Abstract:
Example embodiments include a method of reducing noise include forming a main signal and one or more reference signals at a beam-former based on at least two received audio signals, detecting voice activity at a voice activity detector, where the voice activity detector receives the main and reference signals and outputting a desired voice activity signal, adaptively cancelling noise at an adaptive noise canceller, where the adaptive noise canceller receives the main, reference, and desired voice activity signals and outputs an adaptive noise cancellation signal, and reducing noise at a noise reducer receiving the desired voice activity and adaptive noise cancellation signals and outputting a desired speech signal.

Description:
RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional Application No. 61/780,108, filed on Mar. 13, 2013. This application also claims the benefit of U.S. Provisional Application No. 61/839,211, filed on Jun. 25, 2013. This application also claims the benefit of U.S. Provisional Application No. 61/839,227, filed on Jun. 25, 2013. This application also claims the benefit of U.S. Provisional Application No. 61/912,844, filed on Dec. 6, 2013. 
     This application was co-filed on the same day, Feb. 14, 2014, with “Eye Glasses With Microphone Array” by Dashen Fan, U.S. application Ser. No.: 14/180,994. This application was co-filed on the same day, Feb. 14, 2014, with “Sound Induction Ear Speaker For Eye Glasses” by Dashen Fan, U.S. application Ser. No.: 14/180,986. This application was co-filed on the same day, Feb. 14, 2014, with “Eyewear Spectacle With Audio Speaker In The Temple” by Kenny Chow et al., U.S. application Ser. No.: 14/181,037. 
     The entire teachings of the above applications are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     In many computer and electronic systems that record sound, it is desirable to reduce background noise. Reducing background noise can provide advantages to the user such as outputting a clearer audio signal. Reducing background noise can also provide advantages to processes such as automatic speech recognition. 
     The acoustic characteristics of a noise canceling close-talk microphone are often very useful. Such microphones (also referred to herein as “mics”) often have a long boom form factor, which positions the microphone in front of the user&#39;s mouth. However, such a form factor has drawbacks, including deteriorated performance due to ongoing moisture from the user&#39;s mouth accumulating on the surface of the microphone membrane (ECM microphone) and a form-factor considered inconvenient and annoying by most users. 
     Therefore, there is a need for a noise cancelling microphone apparatus and method of its use that overcomes or minimizes the above-referenced problems. 
     SUMMARY OF THE INVENTION 
     More specifically, some embodiments include shortening the boom, moving the microphone away from the user&#39;s mouth, using acoustic housings such as polymeric or rubber extensions or boots to extend the acoustic ports of the microphones, therefore extending the effective close talk range while maintaining the noise canceling property for faraway noises. 
     Example embodiments of the present invention include a short boom headset, such as an audio headset for telephony suitable for enterprise call centers, industrial, and general mobile usage, an in-line “ear buds” headset with an input line (wire, cable, or other connector), mounted on or within the frame of eyeglasses, a near-to-eye (NTE) headset display or headset computing device, a long boom headset for very noisy environments, such as industry, military, and aviation applications, and a gooseneck desktop-style microphone, which can be used to provide theater or symphony-hall type quality acoustics without the structural costs. 
     Example embodiments as well as further details and benefits of the present invention are presented in more detail following the claims. Features of the invention presented herein which are couple may be physically and/or communicatively coupled (e.g., using wired connections or wirelessly). 
     Example embodiments include a method of reducing noise include forming a main signal and one or more reference signals at a beam-former based on at least two received audio signals, detecting voice activity at a voice activity detector, where the voice activity detector receives the main and reference signals and outputting a desired voice activity signal, adaptively cancelling noise at an adaptive noise canceller, where the adaptive noise canceller receives the main, reference, and desired voice activity signals and outputs an adaptive noise cancellation signal, and reducing noise at a noise reducer receiving the desired voice activity and adaptive noise cancellation signals and outputting a desired speech signal. 
     A further example embodiment of the present invention can include a noise canceling digital signal processor (DSP), including a beam-former configured or communicatively coupled to receive at least two audio signals and output a main signal and one or more reference signals based on the at least two audio signals, a voice activity detector configured or communicatively coupled to receive the main and reference signals and output or produce a desired voice activity signal, and adaptive noise canceller configured or communicatively coupled to receive the main, reference, and desired voice activity signals and to output or produce an adaptive noise cancellation signal, and a noise reducer configured or communicatively coupled to receive the desired voice activity and adaptive noise cancellation signals and output or produce a desired speech signal. 
     Still further example embodiments of the present invention can include a desired voice activity signal configured or communicatively coupled to control the adaptive noise canceller and the noise reducer. The voice activity detector can further include one or more short-time detectors, communicatively coupled to or configured to detect a short-time power of each of the received main and reference signals, respectively, one or more log scalers or amplifiers, communicatively coupled to or configured to convert the short time power detections (to a logarithmic scale (e.g., in dB) of each short-time detector, respectively, and one or more combiners, communicatively coupled or configured to receive the amplified short-time power detections of the main signal and one of the reference signals and produce or output a voice activity difference signal (e.g., in dB) based on the difference between the main and reference signal detections. The short-time detectors may be coupled to receive a reference or main signal as an input and output the detected short-time power to a series amplifier. The short time detectors and amplifiers can be in series for each respective signal. The amplifiers can be logarithmic converters (also referred to as log amplifiers or log scalers). The combiners can combine adjacent signals, such as the main signal and one of the at least one reference signals, to produce a voice activity difference signal by subtracting the detection(s) of the reference signal from the main signal (or vice-versa). 
