111111
`
`1111111111111111111111111111111111111111111111111111111111111
`US007155019B2
`
`c12) United States Patent
`Hou
`
`(10) Patent No.:
`(45) Date of Patent:
`
`US 7,155,019 B2
`Dec. 26, 2006
`
`(54) ADAPTIVE MICROPHONE MATCHING IN
`MULTI-MICROPHONE DIRECTIONAL
`SYSTEM
`
`(75)
`
`Inventor: Zezhang Hou, Cupertino, CA (US)
`
`(73) Assignee: Apherma Corporation, Sunnyvale, CA
`(US)
`
`( *) Notice:
`
`Subject to any disclaimer, the term of this
`patent is extended or adjusted under 35
`U.S.C. 154(b) by 1008 days.
`
`(21) Appl. No.: 09/808,694
`
`(22) Filed:
`
`Mar. 14, 2001
`
`(65)
`
`Prior Publication Data
`US 2002/0034310 AI Mar. 21, 2002
`
`Related U.S. Application Data
`(60) Provisional application No. 60/189,282, filed on Mar.
`14, 2000.
`
`(51)
`
`(56)
`
`Int. Cl.
`(2006.01)
`H04R 3100
`(52) U.S. Cl. ......................................... 381192; 381/313
`(58) Field of Classification Search .................. 381/92,
`381/94.1, 94.9, 313, 71.7, 71.6, 71.11
`See application file for complete search history.
`References Cited
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`
`FOREIGN PATENT DOCUMENTS
`569216 A1
`1111993
`9/1994
`06 269085
`8/1998
`0856833 A2
`8/1999
`982971 A2
`63-002500
`111988
`
`EP
`EP
`EP
`EP
`JP
`
`(Continued)
`OTHER PUBLICATIONS
`Glover, "A review of Cardioid Type Unidirectional Microphones",
`(Jan. 1940) J.A.S.A., vol. 11, pp. 296-302.
`(Continued)
`Primary Examiner-Vivian Chin
`Assistant Examiner-Justin Michalski
`(74) Attorney, Agent, or Firm-Beyer Weaver & Thomas
`LLP
`
`(57)
`
`ABSTRACT
`
`Improved approaches to matching sensitivities of micro-
`phones in multi-microphone directional processing systems.
`These approaches operate to adaptively match microphone
`sensitivities so that directional noise suppression is robust.
`As a result, microphone sensitivities remain matched not
`only over time but also while in actual use. These
`approaches are particularly useful for hearing aid applica-
`tions in which directional noise suppression is important.
`
`14 Claims, 14 Drawing Sheets
`
`{500
`
`MIC1 )-----.---------.f---~
`SUBTRACTION
`UNIT
`
`502
`
`OUTPUT
`SIGNAL
`'------,-----'
`
`506
`
`514
`
`1
`
`Sony v. Jawbone
`
`U.S. Patent No. 8,321,213
`
`Sony Ex. 1008
`
`

