Abstract:
Preprocessing speech signals from an indirect conduction microphone. One exemplary method preprocesses the speech signal in two stages. In stage one, an external speech sample is characterized using an auto regression model, and coefficients from the model are convolved with the internal speech signal from the indirect conduction microphone to produce a pre-conditioned internal speech signal. In stage two, a training sound is received by the indirect conduction microphone and filtered through a low-pass filter. The result is then modeled using auto regression, and inverted to produce an inverted filter model. The pre-conditioned internal speech signal is convolved with the inverted filter model to remove negative or undesirable acoustic characteristics and loss from the speech signal from the indirect conduction microphone.

Description:
BACKGROUND OF THE INVENTION 
     Microphones convert sounds to electrical signals and are used with a variety of devices where voice communication or voice control is desired. For example, microphones may be used in or with mobile telephones, two-way radios, personal audio devices, computers, and the like. In some cases, the microphone is part of a headset that includes, for example, speakers or other transducers for reproducing sound. In such cases, the speakers within the headset are positioned close to a user&#39;s ears. The microphone may be positioned on a boom or arm of the headset which is designed to be located at or near the user&#39;s mouth. In other cases, the microphone is not on a boom or arm. Instead the microphone is positioned within the ear canal and is connected to or included within an earphone or ear bud. Such a microphone is referred to as an in-ear microphone or “ear microphone” and eliminates the need for an arm to position the microphone near the user&#39;s mouth. An ear microphone receives speech sound from the user&#39;s mouth after the sound has propagated through the user&#39;s bones and tissue to the ear canal. The ear microphone generates a speech signal which may, for example, be encoded in a first communication device and then transmitted from the first communication device to another or second communication device. The second communication device receives the encoded signal and then decodes that signal. When a speech signal of poor speech quality is encoded at the first communication device, the decoded speech output at the second communication device can be unintelligible. A poor speech signal can be caused by, among other things, improper placement of the ear microphone in the ear canal and reverberations within the ear canal. The speech signal may also be degraded due to the combined effects on the user&#39;s voice as it propagates through the several biological media within the body, i.e., the bone and various tissues located between the mouth and the ear canal. Improving the speech signal prior to encoding (at the first communication device) could lead to improved decoded speech output (at the second communication device), which is therefore more intelligible. 
     Accordingly, there is a need for a method for preprocessing speech for digital audio quality improvement. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views, together with the detailed description below, are incorporated in and form part of the specification, and serve to further illustrate embodiments of concepts that include the claimed invention, and explain various principles and advantages of those embodiments. 
         FIG. 1  is a schematic illustration of a communication device connected to an ear microphone and a speaker microphone. 
         FIG. 2  illustrates the ear microphone of  FIG. 1  in accordance with some embodiments. 
         FIG. 3  is a flowchart of a method of pre-processing speech for digital audio quality enhancement in accordance with some embodiments. 
         FIG. 4  is a chart illustrating a speech spectrum produced using the method of  FIG. 3 . 
     
    
    
     Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention. 
     The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Some exemplary embodiments of the invention include a method for preprocessing speech signals received from an indirect conduction microphone. In one embodiment, the method includes receiving an external speech sound with a direct conduction microphone. The method further includes estimating an external speech spectral model, including a plurality of coefficients, based on the external speech sound. The method further includes receiving an internal speech signal from the indirect conduction microphone. The method further includes combining the plurality of coefficients with the internal speech signal to produce a preconditioned internal speech signal. The method further includes obtaining a low-frequency training sound signal, and estimating a filter model characteristic based on the low-frequency training sound signal. The method further includes determining an inverted filter model characteristic, and combining the inverted filter model characteristic with the preconditioned internal speech signal to produce a preprocessed internal speech signal. 
       FIG. 1  schematically illustrates a communication device  10 . Embodiments of the invention are described in connection with the communication device  10 . However, the microphones and speech processing techniques described herein may be used with other types of devices not just the exemplary communication device  10  described and illustrated. 
