Patent Publication Number: US-10771887-B2

Title: Anisotropic background audio signal control

Description:
TECHNICAL FIELD 
     The present disclosure relates to audio signal control. 
     BACKGROUND 
     Local participants in conferencing sessions (e.g., online or web-based meetings) often use headsets with an integrated speaker and/or microphone to communicate with remote meeting participants. The microphone detects speech from the local participant for transmission to the remote meeting participants, but frequently picks up undesired anisotropic background audio signals (e.g., background talkers) along with the speech. When transmitted with the speech, the undesired anisotropic background audio signals can prevent the remote meeting participants from understanding the speech. This can be a hindrance to all meeting participants and reduce the effectiveness of the conferencing session. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a system for controlling an anisotropic background audio signal, according to an example embodiment. 
         FIGS. 2A and 2B  illustrate respective arrangements of microphones employed in a headset with a boom, according to an example embodiment. 
         FIG. 3  is a functional signal processing flow diagram illustrating extraction of a reference signal that includes an anisotropic background audio signal, according to an example embodiment. 
         FIG. 4  is a functional signal processing flow diagram illustrating signal selection based on headset position, according to an example embodiment. 
         FIG. 5  is a functional signal processing flow diagram illustrating cancellation of an anisotropic background audio signal, according to an example embodiment. 
         FIG. 6  is a functional signal processing flow diagram illustrating suppression of an anisotropic background audio signal, according to an example embodiment. 
         FIG. 7  is a functional signal processing flow diagram illustrating update control of an adaptive filter configured to extract a reference signal, according to an example embodiment. 
         FIG. 8  is a functional signal processing flow diagram illustrating update control of an adaptive filter configured to cancel an anisotropic background audio signal, according to an example embodiment. 
         FIG. 9  is a flowchart of a method for controlling an anisotropic background audio signal, according to an example embodiment. 
     
    
    
     DESCRIPTION OF EXAMPLE EMBODIMENTS 
     Overview 
     In one example embodiment, a headset obtains, from a first microphone on the headset, a first audio signal including a user audio signal and an anisotropic background audio signal. The headset obtains, from a second microphone on the headset, a second audio signal including the user audio signal and the anisotropic background audio signal. The headset extracts, from the first audio signal and the second audio signal, using a first adaptive filter, a reference audio signal including the anisotropic background audio signal. Based on the reference signal, the headset cancels, using a second adaptive filter, the anisotropic background audio signal from a third audio signal derived from the first and second audio signals to produce an output audio signal. The headset provides the output audio signal to a receiver device. 
     EXAMPLE EMBODIMENTS 
     With reference made to  FIG. 1 , shown is an example system  100  for controlling an anisotropic background audio signal. In the scenario depicted by  FIG. 1 , meeting attendees  105 ( 1 ) and  105 ( 2 ) are attending an online/remote meeting (e.g., audio call) or conference session. System  100  includes communications server  110 , headsets  115 ( 1 ) and  115 ( 2 ), and telephony devices  120 ( 1 ) and  120 ( 2 ). Communications server  110  is configured to host or otherwise facilitate the meeting. Meeting attendee  105 ( 1 ) is wearing headset  115 ( 1 ) and meeting attendee  105 ( 1 ) is wearing headset  115 ( 2 ). Headsets  115 ( 1 ) and  115 ( 2 ) enable meeting attendees  105 ( 1 ) and  105 ( 2 ) to communicate with (e.g., speak and/or listen to) each other in the meeting. Headsets  115 ( 1 ) and  115 ( 2 ) may pair to telephony devices  120 ( 1 ) and  120 ( 2 ) to enable communication with communications server  110 . Examples of telephony devices  120 ( 1 ) and  120 ( 2 ) may include desk phones, laptops, conference endpoints, etc. 
       FIG. 1  shows a block diagram of headset  115 ( 1 ). Headset  115 ( 1 ) includes memory  125 , processor  130 , and wireless communications interface  135 . Memory  125  may be read only memory (ROM), random access memory (RAM), magnetic disk storage media devices, optical storage media devices, flash memory devices, electrical, optical, or other physical/tangible memory storage devices. Thus, in general, memory  125  may comprise one or more tangible (non-transitory) computer readable storage media (e.g., a memory device) encoded with software comprising computer executable instructions and when the software is executed (by the processor  130 ) it is operable to perform the operations described herein. 