`

`US 7,155,019 B2
`Page 2
`
`JP
`wo
`
`FOREIGN PATENT DOCUMENTS
`11-220796
`8/1999
`wo 99/03091
`111999
`OTHER PUBLICATIONS
`Killion et al., "The case of the missing dots: AI and SNR loss",
`(May 1998) The Hearing Journal, vol. 51, No.5, pp. 1-6.
`Gravel et a!., "Children's Speech Recognition in Noise Using
`Onmi-Directional and Dual-Microphone Hearing Aid Technology",
`Ear & Hearing, Feb. 1999, pp. 1-11.
`Ricketts et a!., "Comparison of Performance across Three Direc-
`tional Hearing Aids", J AmAcadAudiol vol. 10, 1999, pp. 180-189.
`Buerkli-Halevy, "The directional microphone advantage", Hearing
`Instruments, vol. 38, No. 8, 1987.
`Preves, "Directional Microphone Use in ITE Hearing Instruments",
`The Hearing Rev., Jul. 1997, pp. 21-27.
`Greenberg et a!., "Evaluation of an adaptive beamforming method
`for hearing aids", J. Acoust. Soc. Am. 91 (3), Mar. 1992, pp.
`1662-1676.
`Agnew, "How multi-microphone arrays can improve directional-
`ity", The Hearing Journal, vol. 50, No. 8, Aug. 1997, pp. 34-46.
`Roberts et a!., "Measurement and Intelligibility Optimization of
`Directional Microphones for Use in Hearing Aid Devices", 103'd
`Convention of Audio Engineering Society (AES), Sep. 1997, 13 pp.
`Desloge et a!., "Microphone-Array Hearing Aids with Binaural
`Output-Part I: Fixed-Processing Systems", IEEE Trans. on Speech
`& Audio Proc., vol. 5, No. 6, Nov. 1997, pp. 529-542.
`Welker et al., "Microphone-Array Hearing Aids with Binaural
`Output-Part II: A Two-Microphone Adaptive System", IEEE
`Trans. On Speech & Audio Proc., vol. 5, No. 6, Nov. 1997, pp.
`543-551.
`Greenberg, "Modified LMS Algorithms for Speech Processing with
`an Adaptive Noise Canceller", IEEE Trans. On Speech & Audio
`Proc., vol. 6, No. 4, Jul. 1998, pp. 338-351.
`Van Tasell, "New DSP instrument designed to maximize binaural
`benefits", , The Hearing Journal, vol. 51, No. 4, Apr. 1998, pp.
`40-49.
`
`Edwards et a!., "New digital processor for hearing loss compensa-
`tion is based on auditory system", The Hearing Journal, vol. 51, No.
`8, Aug. 1998.
`Stadler eta!., "On the potential of fixed arrays for hearing aids", J.
`Acoust. Soc. Am. 94 (3), Pt. 1, Sep. 1993, pp. 1332-1342.
`Zurek et a!., "Prospects and Limitations of Microphone-Array
`Hearing Aids", World Scientific, Aug. 1995, pp. 233-244.
`Killion et al., "Real-world performance of an ITE directional
`microphone", The Hearing Journal., vol. 51, No.4, Apr. 1998, pp.
`1-6.
`Killion, "SNR Loss: 'I Can Hear What People Say, But I Can't
`Understand Them'", The Hearing Rev., vol. 4, No. 12, Dec. 1997.
`Edwards et a!., "Signal-processing algorithms for a new software-
`based, digital hearing device", The Hearing Journal, vol. 51, No.9,
`Sep. 1998.
`Killion, "The SIN report: circuits haven't solved the hearing-in-
`noise problem", The Hearing Journal., vol. 50, No. 10, Oct. 1997,
`pp. 28-31.
`Cadalli eta!., "Wideband Maximum Likelihood Direction Finding
`and Signal Parameter Estimation by Using the Tree-Structured EM
`Algorithm", IEEE Trans. On Signal Proc., vol. 47, No. 1, Jan. 1999,
`pp. 201-206.
`Souza et al., "QuantifYing the Contribution of Audibility to Rec-
`ognition of compression-Amplified Speech", Ear & Hearing, vol.
`20, No. 1, Feb. 1999, pp. 12-20.
`microZOOM™, Marketing pamphlet, Phonak, 2 pgs.
`Senso: The Single Most Advanced Hearing Instrument Ever Devel-
`oped, Marketing pamphlet, Widex, 2 pgs.
`"HJReport", The Hearing Journal, vol. 50, No. 8, Aug. 1997, pp.
`7-8.
`International Search Report, EPO/ISA, re PCT/USOl/08256, dated
`Aug. 2, 2002.
`* cited by examiner
`
`2
`
`

`

`U.S. Patent
`
`Dec. 26, 2006
`
`Sheet 1 of 14
`
`US 7,155,019 B2
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`

`

`U.S. Patent
`
`Dec. 26, 2006
`
`Sheet 2 of 14
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`US 7,155,019 B2
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`

`U.S. Patent
`
`Dec. 26, 2006
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`US 7,155,019 B2
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`Dec. 26, 2006
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`US 7,155,019 B2
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`

`U.S. Patent
`
`Dec. 26, 2006
`
`Sheet 12 of 14
`
`US 7,155,019 B2
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`

`U.S. Patent
`
`Dec. 26, 2006
`
`Sheet 13 of 14
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`US 7,155,019 B2
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`U.S. Patent
`
`Dec. 26, 2006
`
`Sheet 14 of 14
`
`US 7,155,019 B2
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`