     The communication device  10  includes a radio  12  and an ear microphone  14 . In some embodiments, the communication device  10  also includes a speaker microphone  16 . The radio  12  includes a processing unit  18  (e.g., a microprocessor, application specific integrated circuit, etc.), a memory  20 , an input/output interface  22 , a voice encoder  24 , a transceiver  26 , an antenna  28 , and a built-in microphone  30 . The processing unit  18  is connected to the memory  20 , the input/output interface  22 , the voice encoder  24 , and the transceiver  26 . The ear microphone  14 , the speaker microphone  16 , and the built-in microphone  30  are all capable of sensing sound, converting the sound to electrical signals, and transmitting the electrical signals to the processing unit  18  via the input/output interface  22 . Direct conduction microphones, for example, the built-in microphone  30  and the speaker microphone  16  sense sound conducted through air. Indirect conduction microphones, for example, the ear microphone  14 , sense sound conducted partially or wholly through bone and other body tissue. While the systems and methods described herein are described particularly in relation to the ear microphone  14 , it should be noted that they may also be suitable for preprocessing speech signals produced by other indirect conduction microphones, for example, skull microphones and throat microphones. 
     The processing unit  18  processes the electrical signals received from the ear microphone  14 , the speaker microphone  16 , and the built-in microphone  30  via the input/output interface  22 . The processing unit  18  is connected to the voice encoder  24  via the input/output interface  22 , and provides the processed and unprocessed electrical signals to the voice encoder  24  through the input/output interface  22 . The voice encoder  24  encodes the electrical signals and produces a digital output for transmission by the radio  12  to other radio devices. The voice encoder  24  provides the digital output to the processing unit  18  via the input/output interface  22 . The transceiver  26  transmits and receives radio signals using antenna  28 . The processing unit  18 , the voice encoder  24 , and the transceiver  26  may include various digital and analog components, which for brevity are not described herein and which may be implemented in hardware, software, or a combination of both. 
     The memory  20  can include one or more non-transitory computer-readable media, and includes a program storage area and a data storage area. The program storage area and the data storage area can include combinations of different types of memory, as described herein. 
     The processing unit  18  obtains and provides information (e.g., from the memory  20  and/or the input/output interface  22 ), and processes the information by executing one or more software instructions or modules, capable of being stored, for example, in a random access memory (“RAM”) area of the memory  20  (e.g., during execution) or a read only memory (“ROM”) of the memory  20  (e.g., on a generally permanent basis) or another non-transitory computer readable medium. The software can include firmware, one or more applications, program data, filters, rules, one or more program modules, and other executable instructions. The processing unit  18  is configured to retrieve from the memory  20  and execute, among other things, software related to the control processes and methods described herein. The input/output interface  22  obtains information and signals from, and provides information and signals to, (e.g., over one or more wired and/or wireless connections) devices both internal and external to the radio  12 . The processing unit  18 , the memory  20 , and the input/output interface  22 , as well as the other various modules are connected by one or more control or data buses. The use of control and data buses for the interconnection between and exchange of information among the various modules and components would be apparent to a person skilled in the art in view of the description provided herein. It should be understood that although only a single processing unit  18 , input/output interface  22 , and memory  20  are illustrated in  FIG. 1 , the communication device  10  can include multiple processing units, memory modules, and/or input/output interfaces. 
       FIG. 2  illustrates the ear microphone  14  positioned in an ear  31  of a user. The ear  31  includes an outer ear  32 , an ear canal  34 , and an inner ear  36 . The ear microphone  14  includes a microphone element  38 , which is positioned to face the inner ear  36  when the ear microphone  14  is inserted in the ear canal  34 . To use the ear microphone  14 , it is positioned in the ear canal  34 . The ear microphone  14  forms a seal with the ear canal  34 , forming a chamber  40 , which is acoustically isolated from the ambient noise in the user&#39;s environment. When the user speaks, sound radiates from the user&#39;s mouth, producing external speech sounds. Internal speech sounds are produced when some of the sound from the user&#39;s external speech propagates internally through multiple biological media, i.e., the flesh and bone of the user&#39;s head, each with its own acoustic propagation characteristics. This propagation through multiple biological media creates a composite signal, which causes the ear canal wall  42  to vibrate. This vibration produces a sound  44 , in the ear canal, which is received by the microphone element  38 . Microphone element  38  is capable producing electrical signals in response to the sound  44 , and communicating the electrical signals to the processing unit  18  via the cable  46 . 
     Three factors negatively affect the quality of the output signal, produced by the radio  12 , containing audio information representing the sound sensed by the ear microphone  14 . First, the sound  44  experiences loss as it travels from the mouth, through bone and other tissue, to the ear canal wall  42 . Second, the multiple biological propagation media induce frequency-selective attenuation and phase group delay characteristics particular to each propagation medium. Third, the sound  44  produces reverberations in the chamber  40 . 
