Abstract:
Techniques for processing audio signals include removing noise from the audio signals or otherwise clarifying the audio signals prior to outputting the audio signals. The disclosed techniques may employ minimum mean squared error (MMSE) analyses on audio signals received from a primary microphone and at least one reference microphone, and to techniques in which the MMSE analyses are used to reduce or eliminate noise from audio signals received by the primary microphone. Optionally, confidence intervals may be assigned to different frequency bands of an audio signal, with each confidence interval corresponding to a likelihood that its respective frequency band includes targeted audio, and each confidence interval representing a contribution of its respective frequency band in a reconstructed audio signal from which noise has been removed.

Description:
TECHNICAL FIELD 
       [0001]    This disclosure relates generally to techniques for processing audio signals, including techniques for removing noise from audio signals or otherwise clarifying the audio signals prior to outputting the audio signals. More specifically, this disclosure relates to techniques in which minimum mean squared error (MMSE) analyses are conducted on audio signals received from a primary microphone and at least one reference microphone, and to techniques in which the MMSE analyses are used to reduce or eliminate noise from audio signals received by the primary microphone. 
       SUMMARY 
       [0002]    In various aspects, a method according to this disclosure is a clarification process that includes identifying a targeted portion, or component, of an audio signal and reducing or eliminating noise that accompanies the targeted portion of the audio signal. When the clarification process is used, the targeted portion of the primary audio signal, or at least a significant portion of the targeted portion of the primary audio signal, will remain after, or survive, the clarification process. Each portion of the primary audio signal that remains following the clarification process is referred to herein as a “clarified audio signal.” In embodiments where different frequency bands of the primary audio signal are separately clarified, the clarified audio signals may be included in a reconstructed version of the primary audio signal, which is also referred to herein as a “reconstructed audio signal.” In embodiments where the clarification process is used with an audio communication device, such as a mobile telephone, the targeted portion of the primary audio signal may comprise an individual&#39;s voice. Once a primary audio signal has been clarified and the clarified audio signal has optionally been included in a reconstructed audio signal, the clarified and/or reconstructed audio signal may be stored, transmitted to another device and/or audibly output. 
         [0003]    A method for processing an audio signal includes receiving the audio signal, in the form of sound, with at least two microphones in proximity to one another, but providing different orientations or perspectives and, therefore, receiving the audio signal in different ways from one another, or from different perspectives. Such an arrangement is referred to as a “binaural environment.” The microphones include a primary microphone and one or more reference microphones. The primary microphone may be positioned to receive an audio signal from an intended source; for example, the primary microphone may comprise a microphone of a mobile telephone into which an individual speaks while using the mobile telephone. The audio signal from the intended source may comprise targeted audio, or targeted sound. Because of its orientation or perspective, the audio signal received by the primary microphone is referred to herein as a “primary audio signal.” 
         [0004]    Each reference microphone may be positioned somewhat remotely from the intended source of sound, at a location and orientation, or perspective, that enable the reference microphone to receive background sound to the same extent or to a greater extent than the background sound is received by the primary microphone, and to receive targeted audio to a lesser extent than the primary microphone receives targeted audio. The audio signal received from the perspective of each reference microphone is referred to herein as a “reference audio signal.” 
         [0005]    Once an audio signal has been received as a primary audio signal and one or more reference audio signals, the primary audio signal may be clarified. As part of the clarification process, the primary audio signal and each reference audio signal may be subjected to one or more adaptive time domain filters. In a specific embodiment, the primary audio signal and/or each reference audio signal may be subjected to a least mean squares (LMS) filter. 