     Wireless communications interface  135  may be configured to operate in accordance with the Bluetooth® short-range wireless communication technology or any other suitable technology now known or hereinafter developed. Wireless communications interface  135  may enable communication with telephony device  120 ( 1 ). Although wireless communications interface  135  is shown in  FIG. 1 , it will be appreciated that other communication interfaces may be utilized additionally/alternatively. For example, in another embodiment, headset  115 ( 1 ) may utilize a wired communication interface to connect to telephony device  120 ( 1 ). 
     Headset  115 ( 1 ) also includes microphones  140 ( 1 ) and  140 ( 2 ), audio processor  145 , and speaker  150 . Audio processor  145  may include one or more integrated circuits that convert audio detected by microphones  140 ( 1 ) and  140 ( 2 ) to digital signals that are supplied (e.g., as receive signals) to the processor  130  for wireless transmission via wireless communications interface  135  (e.g., when meeting attendee  105 ( 1 ) speaks). Thus, processor  130  is coupled to receive signals derived from outputs of microphones  140 ( 1 ) and  140 ( 2 ) via audio processor  145 . Audio processor  145  may also convert received audio (via wireless communication interface  135 ) to analog signals to drive speaker  150  (e.g., when meeting attendee  105 ( 2 ) speaks). Headset  115 ( 2 ) may include similar functional components as those shown at  120  with reference to headset  115 ( 1 ). 
     Anisotropic background audio signal  155  is present in the local environment of headset  115 ( 1 ). In this example, anisotropic background audio signal  155  originates from person who is loudly speaking near meeting attendee  105 ( 1 ), although it will be appreciated that anisotropic background audio signal  155  may be any noise that reaches microphones  140 ( 1 ) and  140 ( 2 ) at different levels of magnitude. Here, because the person is standing to one side of meeting attendee  105 ( 1 ), the noise from the person reaches microphone  140 ( 1 ) at a different (e.g., lower) level of magnitude than at microphone  140 ( 2 ). 
     Conventionally, anisotropic background audio signal  155  would heavily interfere with the online meeting between meeting attendees  105 ( 1 ) and  105 ( 2 ). For example, in some conventional headsets, the anisotropic background audio signal  155  would drown out any speech from meeting attendee  105 ( 1 ). Other conventional headsets might be configured for traditional noise reduction or suppression, although these are too limited to adequately deal with anisotropic background audio signal  155 . Traditional noise reduction algorithms might not suppress anisotropic background audio signal  155  because anisotropic background audio signal  155  is a speech signal. Moreover, traditional noise suppression algorithms can attempt to suppress the anisotropic background audio signal  155  at some frequency and time, but this often distorts the speech from meeting attendee  105 ( 1 ) because that speech and the anisotropic background audio signal  155  generally have some overlap in time and frequency. Thus, traditional methods often fail because the anisotropic background audio signal  155  and the speech from meeting attendee  105 ( 1 ) can have similar energy signals. 
     Accordingly, in order to alleviate noise interference due to anisotropic background audio signal  155 , anisotropic background audio signal control logic  160  is provided in memory  125 . Briefly, anisotropic background audio signal control logic  160  causes processor  130  to perform operations to cancel (rather than merely reduce or suppress by conventional means) anisotropic background audio signal  155 . Anisotropic background audio signal control logic  160  enables headset  115 ( 1 ) to cancel anisotropic background audio signal  155  without distorting speech from meeting attendee  105 ( 1 ). Headset  115 ( 1 ) may remove anisotropic background audio signal  155  before providing an output audio signal to headset  115 ( 2 ). It will be appreciated that at least a portion of anisotropic background audio signal control logic  160  may be included in devices other than headset  115 ( 1 ), such as at communications server  110 . 
