Abstract:
A synchronous detection and calibration system is provided for expedient calibration of differential acoustic sensors in a manufacturing and testing environment. By processing a series of sequentially received tones, respective portions of a system using differential acoustic sensors are tuned for optimum individual operation, following which corresponding control data are generated and stored for use in selecting among predetermined calibration vectors which establish and maintain optimum system operation.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    The present invention relates to acoustic sensors, including microphone arrays, and in particular, to amplifier circuits for differential microphone arrays. 
         [0003]    2. Description of the Related Art 
         [0004]    With the seemingly ever increasing popularity of cellular telephones, as well as personal digital assistances (PDAs) providing voice recording capability, it has become increasingly important to have noise canceling microphones capable of operating in noisy acoustic environments. Further, even in the absence of excessive background noise, noise canceling microphones are nonetheless highly desirable for certain applications, such as speech recognition devices and high fidelity microphones for studio and live performance uses. 
         [0005]    Such microphones are often referred to as pressure gradient or first order differential (FOD) microphones, and have a diaphragm which vibrates in accordance with differences in sound pressure between its front and rear surfaces. This allows such a microphone to discriminate against airborne and solid-borne sounds based upon the direction from which such noise is received relative to a reference axis of the microphone. Additionally, such a microphone can distinguish between sound originating close to and more distant from the microphone. 
         [0006]    For the aforementioned applications, so called close-talk microphones, i.e., microphones which are positioned as close to the mouth of the speaker as possible, are seeing increasing use. In particular, multiple microphones are increasingly configured in the form of a close-talking differential microphone array (CTDMA), which inherently provide low frequency far field noise attenuation. Accordingly, a CTDMA advantageously cancels far field noise, while effectively accentuating the voice of the close talker, thereby spatially enhancing speech quality while minimizing background noise. (Further discussion of these types of microphones can be found in U.S. Pat. Nos. 5,473,684, and 5,586,191, the disclosures of which are incorporated herein by reference.) 
         [0007]    Optimum performance of a CTDMA system using multiple microphones is obtained when all the microphones have the same frequency characteristics. However, in practice, the frequency characteristics of microphones tend to vary from each other due to process variations in their production. For example, typical electret microphones can have variations of as much as 3 dB in the telephony frequency range. The performance of a CTDMA system degrades greatly if variations among the microphones exceed a range of 0.5-1.0 dB. Thus, extra measures are needed to calibrate such variations. While technically suitable calibration systems and methods are known, they tend to be costly in terms of hardware and time needed for operation, both of which are unacceptable for use in manufacture and test of low cost consumer electronics, such as cellular telephone handsets. Additionally, existing solutions are typically implemented with one or more analog-to-digital converters (ADCs) which couple the microphones to power consuming digital signal processor (DSP) systems performing powerful signal processing algorithms that, in turn, unavoidably degrade battery operating times. 
       SUMMARY OF THE INVENTION 
       [0008]    In accordance with the presently claimed invention, a synchronous detection and calibration system provides for expedient calibration of differential acoustic sensors in a manufacturing and testing environment. By processing a series of sequentially received tones, respective portions of a system using differential acoustic sensors are tuned for optimum individual operation, following which corresponding control data are generated and stored for use in selecting among predetermined calibration vectors which establish and maintain optimum system operation. 
         [0009]    In accordance with one embodiment of the presently claimed invention, a synchronous detection and calibration system for a close-talking differential microphone array (CTDMA) includes: 
         [0010]    a plurality of input electrodes to convey a plurality of microphone signals each of which corresponds to a source audio signal having a plurality of frequencies; 
         [0011]    controllable amplifier circuitry coupled to the plurality of input electrodes and responsive to a plurality of amplifier control signals and the plurality of microphone signals by providing a plurality of selectively amplified signals at each of the plurality of frequencies; 
         [0012]    controllable filter circuitry coupled to the controllable amplifier circuitry and responsive to a plurality of filter control signals and the plurality of selectively amplified signals by providing a plurality of selectively filtered signals at each of the plurality of frequencies; 
         [0013]    signal combining circuitry coupled to the controllable filter circuitry and responsive to the plurality of selectively filtered signals by providing a combination signal at each of the plurality of frequencies, wherein the combination signal has a plurality of values each of which is related to a difference between corresponding ones of the plurality of selectively filtered signals; 
         [0014]    synchronous signal detection circuitry coupled to one of the plurality of input electrodes and the signal combining circuitry, and responsive to one of the plurality of microphone signals and the combination signal by providing an error signal indicative of respective ones of the plurality of combination signal values; and 
         [0015]    calibration circuitry coupled to the synchronous signal detection circuitry, the controllable amplifier circuitry and the controllable filter circuitry, and responsive to the error signal by providing the plurality of amplifier control signals and the plurality of filter control signals such that the error signal, for each of the plurality of frequencies, is indicative of a minimum difference between the corresponding ones of the plurality of selectively filtered signals. 