     In still further example embodiments, the voice activity difference signal can be communicatively coupled to a single signal channel voice activity detector, which outputs the desired voice activity signal. The voice activity detector can further include one or more OR-gates or AND-gates, the selection of OR-gates or AND-gates based on microphone configuration, arranged to receive multiple desired voice activity signals and output one of the multiple desired voice activity signals based on the OR gate truth (or logic) table. The multiple desired voice activity signals can also be final consolidated desired voice activity signals. The short-time detector may be a root-mean-square (RMS) detector, a power detector, energy detector or similar. 
     In yet further example embodiments, the beam-former can include one or more low-pass filters (LPFs) (e.g., de-emphasis filters). The LPFs can be arranged to filter each of the main and reference signals prior to reception by the voice activity detector. A unitary multi-signal LPF can be used or individual LPFs for each signal can be used. The LPFs can have the same frequency response or transfer function characteristics. Alternatively, LPFs may have different frequencies responses and transfer function characteristics for each signal. The LPFs can have a gradual roll-off slope, starting from a frequency between approximately 1 kHz and 4 kHz and continuing to the Nyquist frequency. The beam-former can also include a frequency response matching filter arranged to filter the reference and/or main signals. The frequency response matching filters can be used to adjust the gain, phase, and/or shaping the frequency response of the signal. The frequency response matching filters can be used to match the frequency response of the reference and/or main signals. 
     In a yet further example embodiment, a bi-directional pressure-gradient microphone elements can provide or output the at least two audio signals to the VAD module and the channel noise reduction module. The bi-directional pressure-gradient microphone element can have two acoustic ports. The pressure-gradient microphone element can be sealed within an acoustic housing or acoustic extension or rubber boot such as polymeric or rubber extensions or boots. The term “seal” or “sealed” as used herein generally refers to an air-tight or hermetic seal. The acoustic extension can include an acoustic duct for each acoustic port. The acoustic ducts can extend the range of each acoustic port. Thus, near-field talk range of the microphone can be increased. The pressure gradient microphone element, or with the acoustic housing, can be further mounted airtight within a tube. The tube can be cylindrical, square, or any other shape. The tube can include at least a pair of acoustic openings and wind-screen material. The acoustic openings can be located longitudinally along the tube at distances spaced equal to or greater than the range of each acoustic port. The wind-screen material can be a foam or wind-guard material and can be used to fill the interior of the tube, between the acoustic extension and tube ends. The cylindrical tube can be a short boom coupled to a headset device. 
     In still further example embodiments, an array of microphones can generate the at least two audio signals. The at least two audio signals can be received at a beam-former. The audio signals can be digitized. The array of microphones can include at least two pressure gradient microphone elements, each pressure gradient microphone element having two acoustic ports. The acoustic ports can be the entry points (inputs) for sound waves. The two pressure gradient microphone elements can be bidirectional and identical. The two pressure gradient microphone elements can be further sealed within an acoustic housing, acoustic extension or airtight rubber boot. The acoustic housing, extension or rubber boot can include an acoustic duct for each acoustic port. The acoustic ducts can extend the range of each acoustic port. Thus, the near-field talk range of the microphones can be increased. The pressure gradient microphone elements can further be mounted airtight in series within a substantially cylindrical tube. The cylindrical tube can include at least three acoustic openings and wind-screen material or foam filling material. The acoustic openings can be located longitudinally along the tube at equally spaced distances greater than the range of each acoustic duct, or at a range at least equal to the range of each acoustic duct. The wind-screen or foam filling material can be used to fill the interior tube space between the acoustic openings and the acoustic ports, thus blocking wind and wind noise. The wind-screen can be a foam material or other material (e.g., wind guard sleeves over the rubber boots). The cylindrical tube can be a short boom coupled to a headset device. The cylindrical to can also be coupled to a goose neck desktop microphone device. 
     In still further embodiments, two omni-directional mics and additional beam-forming can be substituted for a pressure gradient microphone with acoustic extension. For example, each pressure gradient microphone element can be replaced by two omni-directional microphone elements where one omni-directional microphone element is located approximately at the position of each acoustic port (at the end of each acoustic extension duct). The output or output audio signal produced by the two omni-directional microphone elements can be received by the beam-former and processed to produce a beam pattern equivalent to the pressure-gradient microphone beam pattern. The beam-former can be an analog beam-former or a digital beam-former (that electronically forms beams). A bi-directional microphone with acoustic port extensions can be replaced by two omni-directional microphones, each being located approximately at the position of an acoustic port at the end of an acoustic extension duct and additional beam-former circuitry. 
     In still further example embodiments, the array of microphones can be coupled to a long boom headset device. Such a long boom headset can appear to be a conventional close-talk mic; however, it is a big boom mic with two mics in parallel. The end of the microphone boom can be arranged for positioning in front of the user&#39;s mouth while remaining microphone elements are arranged for positioning at the side(s) of the user&#39;s mouth. The end of the microphone therefore remains a short distance from the user&#39;s mouth. Such a close talk long boom design can be used in very heavy noise environments, including military, aviation, and industrial environments. Such a device can provide useful noise cancellation performance. The array of mics can include two pressure gradient noise cancellation microphones, wherein one of the microphones is positioned directly in front of the mouth of the user, while the other microphone is located at the side of the user&#39;s mouth. The two mics can be identical in a single housing (casing) or identical housings. The microphone patterns can be directionally parallel to each other and perpendicular to the boom. Each mic within the housing might can have a front and back opening. The digital signal processing circuitry can be located within the housing between the mics. The array can include bi-directional microphones replacing the pressure gradient noise cancellation microphones. The array can include omni-directional microphones as well. The array can include two to four microphones. 