       FIG. 3  illustrates an exemplary method  100 , which the processing unit  18  can use to preprocess the internal speech signal produced by ear microphone  14 . This preprocessing reduces the effects of the loss and reverberation before the internal speech signals are digitally encoded by the voice encoder  24 . In the example illustrated, the method includes two stages. As will be explained in greater detail below, the first stage, preconditions the internal speech signals by enhancing the signal so it better approximates an external speech signal. The second stage further enhances the internal speech signals by subtracting the effects of the reverberation and the loss generated when the external speech propagates internally to the ear canal  34 . 
     Stage one begins at block  101 , where the processing unit  18  receives a sample of external speech produced by the user using an external microphone, i.e., a microphone other than the ear microphone  14 . External speech (i.e., speech sensed via direct conduction) has higher audio quality compared to internal speech (i.e., speech sensed via indirect conduction). In some embodiments, the external microphone is the speaker microphone  16 . In other embodiments, the external microphone is the built-in microphone  30  of the communication device  10 . In some embodiments, the processing unit  18  can select to use either the speaker microphone  16  or the built-in microphone  30 , or use both. In some embodiments, the communication device  10  prompts the user to produce an external speech sample. In other embodiments, the processing unit  18  takes an external speech sample at a suitable point during radio transmission of voice signals by the user. For example, while the user is making a voice transmission using the communication device  10 , the processing unit  18  may activate the built-in microphone  30  to take an external speech sample. 
     In block  103 , the processing unit  18  uses an autoregressive filter to estimate a spectral model for the external speech sample. The external speech spectral model includes coefficients, which characterize the external speech. 
     In block  105 , processing unit  18  stores the external speech spectral model in the memory  20 . Once the external speech spectral model is stored, it can be used continuously. In some embodiments, the external speech spectral model is updated periodically. In other embodiments, the external speech spectral model is updated in response to a prompt from the user, or from a remote system. Blocks  101 - 105  prepare the external speech spectral model, which will be used to preconditions the internal speech signal received in block  107 . This will enhance the internal speech signal by supplying it with the high-frequency components that were attenuated during transmission through the biological propagation media. The resulting preconditioned internal speech signal will better approximate an external speech signal. As described more particularly below, this will, when combined with stage two, produce a preprocessed internal speech signal that is an improvement over the original internal speech signal in both the high and low frequencies. 
     In block  107 , the processing unit  18  receives the internal speech signal from the ear microphone  14 . The internal speech signal is produced when the user speaks during routine usage of the communication device  10 , for example, when the communication device  10  is a two-way radio, and the user wishes to transmit a voice message to another user using a second two-way radio. In block  109 , the processing unit  18  preconditions the internal speech signal by mathematically convolving the internal speech signal with the external speech spectral model. The convolution in block  109  preconditions the internal speech signal to produce a preconditioned internal speech signal. The processing unit  18  outputs the preconditioned internal speech signal in block  111 . 
     Stage two begins with the processing unit  18  receiving a training sound produced by the user of the ear microphone  14 . For example, the user can produce the training sound by making a low, continuous sound with the mouth closed, i.e., by humming. The training sound serves as a wide-band forcing function to estimate the transfer function of the acoustic path through the bone and tissue between the mouth and the ear canal  34 . The training sound is also used to capture the characteristics of the ear canal  34  acoustics, e.g., the reverberation. In some embodiments, the communication device  10  prompts the user to produce the training sound. In other embodiments, the user prompts the communication device  10  that the user will be producing a training sound. 
     In block  115 , a low-pass filter is applied to the training sound signal to obtain a low-frequency training sound signal. The low-frequency training sound signal captures effects of the loss generated when the external speech propagates internally to the ear canal  34 . 
     In block  117 , the processing unit  18  uses an autoregressive filter to create a spectral model for the low-frequency training sound signal. The low-frequency training sound spectral model includes a filter model characteristic. In block  119 , processing unit  18  stores the low-frequency training sound spectral model in the memory  20 . Once the low-frequency training sound model is stored, it can be used continuously. 
     In some embodiments, the processing unit  18  causes the ear microphone  14  to artificially generate the internal excitation signal in lieu of the user-generated training sound. The excitation signal can be used by method  100  to mitigate the effects of the reverberation of the chamber  40 , but not the effects of the loss. 
     In block  121 , the processing unit  18  inverts the filter model characteristic to produce an inverted filter model characteristic. As described more particularly below, the inverted filter model is beneficially used to mitigate the reverberation, by removing excess low-frequency components (caused by the reverberation) still present in the preconditioned internal speech signal. This will further enhance the internal speech signal, producing a preprocessed internal speech signal that is an improvement over the original internal speech signal in both the high and low frequencies. 
