Patent Publication Number: US-2021185428-A1

Title: Method and System for Hybrid Noise Cancellation

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application No. 13/646,921, filed Oct. 8, 2012, which claims priority to U.S. Provisional Patent Application Ser. No. 61/544,864, filed Oct. 7, 2011, entitled HYBRID ANALOG DIGITAL ACTIVE NOISE CANCELLER, naming Nitish K. Murthy et al. as inventors, both are hereby fully incorporated herein by reference for all purposes. 
    
    
     BACKGROUND 
     The disclosures herein relate in general to audio signal processing, and in particular to a method and system for hybrid active noise cancellation. 
     A user may hear noise from a surrounding environment. A mechanical structure can attempt to physically buffer the user&#39;s ears against some of the noise, but the mechanical structure has limits. In addition to the mechanical structure, an active noise cancellation system can attempt to generate signals for cancelling at least some of the noise. Nevertheless, different techniques for active noise cancellation have respective shortcomings and trade-offs. 
     SUMMARY 
     From a first microphone, first microphone signals are received that represent first sound waves. From a second microphone, second microphone signals are received that represent second sound waves. In response to the first microphone signals, analog processing is performed to estimate noise in the first sound waves, and first analog signals are generated for cancelling at least some of the estimated noise in the first sound waves. In response to the second microphone signals, digital processing is performed to estimate noise in the second sound waves, and digital information is generated for cancelling at least some of the estimated noise in the second sound waves. The digital information is converted into second analog signals that represent the digital information. The first and second analog signals are combined into third analog signals for cancelling at least some of the estimated noise in the first and second sound waves. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a system of the illustrative embodiments. 
         FIG. 2  is a graph of an example noise signal and an example noise cancellation signal. 
         FIG. 3  is a block diagram of an active noise cancellation (“ANC”) unit of the system of  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a block diagram of a system, indicated generally at  100 , of the illustrative embodiments. A human user  102  has a left ear  104  and a right ear  106  for hearing. An earset  108 , which at least partially fits over and/or into the ear  104 , has: (a) a right side, which faces the ear  104 , and which has a built-in speaker for outputting sound waves to the ear  104 ; and (b) a left side (opposite from the right side), which faces away from the ear  104  toward an environment around the left side of the earset  108  (“left surrounding environment”). Similarly, an earset  110 , which at least partially fits over and/or into the ear  106 , has: (a) a left side, which faces the ear  106 , and which has a built-in speaker for outputting sound waves to the ear  106 ; and (b) a right side (opposite from the left side), which faces away from the ear  106  toward an environment around the right side of the earset  110  (“right surrounding environment”). In one example, the earsets  108  and  110  include mechanical structures that physically buffer the ears  104  and  106 , respectively, against some noise from within the left and right surrounding environments. 
     The earset  108  is integral with: (a) an error microphone  112 , which is located on the right (interior) side of the earset  108 ; and (b) a reference microphone  114 , which is located on the left (exterior) side of the earset  108 . The error microphone  112 : (a) converts, into analog signals, sound waves from a space between the ear  104  and the right side of the earset  108  (e.g., including sound waves from the built-in speaker of the earset  108 ); and (b) outputs those signals. The reference microphone  114 : (a) converts, into analog signals, sound waves from the left surrounding environment (e.g., ambient noise around the reference microphone  114 ); and (b) outputs those signals. 
     The earset  110  is integral with: (a) an error microphone  116 , which is located on the left (interior) side of the earset  110 ; and (b) a reference microphone  118 , which is located on the right (exterior) side of the earset  110 . The error microphone  116 : (a) converts, into analog signals, sound waves from a space between the ear  106  and the left side of the earset  110  (e.g., including sound waves from the built-in speaker of the earset  110 ); and (b) outputs those signals. The reference microphone  118 : (a) converts, into analog signals, sound waves from the right surrounding environment (e.g., ambient noise around the reference microphone  118 ); and (b) outputs those signals. 