     In still further example embodiments, the array of microphones can be located in-line with a headphone feed connector. The headphone feed connector can be a pair of ear-buds, such as the type that are typically used with a cell phone for hands-free calling, or other similar audio headset device. Microphones of the array of microphones can be pressure gradient microphones or omni-directional microphones or some other microphone type. Such an array of microphones can be located along the connector (e.g., wire, cable, etc.) at various points, such as close to the user&#39;s mouth or in proximity of the Y split, above, at or below the split (the “Y” split is where the left and right ear bud cords split from the input cord connector). 
     In still further example embodiments, the array of microphones can be located within or mounted on the housing of an eyeglasses frame. A first microphone can be located near the bridge support (the bridge support separates the lenses of the classes and typically sits on or above the user&#39;s nose). The first microphone can have top and bottom acoustic ports. A second microphone can be located near an end-point of the glasses frames (near a user&#39;s temple, between the lens and a support arm). The second microphone can have top and bottom acoustic ports. A yet further example embodiment can include a third microphone, located at the opposite end-point of the glasses from the second mic and have top and bottom acoustic ports. 
     The array of microphones, in a still further embodiment, can include three or more omni-directional microphone elements. The beam-former can be further configured to receive an audio signal for each respective microphone element. Thus, there are three or more audio signals input to the beam-former. The beam-former can include splitters, combiners, amplifiers, and phase shifters. The amplifiers and phase shifters can be located in series along branches or signals of the beam-forming network, where the splitters and combiners are used to form branches or signals of the beam-forming network originating from the microphone elements. The beam-former can be further arranged such that adjacent audio signals are combined to produce two or more audio difference signals. The two or more audio difference signals can have equivalent phase lengths. 
     In general, alternate embodiments can be realized by replacing each bi-directional microphone element with two omni-directional microphone elements electrically coupled together using a beam-former. Such substitution can achieve an identical beam pattern. In certain embodiments, two bi-directional microphone elements with two omni-directional elements, alternative embodiments can result by combining the eliminating one of the two middle positional microphone elements, such that three microphone elements in series, and adjusting the beam-forming accordingly. In the three microphone element example the middle microphone element is used with beam-forming to produce equivalent beam patterns of both the first bi-directional microphone beam pattern, forming the main signal, and the second bi-directional microphone beam pattern, forming the reference signal. 
     Example embodiments of the digital signal processor (DSP) can be implemented using a system on a chip (SOC), a Bluetooth chip, a DSP chip, or codec with the DSP integrated circuits (ICs). 
     In a still further example process for reducing noise can be executed on a non-transitory computer program product, including a computer readable medium having computer readable instructions stored thereon. The computer readable instructions when loaded and executed by a processor can cause the processor to form beams based on at least two audio signal inputs and produce a main signal and one or more reference signals, detect voice activity based on the main and reference signals and produce a desired voice activity signal, adaptively cancel noise based on the main, reference, and desired voice activity signals and produce an adaptive noise cancellation signal, and reduce noise based on the desired voice activity and adaptive noise cancellation signals and output a desired speech signal. 
     Further example embodiments of the present invention may be configured using a computer program product; for example, controls may be programmed in software for implementing example embodiments of the present invention. Further example embodiments of the present invention may include a non-transitory computer readable medium containing instruction that may be executed by a processor, and, when executed, cause the processor to complete methods described herein. It should be understood that elements of the block and flow diagrams described herein may be implemented in software, hardware, firmware, or other similar implementation determined in the future. In addition, the elements of the block and flow diagrams described herein may be combined or divided in any manner in software, hardware, or firmware. If implemented in software, the software may be written in any language that can support the example embodiments disclosed herein. The software may be stored in any form of computer readable medium, such as random access memory (RAM), read only memory (ROM), compact disk read only memory (CD-ROM), “Flash” memory and so forth. In operation, a general purpose or application specific processor loads and executes software in a manner well understood in the art. It should be understood further that the block and flow diagrams may include more or fewer elements, be arranged or oriented differently, or be represented differently. It should be understood that implementation may dictate the block, flow, and/or network diagrams and the number of block and flow diagrams illustrating the execution of embodiments of the invention. 
     In another embodiment, a handheld device for recording audio includes a top portion and a bottom portion. A first of the array of microphones is housed in the top portion and a second of the array of microphones is in the bottom portion. The top portion can also house at least two microphones and the bottom portion can house at least two microphones. 
     In an embodiment, a noise cancelling microphone further includes a headset, and a short boom housing the noise cancelling microphone. The short boom can also house two noise cancelling microphones. 
     The noise cancelling microphone can also include at least one earphone, the earphone housing the noise cancelling microphone. The noise cancelling microphone can also include eye-glasses configured to house at least one microphone. 
     The noise cancelling microphone can also include a headset, the headset configured to house a close-talk dual-microphone long boom. 
     The noise cancelling microphone can also include a gooseneck podium configured to house at least two microphone elements. 
     This invention has many advantages. For example, the audio device of the invention, by virtue of the microphone array, improves accurate recognition of speech by minimizing unwanted noise, particularly in those embodiments that employ a digital signal processor that actively cancels unwanted noise, thereby decreasing arrays in such speech recognition. Further, the present invention integrates the microphone array and digital signal processor in a convenient and comfortable format for everyday use. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram illustrating an example embodiment of a noise cancellation circuit of the present invention. 
         FIG. 2  is a block diagram illustrating an example embodiment of a beam-forming module of the invention that can be employed in the noise cancelling circuit. 
         FIG. 3  is a block diagram illustrating an example embodiment of a Desired Voice Activity Detection Module of the invention. 
         FIG. 4  is a block diagram illustrating an example embodiment of a noise cancellation circuit of the invention employed to receive a closer microphone signal and a first and second further microphone signal respectively. 
