Abstract:
In accordance with embodiments of the present disclosure, a method and apparatus for providing a digitized microphone signal to a digital processing device may include an analog signal path portion, a digital signal path portion, and a control circuit. The analog signal path portion may have an audio input configured to receive an analog input signal indicative of audio sounds incident upon an audio transducer. The digital signal path portion may have an analog-to-digital converter for converting the analog microphone signal to the digitized microphone signal. The control circuit may be configured to control a magnitude of the analog input signal or a derivative thereof in order to reduce audio distortion occurring in either or both of the analog signal path portion and the digital signal path portion.

Description:
RELATED APPLICATIONS 
     The present disclosure claims priority to U.S. Provisional Patent Application Ser. No. 61/810,075, filed Apr. 9, 2013, which is incorporated by reference herein in its entirety. 
    
    
     FIELD OF DISCLOSURE 
     The present disclosure relates in general to audio systems, and more particularly, to reducing distortion of a microphone signal. 
     BACKGROUND 
     Microphones are ubiquitous on many devices used by individuals, including computers, tablets, smart phones, and many other consumer devices. Generally speaking, a microphone is an electroacoustic transducer that produces an electrical signal in response to deflection of a portion (e.g., a membrane or other structure) of a microphone caused by sound incident upon the microphone. To process audio signals generated by a microphone, microphones are often coupled to an audio system. However, many traditional audio system topologies may have disadvantages, as is illustrated with reference to  FIG. 1 . 
       FIG. 1  illustrates a block diagram of selected components of an example audio system  100 , as is known in the art. As shown in  FIG. 1 , audio system  100  may include an analog signal path portion comprising bias voltage source  102 , a microphone transducer  104 , analog pre-amplifier  108 , a digital path portion comprising an analog-to-digital converter (ADC)  110 , a driver  112 , and a digital audio processor  114 . 
     Bias voltage source  102  may comprise any suitable system, device, or apparatus configured to supply microphone transducer  104  with a direct-current bias voltage V BIAS , such that microphone transducer  104  may generate an electrical audio signal. Microphone transducer  104  may comprise any suitable system, device, or apparatus configured to convert sound incident at microphone transducer  104  to an electrical signal, wherein such sound is converted to an electrical analog input signal using a diaphragm or membrane having an electrical capacitance (modeled as variable capacitor  106  in  FIG. 1 ) that varies as based on sonic vibrations received at the diaphragm or membrane. Microphone transducer  104  may include an electrostatic microphone, a condenser microphone, an electret microphone, a microelectromechanical systems (MEMs) microphone, or any other suitable capacitive microphone. Pre-amplifier  108  may receive the analog input signal output from microphone transducer  104  and may comprise any suitable system, device, or apparatus configured to condition the analog audio signal for processing by ADC  110 . 
     ADC  110  may receive a pre-amplified analog audio signal output from pre-amplifier  108 , and may comprise any suitable system device or apparatus configured to convert the pre-amplified analog audio signal received at its input to a digital signal representative of the analog audio signal generated by microphone transducer  104 . ADC  110  may itself include one or more components (e.g., delta-sigma modulator, decimator, etc.) for carrying out the functionality of ADC  110 . Driver  112  may receive the digital signal output by ADC  110  and may comprise any suitable system, device, or apparatus configured to condition such digital signal (e.g., encoding into Audio Engineering Society/European Broadcasting Union (AES/EBU), Sony/Philips Digital Interface Format (S/PDIF), or other suitable audio interface standards), in the process generating a digitized microphone signal for transmission over a bus to digital audio processor  114 . 
     Once converted to the digitized microphone signal, the digitized microphone signal may be transmitted over significantly longer distances without being susceptible to noise as compared to an analog transmission over the same distance. In some embodiments, one or more of bias voltage source  102 , pre-amplifier  108 , ADC  110 , and driver  112  may be disposed in close proximity with microphone transducer  104  to ensure that the length of the analog signal transmission lines are relatively short to minimize the amount of noise that can be picked up on such analog output lines carrying analog signals. For example, in some embodiments, one or more of bias voltage source  102 , microphone transducer  104 , pre-amplifier  108 , ADC  110 , and driver  112  may be formed on the same integrated circuit die or substrate. 