     Accordingly, the signals from the error microphone  112  and the reference microphone  114  represent various sound waves. An active noise cancellation (“ANC”) unit  120 : (a) receives and processes the signals from the error microphone  112  and the reference microphone  114 ; and (b) in response thereto, outputs analog signals for cancelling at least some noise in those sound waves. The built-in speaker of the earset  108 : (a) receives the signals from the ANC unit  120 ; and (b) in response thereto, outputs additional sound waves for achieving the noise cancellation. 
     Similarly, the signals from the error microphone  116  and the reference microphone  118  represent sound waves. An ANC unit  122 : (a) receives and processes the signals from the error microphone  116  and the reference microphone  118 ; and (b) in response thereto, outputs analog signals for cancelling at least some noise in those sound waves. The built-in speaker of the earset  110 : (a) receives the signals from the ANC unit  122 ; and (b) in response thereto, outputs additional sound waves for achieving the noise cancellation. 
     In one example, the ANC unit  120  optionally: (a) receives digital audio information from a left channel of an audio source  124 ; and (b) combines the left channel&#39;s audio into the signals that the ANC unit  120  outputs to the built-in speaker of the earset  108 . Accordingly, in this example: (a) the built-in speaker of the earset  108  further outputs sound waves (e.g., music and/or speech) that are represented by the left channel&#39;s digital audio information, so that those sound waves are audible to the ear  104 ; and (b) the ANC unit  120  suitably accounts for those sound waves in its further processing of the signals from the error microphone  112  for cancelling at least some noise in those sound waves. 
     Similarly, the ANC unit  122  optionally: (a) receives digital audio information from a right channel of the audio source  124 ; and (b) combines the right channel&#39;s audio into the signals that the ANC unit  122  outputs to the built-in speaker of the earset  110 . Accordingly, in this example: (a) the built-in speaker of the earset  110  further outputs sound waves (e.g., music and/or speech) that are represented by the right channel&#39;s digital audio information, so that those sound waves are audible to the ear  106 ; and (b) the ANC unit  122  suitably accounts for those sound waves in its further processing of the signals from the error microphone  116  for cancelling at least some noise in those sound waves. 
       FIG. 2  is a graph of: (a) an example noise signal  202 , such as a signal from the error microphone  112  or the reference microphone  114 ; and (b) an example noise cancellation signal  204 , such as a signal from the ANC unit  120  to the built-in speaker of the earset  108 . As shown in  FIG. 2 , the signal  204  is substantially inverted from the signal  202 , so that a phase of the signal  204  is shifted (relative to a phase of the signal  202 ) by ˜180 degrees (e.g., 180 degrees plus a latency) across a bandwidth of the signals  202  and  204 . For example, the latency may result from a processing cycle of the ANC unit  120 . In this manner, the signal  204  is effective for cancelling at least some noise in a sound wave that is represented by the signal  202 . 
       FIG. 3  is a block diagram of the ANC unit  120 , which is a representative one of the substantially identical ANC units  120  and  122 . The error microphone  112  is coupled through an analog-to-digital converter (“ADC”)  302  to a digital feedback controller  304 , so that the ADC  302 : (a) from the error microphone  112 , receives the analog signals that the error microphone  112  outputs in response to sound waves from the space between the ear  104  and the right side of the earset  108 ; (b) converts those analog signals into corresponding digital data that represent those sound waves; and (c) outputs such digital data to the digital feedback controller  304 . Optionally (e.g., programmably), the reference microphone  114  is coupled through an ADC  306  to the digital feedback controller  304 , so that the ADC  306 : (a) from the reference microphone  114 , receives the analog signals that the reference microphone  114  outputs in response to sound waves from the left surrounding environment; (b) converts those analog signals into corresponding digital data that represent those sound waves; and (c) outputs such digital data to the digital feedback controller  304 . 