         FIG. 5  is an embodiment of a boom tube housing three microphones in an arrangement of one embodiment of the invention. 
         FIG. 6  is an embodiment of a boom tube housing four microphones in an arrangement of one embodiment of the invention 
         FIG. 7  is a block diagram illustrating an example embodiment of a beam-forming module accepting three signals of the invention. 
         FIG. 8  is a block diagram illustrating an example embodiment of a desired voice activity detection (VAD) module accepting three signals of the invention. 
         FIGS. 9A-B  are diagrams illustrating an example embodiment of the invention including a display and first and second microphones. 
         FIG. 10  is an illustration of an embodiment of eye-glasses of the invention having two embedded microphones. 
         FIG. 11  is an illustration of an embodiment of eyeglasses of the invention having three embedded microphones. 
         FIGS. 12A-B  are diagrams illustrating an example embodiment of a rubber boot and microphone assembly of the invention. 
         FIG. 13  is a diagram illustrating example positions of placements of the microphones of the invention. 
         FIG. 14  is a block diagram illustrating an example embodiment of a noise cancellation circuit of the present invention employing a single microphone. 
         FIGS. 15A-E  are diagrams of headsets having a dual-microphone attached. 
         FIGS. 16A-B  are diagrams illustrating example embodiments of a headset having a short boom. 
         FIGS. 17A-B  are diagrams illustrating example embodiments of a headset having a short boom. 
         FIGS. 18A-B  are diagrams illustrating example embodiments of two-way radios. 
         FIG. 19  is a diagram illustrating an example embodiment of a two-way radio. 
         FIG. 20  is a diagram illustrating an example embodiment of a two-way radio having a microphone in a bottom portion of the device and a microphone in the top portion of the device. 
         FIG. 21  is a diagram illustrating an example embodiment of a two-way radio having four microphones. 
         FIG. 22  is a diagram of a cellphone includes microphones. 
         FIG. 23  is a diagram illustrating an example embodiment of a cell phone  2302  having four microphones. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention. 
     In a head mounted computer, a user can desire a noise-canceling close-talk microphone without a boom microphone in front of his or her mouth. The microphone in front of the user&#39;s mouth can be viewed as annoying. In addition, moisture from the user&#39;s mouth can condense on the surface of the Electret Condenser Microphone (ECM) membrane, which after long usage can deteriorate microphone sensitivity. 
     In an embodiment, a short tube boom headset can solve these problems by shortening the boom, moving the ECM away from the user&#39;s mouth and using a rubber boot to extend the acoustic port of the noise-canceling microphone. This can extend the effective close-talk range of the ECM. This maintains the noise-canceling ECM property for far away noises. In addition, the boom tube can be lined with wind-screen form material. This solution further allows the headset computer to be suitable for enterprise call center, industrial, and general mobile usage. In an embodiment with identical dual-microphones within the tube boom, the respective rubber boots of each microphone can also be identical. 
     In an embodiment, the short tube boom headset can be a wired or wireless headset. The headset includes the short microphone (e.g., and ECM) tube boom. The tube boom can extend from the housing of the headset along the user&#39;s cheek, where the tube boom is either straight or curved. The tube boom can extend the length of the cheek to the side of the user&#39;s mouth, for instance. The tube boom can include a single noise-cancelling microphone on its inside. 
     The tube boom can further include a dual microphone inside of the tube. A dual microphone can be more effective in cancelling out non-stationary noise, human noise, music, and high frequency noises. A dual microphone can be more suitable for mobile communication, speech recognition, or a Bluetooth headset. The two microphones can be identical, however a person of ordinary skill in the art can also design a tube boom having microphones of different models. 
     In an embodiment having dual-microphones, the two microphones enclosed in their respective rubber boats are placed in series along the inside of the tube. 
     The tube can have a cylindrical shape, although other shapes are possible (e.g., a rectangular prism, etc.). The short tube boom can have two openings, one at the tip, and a second at the back. The tube surface can be covered with a pattern of one or more holes or slits to allow sound to reach the microphone inside the tube boom. In another embodiment, the short tube boom can have three openings, one at the tip, another in the middle, and another in the back. The openings can be equally spaced, however, other a person of ordinary skill in the art can design other spacings. 
     The microphone in the tube boom is a bi-directional noise-cancelling microphone having pressure-gradient microphone elements. The microphone can be enclosed in a rubber boot extending acoustic port on the front and the back side of the microphone with acoustic ducts. Inside of the boot, the microphone element is sealed in the air-tight rubber boot. 
     Within the tube, the microphone with the rubber boot is placed along the inside of the tube. An acoustic port at the tube tip aligns with the boom opening, and an acoustic port at the tube back aligns with boom opening. The rubber boot can be offset from the tube ends to allow for spacing between the tube ends and the rubber boot. The spacing further allows breathing room and for room to place a wind-screen of appropriate thickness. The rubber boot and inner wall of the tube remain air-tight, however. A wind-screen foam material (e.g., wind guard sleeves over the rubber boot) fills the air-duct and the open space between acoustic port and tube interior/opening. 
       FIG. 1  is a block diagram  100  illustrating an example embodiment of a noise cancellation circuit of the present invention. 
     Signals  110  and  112  from two microphones are digitized and fed into the noise cancelling circuit  101 . The noise cancelling circuit  101  can be a digital signal processing (DSP) unit (e.g., software executing on a processor, hardware block, or multiple hardware blocks). In an embodiment, the noise cancellation circuit  101  can be a digital signal processing (DSP) chip, a system-on-a-chip (SOC), a Bluetooth chip, a voice CODEC with DSP chip, etc. The noise cancellation circuit  101  can be located in a Bluetooth headset near the user&#39;s ear, in an inline control case with battery, or inside the connector, etc. The noise cancellation circuit  101  can be powered by a battery or by a power source of the device that the headset is connected to, such as the device&#39;s batter, or power from a USB, micro-USB, or Lightening connector. 
