 208 of fullband
`ADMaA200; subtraction nodes 810, 812, and 816 of ADMA
`800 are analogousto subtraction nodes 210, 212, and 216 of
`ADMA200; amplifier 814 of ADMA800 is analogous to
`amplifier 214 of ADMA 200; and lowpass filter 818 of
`ADMA800is 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
`1, and amplifier 814 can apply a different gain B(1,i) to each
`different subband | in the backward-facing cardioid signal
`c,(i). In addition, synthesis filter bank 824 combines the
`different subbandsignals 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
`ADMA800.
`
`[0056] By using this algorithm, multiple spatially distinct
`noise sources with non-overlapping spectra located in the
`back half plane of the ADMAcan betracked and attenuated
`simultaneously.
`[0057]
`Implementation and Measurements
`[0058] PC-based real-time implementations running under
`the Microsoft™ Windows™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 aswell as two dipole elements of
`the type Panasonic WM-55D103 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
`ADMAofFIG.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 ADMAof FIG.5 were obtained by choos-
`ing the spacing d, between the dipole microphones suchthat
`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.
`
`, where 1 denotes the
`[0052] The gain parameter B(1,i)
`subband bin and1 is the discrete time instance, is preferably
`updated by an adaptive algorithm that minimizes the output
`powerof 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;
`
`y(bi=co(l)-BG idea),
`
`(13a)
`
`[0053]
`
`the fullband ADMA
`[0060] FIGS. 9A and 9B depict
`directivity patterns for first-order and second-order arrays,
`respectively. These measurements were performed byplac-
`ing a broadband jammer(noise source) at approximately 90°
`with respect to the array’s axis (i.e., 6, for the first-order
`array and 6.,, for the second-orderarray)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-
`Bu,i+ lL = pb d+
`lIca(Z, dP? +4
`order subband implementation, four loudspeakers simulta-
`neously played sinusoidal signals while positioned in the
`back half plane of the arrays at 8,, 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.
`
`Hy, Deg, i)HYGDealt (13b)
`
`[0054] where
`
`BLD=5
`
`BU), 9< BLD <1
`0,
`BU, i) <0
`1,
`BL D>1
`
`(14)
`
`[0055]
`constant.
`
`and uw is the update parameter and @ is a positive
`
`In order to combat the noise amplification proper-
`[0062]
`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,JEEE 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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`

`US 2003/0031328 Al
`
`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 shownthat, 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 showsthe practicability of using
`these arrays as acoustic front-ends for a variety of applica-
`tions including telephony, automatic speech recognition, and
`teleconferencing.
`invention may be implemented as
`[0065] The present
`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.
`
`[0066] 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 orcarrier,
`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.
`Whatis claimedis:
`
`1. Asecond-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 thefirst 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 secondelectrical
`signal and the delayedfirst 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 ofthefirst and
`second first-order elementsis a first-order differential micro-
`phonearray (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) asecond 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
`ADMAto 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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`

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`US 2003/0031328 Al
`
`Feb. 13, 2003
`
`first first-order element into two or more subbandelectrical
`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 into two or more sub-
`
`band electrical signals corresponding to the two or
`more different frequency subbands; and
`
`(k) a synthesis filter bank (e.g., 824 of FIG. 8) configured
`to combine two or more different subband difference
`
`