`

`US 7,155,019 B2
`
`1
`ADAPTIVE MICROPHONE MATCHING IN
`MULTI-MICROPHONE DIRECTIONAL
`SYSTEM
`
`CROSS-REFERENCE TO RELATED
`APPLICATIONS
`
`This application claims the benefit of U.S. Provisional
`Application No. 60/189,282, filed Mar. 14, 2000, and
`entitled "METHODS FOR ADAPTIVE MICROPHONE
`MATCHING IN MULTI-MICROPHONE DIRECTIONAL
`SYSTEM", the contents of which is hereby incorporated by
`reference. This application is also related to U.S. application
`Ser. No. 09/788,271, filed Feb. 16, 2001, and entitled
`"NULL ADAPTATION
`IN MULTI-MICROPHONE
`DIRECTIONAL SYSTEM", the contents of which is hereby
`incorporated by reference.
`
`BACKGROUND OF THE INVENTION
`
`2
`good match of sensitivity between microphones is very
`important to good directionality.
`the
`Conventionally, manufacturers manually match
`microphone for their multi-microphone directional process-
`ing systems. While manual matching of the microphones
`provides for improved directionality, the operational or
`manufacturing costs are substantial. Besides cost-effective-
`ness, manual matching has other problems that compromise
`manual matching. One problem is that microphone sensi-
`10 tivity tends to drift over time. Hence, once matched micro-
`phones can become mismatched over time. Another problem
`is that the sensitivity difference can depend on how the
`multi-microphone directional processing systems is used.
`For example, in hearing aid applications, a microphone pair
`15 that is perfectly matched as determined by measurements at
`manufacture may become mismatched when the hearing aid
`is put on a patient. This can occur because at manufacture
`the microphones are measured in a field where sound
`pressure level is the same everywhere (free field), while in
`20 real life situation (in situ) sound pressure may not distribute
`uniformly at microphone locations. Hence, when such pres-
`sure differences result, the microphones are in effect mis-
`matched. In another word, because the microphones are
`matched in free field, not in situ, the microphones can
`25 actually be mismatched when used in real life, which
`degrades directionality.
`Some manufacturers have used a fixed filter in their
`designs of multi-microphone directional processing sys-
`tems. FIG. 4 illustrates a conventional two-microphone
`30 directional processing system 400 having a first microphone
`402, a second microphone 404, a delay 406, a fixed filter
`408, and a subtraction unit 410. The fixed filter 408 can
`serve to compensate for a mismatch in microphone sensi-
`tivity. The fixed filter approach is more cost-effective that the
`35 manual matching. However, the other problems (e.g., drift
`over time and in-situ mismatch) of manual matching are still
`present with the fixed filter approach.
`Thus, there is a need for improved approaches to match
`sensitivities of microphones in multi-microphone directional
`40 processing systems.
`
`1. Field of the Invention
`The present invention relates to multi-microphone sound
`pick-up systems and, more particularly, to matching micro-
`phone sensitivity in multi-microphone sound pick-up sys-
`tems.
`2. Description of the Related Art
`Suppressing interfering noise is still a major challenge for
`most communication devices involving a sound pick up
`system such as a microphone or a multi-microphone array.
`The multi-microphone array can selectively enhance sounds
`coming from certain directions while suppressing interfer-
`ence coming from other directions.
`FIG. 1 shows a typical direction processing system in a