         [0016]    In accordance with another embodiment of the presently claimed invention, a synchronous detection and calibration system for a close-talking differential microphone array (CTDMA) includes: 
         [0017]    input means for conveying a plurality of microphone signals each of which corresponds to a source audio signal having a plurality of frequencies; 
         [0018]    controllable amplifier means for responding to a plurality of amplifier control signals and the plurality of microphone signals by providing a plurality of selectively amplified signals at each of the plurality of frequencies; 
         [0019]    controllable filter means for responding to a plurality of filter control signals and the plurality of selectively amplified signals by providing a plurality of selectively filtered signals at each of the plurality of frequencies; 
         [0020]    signal combiner means for responding to the plurality of selectively filtered signals by providing a combination signal at each of the plurality of frequencies, wherein the combination signal has a plurality of values each of which is related to a difference between corresponding ones of the plurality of selectively filtered signals; 
         [0021]    synchronous signal detector means for responding to one of the plurality of microphone signals and the combination signal by providing an error signal indicative of respective ones of the plurality of combination signal values; and 
         [0022]    calibration means for responding to the error signal by providing the plurality of amplifier control signals and the plurality of filter control signals such that the error signal, for each of the plurality of frequencies, is indicative of a minimum difference between the corresponding ones of the plurality of selectively filtered signals. 
         [0023]    In accordance with another embodiment of the presently claimed invention, a synchronous detection and calibration system for a close-talking differential microphone array (CTDMA) includes: 
         [0024]    a plurality of input electrodes to convey a plurality of microphone signals, including a selected input electrode to convey a selected microphone signal, wherein each one of the plurality of microphone signals corresponds to a source audio signal having a plurality of frequencies; 
         [0025]    first controllable amplifier circuitry coupled to at least one of the plurality of input electrodes and responsive to at least a first amplifier control signal and at least one the plurality of microphone signals by providing at least a first selectively amplified signal at each of the plurality of frequencies; 
         [0026]    second controllable amplifier circuitry coupled to the first controllable amplifier circuitry and responsive to at least a second amplifier control signal and the first selectively amplified signal by providing a second selectively amplified signal at each of the plurality of frequencies; 
         [0027]    signal combining circuitry coupled to the selected input electrode and the second controllable amplifier circuitry, and responsive to the selected microphone signal and the second selectively amplified signal by providing a combination signal at each of the plurality of frequencies, wherein the combination signal has a plurality of values each of which is related to a difference between corresponding ones of the selected microphone signal and second selectively amplified signal; 
         [0028]    synchronous signal detection circuitry coupled to the selected input electrode and the signal combining circuitry, and responsive to the selected microphone signal and the combination signal by providing an error signal indicative of respective ones of the plurality of combination signal values; and 
         [0029]    calibration circuitry coupled to the synchronous signal detection circuitry, the first controllable amplifier circuitry and the second controllable amplifier circuitry, and responsive to the error signal by providing the at least a first amplifier control signal and the at least a second amplifier control signal such that the error signal, for each of the plurality of frequencies, is indicative of a minimum difference between the corresponding ones of the selected microphone signal and second selectively amplified signal. 
         [0030]    In accordance with another embodiment of the presently claimed invention, a synchronous detection and calibration system for a close-talking differential microphone array (CTDMA) includes: 
         [0031]    input means for conveying a plurality of microphone signals, including a selected input electrode to convey a selected microphone signal, wherein each one of the plurality of microphone signals corresponds to a source audio signal having a plurality of frequencies; 
         [0032]    first controllable amplifier means for responding to at least a first amplifier control signal and at least one the plurality of microphone signals by providing at least a first selectively amplified signal at each of the plurality of frequencies; 
         [0033]    second controllable amplifier means for responding to at least a second amplifier control signal and the first selectively amplified signal by providing a second selectively amplified signal at each of the plurality of frequencies; 
         [0034]    signal combiner means for responding to the selected microphone signal and the second selectively amplified signal by providing a combination signal at each of the plurality of frequencies, wherein the combination signal has a plurality of values each of which is related to a difference between corresponding ones of the selected microphone signal and second selectively amplified signal; 
         [0035]    synchronous signal detector means for responding to the selected microphone signal and the combination signal by providing an error signal indicative of respective ones of the plurality of combination signal values; and 
         [0036]    calibration means for responding to the error signal by providing the at least a first amplifier control signal and the at least a second amplifier control signal such that the error signal, for each of the plurality of frequencies, is indicative of a minimum difference between the corresponding ones of the selected microphone signal and second selectively amplified signal. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0037]      FIG. 1  is a functional block diagram of a synchronous detection and calibration system in accordance with one embodiment of the presently claimed invention. 