     The noise cancellation circuit  101  includes four functional blocks: a beam-forming (BF) module  102 , a Desired Voice Activity Detection (VAD) Module  108 , an adaptive noise cancellation (ANC) module  104  and a single signal noise reduction (NR) module  106 . The two signals  110  and  112  are fed into the BF module  102 , which generates a main signal  130  and a reference signal  132  to the ANC module  104 . A closer (i.e., relatively close to the desired sound) microphone signal  110  is collected from a microphone closer to the user&#39;s mouth and a further (i.e., relatively distant to the desired sound) microphone signal is collected from a microphone further from the user&#39;s mouth, relatively. The BF module  102  also generates a main signal  120  and reference signal  122  for the desired VAD module  108 . The main signal  120  and reference signal  122  can, in certain embodiments, be different from the main signal  130  and reference signal  132  generated for the for ANC module  104 . 
     The ANC module  104  processes the main signal  130  and the reference signal  132  to cancel out noises from the two signals and output a noise cancelled signal  142  to the single channel NR module  106 . The single signal NR module  106  post-processes the noise cancelled signal  142  from the ANC module  104  to remove any further residue noises. Meanwhile, the VAD module  108  derives, from the main signal  120  and reference signal  122 , a desired voice activity detection (DVAD) signal  140  that indicates the presence or absence of speech in the main signal  120  and reference signal  122 . The DVAD signal  140  can then be used to control the ANC module  104  and the NR module  106  from the result of BF module  102 . The DVAD signal  140  indicates to the ANC module  104  and the Single Channel NR module  106  which sections of the signal have voice data to analyze, which can increase the efficiency of processing of the ANC module  104  and single channel NR module  106  by ignoring sections of the signal without voice data. Desired speech signal  144  is generated by single channel NR module  106 . 
     In an embodiment, the BF module  102 , ANC module  104 , single NR reduction module  106 , and desired VAD module  108  employ linear processing (e.g., linear filters). A linear system (which employs linear processing) satisfies the properties of superposition and scaling or homogeneity. The property of superposition means that the output of the system is directly proportional to the input. For example, a function F(x) is a linear system if:
 
 F ( x   1   +x   2   +â                       )= F ( x   1 )+ F ( x   2 )+{circumflex over ( a )}                   

     A satisfies the property of scaling or homogeneity of degree one if the output scales proportional to the input. For example, a function F(x) satisfies the properties of scaling or homogeneity if, for a scalar Î±:
 
 F ( Î±x )= Î±F ( x )
 
     In contract, a non-linear function does not satisfy both of these conditions. 
     Prior noise cancellation systems employ non-linear processing. By using linear processing, increasing the input changes the output proportionally. However, in non-linear processing, increasing the input changes the output non-proportionally. Using linear processing provides an advantage for speech recognition by improving feature extraction. Speaker recognition algorithm is developed based on noiseless voice recorded in quiet environment with no distortion. A linear noise cancellation algorithm does not introduce nonlinear distortion to noise cancelled speech. Speech recognition can deal with linear distortion on speech, but not non-linear distortion of speech. Linear noise cancellation algorithm is “transparent” to the speech recognition engine. Training speech recognition on the variations of nonlinear distorted noise is impossible. Non-linear distortion can disrupt the feature extraction necessary for speech recognition. 
     An example of a linear system is a Weiner Filter, which is a linear single channel noise removal filter. The Wiener filter is a filter used to produce an estimate of a desired or target random process by linear time-invariant filtering an observed noisy process, assuming known stationary signal, noise spectra, and additive noise. The Wiener filter minimizes the mean square error between the estimated random process and the desired process. 
       FIG. 2  is a block diagram  200  illustrating an example embodiment of a beam-forming module  202  that can be employed in the noise cancelling circuit  101 . The BF module  202  receives the closer microphone signal  210  and further microphone signal  212 . 
     A further microphone signal  212  is inputted to a frequency response matching filter  204 . The frequency response matching filter  204  adjusts gain, phase, and shapes the frequency response of the further microphone signal  212 . For example, the frequency response matching filter  204  can adjust the signal for the distance between the two microphones, such that an outputted reference signal  232  representative of the further microphone signal  212  can be processed with the main signal  230 , representative of the closer microphone signal  210 . The main signal  230  and reference signal  232  are sent to the ANC module. 
     A closer microphone signal  210  is outputted to the ANC module as a main signal  230 . The closer microphone signal  210  is also inputted to a low-pass filter  206 . The reference signal  232  is inputted to a low-pass filter  208  to create a reference signal  222  sent to the Desired VAD module. The low-pass filters  206  and  208  adjust the signal for a “close talk case” by, for example, having a gradual low off from 2 kHz to 4 kHz, in one embodiment. Other frequencies can be used for different designs and distances of the microphones to the user&#39;s mouth, however. 
       FIG. 3  is a block diagram illustrating an example embodiment of a Desired Voice Activity Detection Module  302 . The DVAD module  302  receives a main signal  320  and a reference signal  322  from the beam-forming module. The main signal  320  and reference signal  322  are processed by respective short-time power modules  304  and  306 . The short-time power modules  304  and  306  can include a root mean square (RMS) detector, a power (PWR) detector, or an energy detector. The short-time power modules  304  and  306  output signals to respective amplifiers  308  and  310 . The amplifiers can be logarithmic converters (or log/logarithmic amplifiers). The logarithmic converters  308  and  310  output to a combiner  312 . The combiner  312  is configured to combine signals, such as the main signal and one of the at least one reference signals, to produce a voice activity difference signal by subtracting the detection(s) of the reference signal from the main signal (or vice-versa). The voice activity difference signal is inputted into a single channel VAD module  314 . The single channel VAD module can be a conventional VAD module. The single channel VAD  314  outputs the desired voice activity signal. 