`two-microphone hearing aid. The two microphones pick-up
`sounds and convert them into electronic or digital signals.
`The output signal form the second microphone is delayed
`and subtracted from the output signal of the first micro-
`phone. The result is a signal with interference from certain
`directions being suppressed. In other words, the output
`signal is dependent on which directions the input signals
`come from. Therefore, the system is directional. The physi-
`cal distance between the two microphones and the delay are
`two variables that control the characteristics of the direc-
`tionality. For hearing aid applications, the physical distance
`is limited by the physical dimension of the hearing aid. The
`delay can be set in a delta-sigma analog-to-digital converter
`(AID) or by use of an all-pass filter.
`The sensitivity of the microphones of the sound pick up
`system must be matched in order to achieve good direction-
`ality. When the sensitivities of the microphones are not
`properly matched, then the directionality is substantially
`degraded and thus the ability to suppress interference com-
`ing from a particular direction is poor. FIGS. 2(a), 2(b), 2(c)
`and 2(d) illustrate representative polar patterns for micro-
`phone sensitivity discrepancies of 0, 1, 2, and 3 dB, respec-
`tively. Note that the representative polar pattern shown in
`FIG. 2(a) is the desired polar pattern which offers maxi-
`mized directionality. The representative polar patterns
`shown in FIGS. 2(b)-2(d) are distorted polar patterns that
`respectively illustrate directionality becoming progressively 60
`worse as the sensitivity discrepancy increases respectively
`from 1, 2 and 3 dB. FIGS. 3(a), 3(b), 3(c) and 3(d) illustrate
`representative spectrum response for microphone sensitivity
`discrepancies of 0, 1, 2, and 3 dB, respectively, with
`reference to a 1 kHz pure tone in white noise. Note that the 65
`Signal-to-Noise Ratio of the spectrum shown in FIGS.
`3(a)-3(d) is 14, 11, 9 and 7 dB, respectively. Accordingly, a
`
`SUMMARY OF THE INVENTION
`
`Broadly speaking, the invention relates to improved
`45 approaches to matching sensitivities of microphones in
`multi-microphone directional processing systems. These
`approaches operate to adaptively match microphone sensi-
`tivities so that directional noise suppression is robust. As a
`result, microphone sensitivities remain matched not only
`50 over time but also while in actual use. These approaches are
`particularly useful for hearing aid applications in which
`directional noise suppression is important.
`The invention can be implemented in numerous ways
`including as a method, system, apparatus, device, and com-
`55 puter readable medium. Several embodiments of the inven-
`tion are discussed below.
`As an adaptive directional sound processing system, one
`embodiment of the invention includes at least: at least first
`and second microphones spaced apart by a distance, the first
`microphones producing a first electronic sound signal and
`the second microphone producing a second electronic sound
`signal; means for processing the second electronic sound
`signal to adaptively produce a compensation scaling amount
`that compensates for sensitivity differences between the first
`and second microphones; a scaling circuit operatively con-
`nected to the means for scaling and the second microphone,
`the scaling circuit operates to scale the second electronic
`
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`