     Digital audio processor  114  may comprise any suitable system, device, or apparatus configured to process the digitized microphone signal for use in a digital audio system. For example, digital audio processor  114  may comprise a microprocessor, microcontroller, digital signal processor (DSP), application specific integrated circuit (ASIC), or any other device configured to interpret and/or execute program instructions and/or process data, such as the digitized microphone signal output by driver  112 . 
     Despite the various advantages of digital microphone systems such as those shown in  FIG. 1 , such digital microphone systems may have disadvantages. For example, many components of the analog path portion, and in particular pre-amplifier  108 , may be susceptible to noise and may consume significant amounts of power during operation. 
     SUMMARY 
     In accordance with the teachings of the present disclosure, certain disadvantages and problems associated with existing audio systems including microphones may be reduced or eliminated. 
     In accordance with embodiments of the present disclosure, an input network for a delta-sigma modulator having a feedback digital-to-analog stage and at least one integrator stage including an input integrator stage may include a plurality of sampling capacitors. At least one of the plurality of sampling capacitors may include a microphone capacitance of a microphone transducer, wherein the microphone capacitance may be variable and indicative of audio sounds incident upon the microphone transducer. During a first phase of a clock signal, the one or more sampling capacitors may sample an input signal wherein the input signal is a function of the microphone capacitance. During a second phase of the clock signal, the input network may charge transfer the input signal onto an input of the input integrator stage. 
     In accordance with these and other embodiments of the present disclosure, a method may include, during a first period of a first phase of a clock signal, sampling an input signal with one or more sampling capacitors of input network for a delta-sigma modulator having a feedback digital-to-analog stage, at least one integrator stage including an input integrator stage, and a plurality of sampling capacitors. At least one of the plurality of sampling capacitors may comprise a microphone capacitance of a microphone transducer, wherein the microphone capacitance is variable and indicative of audio sounds incident upon the microphone transducer, and further wherein the input signal is a function of the microphone capacitance. The method may also include transferring the input signal onto an input of the input integrator stage during a second phase of the clock signal. 
     Technical advantages of the present disclosure may be readily apparent to one having ordinary skill in the art from the figures, description and claims included herein. The objects and advantages of the embodiments will be realized and achieved at least by the elements, features, and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are explanatory examples and are not restrictive of the claims set forth in this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present embodiments and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings, in which like reference numbers indicate like features, and wherein: 
         FIG. 1  illustrates a block diagram of selected components of an example audio system, as is known in the art; 
         FIG. 2  illustrates a block diagram of selected components of an example audio system, in accordance with embodiments of the present disclosure; 
         FIG. 3  illustrates a block diagram of an example delta-sigma feedforward modulator, in accordance with embodiments of the present disclosure; 
         FIG. 4A  illustrates a block diagram of an input sampling network for a delta-sigma modulator, in accordance with embodiments of the present disclosure; and 
         FIG. 4B  illustrates a block diagram of another input sampling network for a delta-sigma modulator, in accordance with embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  illustrates a block diagram of selected components of an example audio system  200 , in accordance with embodiments of the present disclosure. As shown in  FIG. 2 , audio system  200  may include an analog-to-digital converter (ADC)  210 , a driver  212 , and a digital audio processor  214 . 
     As shown in  FIG. 2 , ADC  210  may include an input network  400  and a modulator  300 . As described in greater detail below, input network  400  may include a switched capacitor network wherein at least one switched capacitor of the switched capacitor network comprises a microphone capacitance of a microphone transducer. Such microphone capacitance may be variable based on sounds incident upon the microphone transducer, and accordingly, such microphone transducer may generate an analog signal which may then be converted into the digital domain by remaining components of ADC  210 , including modulator  300 . Driver  212  may receive the digital signal output by ADC  210  and may comprise any suitable system, device, or apparatus configured to condition such digital signal (e.g., encoding into AES/EBU, S/PDIF, or other suitable audio interface standards), in the process generating a digitized microphone signal for transmission over a bus to digital audio processor  214 . Digital audio processor  214  may comprise any suitable system, device, or apparatus configured to process the digitized microphone signal for use in a digital audio system. For example, digital audio processor  214  may comprise a microprocessor, microcontroller, digital signal processor (DSP), application specific integrated circuit (ASIC), or any other device configured to interpret and/or execute program instructions and/or process data, such as the digitized microphone signal output by driver  212 . 