     Headset  115 ( 1 ) may have a boom design or a boomless design. In a boom design, headset  115 ( 1 ) includes a boom that houses microphones  140 ( 1 ) and  140 ( 2 ).  FIGS. 2A and 2B  respectively illustrate example arrangements  200 A and  200 B of microphones  140 ( 1 ) and  140 ( 2 ) employed in headset  115 ( 1 ) with a boom. In both arrangements  200 A and  200 B, microphones  140 ( 1 ) and  140 ( 2 ) are separated by a distance D. Distance D may vary depending on the specific use case, but may be large enough to enable implementation of the techniques described herein. Furthermore, in both arrangements  200 A and  200 B, microphone  140 ( 1 ) is a directional microphone oriented toward a source of a user audio signal (e.g., the mouth of meeting attendee  105 ( 1 )). In arrangement  200 A, microphone  140 ( 2 ) is a directional microphone oriented away from the source of the user audio signal. In arrangement  200 B, microphone  140 ( 2 ) is an omnidirectional microphone. 
     In a boomless design, headset  115 ( 1 ) includes a first earpiece that houses microphone  140 ( 1 ) and a second earpiece that houses microphone  140 ( 1 ). One of the first and second earpieces may be configured for the left ear of meeting attendee  105 ( 1 ), and the other of the first and second earpieces may be configured for the right ear of meeting attendee  105 ( 1 ). Microphones  140 ( 1 ) and  140 ( 2 ) may both be oriented toward the source of the user audio signal, and may be unidirectional or omnidirectional. It will be appreciated that microphones  140 ( 1 ) and  140 ( 2 ) may be physical microphones or virtual microphones comprising an array of physical microphones. In either design, the relative position between microphones  140 ( 1 ) and  140 ( 2 ) and the mouth of meeting attendee  105 ( 1 ) does not change. Moreover the distances between the mouth and microphones  140 ( 1 ) and  140 ( 2 ) are relatively short, and therefore audio signals from the direct acoustic path tend to dominate. 
       FIG. 3  is an example functional signal processing flow diagram  300  illustrating extraction of a reference audio signal  305  that includes anisotropic background audio signal  155 . Reference is also made to  FIG. 1  for purposes of the description of  FIG. 3 . Headset  115 ( 1 ) obtains, from microphone  140 ( 1 ), a first audio signal  310  including a user audio signal (e.g., speech from meeting attendee  105 ( 1 )) and anisotropic background audio signal  155 . Headset  115 ( 1 ) further obtains, from microphone  140 ( 2 ), a second audio signal  315  including the user audio signal and anisotropic background audio signal  155 . In other words, first audio signal  310  and second audio signal  315  both include the (desired) user audio signal and the (undesired) anisotropic background audio signal  155 . In this example, the relative magnitude of anisotropic background audio signal  155  is greater at microphone  140 ( 2 ), and the relative magnitude of the user audio signal is greater at microphone  140 ( 1 ). As such, first audio signal  310  includes a stronger user audio signal, and second audio signal  315  includes a stronger anisotropic background audio signal  155 . 
     Headset  115 ( 1 ) extracts, from first audio signal  310  and second audio signal  315 , reference audio signal  305 . Reference signal  305  may include anisotropic background audio signal  155  and any (isotropic) background noise, but may exclude most or all of the user audio signal. Headset  115 ( 1 ) uses adaptive filter  320  (e.g., time domain element filter) to extract the reference audio signal  305 . In this example, first audio signal  310  is the primary input for adaptive filter  320 , second audio signal  315  is the reference input for adaptive filter  320 , and reference signal  305  is the error output of adaptive filter  320 . Adder  322  generates reference signal  305  based on an output signal  325  of adaptive filter  320  and first audio signal  310  (e.g., by subtracting output signal  325  from first audio signal  310 ). 
     As shown in  FIG. 3 , in a boomless design, adder  330  may combine output signal  325  with first audio signal  310  to produce a combined signal  335 . Scaling node  340  may scale the combined signal by one-half to produce third audio signal  345 . Thus, third audio signal  345  may include an enhanced user audio signal. In a boom design (not shown), the first audio signal  310  may be used as reference signal  305  because microphone  140 ( 1 ) picks up the user audio signal better than microphone  140 ( 2 ). 