         [0006]    Regardless of whether or not the primary audio signal or any reference audio signal is subjected to one or more adaptive time domain filters, a noise estimate is obtained. The noise estimate may be obtained from one or more reference audio signals. More specifically, the noise estimate may be obtained from one or more frequency bands in which one or more parts of at least one targeted audio (e.g., formants, or the spectral peaks of the human voice; etc.) are known to be present. The noise estimate may be obtained from the reference audio signal(s) alone, or by comparing appropriate portions (e.g., each frequency band of interest, etc.) of the reference audio signal(s) to corresponding portions of the primary audio signal, which, in addition to noise, will include the target audio. Even more specifically, a sample of a particular frequency band of the primary audio signal may be compared with a simultaneously obtained sample of the same particular frequency band of one or more reference audio signals to identify suspected, or likely, noise present in that frequency band of the primary audio signal (i.e., a noise estimate). Regardless of how it is obtained, each noise estimate may be used to identify suspected noise, or likely noise, present in the primary audio signal or in one or more frequency bands of the primary audio signal. By analyzing audio signals in a binaural environment, noise estimation may be conducted without a voice activity detector, as is required when noise is estimated without the use of a reference audio signal. 
         [0007]    Each noise estimate may be considered while conducting a minimum mean square error (MMSE) analysis on the primary audio signal or on one or more frequency bands of the primary audio signal. The MMSE analysis may be used to minimize error, defined by a function of noise estimates and the frequency decomposition of the primary audio signals. The result of that minimization may be used to modify one or more frequency bands of the primary audio signal. In some embodiments, the MMSE analysis may be tailored based on one or more noise estimates. Alternatively, one or more noise estimates may be accounted for or incorporated into the MMSE analysis of the primary audio signal or one or more frequency bands of the primary audio signal. The MMSE analysis at least partially eliminates the noise from the primary audio signal or from one or more frequency bands of the primary audio signal, providing one or more clarified audio signals. Stated another way, the overall presence of noise in one or more frequency bands of the clarified audio signal(s) may be reduced, or, in the case of each frequency band that includes noise but lacks targeted audio, the overall presence of the frequency band in the reconstructed output signal may be reduced. 
         [0008]    In some embodiments, including those where a primary audio signal has been separated into a plurality of different frequency bands, as well as those where an MMSE analysis performed on different frequency bands has resulted in a plurality of clarified audio signals, with each clarified audio signal corresponding to a frequency band of the plurality of frequency bands, a confidence interval may be assigned to each frequency band or clarified audio signal. The confidence level for each frequency band, or clarified audio signal, may correspond to the degree to which that frequency band, or clarified audio signal, will be included in a reconstructed audio signal. Each confidence interval may be based on real-time analysis and/or, in some embodiments, on historical data. More specifically, the confidence interval for each frequency band or clarified audio signal may correspond to information gleaned from the primary audio signal and each reference audio signal (e.g., a noise estimate for the corresponding frequency band, results of the MMSE analysis on the corresponding frequency band, etc.). 
         [0009]    The confidence interval may at least partially correspond to a likelihood that its corresponding frequency band or clarified audio signal includes at least a portion of the targeted audio of the primary audio signal, such as a human voice, music, or the like. In some embodiments, the confidence interval for a particular frequency band or clarified audio signal may correspond to the likelihood that the frequency band or clarified audio signal includes at least a portion of the targeted audio. Alternatively, or in addition, the confidence interval for a particular frequency band or clarified audio signal may correspond to an amount of noise (e.g., a percentage of noise, etc.) removed from the clarified audio signal when compared with the noise present in the corresponding frequency band of a corresponding portion of a reference audio signal. 
         [0010]    Each confidence interval may be embodied as a gain value; e.g., a value between zero (0) and one (1), which may be used as a multiplier for its corresponding predetermined frequency band and, thus, to control the extent to which that corresponding predetermined frequency band is included in the reconstructed output audio signal. As an example, if there is a high level of confidence that a frequency band or a clarified audio signal corresponds to a portion of the targeted audio of the primary audio signal (e.g., from the MMSE analysis on that frequency band, etc.), a relatively high gain value (e.g., greater than 0.5, between 0.6 and 1, etc.) may be assigned to that frequency band. If a frequency band is less likely to correspond to a portion of the target audio of the primary audio signal, the corresponding confidence interval may be low, and a correspondingly low gain value (e.g., a gain value of 0.5 or less, etc.) may be assigned to that particular frequency band. If there is a very low level of confidence that a frequency band corresponds to a portion of the targeted audio, or that the frequency band is very likely to be primarily made up of noise, a very low gain value (e.g., less than 0.3, etc.) may be assigned to that particular frequency band. 