       FIG. 3  illustrates a block diagram of an example delta-sigma feed-forward modulator  300 , in accordance with embodiments of the present disclosure. Modulator  300  may be a suitable modulator that may be used as or as part of ADC  210  depicted in  FIG. 2 . Delta-sigma modulator  300  may include an input summer  301  and one or more integrator stages  302 . Although any suitable number of integrator stages may be used, in the embodiments represented by  FIG. 3 , delta-sigma modulator  300  includes five integrator stages  302   a - 302   e , and thus delta-sigma modulator  300  depicted in  FIG. 3  is a fifth-order delta-sigma modulator. Delta-sigma modulator  300  may include a weighted feed-forward design in which the outputs of each of the integrator stages may be passed through a respective gain stage (amplifier)  303  (e.g., amplifiers  303   a - 303   e ) to an output summer  304 . Amplifiers  303   a - 303   e  may allow the outputs of the integrator stages to be weighted at the input of summer  304 . The output from summer  304  may be quantized by a multiple-bit quantizer  305 , which may generate a multiple-bit digital output signal labeled as ADC in  FIG. 3 . Additionally, the output from quantizer  305  may be fed back to the inverting input of summer  301  through dynamic element matching (DEM) circuitry  306  and digital-to-analog converter (DAC)  307 . 
       FIG. 3  also depicts an additional feed-forward path, including amplifier  308 , between modulator input  310  and summer  304 . The gain of amplifier stage  308  may be given by the equation gain=(1/Quantizer gain)(1/DAC gain), where “Quantizer gain” is a signal gain applied by quantizer  305  and “DAC gain” is a signal gain applied by DAC  307 . The purpose of this additional feed-forward path is to cancel as much of the input signal energy from the delta-sigma loop as possible. Consequently, most of the voltage swing within the modulator may be quantization noise. In turn, the design constraints on the sub-circuits within modulator  300  may be relaxed. For example, the first integrator stage  302   a  is typically the major contributor to the noise performance of the entire modulator. This feed-forward technique results in less signal energy at the outputs of the integrator stages, and hence such parameters as the stage operation amplifier (opamp) DC gain may be reduced. In turn, the power consumption of the device as well as the die size may be reduced. 
     A fifth-order feed-forward design was selected for discussion purposes; in actual implementation, the order as well as the configuration of modulator  300  may vary. 
       FIG. 4A  illustrates a block diagram of an input sampling network  400   a , in accordance with embodiments of the present disclosure. In some embodiments, input sampling network  400   a  may be utilized as input summer  301  and first stage  302   a  of delta-sigma modulator  300  of  FIG. 3 , although its utility is not limited thereto. 
     Input network  400   a  may include sampling capacitors  410  and  411 . As depicted in  FIG. 4A , a capacitance of sampling capacitor  410  may be variable. Such variable capacitance C M  may be a microphone capacitance of a microphone transducer  409 , wherein the microphone capacitance is indicative of audio sounds incident upon microphone transducer  409 . Microphone transducer  409  may comprise any suitable system, device, or apparatus configured to convert sound incident at microphone transducer  409  to an electrical signal, wherein such sound is converted to an electrical analog input signal using a diaphragm or membrane having an electrical capacitance (e.g., sampling capacitor  410  in  FIGS. 4A and 4B ) that varies as based on sonic vibrations received at the diaphragm or membrane. Microphone transducer  409  may include an electrostatic microphone, a condenser microphone, an electret microphone, a microelectromechanical systems (MEMs) microphone, or any other suitable capacitive microphone. 
     As shown in  FIG. 4A , sampling capacitors  410  and  411  may be biased by a bias voltage source  401 . Bias voltage source  401  may comprise any suitable system, device, or apparatus configured to supply sampling capacitors  410  and  411  a direct-current bias voltage V BIAS , such that microphone transducer  409  may generate an electrical audio signal as its capacitance varies in response to incident audio sounds. 
     Input network  400   a  of  FIG. 4A  may generally operate in accordance with a clock signal CLK, the complement of which is a signal CLK′. Each of clock signals CLK and CLK′ may comprise a square-wave signal, as shown in  FIG. 4A . Clock signals CLK and CLK′ may define clock cycles operating at a sampling rate wherein each clock cycle includes a first phase when clock signal CLK is high and clock signal CLK′ is low and a second phase when clock signal CLK is low and clock signal CLK′ is high. Generally, during the first phase of each cycle, switches  402  and  408  may close and a charge stored by sampling capacitor  410  as a result of the change in capacitance C M  of sampling capacitor  410  in response to incident audio sounds may be sampled onto cross-coupled sampling capacitors  410  and  411  as a differential signal. During the second phase of each cycle, switches  404  and  406  may close, effectively changing the polarity of bias voltage source  401 . Consequently the charges sampled onto sampling capacitors  410  and  411  during the first phase may be respectively charge transferred onto integration capacitors  414   a  and  414   b  which are each coupled between inputs and outputs of an integrator  412 . 