     In one example, delay node  350  may delay the first audio signal  310  by a length of time equal to a difference between a time at which the user audio signal reaches microphone  140 ( 1 ) and a time at which the user audio signal reaches microphone  140 ( 2 ). Delaying the first audio signal  310  may ensure that adaptive filter  320  converges when the user audio signal is present. The length of time may correspond to distance D ( FIG. 2 ) and the way in which meeting attendee  105 ( 1 ) is wearing headset  115 ( 1 ). For example, in a boomless design, meeting attendee  105 ( 1 ) may place the left or right earpiece relatively far forward or backward such that the user audio signal reaches the left and right earpieces at different times. In this example, the length of time of the delay may be the maximum possible time difference at which the user audio signal reaches the left and right earpieces. The delay may be on the order of hundreds of microseconds. The tail length of adaptive filter  320  may approximately double the delay, and may be less than one millisecond. 
       FIG. 4  is an example functional signal processing flow diagram  400  illustrating signal selection based on headset position. Reference is also made to  FIGS. 1 and 3  for purposes of the description of  FIG. 4 . The anisotropic background audio signal control logic  160  of headset  115 ( 1 ) may include earpiece position estimation function  410 , which estimates earpiece position on meeting attendee  105 ( 1 ). Earpiece position estimation function  410  may perform earpiece position estimation based on the envelop  420  of adaptive filter  320 , Signal-to-Noise Ratio (SNR) 430 of first audio signal  310 , SNR  440  of second audio signal  315 , and SNR  445  of third audio signal  345 . Envelope  420  (e.g., in the time domain) may provide a strong indication of earpiece position. In an ideal case, the user audio signal reaches the left and right earpieces at the same time, meaning that adaptive filter  320  should have only one peak (at the delay of delay node  350 ) with the other taps at almost zero. When the earpieces are not in the correct position, envelop  420  may include other peaks. In the non-ideal case, envelop  420 , along with SNRs  430 ,  440 , and  445 , may be used to determine earpiece position estimation. When earpiece position estimation function  410  indicates that the earpieces are not ideally positioned, one of the first audio signal  310 , second audio signal  315 , and third audio signal  345  having the highest SNR may be selected. 
     Thus, first audio signal  310 , second audio signal  315 , and third audio signal  345  are candidate audio signals. Based on earpiece position estimation function  410 , candidate signal selection function  450  selects one of the candidate audio signals (here, third audio signal  345 ). Candidate signal selection function  450  may make the selection based on SNRs  430 ,  440 , and/or  445  (e.g., by selecting the highest SNR), and/or based on envelop  420 . For example, in a boomless design, when meeting attendee  105 ( 1 ) has not placed the earpieces at the optimal positions, the signal from one of microphones  140 ( 1 ) and  140 ( 2 ) may have a significantly lower level of the user audio signal than the other of microphones  140 ( 1 ) and  140 ( 2 ). Accordingly, in certain situations it may be preferable to intelligently select a signal with the highest SNR instead of, for example, the third audio signal  345 . 
       FIG. 5  is an example functional signal processing flow diagram  500  illustrating cancellation of anisotropic background audio signal  155 . Reference is also made to  FIGS. 1, 3 and 4  for purposes of the description of  FIG. 5 . The anisotropic background audio signal control logic  160  of headset  115 ( 1 ) may use adaptive filter  510  to cancel anisotropic background audio signal  155  from the third audio signal  345  based on reference signal  305 . The third audio signal  345 , having been selected by candidate signal selection function  450 , is the primary input for adaptive filter  510 . Reference signal  305  is the reference input for adaptive filter  510 . Fourth audio signal  520  is the error output of adaptive filter  510 . Delay node  530  may delay the third audio signal  345  to ensure that adaptive filter  510  converges. 
     Because adaptive filter  320  ( FIG. 3 ) already removed the user audio signal from reference signal  305 , adaptive filter  510  may not distort the user audio signal in the third audio signal  345 . Adaptive filter  510  may be a time or frequency domain element filter, although a frequency domain implementation may be particularly computation efficient. The tail length of adaptive filter  510  may be in the range of 10 to 50 milliseconds, since the anisotropic background audio signal  155  received by microphones  140 ( 1 ) and  140 ( 2 ) may have reflections due to the acoustic environment (e.g., the head of meeting attendee  105 ( 1 )). 