     In response to such digital data from the ADC  302 , and optionally in response to such digital data from the ADC  306 , the digital feedback controller  304 : (a) performs digital processing to estimate noise in those sound waves; and (b) generates digital information for cancelling at least some of the estimated noise (“noise cancellation information”). A digital mixer  308  combines the noise cancellation information and the digital audio information (if any) that the digital mixer  308  receives from the left channel of the audio source  124 . A digital-to-analog converter (“DAC”)  310 : (a) receives such combined information from the digital mixer  308 ; (b) converts such combined information into corresponding analog signals that represent such combined information; and (c) outputs those analog signals to an analog mixer  312 . 
     The reference microphone  114  is connected to an analog feed-forward controller  314 , so that the analog feed-forward controller  314 : (a) from the reference microphone  114 , receives the analog signals that the reference microphone  114  outputs in response to sound waves from the left surrounding environment; (b) in response to such analog signals, performs analog processing to estimate noise in those sound waves; and (c) generates analog signals for cancelling at least some of the estimated noise (“noise cancellation signals”). For that purpose, in one embodiment, the analog feed-forward controller  314  includes at least one inverting operational amplifier. In the illustrative embodiments, the analog feed-forward controller  314  outputs the noise cancellation signals in a manner that accounts for physical buffering (e.g., filtering) by a mechanical structure of the earset  108 , so that: (a) the analog feed-forward controller  314  estimates noise that such physical buffering fails to exclude from the space between the ear  104  and the right side of the earset  108  (“remaining noise”); (b) the noise cancellation signals are for cancelling at least some of the remaining noise; and (c) accordingly, the noise cancellation signals are substantially inverted (and their phases are shifted by ˜180 degrees) from the remaining noise across a bandwidth thereof. 
     The analog mixer  312 : (a) combines the noise cancellation signals and the analog signals that the analog mixer  312  receives from the DAC  310 ; and (b) outputs such combined signals to the earset  108 . The built-in speaker of the earset  108 : (a) receives such combined signals from the analog mixer  312 ; and (b) in response thereto, outputs additional sound waves for achieving the noise cancellation. 
     In comparison to a feed-forward controller, a feedback controller&#39;s efficacy is especially improved if its operations are performed by digital processing, which enhances precision of such operations. Accordingly, in the ANC unit  120 : (a) the feedback controller  304  performs its operations by digital processing, with oversampling, in either an adaptive manner (e.g., in a first embodiment) or a non-adaptive manner (e.g., in a second embodiment); and (b) the feed-forward controller  314  perform its operations by analog processing. 
     In that manner, the ANC unit  120  implements a hybrid analog-digital ANC technique whose advantages include: (a) with the analog feed-forward controller  314 , relatively good noise cancellation at lower frequencies; (b) with the digital feedback controller  304 , digital tuneability, and cancellation of at least some residual noise that would have otherwise remained uncancelled by the analog feed-forward controller  314 ; and (c) aggregately, better noise cancellation over a wider range of frequencies. For example, in comparison to the digital feedback controller  304 , the analog operations of the analog feed-forward controller  314  are less precise (which may allow residual noise to remain uncancelled) and more cumbersome to tune, but those analog operations achieve: (a) reduced latency for supporting higher frequency bandwidths at lower sampling rates; (b) more stability; and (c) better noise cancellation at lower frequencies. In comparison to the analog feed-forward controller  314 , the digital operations of the digital feedback controller  304  have more latency (which may reduce phase margin and diminish stability) and less noise cancellation at lower frequencies, but those digital operations achieve a bandwidth of cancellation that is: (a) digitally tuneable (e.g., programmable coefficients of noise filtering); and (b) relatively large at high feedback loop gains. 