         [0011]    When a plurality of frequency bands have been separated, or extracted, from a primary audio signal and a confidence interval has been assigned to each frequency band, the confidence intervals may then be used to determine the extent to which each of the frequency bands will be included in a reconstructed audio signal; i.e., the presence of each frequency band of the reconstructed audio output signal may correspond to its confidence interval. More specifically, each confidence interval may be used to dynamically adjust a magnitude of its corresponding frequency band to improve signal-to-noise ratio (SNR) of the resulting reconstructed signal. Frequency bands with higher confidence intervals will have a greater presence than frequency bands with lower confidence intervals, making the frequency bands with high confidence intervals more pronounced in the reconstructed audio signal than the frequency bands with low confidence intervals. Once confidence intervals have been assigned, the frequency bands may be recompiled to generate the reconstructed audio signal. 
         [0012]    The disclosed clarification process may be conducted on a continuous or substantially continuous basis (e.g., in a series of time segments, etc.). 
         [0013]    Any embodiment of a clarification process according to this disclosure may be embodied as a program (e.g., a software application, or “app”; firmware; etc.) that controls operation of a processing element of an electronic device. Accordingly, an electronic device of this disclosure may be configured to provide a clarified audio signal and/or a reconstructed audio signal with little or no noise, regardless of the degree to which noise was present in a source audio signal. The electronic device may then be configured to store, transmit and/or provide an audible output of the clarified audio signal and/or the reconstructed audio signal. 
         [0014]    In a specific, but non-limiting embodiment, such an electronic device may comprise a mobile telephone or other audio communication device. In addition to including the program and a processor, the audio communication device may include a primary microphone and one or more reference microphones. The audio communication device may also include a transmission element, such as an antenna that transmits an audio signal. The primary microphone and each reference microphone are configured to receive an audio signal and to communicate the audio signal to the processor. The processor processes a primary audio signal from the primary microphone and a reference audio signal from each reference microphone in accordance with an embodiment of an above-described method, and generates a clarified audio signal and/or a reconstructed audio signal. The clarified audio signal and/or the reconstructed audio signal may then be transmitted by the output element of the audio communication device; for example, to a cellular carrier network, from which the clarified audio signal and/or the reconstructed audio signal may be ultimately received by a recipient device, such as another telephone. 
         [0015]    Other aspects, as well as features and advantages of various aspects, of the disclosed subject matter will become apparent to those of ordinary skill in the art through consideration of the ensuing description, the accompanying drawings and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    In the drawings: 
           [0017]      FIG. 1  is a flow chart showing an embodiment of a method for clarifying audio signals; 
           [0018]      FIG. 2  is a flow chart illustrating an embodiment of use of adaptive least mean squares (LMS) filtering in an embodiment of a method for clarifying audio signals in accordance with teachings of this disclosure; and 
           [0019]      FIG. 3  schematically depicts an embodiment of an electronic device configured to execute an embodiment of a method for clarifying audio signals in accordance with teachings of this disclosure. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    With reference to  FIG. 1 , an embodiment of a method for clarifying an audio signal is illustrated and described. In general, the method includes three components: receiving an audio signal, at reference  10 ; processing the audio signal, at reference  20 , to provide a clarified audio signal and/or a reconstructed audio signal; and outputting the clarified audio signal and/or the reconstructed audio signal, at reference  40 . 
         [0021]    The act of receiving an audio signal, at reference  10 , may include receiving a plurality of audio signals. At reference  12 , a primary audio signal may be received from a first source, such as a primary microphone  112  of a mobile telephone or other audio communication device  100 , as shown in  FIG. 3 . At reference  14  of  FIG. 1 , one or more reference microphones  114  of the audio communication device  100  may receive a reference audio signal. The primary microphone  112  and each reference microphone  114  may respectively receive the primary audio signal and each reference audio signal simultaneously and in phase. In some embodiments, the components of the primary audio signal and each reference audio signal may be substantially the same, but in different amounts, due to an intraaural level difference (ILD) between the different orientations, or perspectives, of the respective primary microphone  112  and reference microphone(s)  114  by which the primary audio signal and the reference audio signal(s) were obtained. 