           [0038]      FIG. 2  is a functional block diagram of a synchronous detection and calibration system in accordance with another embodiment of the presently claimed invention. 
           [0039]      FIG. 3  is a functional block diagram of one example embodiment of a synchronous energy detector suitable for use in the systems of  FIGS. 1 and 2 . 
       
    
    
     DETAILED DESCRIPTION 
       [0040]    The following detailed description is of example embodiments of the presently claimed invention with references to the accompanying drawings. Such description is intended to be illustrative and not limiting with respect to the scope of the present invention. Such embodiments are described in sufficient detail to enable one of ordinary skill in the art to practice the subject invention, and it will be understood that other embodiments may be practiced with some variations without departing from the spirit or scope of the subject invention. 
         [0041]    Throughout the present disclosure, absent a clear indication to the contrary from the context, it will be understood that individual circuit elements as described may be singular or plural in number. For example, the terms “circuit” and “circuitry” may include either a single component or a plurality of components, which are either active and/or passive and are connected or otherwise coupled together (e.g., as one or more integrated circuit chips) to provide the described function. Additionally, the term “signal” may refer to one or more currents, one or more voltages, or a data signal. Within the drawings, like or related elements will have like or related alpha, numeric or alphanumeric designators. Further, while the present invention has been discussed in the context of implementations using discrete electronic circuitry (preferably in the form of one or more integrated circuit chips), the functions of any part of such circuitry may alternatively be implemented using one or more appropriately programmed processors, depending upon the signal frequencies or data rates to be processed. 
         [0042]    Referring to  FIG. 1 , a synchronous detection and calibration system  100   a  in accordance with one embodiment of the presently claimed invention processes audio signals received form acoustic sensors in the form of microphones based upon calibration data generated in accordance with the presently claimed invention. This system  100   a  includes microphones  102   a ,  102   b , variable gain amplifiers  104   a ,  104   b , biquad filters  106   a ,  106   b , summing circuitry  108 , a synchronous energy detector  112 , a calibration controller  114 , a lookup table (e.g., a read only memory)  116 , and a programmable memory (e.g., an electrically erasable programmable read only memory)  118 , all interconnected substantially as shown. 
         [0043]    During normal operation, incoming acoustic signals  101  are received by the microphones  102   a ,  102   b  and converted to corresponding electrical signals  103   a ,  103   b . These signals  103   a ,  103  are amplified with variable gain amplifiers  104   a ,  104   b , the gains for which are controlled in accordance with control signals  117   a ,  117   b  from the lookup table  116 . The resulting amplified signals  105   a ,  105   b  are filtered by the biquad filters  106   a ,  106   b , the characteristics (e.g., gain Gn, center frequency Fe and quality factor Q) are controlled in accordance with additional control signals  117   c ,  117   d  from the lookup table  116 . The filtered signals  107   a ,  107   b  are differentially summed in the summing circuit  108 . The resulting sum signal  109  is further amplified with a variable gain amplifier  110 , the gain for which is controlled in accordance with another control signal  117   e  from the lookup table  116  (e.g., to compensate for other losses elsewhere within the host system) to produce the final output signal  111 . 
         [0044]    During calibration of the system  100   a , a series of sequential tones are provided as the acoustic signals  101 , e.g., from a loudspeaker. In accordance with one embodiment, three test tones are used, e.g., 300, 1,000 and 3,000 Hertz. However, any number of tones at any desired frequency can be used for calibrating this system  100   a . During calibration, the center frequencies of the biquad filters  106   a ,  106   b  are set to the frequency of the test tone being used at that time, and the degree of frequency dependent gain is necessarily set to a minimum to avoid altering the frequency dependent gain mismatch realized between any chosen pair of aforesaid microphones. The sum signal  109 , which serves as an error signal (i.e., the difference between the filtered signals  107   a ,  107   b ), is processed by the synchronous energy detector  112  in synchronization with one of the incoming microphone signals  103   b  (discussed in more detail below). 
         [0045]    While monitoring the processed error signal  113 , the calibration controller  114  provides control signals  115   b  to the lookup table  116  so as to cause appropriate control signals  117   a ,  117   b  to be provided to one or both of the variable gain amplifiers  104   a ,  104   b  such that the magnitude of the processed error signal  113 , which corresponds to the input error signal  109 , to be minimized. This operation is performed for each of the test tones. (The control data for the control signals  117   a ,  117   b ,  117   c ,  117   d  is based on prior characterization or testing of the system  100   a  and has been preprogrammed into the lookup table  116 .) 