`US 7,155,019 B2
`
`4
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`The invention will be readily understood by the following
`detailed description in conjunction with the accompanying
`drawings, wherein like reference numerals designate like
`structural elements, and in which:
`FIG. 1 shows a typical direction processing system in a
`two-microphone hearing aid;
`FIGS. 2( a )-2( d) illustrate representative polar patterns for
`10 various microphone sensitivity discrepancies;
`FIGS. 3(a)-3(d) illustrate representative Signal-to-Noise
`Ratio spectrums respectively corresponding to the represen-
`tative polar patterns shown in FIGS. 2(a)-2(d);
`FIG. 4 illustrates a conventional two-microphone direc-
`tional processing system;
`FIG. 5 is a block diagram of a two-microphone directional
`processing system according to one embodiment of the
`invention;
`FIG. 6 is a block diagram of a two-microphone directional
`processing system according to another embodiment of the
`invention;
`FIG. 7 is a block diagram of a minimum estimate unit
`according to one embodiment of the invention;
`FIG. 8 is a block diagram of a minimum estimate unit
`according to another embodiment of the invention;
`FIG. 9 is a block diagram of a multi-microphone direc-
`tional processing system that operates to perform multi-band
`adaptive compensation for microphone mismatch;
`FIG. 10 is a block diagram of a multi-microphone direc-
`tional processing system according to one embodiment of
`the invention; and
`FIG. 11 is a block diagram of a multi-microphone direc-
`tional processing system according to another embodiment
`35 of the invention.
`
`3
`sound signal in accordance with the compensation scaling
`amount; and a subtraction circuit operatively connected to
`the scaling circuit and the first microphone, the subtraction
`circuit producing an output difference signal by subtracting
`the scaled second electronic sound signal from the first
`electronic sound signal.
`As an adaptive directional sound processing system,
`another embodiment of the invention includes at least: at
`least first and second microphones spaced apart by a pre-
`determined distance, the first microphones producing a first
`electronic sound signal and the second microphone produc-
`ing a second electronic sound signal; a first minimum
`estimate circuit operatively coupled to the first microphone,
`the first minimum estimate circuit produces a first minimum
`estimate for the first electronic sound signal from the first 15
`microphone; a second minimum estimate circuit operatively
`coupled to the second microphone, the second minimum
`estimate circuit produces a second minimum estimate for the
`second electronic sound signal from the second microphone;
`a divide circuit operatively connected to the first and second 20
`minimum estimate circuits, the divide circuit operates to
`produce a scaling signal from the first and second minimum
`estimates; a multiply circuit operatively connected to the
`divide circuit and the second microphone, the multiply
`circuit operates to multiply the second electronic sound 25
`signal by the scaling signal to produce a scaled second
`electronic sound signal; and a subtraction circuit operatively
`connected to the multiply circuit and the first microphone,
`the subtraction circuit producing an output difference signal
`by subtracting the scaled second electronic sound signal 30
`from the first electronic sound signal.
`As a hearing aid device having an adaptive directional
`sound processing, one embodiment of the invention includes
`at least: at least first and second microphones spaced apart
`by a distance, the first microphones producing a first elec-
`tronic sound signal and the second microphone producing a
`second electronic sound signal; sensitivity difference detec-
`tion circuitry operatively connected to the first and second
`microphones, the sensitivity difference detection circuitry 40
`adaptively produces a compensation scaling amount corre-
`sponding to sensitivity differences between the first and
`second microphones; a scaling circuit operatively connected
`to the sensitivity difference detection circuitry and the
`second microphone, the scaling circuit operates to scale the 45
`second electronic sound signal in accordance with the com-
`pensation scaling amount; and a subtraction circuit opera-
`tively connected to the scaling circuit and the first micro-
`phone, the subtraction circuit producing an output difference
`signal by subtracting the scaled second electronic sound 50
`signal from the first electronic sound signal.
`As a method for adaptively measuring and compensating
`for acoustical differences between sound signals picked up
`by microphones, one embodiment of the invention includes
`at least the acts of: receiving first and second electronic
`sound signals from first and second microphones, respec-
`tively; determining a compensation scaling amount that
`compensates for acoustic differences with respect to the first
`and second microphones; scaling the second electronic
`sound signal in accordance with the compensation scaling
`amount; and producing a differential electronic sound signal
`by subtracting the scaled second electronic sound signal
`from the first electronic sound signal.
`Other aspects and advantages of the invention will
`become apparent from the following detailed description 65
`taken in conjunction with the accompanying drawings which
`illustrate, by way of example, the principles of the invention.
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`The invention relates to improved approaches to matching
`sensitivities of microphones in multi-microphone directional
`processing systems. These approaches operate to adaptively
`match microphone sensitivities so that directional noise
`suppression is robust. As a result, microphone sensitivities
`remain matched not only over time but also while in actual
`use. These approaches are particularly useful for hearing aid
`applications in which directional noise suppression is impor-
`tant.
`According to one aspect, the invention operates to adap-
`tively measure a sensitivity difference between microphones
`in a multi-microphone directional processing system, and
`then compensate (or correct) an electronic sound signal from
`one or more of the microphones. As a result of the adaptive
`55 processing, the microphones "effectively" become matched
`and remain matched over time and while in use.
`Consequently, the invention enables multi-microphone
`directional processing systems to achieve superior direction-
`ality and consistent Signal-to-Noise Ratio (SNR) across all
`60 conditions. The invention is described below with respect to
`embodiments particularly well suited for use with hearing
`aid applications. However, it should be recognized that the
`invention is not limited to hearing aid applications, but is
`applicable to other sound pick-up systems.
`Embodiments of this aspect of the invention are discussed
`below with reference to FIGS. 5-11. However, those skilled
`in the art will readily appreciate that the detailed description
`
`18
`
`