         [0022]    Upon receiving the primary audio signal and each reference audio signal, the primary microphone  112  and each reference microphone  114  of the audio communication device  100  shown in  FIG. 3  may, at reference  16  of  FIG. 1 , communicate these signals to a processor  120  of the audio communication device  100 . 
         [0023]    At reference  20  of  FIG. 1 , the primary audio signal and each reference audio signal may be processed in a manner that will provide a clarified audio signal. This clarification process may include a number of acts, which are set forth in detail in  FIG. 2 . At reference  22  of  FIG. 2 , the primary audio signal and, optionally, each reference audio signal, may be subjected to one or more adaptive time domain filters. Such a filter, which may comprise a low pass filter, may remove error, or likely noise, from the filtered signals, resulting in a more refined signal, or a clearer signal, following further processing. In a specific embodiment, a least mean squares filter (LMS) may be used as the adaptive time domain filter. The adaptive time domain filter may provide a rough, or passive, filter that removes some noise and/or other undesired artifacts from each filtered signal. 
         [0024]    At reference  24  of  FIG. 2 , a noise estimate may be obtained. More specifically, the reference audio signal or, in embodiments where a plurality of reference audio signals are received, the reference audio signals may be processed in a manner that provides a noise estimate. Such processing may include evaluation of one or more frequency bands that likely include target audio, such as a formant making up part of the voice of an individual speaking into the primary microphone  112  of the audio communication device  100  ( FIG. 3 ). The noise estimate provided by such processing may be based solely upon audio signals from each evaluated frequency band of each reference audio signal. Alternatively, the noise estimate may be based on differences between each evaluated frequency band of each reference audio signal and each corresponding frequency band of a primary audio signal that corresponds to the reference audio signal(s). In a specific embodiment, if a particular frequency band from a reference audio signal has substantially the same power or greater power than the same frequency band of a corresponding primary audio signal, that frequency band is most likely to be made up primarily of noise and, therefore, may be considered to be made up primarily of noise. If a frequency band from the primary audio signal has a greater power than the same frequency band in a corresponding reference audio signal, it is likely to include at least a portion of the targeted audio and may, therefore, be considered to include at least a portion of the targeted audio. 
         [0025]    Once a noise estimate has been obtained, the noise estimate may be used in conjunction with a minimum mean square error (MMSE) analysis of the primary audio signal, as set forth at reference  26  of  FIG. 2 . In some embodiments, the MMSE analysis may account for the noise estimate. More specifically, the MMSE analysis may be tailored based on the noise estimate. For example, the noise estimate may be incorporated into the MMSE analysis. The MMSE analysis may then be applied to the primary audio signal in a manner know in the art to provide at least one clarified audio signal. In embodiments where the primary audio signal has been subjected to an adaptive time domain filter, the spectral characteristics of the primary audio signal have been modified, and the MMSE analysis may be modified accordingly. In some embodiments, the MMSE analysis may be separately applied to different frequency bands of the primary audio signal to provide a plurality of clarified audio signals, each corresponding to one of the frequency bands of the primary audio signal. 
         [0026]    At reference  28  of  FIG. 2 , a confidence interval may be assigned to each frequency band of the primary audio signal. Confidence intervals may be applied to unprocessed frequency bands of a primary audio signal, to filtered frequency bands of the primary audio signal or to clarified audio signals resulting from MMSE analyses on the frequency bands of the primary audio signal. Each confidence interval may provide an indicator of the likelihood that a corresponding frequency band of the primary audio signal corresponds to at least a portion of the targeted audio. In some embodiments, the primary audio signal and each reference audio signal, or information obtained from either or both of those signals (e.g., the noise estimate for each frequency band, the results of the MMSE analysis on each frequency band, etc.) may be considered while assigning the confidence interval to each frequency band of the primary audio signal. 