         [0046]    Following completion of these tests, i.e., after the appropriate gain control data  117   a ,  117   b  have been determined for minimizing the error signal  109  at each test tone, the corresponding control data  115   b  are provided as index data  115   c  to the programmable memory  118 . This index data  115   c  is stored in the programmable memory  118  for later use as the control data  119  for the lookup table during normal operation of the system  100   a . As will be readily understood by one of ordinary skill in the art, coordination and timing of all operations are controlled using system control data  199  provided by a host system controller (not shown). 
         [0047]    Referring to  FIG. 2 , an alternative embodiment  100   b  includes most elements of the system of  100   a  of  FIG. 1 , plus a variable gain calibration amplifier  104   c  and summing circuit  120 , all interconnected substantially as shown. In this embodiment  100   b , one of the amplified microphone signals  105   a  is further amplified by the calibration amplifier  104   c  in accordance with control signals  115   d  from the calibration controller  114 . The resulting amplified signal  105   c  is differentially summed with the other microphone signal  103   b  to produce the error signal  121  to be processed by the synchronous energy detector  112 . During calibration, the center frequencies of the biquad filters  106   a ,  106   b  are set to the frequency of the test tone being processed at the time, and the gain G 2  of the calibration amplifier  104   c  is set and maintained at a predetermined value (e.g., zero decibels). In this system  100   b , an odd number of test tones are used, with the middle test tone applied first. 
         [0048]    Applying the middle test tone (e.g., 1,000 Hertz), the error signal  121  is minimized by varying the gain G 1  of the input amplifier  104   a  in accordance with its control data  117   a , as selected by the control data  115   b  from the calibration controller  114  based on the processed error signal  113 , as discussed above. The gain G 1  at which the error signal  121  is minimized is maintained for subsequent testing using the remaining test tones (e.g., 300 and 3,000 Hertz). The remaining test tones are then applied sequentially, as discussed above, with the gain G 2  of the calibration amplifier  104   c  now being controlled, in accordance with its control data  115   d , to minimize the error signal  121  for each test tone. Based upon these tests, a gain G 2  of the calibration amplifier  104   c  can be determined that provides for minimization of the error signal  121  for all test tones other than the middle test tone. This gain value G 2  can then be mapped into corresponding appropriate gain values for amplifiers within the biquad filters  106   a ,  106   b  by selecting the appropriate control data  117   c ,  117   d  within the lookup table  116 . 
         [0049]    Following completion of these calibration tests using the test tones, the calibration control data  115   b  which produces the desired control data  117   a ,  117   c ,  117   d  for the input amplifier  104   a  and biquad filters  106   a ,  106   b , as discussed above, is provided as index data  115   c  to the programmable memory  118  for storage and use as control data  119  for the lookup table  116  during normal operation of the system  100   b.    
         [0050]    Referring to  FIG. 3 , one example embodiment  112   a  of the synchronous energy detector can be implemented using a limiter (e.g., a signal slicer)  202 , a signal multiplier (e.g., a mixer)  204 , and a signal integrator  206 , interconnected substantially as shown. The microphone signal  103   b  used for synchronizing the detector  112   a  is limited by the limiter  202 . The limited signal  203  is multiplied with the error signal  109 / 121  to produce a product signal  205  that is independent of polarity changes in the original input signals  107   a ,  107   b  ( FIG. 1 ),  105   c ,  103   b  ( FIG. 2 ) that produce the error signal  109 / 121 . The polarity of the product signal  205  is determined by the relative magnitudes of the original input signals  107   a ,  107   b ,  105   c ,  103   b , which reflect the mismatches in the input sensors  102   a ,  102   b . Accordingly, by analyzing the product signal  205  at various gain steps, as discussed above, the degree of mismatch between the sensors  102   a ,  102   b  can be determined. 
         [0051]    To track the polarity of the product signal  205  more effectively, it is integrated within the integrator  206  which attenuates random variations and circuit noise present within the product signal  205 . This integrator  206  operates in a periodic manner in accordance with the control data  115   a  from the calibration controller  114 , with the duration of each integration cycle being controlled by the calibration controller  114  (e.g., in accordance with an oscillator). At the beginning of each test cycle, the gain steps are established, as discussed above, and the output  113  of the integrator  206  is reset to a predetermined value (e.g., zero). The product signal  205  is then integrated throughout the remainder of the test cycle. As discussed above, these test cycles are repeated until the optimum gain steps are determined. 
         [0052]    Various other modifications and alternations in the structure and method of operation of this invention will be apparent to those skilled in the art without departing from the scope and the spirit of the invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. It is intended that the following claims define the scope of the present invention and that structures and methods within the scope of these claims and their equivalents be covered thereby.