       FIG. 4  is a block diagram  400  illustrating an example embodiment of a noise cancellation circuit  401  employed to receive a closer microphone signal  410  and a first and second further microphone signal  412  and  414 , respectively. The noise cancellation circuit  401  is similar to the noise cancellation circuit  101  described in relation to  FIG. 1 , however, the noise cancellation circuit  401  is employed to receive three signals instead of two. A beam-forming (BF) module  402  is arranged to receive the signals  410 ,  412  and  414  and output a main signal  430 , a first reference signal  432  and second reference signal  434  to an adaptive noise cancellation module  404 . The beam-forming module is further configured to output a main signal  422 , first reference signal  420  and second reference signal  424  to a voice activity detection (VAD) module  408 . 
     The ANC module  404  produces a noise cancelled signal  442  to a Single Channel Noise Reduction (NR) module  406 , similar to the ANC module  104  of  FIG. 1 . The single NR module  406  then outputs desired speech  444 . The VAD module  408  outputs the DVAD signal to the ANC module  404  and the single channel NR module  406 . 
       FIG. 5  is an example embodiment of beam-forming from a boom tube  502  housing three microphones  506 ,  508 , and  510 . A first microphone  506  is arranged closest to a tip  504  of the boom tube  502 , a second microphone  508  is arranged in the boom tube  502  further away from the tip  504 , and a third microphone  510  is arranged in the boom tube  502  even further away from the tip  504 . The first microphone  506  and second microphone  508  are arranged to provide data to output a left signal  526 . The first microphone is arranged to output its signal to a gain module  512  and a delay module  514 , which is outputted to a combiner  522 . The second microphone is connected directly to the combiner  522 . The combiner  522  subtracts the two provided signals to cancel noise, which creates the left signal  526 . 
     Likewise, the second microphone  508  is connected to a gain module  516  and a delay module  518 , which is outputted to a combiner  520 . The third microphone  510  is connected directly to the combiner  520 . The combiner  520  subtracts the two provided signals to cancel noise, which creates the right signal  520 . 
       FIG. 6  is an example embodiment of beam-forming from a boom tube  652  housing four microphones  656 ,  658 ,  660  and  662 . A first microphone  656  is arranged closest to a tip  654  of the boom tube  652 , a second microphone  658  is arranged in the boom tube  652  further away from the tip  654 , a third microphone  660  is arranged in the boom tube  652  even further away from the tip  654 , and a fourth microphone  662  is arranged in the boom tube  652  away from the tip  654 . The first microphone  656  and second microphone  658  are arranged to provide data to output a left signal  686 . The first microphone is arranged to output its signal to a gain module  672  and a delay module  674 , which is outputted to a combiner  682 . The second microphone is connected directly to the combiner  658 . The combiner  682  subtracts the two provided signals to cancel noise, which creates the left signal  686 . 
     Likewise, the third microphone  660  is connected to a gain module  676  and a delay module  678 , which is outputted to a combiner  680 . The fourth microphone  662  is connected directly to the combiner  680 . The combiner  680  subtracts the two provided signals to cancel noise, which creates the right signal  684 . 
       FIG. 7  is a block diagram  700  illustrating an example embodiment of a beam-forming module  702  accepting three signals  710 ,  712  and  714 . A closer microphone signal  710  is output as a main signal  730  to the ANC module and also inputted to a low-pass filter  717 , to be outputted as a main signal  720  to the VAD module. A first further microphone signal  712  and second closer microphone signal  714  are inputted to respective frequency response matching filters  706  and  704 , the outputs of which are outputted to be a first reference signal  732  and second reference signal  734  to the ANC module. The outputs of the frequency response matching filters  706  and  704  are also outputted to low-pass filters  716  and  718 , respectively, which output a first reference signal  722  and second reference signal  724 , respectively. 
       FIG. 8  is a block diagram  800  illustrating an example embodiment of a desired voice activity detection (VAD) module  802  accepting three signals  820 ,  822  and  824 . The VAD module  802  receives a main signal  820 , a first reference signal  822  and a second reference signal  824  at short-time power modules  804 ,  805  and  806 , respectively. The short-time power modules  804 ,  805 , and  806  are similar to the short-time power modules described in relation to  FIG. 3 . The short-time power modules  804 ,  805 , and  806  output to respective amplifiers  808 ,  809  and  810 , which can each be a logarithmic converter. Amplifiers  808  and  809  output to a combiner module  811 , which subtracts the two signals and outputs the difference to a single channel VAD module  814 . Amplifiers  810  and  808  output to a combiner module  812 , which subtracts the two signals and outputs the difference to a single channel VAD module  816 . The single channel VAD modules  814  and  816  output to a logical OR-gate  818 , which outputs a DVAD signal  840 . 
       FIG. 9A  is a diagram  900  illustrating an example embodiment of a display  902  having a first microphone  902  and second microphone  904 . The first microphone  902  is arranged to be closer to the user&#39;s mouth than the second microphone  904 , which is further from the user&#39;s mouth. In an embodiment, the microphones  902  and  904  are arranged in cylindrical holes in the display&#39;s  902  housing. 