     In block  123 , the processing unit  18  receives the preconditioned internal speech signal produced in block  111 , and mathematically convolves, or combines, it with the inverted filter model characteristic produced in block  121 . By combining the preconditioned internal speech signal with the inverted filter model characteristic, the convolution in block  123  subtracts the effects of the reverberation and the loss to produce a preprocessed internal speech signal. The processing unit  18  outputs the preprocessed internal speech signal to the voice encoder  24  in block  125 . 
     As noted above, the external speech spectral model created in block  103  can be continuously used by the processing unit  18  in block  109  to precondition the internal speech signal. Similarly, the low-frequency training sound spectral model created in block  117  can be used continuously by the processing unit  18  in blocks  121  and  123  to enhance the preconditioned internal speech signal. Accordingly, embodiments of the invention utilize method  100  by performing blocks  101 - 105  and  113 - 119  once to generate the external speech and low-frequency training sound spectral models, and continuously perform blocks  107 - 125  (indicated by the area  127  bounded by the dashed line in  FIG. 3 ) to produce a preprocessed internal speech signal input to the voice encoder  24 . The continuously performed blocks of method  100  are performed during transmission, when the user&#39;s speech causes the ear microphone  14  to generate a speech signal. 
       FIG. 4  is a chart illustrating experimental results of using the method  100  to preprocess speech signals. The chart illustrates the spectral characteristics of the preprocessed internal speech (line  201 ), compared to the original internal speech (line  203 ), and the original external speech (line  205 ). The spectral characteristic curves for the preprocessed internal speech and the external speech are similar. This demonstrates how the method  100 , when applied to the internal speech, produces a preprocessed speech signal that is closer in form to the external speech. 
     In the foregoing specification, specific embodiments have been described. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present teachings. 
     The benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential features or elements of any or all the claims. The invention is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims as issued. 
     Moreover in this document, relational terms such as first and second, top and bottom, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms “comprises,” “comprising,” “has”, “having,” “includes”, “including,” “contains”, “containing” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises, has, includes, contains a list of elements does not include only those elements but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by “comprises . . . a”, “has . . . a”, “includes . . . a”, “contains . . . a” does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises, has, includes, contains the element. The terms “a” and “an” are defined as one or more unless explicitly stated otherwise herein. The terms “substantially”, “essentially”, “approximately”, “about” or any other version thereof, are defined as being close to as understood by one of ordinary skill in the art, and in one non-limiting embodiment the term is defined to be within 10%, in another embodiment within 5%, in another embodiment within 1% and in another embodiment within 0.5%. The term “coupled” as used herein is defined as connected, although not necessarily directly and not necessarily mechanically. A device or structure that is “configured” in a certain way is configured in at least that way, but may also be configured in ways that are not listed. 
     It will be appreciated that some embodiments may be comprised of one or more generic or specialized processors (or “processing devices”) such as microprocessors, digital signal processors, customized processors and field programmable gate arrays (FPGAs) and unique stored program instructions (including both software and firmware) that control the one or more processors to implement, in conjunction with certain non-processor circuits, some, most, or all of the functions of the method and/or apparatus described herein. Alternatively, some or all functions could be implemented by a state machine that has no stored program instructions, or in one or more application specific integrated circuits (ASICs), in which each function or some combinations of certain of the functions are implemented as custom logic. Of course, a combination of the two approaches could be used. 
     Moreover, an embodiment can be implemented as a computer-readable storage medium having computer readable code stored thereon for programming a computer (e.g., comprising a processor) to perform a method as described and claimed herein. Examples of such computer-readable storage mediums include, but are not limited to, a hard disk, a CD-ROM, an optical storage device, a magnetic storage device, a ROM (Read Only Memory), a PROM (Programmable Read Only Memory), an EPROM (Erasable Programmable Read Only Memory), an EEPROM (Electrically Erasable Programmable Read Only Memory) and a Flash memory. Further, it is expected that one of ordinary skill, notwithstanding possibly significant effort and many design choices motivated by, for example, available time, current technology, and economic considerations, when guided by the concepts and principles disclosed herein will be readily capable of generating such software instructions and programs and ICs with minimal experimentation. 
     The Abstract of the Disclosure is provided to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. In addition, in the foregoing Detailed Description, it can be seen that various features are grouped together in various embodiments for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separately claimed subject matter.