         [0027]    Each confidence interval may control the extent to which a corresponding predetermined frequency band is included in the reconstructed output audio signal. The practical effect of each confidence interval is to attenuate frequency bands that are not believed to contribute to the targeted audio. The confidence interval for a particular, predetermined frequency band may be applied to that predetermined frequency band in any suitable manner. Without limitation, the confidence interval may comprise a multiplier for its corresponding predetermined frequency band. In a specific embodiment, each confidence interval may be embodied as a gain value; i.e., a value between zero (0) and one (1). For example, if a particular frequency band is likely to a portion of the targeted audio of the primary audio signal, a relatively high gain value (e.g., greater than 0.5, between 0.6 and 1, etc.) may be assigned to that frequency band. If a particular frequency band is at least as likely to include noise as the likelihood that it includes a portion of the targeted audio, the confidence interval for that frequency band may be low, and a correspondingly low gain value (e.g., a gain value of 0.5 or less, etc.) may be assigned to that frequency band. If it is unlikely that a particular frequency band includes a portion of the targeted audio, or that the particular frequency band is very likely to be the result of noise, a very low confidence interval and a very low gain value (e.g., less than 0.3, etc.) may be assigned to that frequency band. 
         [0028]    With an appropriate confidence interval assigned to each frequency band of the primary audio signal, that frequency band may be adjusted in an appropriate manner, at reference  30  of  FIG. 2 . In embodiments where the confidence interval corresponds to a gain value, the gain value may be applied to the frequency band. 
         [0029]    At reference  32  of  FIG. 2 , a reconstructed audio signal may be constructed by combining one or more frequency bands that have been modified. The frequency bands that are combined may be modified by the above-described MMSE analysis, using a confidence interval, or by a combination of MMSE analysis and confidence intervals. 
         [0030]    The reconstructed audio signal may then be output at reference  40  of  FIG. 1 . In embodiments where a process of the type that has been described in reference to  FIGS. 1 and 2  is used to modify audio that has been received by a primary microphone and one or more reference microphones of an audio communication device  100 , such as the mobile telephone depicted by  FIG. 3 , the modified primary audio signal may be communicated by a processor  110  of the audio communication device  100  to an antenna  130  of the audio communication device  100 , which then transmits the modified primary audio signal to another audio communication device or to a network, which may then transmit the modified primary audio signal to another audio communication device. The audio communication device that receives the modified primary audio signal may then process that signal in a manner that provides an audible output with little or no noise. 
         [0031]    While the preceding disclosure has been provided primarily in the context of audio communication devices, the disclosed subject matter may be applied to audio signals in a variety of other contexts as well. Without limitation, the disclosed subject matter may be useful with apparatuses that are used to receive and amplify sound (e.g., systems that include microphones, amplifiers and, optionally, mixers, etc.), with apparatuses that receive and record audio (e.g., voice recorders, video recorders, sound studios, etc.), with audio headsets (e.g., wired, wireless (e.g., BLUETOOTH®, etc.), etc.) and in a variety of other contexts. More specifically, as illustrated by  FIG. 3 , the reconstructed audio signal may be stored by memory  120  associated with the processor  110  of an electronic device, such as the audio output device  100  or another device that is configured to receive and store audio (e.g., a voice recorder, an audio recorder, a video camera, etc.). Alternatively, the reconstructed audio signal may be audibly output by a speaker  140  of an electronic device, such as a loud speaker of a stereo, a portable electronic device, a computer, a sound system or the like. 
         [0032]    In embodiments where the primary audio signal comprises a signal that is obtained (e.g., by a primary microphone  112  of an audio communication device  100 — FIG. 3 ) and stored (e.g., by memory  120  associated with a processor  110  of the audio communication device  100 , etc.), transmitted (e.g., by the antenna  130  of the audio communication device  100 , etc.) or output (e.g., by a speaker  140  of the audio communication device  100 , etc.) in real-time or substantially in real-time, the processes that have been described in reference to  FIGS. 1 and 2  may be conducted repeatedly. 
         [0033]    Repetition of the clarification process(es) may provide for continuous modification of the primary audio signal, and for quick adjustments that account for changes in the relative levels of noise and targeted audio in the primary audio signal. 
         [0034]    Although the foregoing disclosure provides many specifics, these should not be construed as limiting the scope of any of the ensuing claims. Other embodiments may be devised which do not depart from the scopes of the claims. Features from different embodiments may be employed in combination. The scope of each claim is, therefore, indicated and limited only by its plain language and the full scope of available legal equivalents to its elements.