     In a first alternative embodiment, the error microphone  112  and the reference microphone  114  remain located on opposite sides (of the earset  108 ) from one another, but the reference microphone  114  is spaced a farther distance (e.g., several inches or feet) away from the earset  108 . In a second alternative embodiment, the error microphone  112  and the reference microphone  114  are located on the same side (of the earset  108 ) as one another, so that they convert sound waves that may be similar to (or even identical) to one another. In one example of the second alternative embodiment, the error microphone  112  and the reference microphone  114  are both located on the right side of the earset  108 . Even in the first and second alternative embodiments, many of the hybrid analog-digital ANC technique&#39;s advantages (discussed hereinabove) are still achieved, because: (a) the error microphone  112  remains coupled through the ADC  302  to the digital feedback controller  304 ; and (b) the reference microphone  114  remains connected to the analog feed-forward controller  314  and is optionally coupled through the ADC  306  to the digital feedback controller  304 . 
     The system  100  is formed by electronic circuitry components for performing the system  100  operations, implemented in a suitable combination of software, firmware and hardware. In one embodiment, such components include a digital signal processor (“DSP”), which is a computational resource for executing instructions of computer-readable software programs to process data (e.g., a database of information) and perform additional operations (e.g., communicating information) in response thereto. For operations of the DSP, such programs and data are stored in a memory of the DSP and/or in another computer-readable medium (e.g., hard disk drive, flash memory card, or other nonvolatile storage device) of the system  100 . 
     In the illustrative embodiments, a single DSP is suitably programmed to perform certain operations of both ANC units  120  and  122 , so that the single DSP implements portions of both ANC units  120  and  122 . In one example, the single DSP is a suitably programmed stereo audio codec with embedded miniDSP, such as part number TLV 320 AIC 3254  available from TEXAS INSTRUMENTS INCORPORATED of Dallas, Tex.. In that example, the single DSP is suitably programmed to implement: (a) portions indicated by a dashed enclosure  316  of the ANC unit  120 ; and (b) substantially identical portions of the ANC unit  122 . 
     In the illustrative embodiments, a computer program product is an article of manufacture that has: (a) a computer-readable medium; and (b) a computer-readable program that is stored on such medium. Such program is processable by an instruction execution apparatus (e.g., system or device) for causing the apparatus to perform various operations discussed hereinabove (e.g., discussed in connection with a block diagram). For example, in response to processing (e.g., executing) such program&#39;s instructions, the apparatus (e.g., programmable information handling system) performs various operations discussed hereinabove. Accordingly, such operations are computer-implemented. 
     Such program (e.g., software, firmware, and/or microcode) is written in one or more programming languages, such as: an object-oriented programming language (e.g., C++); a procedural programming language (e.g., C); and/or any suitable combination thereof. In a first example, the computer-readable medium is a computer-readable storage medium. In a second example, the computer-readable medium is a computer-readable signal medium. 
     A computer-readable storage medium includes any system, device and/or other non-transitory tangible apparatus (e.g., electronic, magnetic, optical, electromagnetic, infrared, semiconductor, and/or any suitable combination thereof) that is suitable for storing a program, so that such program is processable by an instruction execution apparatus for causing the apparatus to perform various operations discussed hereinabove. Examples of a computer-readable storage medium include, but are not limited to: an electrical connection having one or more wires; a portable computer diskette; a hard disk; a random access memory (“RAM”); a read-only memory (“ROM”); an erasable programmable read-only memory (“EPROM” or flash memory); an optical fiber; a portable compact disc read-only memory (“CD-ROM”); an optical storage device; a magnetic storage device; and/or any suitable combination thereof 
     A computer-readable signal medium includes any computer-readable medium (other than a computer-readable storage medium) that is suitable for communicating (e.g., propagating or transmitting) a program, so that such program is processable by an instruction execution apparatus for causing the apparatus to perform various operations discussed hereinabove. In one example, a computer-readable signal medium includes a data signal having computer-readable program code embodied therein (e.g., in baseband or as part of a carrier wave), which is communicated (e.g., electronically, electromagnetically, and/or optically) via wireline, wireless, optical fiber cable, and/or any suitable combination thereof. 
     Although illustrative embodiments have been shown and described by way of example, a wide range of alternative embodiments is possible within the scope of the foregoing disclosure.