     Feedback voltages DAC +  and DAC −  may also be applied to the feedback capacitors  430   a  and  430   b  during each phase via switches  426  and  428 , thus performing sampling for the feedback voltage signals and the function of summer  301  in applying negative feedback of the DAC +  and DAC −  signals. In some embodiments, the relative sizes of the capacitances C F  of feedback capacitors  430   a  and  430   b  and the capacitances C M  and C S  of sampling capacitors  410  and  411  may be selected to provide for a desired feedback gain. 
       FIG. 4B  illustrates a block diagram of another input sampling network  400   b , in accordance with embodiments of the present disclosure. In some embodiments, input sampling network  400   b  may be utilized as input summer  301  and first stage  302   a  of delta-sigma modulator  300  of  FIG. 3 , although its utility is not limited thereto. Sampling network  400   b  as depicted in  FIG. 4B  is identical to sampling network  400   b  depicted in  FIG. 4A , except that fixed-capacitance capacitor  411  of input network  400  is replaced with variable-capacitance capacitor  411   b . Accordingly, operation and functionality of input network  400   b  may be similar to that of input network  400   a , except that the variable capacitance of capacitor  411   b  may allow for larger differential input signals than if such capacitance was fixed, which may improve performance characteristics (e.g., noise, signal integrity) of input network  400   b.    
     For example, in some embodiments sampling capacitor  411   b  may comprise a second microphone capacitance of a second microphone transducer, wherein the second microphone capacitance varies as a function of audio sounds incident upon the second microphone transducer. In such embodiments, the second microphone transducer may be physically arranged with respect to microphone transducer  409  such that charge forming on sampling capacitors  410  and  411   b  is of substantially equal magnitude but with opposite polarity, thus potentially increasing the voltage swing of differential input signal as compared to embodiments in which sampling capacitor  411  is fixed. 
     As another example, in some embodiments, input network  400   b  may be configured (e.g., may comprise additional components than that depicted in  FIG. 4B ) such that the variable capacitance of sampling capacitor  411   b  is a function of the analog feedback signal (e.g., differential output signal represented by DAC+ and DAC−) generated by a feedback digital-to-analog stage of ADC  210 . 
     In some embodiments of the present disclosure, an input network (e.g., input network  400   a  or input network  400   b ), including a microphone transducer integral thereto, may be formed along with other components of modulator  300  upon a single substrate. In other embodiments, one or more components of an input network (e.g., input network  400   a  or input network  400   b ), including a microphone transducer integral thereto, and other components of modulator  300  may be formed on different substrates packaged within the same integrated circuit package. 
     The methods and systems disclosed herein may provide one or more advantages over traditional approaches. For example, by integrating the capacitance of a microphone transducer into the input network of a delta-sigma modulator, a digital microphone system may not require a separate analog pre-amplifier (e.g., analog pre-amplifier  108 ), which may potentially reduce circuit size, reduce power consumption, and reduce noise present in an analog portion of a digital microphone system. 
     This disclosure encompasses all changes, substitutions, variations, alterations, and modifications to the example embodiments herein that a person having ordinary skill in the art would comprehend. Similarly, where appropriate, the appended claims encompass all changes, substitutions, variations, alterations, and modifications to the example embodiments herein that a person having ordinary skill in the art would comprehend. Moreover, reference in the appended claims to an apparatus or system or a component of an apparatus or system being adapted to, arranged to, capable of, configured to, enabled to, operable to, or operative to perform a particular function encompasses that apparatus, system, or component, whether or not it or that particular function is activated, turned on, or unlocked, as long as that apparatus, system, or component is so adapted, arranged, capable, configured, enabled, operable, or operative. 
     All examples and conditional language recited herein are intended for pedagogical objects to aid the reader in understanding the disclosure and the concepts contributed by the inventor to furthering the art, and are construed as being without limitation to such specifically recited examples and conditions. Although embodiments of the present disclosure have been described in detail, it should be understood that various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the disclosure.