`

`US 7,155,019 B2
`
`5
`given herein with respect to these figures is for explanatory
`purposes as the invention extends beyond these limited
`embodiments.
`As noted above, microphone matching is important for
`multi-microphone directional systems. Different and undes-
`ired responses will result when the sensitivities of the
`microphones are not matched. The acoustic delay between
`the microphones further complicates matching problems.
`For example, even if the microphones are perfectly matched,
`the instantaneous response of the microphones can be dif-
`ferent because of the delay and/or fluctuation in the acoustic
`signals. Therefore, it is not enough to simply use the
`difference of the responses to correct the problem. More
`complex processing is necessary to eliminate the effects of
`acoustic delay between the microphones and/or the fluctua-
`tion in the acoustic signals.
`According to one aspect of the invention, responses from
`each microphone are processed such that the resulting
`processed signals are not sensitive to the acoustic delay
`between the microphones and the fluctuation of acoustic
`conditions. A difference between the processed signals from
`the microphone channels can then be used to scale at least
`one microphone's response so as to compensate or correct
`for sensitivity differences between the microphones.
`FIG. 5 is a block diagram of a two-microphone directional
`processing system 500 according to one embodiment of the
`invention. The two-microphone directional processing sys-
`tem 500 includes a first microphone 502 and a second
`microphone 504. The first microphone 502 produces a first
`electronic sound signal and the second microphone 504
`produces a second electronic sound signal. A delay unit 506
`delays the second electronic sound signal. The two-micro-
`phone directional processing system 500 also includes a first
`minimum estimate unit 508, a second minimum estimate
`unit 510 and a divide unit 512. The first minimum estimate
`unit 508 estimates the minimum for the first electronic sound
`signal. The second minimum estimate unit 510 estimates the
`minimum of the second electronic sound signal. Typically,
`these minimums are measured over a time constant duration,
`such that the minimum is a relatively long-term minimum.
`The divide unit 512 produces a quotient by dividing the first
`minimum estimate by the second minimum estimate. The
`quotient represents a scaling amount that is sent to a mul-
`tiplication unit 514. The second electronic sound signal is
`then multiplied with the scaling amount to produce a com-
`pensated sound signal. The compensated sound signal is thus
`compensated (or corrected) for the relative difference in
`sensitivity between the mismatched first and second micro-
`phones 502 and 504. A subtraction unit 516 then subtracts
`the compensated electronic sound signal from the first
`electronic sound signal to produce an output signal. At this
`point, the output signal has been processed by the two-
`microphone directional processing system 500 to have
`robust directionality despite a mismatch between the first
`and second microphones 502 and 504.
`The two-microphone directional processing system 500
`uses a single-band adaptive compensation scheme to com-
`pensate for sensitivity differences between the microphones.
`In this embodiment, minimum estimates and division cal-
`culations are performed. The minimum estimates can, for
`example, be performed by minimum estimate units shown in
`more detail below with respect to FIGS. 7 and 8. It should
`also be noted that the delay unit 506 can be positioned within
`the two-microphone directional processing system 500 any-
`where in the channel associated with the second electronic
`sound signal prior to the subtraction unit 516. Still further,
`
`6
`it should be noted that a multiple-band adaptive compensa-
`tion scheme could alternatively be utilized.
`Moreover, although the two-microphone directional pro-
`cessing system 500 uses minimum estimates of the elec-
`tronic sound signals produced by the first and second
`microphones 502 and 504, other signal characteristics can
`alternatively be used. For example, Root-Mean-Square
`(RMS) average of the electronic sound signals produced by
`the microphones could be used. With such an approach, the
`10 RMS average could be measured over a time constant
`duration. The time constant can be set such that the average
`is relatively long-term so as to avoid impact of signal
`fluctuations. The time constant with an RMS approach is
`likely to be longer than the time constant for the minimum
`15 approach.
`The two-microphone directional processing system 500
`operates to scale the intensity of an electronic sound signal
`from one or more of the microphones. With respect to the
`two-microphone directional processing system 500, the pro-
`20 cessing (including the scaling) is performed in a linear
`domain. However, the scaling or other processing can also
`be performed in a logarithm (or dB) domain.
`FIG. 6 is a block diagram of a two-microphone directional
`processing system 600 according to another embodiment of
`25 the invention. The two-microphone directional processing
`system 600 includes a first microphone 602 and a second
`microphone 604. The first microphone 602 produces a first
`electronic sound signal and the second microphone 604
`produces a second electronic sound signal. A delay unit 606
`30 delays the second electronic sound signal. The two-micro-
`phone directional processing system 600 also includes a first
`minimum estimate unit 608 and a second minimum estimate
`unit 610. The first minimum estimate unit 608 estimates the
`minimum for the first electronic sound signal. The second
`35 minimum estimate unit 610 estimates the minimum of the
`second electronic sound signal. Typically, these minimums
`are measured over a time constant duration, such that the
`minimum is a relatively long-term minimum.
`The two-microphone directional processing system 600
`40 also includes a first linear-to-log conversion unit 612, a
`second linear-to-log conversion unit 614, a subtraction unit
`616, and a log-to-linear conversion unit 618. The first
`minimum estimate is converted from the linear domain to
`the logarithm domain by the first linear-to-log conversion
`45 unit 612, and the second minimum estimate is converted
`from the linear domain to the logarithm domain by the
`second linear-to-log conversion unit 614. The subtraction
`unit 616 then subtracts the second minimum estimate from
`the first minimum estimate to produce a difference amount.
`50 The log-to-linear conversion unit 614 then converts the
`difference amount to the linear domain.
`The converted difference amount produced by the log-to-
`linear conversion unit 614 represents a scaling amount that
`is sent to a multiplication unit 620. The second electronic
`55 sound signal is then multiplied with the scaling amount to
`produce a compensated sound signal. The compensated
`sound signal is thus compensated (or corrected) for the
`relative difference in sensitivity between the mismatched
`first and second microphones 602 and 604. A subtraction
`60 unit 622 then subtracts the compensated electronic sound
`signal from the first electro