       FIG. 6  is an example functional signal processing flow diagram  600  illustrating suppression of an anisotropic background audio signal. Reference is also made to  FIGS. 1, 3, and 5  for purposes of the description of  FIG. 6 . In certain cases, fourth audio signal  520  may still include a remaining anisotropic background audio signal (e.g., residual from anisotropic background audio signal  155 ). To fully remove anisotropic background audio signal  155  from output audio signal  610 , the anisotropic background audio signal control logic  160  may include a suppression function  620  that performs noise suppression on the fourth audio signal  520 . Suppression function  620  may calculate (e.g., in the frequency domain) a suppression gain for the fourth audio signal  520  based on the user audio signal and anisotropic background audio signal  155 . More specifically, suppression function  620  may calculate the suppression gain based on an estimated signal strength of the user audio signal, an estimated signal strength of anisotropic background audio signal  155 , and cancellation performance of anisotropic background audio signal  155  to produce output audio signal  610 . Suppression function  620  may produce output audio signal  610  by applying the suppression gain to the fourth audio signal  520 , thereby removing any remaining anisotropic background audio signal. Headset  115 ( 1 ) may provide output audio signal  610  to a receiver device (e.g., telephony device  120 ( 1 ), which in turn communicates to telephony device  120 ( 2 ) via communications server  110 )). 
     Suppression function  620  may determine the estimated signal strength of the user audio signal by comparing the signal strengths between reference signal  305  and the third audio signal  345 . In particular, the third audio signal  345  includes the user audio signal, anisotropic background audio signal  155 , and any (isotropic) background/environmental noise, while reference signal  305  includes anisotropic background audio signal  155  and the (isotropic) background/environmental noise, with the user audio signal removed. Moreover, suppression function  620  may use the SNR of reference signal  305  as the estimated signal strength of anisotropic background audio signal  155 . 
     Performance estimation function  630  may provide a performance estimation of adaptive filter  510 , and performance estimation function  640  may provide a performance estimation of adaptive filter  320 . If there is strong performance from adaptive filter  320  (as indicated by performance estimation node  640 ), a user audio signal may be present, and therefore suppression may be limited (or nonexistent) so as to avoid distorting the user audio signal. For example, if there is a strong user audio signal, the first audio signal  310  and the third audio signal  345  would be relatively high, and reference signal  305  would be relatively low. Meanwhile, a strong performance from adaptive filter  510  (as indicated by performance estimation function  630 ) indicates that adaptive filter  510  is cancelling a large quantity of anisotropic background audio signal  155 , and therefore suppression may be warranted. For example, when the estimated signal strength of the user audio signal is low, performance estimation function  630  may determine the cancellation performance of anisotropic background audio signal  155  by comparing the respective signal strengths of the third audio signal  345  and the fourth audio signal  520 . With anisotropic background audio signal  155  removed from the third audio signal  345 , the fourth audio signal  520  has the user audio signal and environmental noise. When meeting attendee  105 ( 1 ) is not talking (i.e., the estimated signal strength of the user audio signal is low), the fourth audio signal  520  is mainly environment noise. 
     When the estimated user audio signal strength is relatively low, the suppression gain should be low if the estimated signal strength of anisotropic background audio signal  155  is relatively high and there is strong cancellation performance of anisotropic background audio signal  155 . Low suppression gain attenuates anisotropic background audio signal  155  residue in the fourth audio signal  520 . When the estimated signal strength of the user audio signal is relatively high, the suppression gain should be calculated based on the mask effect of the user audio signal and anisotropic background audio signal  155 . When the estimated signal strength of the user audio signal is much higher than that of anisotropic background audio signal  155 , anisotropic background audio signal  155  is masked by the user audio signal, and as such the suppression gain may be relatively high. When the estimated signal strength of anisotropic background audio signal  155  is high relative to the estimated signal strength of the user audio signal, more attenuation is necessary, and therefore the suppression gain should be relatively low. 