       FIG. 9B  is a diagram  950  illustrating an example embodiment of a display  952  having a first microphone  952  and second microphone  954 . The first microphone  902  is arranged to be closer to the user&#39;s mouth than the second microphone  954 , which is further from the user&#39;s mouth. In an embodiment, the microphones  952  and  954  are arranged in cylindrical holes in the display&#39;s  952  housing. 
       FIG. 10  is a diagram  1000  illustrating an example embodiment of eye-glasses  1002  having embedded microphones. The eye-glasses  1002  have two microphones  1004  and  1006 , a first microphone  1004  being arranged in the middle of the eye-glasses  1002  frame and a second microphone  1006  being arranged on the side of the eye-glasses  1002  frame. The microphones  1004  and  1006  can be pressure-gradient microphone elements, either bi- or uni-directional. Each microphone  1004  and  1006  is within a rubber boot. The rubber boot provides an acoustic port on the front and the back side of the microphone with acoustic ducts. The two microphones  1004  and  1006  and their respective boots can be identical. The microphone elements  1004  and  1006  can be sealed air-tight (e.g., hermetically sealed) inside the rubber boots. The acoustic ducts are filled with wind-screen material. The ports are sealed with woven fabric layers. The lower and upper acoustic ports are sealed with a water-proof membrane. The microphones can be built into the structure of the eye glasses frame. Each microphone has top and bottom holes, being acoustic ports. In an embodiment, the two microphones  1004  and  1006 , which can be pressure-gradient microphone elements, can each be replaced by two omni-directional microphones. 
       FIG. 11  is a diagram  1150  illustrating an example embodiment of eye-glasses  1152  having three embedded microphones. The eye-glasses  1152  of  FIG. 11  are similar to the eye-glasses  1002  of  FIG. 10 , but instead employ three microphones instead of two. The eye-glasses  1152  of  FIG. 11  have a first microphone  1154  arranged in the middle of the eye-glasses  1152 , a second microphone  1156  arranged on the left side of the eye-glasses  1152 , and a third microphone  1158  arranged on the right side of the eye-glasses  1152 . The three microphones can be employed in the three-microphone embodiment described above. 
       FIG. 12A  is an exploded view of a microphone assembly  1200  of the invention. As shown therein, rubber boot  1202   a - b  is separated into a first half of the rubber boot  1202   a  and a second half of the rubber boot  1202   b.  Microphone  501  is between the rubber boot halves. Each rubber boot  1202   a - b  is lined by a wind-screen  1208  material, however  FIG. 12A  shows the wind-screen in the second half of the rubber boot  1202   b.  In the case of a pressure-gradient microphone, the air-duct and the open space between acoustic port and boom interior is filled with wind-screen foam material, such as wind guard sleeves over the rubber boots. 
     A microphone  1204  is arranged to be played between the two halves of the rubber boot  1202   a - b.  The microphone  1204  and rubber boot  1202   a - b  are sized such that the microphone  1204  fits in a cavity within the halves of the rubber boot  1202   a - b.  The microphone is coupled with a wire  1206 , that extends out of the rubber boot  1202   a - b  and can be connected to, for instance, the noise cancellation circuit described above. 
       FIG. 12B  is a perspective view of microphone assembly  1200  when assembled. The rubber boot  1252  of  FIG. 12B  is shown to have both halves  1202   a - b  joined together, where a microphone (not shown) is inside. A wire  1256  coupled to the microphone exist the rubber boot  1252  such that it can be connected to, for instance, the noise cancellation circuit described above. 
       FIG. 13  is an illustration of an embodiment of the invention  1300  showing various optional positions of placement of the microphones  1304   a - e.  As described above, the microphones are pressure-gradient. In an embodiment, microphones can be placed in any of the locations shown in  FIG. 13 , or any combination of the locations shown in  FIG. 13 . In a two-microphone system, the microphone closest to the user&#39;s mouth is referred to as MIC 1 , the microphone further from the user&#39;s mouth is referred to as MIC 2 . In an embodiment, both MIC 1  &amp; MIC 2  can be inline at position  1   1304   a.  In other embodiments, the microphones can be positioned as follows:
         MIC 1  at position  1   1304   a  and MIC 2  at position  2   1304   b;      MIC 1  at position  1   1304   a  and MIC 2  at position  3   1304   c;      MIC 1  at position  1   1304   a  and MIC 2  at position  4   1304   d;      MIC 1  at position  4   1304   d  and MIC 2  at position  5   1304   e;      Both MIC 1  and MIC 2  at position  4   1304   d.          

     If position  4   1304   d  has a microphone, it is employed within a pendant. 
     The microphones can also be employed at other combinations of positions  1304   a - e,  or at positions not shown in  FIG. 13 . 
     Each pressure-gradient microphone element can be replaced with two omni-directional microphones at the location of each acoustic port, resulting in four total microphones. The signal from these two omni-directional microphone can be processed by electronic or digital beam-forming circuitry described above to produce a pressure gradient beam pattern. This pressure gradient beam pattern replaces the equivalent pressure-gradient microphone. 
     In an embodiment of the present invention, if a pressure-gradient microphone is employed, each microphone is within a rubber boot that extends an acoustic port on the front and the back side of the microphone with acoustic ducts. At the end of rubber boot, the new acoustic port is aligned with the opening in the tube, where empty space is filled with wind-screen material. If two omni-directional microphones are employed in place of one pressure-gradient microphone, then the acoustic port of each microphone is aligned with the opening. 