     The suppression gain calculation may consider both global spectrum (for all frequencies) and local spectrum (for specific frequency bins) of the user audio signal and the anisotropic background audio signal  155  signal strength. When global anisotropic background audio signal  155  signal strength is high, even if anisotropic background audio signal  155  signal strength is low for a specific frequency, gain for that frequency may be lower than it would otherwise be when the global anisotropic background audio signal  155  signal strength is low. 
       FIG. 7  is an example functional signal processing flow diagram  700  illustrating update control of adaptive filter  320 . Reference is also made to  FIGS. 1 and 3  for purposes of the description of  FIG. 7 . The anisotropic background audio signal control logic  160  may include update control function  710 , which controls coefficient updates to adaptive filter  320  based on SNR estimations  720 ( 1 ) and  720 ( 2 ) associated with first and second audio signals  310  and  315 . SNR estimations  720 ( 1 ) and  720 ( 2 ) may be based on noise floor estimations  730 ( 1 ) and  730 ( 2 ) of first and second audio signals  310  and  315 , respectively. Adaptive filter  320  may have a very fast convergence time with a short tail length. Since the relative distances between microphones  140 ( 1 ) and  140 ( 2 ) and the mouth of meeting attendee  105 ( 1 ) is fairly constant, adaptive filter  320  need not update constantly/continuously. Update control function  710  may update coefficients of adaptive filter  320  when the SNR of first audio signal  310  is greater than a first predefined threshold, and when the SNR of second audio signal  315  is greater than a second predefined threshold. In one example, the predefined thresholds are set such that adaptive filter  320  is only updated when meeting attendee  105 ( 1 ) is speaking. 
       FIG. 8  is an example functional signal processing flow diagram  800  illustrating update control of adaptive filter  510 . Reference is also made to  FIGS. 1, 3, and 5  for purposes of the description of  FIG. 8 . The anisotropic background audio signal control logic  160  may include update control function  810 , which controls coefficient updates to adaptive filter  510  based on SNR estimations  820 ( 1 ) and  820 ( 2 ) of reference signal  305  and the third audio signal  345 . SNR estimations  820 ( 1 ) and  820 ( 2 ) may be based on noise floor estimations  830 ( 1 ) and  830 ( 2 ) of reference signal  305  and the third audio signal  345 , respectively. Adaptive filter  510  may update when the SNR of reference signal  305  is greater than a third predefined threshold, and when the SNR of the third audio signal  345  is between a fourth predefined threshold and a fifth predefined threshold. When both the user audio signal and anisotropic background audio signal  155  are present simultaneously, the third audio signal  345  may have a higher strength than reference signal  305 . In this case, the fourth audio signal  520  is relatively large, and update control function  810  may cease coefficient updating. 
       FIG. 9  is a flowchart of an example method  900  for controlling an anisotropic background audio signal. Reference is made to  FIG. 1  for purposes of the description of  FIG. 9 . Method  900  may be performed by headset  115 ( 1 ). At  910 , headset  115 ( 1 ) obtains, from a first microphone on a headset, a first audio signal including a user audio signal and an anisotropic background audio signal. At  920 , headset  115 ( 1 ) obtains, from a second microphone on the headset, a second audio signal including the user audio signal and the anisotropic background audio signal. At  930 , headset  115 ( 1 ) extracts, from the first audio signal and the second audio signal, using a first adaptive filter, a reference audio signal including the anisotropic background audio signal. At  940 , based on the reference signal, headset  115 ( 1 ) cancels, using a second adaptive filter, the anisotropic background audio signal from a third audio signal derived from the first and second audio signals to produce an output audio signal. At  950 , headset  115 ( 1 ) provides the output audio signal to a receiver device. 
     Techniques are presented to remove an anisotropic background audio signal from a microphone audio signal before sending an output audio signal to remote side in a conference call. A method that combines anisotropic background audio signal cancellation and suppression may optimize the audio experience for headsets. Multiple microphones may be used in these methods. Two adaptive filters may be used: one for reference signal extraction, and the other for anisotropic background audio signal cancellation. Techniques described herein may apply in boom or boomless headsets. 
     In one form, an apparatus is provided. The apparatus comprises: a first microphone; a second microphone; and a processor coupled to receive signals derived from outputs of the first microphone and the second microphone, wherein the processor is configured to: obtain, from the first microphone, a first audio signal including a user audio signal and an anisotropic background audio signal; obtain, from the second microphone, a second audio signal including the user audio signal and the anisotropic background audio signal; extract, from the first audio signal and the second audio signal, using a first adaptive filter, a reference audio signal including the anisotropic background audio signal; based on the reference signal, cancel, using a second adaptive filter, the anisotropic background audio signal from a third audio signal derived from the first and/or second audio signals to produce an output audio signal; and provide the output audio signal to a receiver device. 
     In one example, the apparatus further comprises a first earpiece that houses the first microphone and a second earpiece that houses the second microphone. In a further example, the processor is further configured to: select the third audio signal from a plurality of candidate audio signals, wherein the plurality of candidate audio signals includes the first audio signal, the second audio signal, and the third audio signal. In a still further example, the processor is configured to select the third audio signal based on a signal-to-noise ratio of the first audio signal, a signal-to-noise ratio the second audio signal, and/or a signal-to-noise ratio of the combined signal. In another still further example, the processor is configured to select the third audio signal based on an envelope of the output of the first adaptive filter. 
     In one example, the apparatus further comprises: a boom that houses the first microphone and the second microphone, wherein the first microphone is a directional microphone oriented toward a source of the user audio signal. In a further example, the third audio signal is the first audio signal. In another further example, the second microphone is a directional microphone oriented away from the source of the user audio signal. In yet another further example, the second microphone is an omnidirectional microphone. 
     In one example, the processor is configured to cancel the anisotropic background audio signal to produce a fourth audio signal, and the processor is further configured to: calculate a suppression gain based on the user audio signal and the anisotropic background audio signal; and remove a remaining anisotropic background audio signal from the fourth audio signal by applying the suppression gain to the fourth audio signal to produce the output audio signal. 
     In one example, the processor is further configured to: update coefficients of the first adaptive filter when a signal-to-noise ratio of the first audio signal is greater than a first predefined threshold, and when a signal-to-noise ratio of the second audio signal is greater than a second predefined threshold. 
     In one example, the processor is further configured to: update coefficients of the second adaptive filter when a signal-to-noise ratio of the reference signal is greater than a first predefined threshold, and when a signal-to-noise ratio of the third audio signal is between a second predefined threshold and a third predefined threshold. 
     In one example, the processor is further configured to: delay the first audio signal by a length of time substantially equal to a difference between a time at which the user audio signal reaches one of the first microphone and the second microphone and a time at which the user audio signal reaches the other of the first microphone and the second microphone. 
     In another form, a method is provided. The method comprises: obtaining, from a first microphone on a headset, a first audio signal including a user audio signal and an anisotropic background audio signal; obtaining, from a second microphone on the headset, a second audio signal including the user audio signal and the anisotropic background audio signal; extracting, from the first audio signal and the second audio signal, using a first adaptive filter, a reference audio signal including the anisotropic background audio signal; based on the reference signal, cancelling, using a second adaptive filter, the anisotropic background audio signal from a third audio signal derived from the first and second audio signals to produce an output audio signal; and providing the output audio signal to a receiver device. 
     In another form, one or more non-transitory computer readable storage media are provided. The non-transitory computer readable storage media are encoded with instructions that, when executed by a processor, cause the processor to: obtain, from a first microphone on a headset, a first audio signal including a user audio signal and an anisotropic background audio signal; obtain, from a second microphone on the headset, a second audio signal including the user audio signal and the anisotropic background audio signal; extract, from the first audio signal and the second audio signal, using a first adaptive filter, a reference audio signal including the anisotropic background audio signal; based on the reference signal, cancel, using a second adaptive filter, the anisotropic background audio signal from a third audio signal derived from the first and second audio signals to produce an output audio signal; and provide the output audio signal to a receiver device. 
     The above description is intended by way of example only. Although the techniques are illustrated and described herein as embodied in one or more specific examples, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made within the scope and range of equivalents of the claims.