     In an embodiment, a long boom dual-microphone headset can look like a conventional close-talk boom microphone, but is a big boom with two-microphones in parallel. An end microphone of the boom is placed in front of user&#39;s mouth. The close-talk long boom dual-microphone design targets heavy noise usage in military, aviation, industrial and has unparalleled noise cancellation performance. For example, one main microphone can be positioned directly in front of mouth. A second microphone can be positioned at the side of the mouth. The two microphones can be identical with identical casing. The two microphones can be placed in parallel, perpendicular to the boom. Each microphone has front and back openings. DSP circuitry can be in the housing between the two microphones. 
     Microphone is housed in a rubber or silicon holder (e.g., the rubber boot) with an air duct extending to the acoustic ports as needed. The housing keeps the microphone in an air-tight container and provides shock absorption. The microphone front and back ports are covered with a wind-screen layer made of woven fabric layers to reduce wind noise or wind-screen foam material. The outlet holes on the microphone plastic housing can be covered with water-resistant thin film material or special water-resistant coating. 
     In another embodiment, a conference gooseneck microphone can provide noise cancellation. In large conference hall, echoes can be a problem for sound recording. Echoes recorded by a microphone can cause howling. Severe echo prevents the user from tuning up speaker volume and causes limited audibility. Conference hall and conference room can be decorated with expensive sound absorbing materials on their walls to reduce echo to achieve higher speaker volume and provide an even distribution of sound field across the entire audience. Electronic echo cancellation equipment is used to reduce echo and increase speaker volume, but such equipment is expensive, can be difficult to setup and often requires an acoustic expert. 
     In an embodiment, a dual-microphone noise cancellation conference microphone can provide an inexpensive, easy to implement solution to the problem of echo in a conference hall or conference room. The dual-microphone system described above can be placed in a desktop gooseneck microphone. Each microphone in the tube is a pressure-gradient bi-directional, uni-directional, or super-directional microphone. 
       FIG. 14  is a block diagram  1400  illustrating an example embodiment of a noise cancellation circuit of the present invention employing a single microphone. A single microphone signal  1402  is received at an activity detection module (VAD)  1404  and a single channel noise reduction module (NR)  1406 . The activity detection module (VAD)  1404  determines the signal microphone signal  1402  contains speech, and notifies the single channel noise reduction module (NR)  1406 . The single channel noise reduction module (NR)  1406 , responsive to the signal from the activity detection module (VD)  1404 , reduces noise on the single microphone signal  1402  and outputs desired speech  1408 . 
       FIG. 15  is a diagram  1500  of a headset  1502  having a dual-microphone  1503  attached. The dual-microphones  1503  are contained in a housing, but the individual microphones within the housing are shown by pictures of microphone  1504  and  1506 . 
       FIG. 16  is a diagram  1600  illustrating an example embodiment of a headset  1602  having a short boom  1604 . The short boom  1604  houses a single microphone  1606  which is enclosed in a rubber boot, described herein above. 
       FIG. 17  is a diagram  1700  illustrating an example embodiment of a headset  1702  having a short boom  1704 . The short boom  1704  houses dual microphones  1706 , comprised of microphone  1706   a  and  1706   b.  Both microphones  1706   a - b  are enclosed in a rubber boot, described herein above. 
       FIG. 18  is a diagram  1800  illustrating example embodiments of two-way radios  1802  and  1804 . Two-way radios are widely used for public safety, enterprise and industrial applications, and consumer applications. 
       FIG. 19  is a diagram  1900  illustrating an example embodiment of a two-way radio  1902 . The two-way radio includes a microphone  1904  in a bottom portion of the two-way radio  1902  and a microphone  1906  in a top portion of the two-way radio  1902 . Traditionally, a two-way radio only has a microphone in the top part of the device. In an embodiment of the present invention, a second microphone is employed at the bottom of the two-way radio  1902  to provide a main microphone at the top and a reference microphone at the bottom. The user employs a push-to-talk button or feature near the top of the device. 
       FIG. 20  is a diagram  2000  illustrating an example embodiment of a two-way radio  2002  having a microphone  2004  in a bottom portion of the device and a microphone  2006  in the top portion of the device. The microphones  2004  and  2006  can be bi-directional microphones with an acoustic extension to the ports in the front and back case surface of the device. 
       FIG. 21  is a diagram  2100  illustrating an example embodiment of a two-way radio  2100  having four microphones. The two-way radio  2102  has two microphones  2104  and  2106  in the bottom portion and two microphones  2108  and  2110  in the top portion. Each bi-directional microphone with an extension shown in previous embodiments can be replaced with two omni-directional microphones (e.g., microphones  2104  and  2106  and microphones  2108  and  2110 ) at each port. The four omni-directional microphone configuration can occupy less space and therefore fit into a smaller device. The omni-directional microphone can be a MEMS microphone. Four microphone is more flexible for speech recorded from further away. The two microphones of the top portion can electronically form a uni-directional beam for far field talk or video recording. 
       FIG. 22  is a diagram  2200  of a cellphone  2202  includes microphones  2204  and  2206 . Handheld smartphones traditionally have a microphone on the bottom part of the phone. The user talks closely to the bottom part of the device while holding it. The same bi-directional microphone with an acoustic extension to the ports can be in the front and back case surface of the device. The main microphone can be in the bottom portion of the cell phone  2202  and reference microphone can be at the top portion. 
       FIG. 23  is a diagram  2300  illustrating an example embodiment of a cell phone  2302  having four microphones. Each bi-directional microphone with extension can be replaced with two omni-directional microphones at each port location. The four omni-directional microphone configuration can fit into a smaller device and therefore occupy less space. The omni-directional microphone can be a MEMS microphone. Four-microphones can be more flexible for a far talk scenario. Uppor two microphones can electronically form a uni-directional beam for far field talk or video recording. 
     The relevant teachings of all patents, published applications and references cited herein are incorporated by reference in their entirety. 
     While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.