Patent Publication Number: US-10313790-B1

Title: Polymorphic playback system with switching oscillation prevention

Description:
FIELD OF DISCLOSURE 
     The present disclosure relates in general to circuits for audio devices, including without limitation personal audio devices, such as wireless telephones and media players, and more specifically, to systems and methods for preventing oscillation or frequent switching between polymorphic modes of a polymorphic playback system. 
     BACKGROUND 
     Personal audio devices, including wireless telephones, such as mobile/cellular telephones, cordless telephones, mp3 players, and other consumer audio devices, are in widespread use. Such personal audio devices may include circuitry for driving a pair of headphones or one or more speakers. Such circuitry often includes a power amplifier for driving an audio output signal to headphones or speakers. 
     A personal audio device may include a polymorphic playback system. A polymorphic playback system may include a playback system in which one or more parameters of a signal path of the playback system are varied based on one or more characteristics of a playback signal processed by the signal path. Examples of a polymorphic playback system include a dynamic range enhancement playback system in which signal gains of a playback path are varied based on one or more characteristics (e.g., signal magnitude) of the playback signal, a multiple processing path playback system in which a processing path for processing the playback signal is selected based on one or more characteristics (e.g., signal magnitude) of the playback signal, an amplifier with a configurable output stage (e.g., configurable as a Class AB output stage or Class D output stage) in which the configuration of the output stage is based on one or more characteristics (e.g., signal magnitude) of the playback signal, or any other suitable polymorphic playback system. 
     Many playback systems, such as those that perform adaptive noise cancellation, are required to have low latency in their playback paths. In addition, many playback systems include playback signals with a significant amount of out-of-band noise. In some instances, the presence of out-of-band noise may cause switching of a polymorphic playback system from a first polymorphic mode in which a first set of one or more parameters are applied to the signal path based on one or more characteristics of a playback signal to a second polymorphic mode in which a second set of one or more parameters are applied to the signal path based on the one more characteristics of a playback signal, and vice versa. In some instances, such out-of-band noise may lead to frequent switching or oscillation between the polymorphic modes. Such frequent switching may occur because thresholds in a control subsystem for switching between modes may be optimized for a particular data source. However, if the data source is changed, or its noise performance drifts over time, noise fluctuation may cause random crossing of the switching threshold and thus, oscillation between polymorphic modes. 
     Such out-of-band noise may influence an in-band signal received by a level detector for detecting a playback signal magnitude in a polymorphic playback system that utilizes the playback signal magnitude as a characteristic for setting one or more parameters of the signal path of the playback system. While a strong low-pass filter could be used to filter out the out-of-band noise, use of a strong low-pass filter may add too much undesirable latency to signal detection. 
     SUMMARY 
     In accordance with the teachings of the present disclosure, one or more disadvantages and problems associated with existing approaches to frequent switching between polymorphic modes in a polymorphic playback system may be reduced or eliminated. 
     In accordance with embodiments of the present disclosure, a polymorphic playback system in which one or parameters of a signal path of the polymorphic playback system are varied based on one or more characteristics of a playback signal processed by the signal path may include a control subsystem configured to detect an out-of-band noise profile of the playback signal and set one or more playback signal magnitude thresholds for switching between polymorphic modes of the polymorphic playback system based on the out-of-band noise profile, wherein the polymorphic modes comprise at least a first polymorphic mode in which one or more first parameters are applied to the signal path and a second polymorphic mode in which one or more second parameters are applied to the signal path. 
     In accordance with these and other embodiments of the present disclosure, a method may be provided for use in a polymorphic playback system in which one or more parameters of a signal path of the polymorphic playback system are varied based on one or more characteristics of a playback signal processed by the signal path, the method comprising detecting an out-of-band noise profile of the playback signal and setting one or more playback signal magnitude thresholds for switching between polymorphic modes of the polymorphic playback system based on the out-of-band noise profile, wherein the polymorphic modes comprise at least a first polymorphic mode in which one or more first parameters are applied to the signal path and a second polymorphic mode in which one or more second parameters are applied to the signal path. 
     Technical advantages of the present disclosure may be readily apparent to one skilled 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 examples and explanatory 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  is an illustration of an example personal audio device, in accordance with embodiments of the present disclosure; 
         FIG. 2A  illustrates a block diagram of selected components of an example audio integrated circuit of a personal audio device, in accordance with embodiments of the present disclosure; 
         FIG. 2B  illustrates a block diagram of selected components of another example audio integrated circuit of a personal audio device, in accordance with embodiments of the present disclosure; 
         FIG. 3  illustrates a block diagram of selected components of an example amplifier, in accordance with embodiments of the present disclosure; 
         FIG. 4  illustrates a block diagram of selected components of a digital microphone integrated circuit, in accordance with embodiments of the present disclosure; 
         FIG. 5A  illustrates a block diagram of selected components of a generalized polymorphic playback system, in accordance with embodiments of the present disclosure; and 
         FIG. 5B  illustrates a block diagram of selected components of another generalized polymorphic playback system, in accordance with embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is an illustration of an example personal audio device  1 , in accordance with embodiments of the present disclosure.  FIG. 1  depicts personal audio device  1  coupled to a headset  3  in the form of a pair of earbud speakers  8 A and  8 B. Headset  3  depicted in  FIG. 1  is merely an example, and it is understood that personal audio device  1  may be used in connection with a variety of audio transducers, including without limitation, headphones, earbuds, in-ear earphones, and external speakers. A plug  4  may provide for connection of headset  3  to an electrical terminal of personal audio device  1 . Personal audio device  1  may provide a display to a user and receive user input using a touch screen  2 , or alternatively, a standard liquid crystal display (LCD) may be combined with various buttons, sliders, and/or dials disposed on the face and/or sides of personal audio device  1 . As also shown in  FIG. 1 , personal audio device  1  may include an audio integrated circuit (IC)  9  for generating an analog audio signal for transmission to headset  3  and/or another audio transducer. 
       FIG. 2A  illustrates a block diagram of selected components of an example audio IC  9 A of a personal audio device, in accordance with embodiments of the present disclosure. In some embodiments, example audio IC  9 A may be used to implement audio IC  9  of  FIG. 1 . As shown in  FIG. 2A , a microcontroller core  18  may supply a digital audio input signal DIG_IN to a digital-to-analog converter (DAC)  14 , which may convert the digital audio input signal to an analog input signal V IN . DAC  14  may supply analog input signal V IN  to an amplifier  16 A which may amplify or attenuate analog input signal V IN  to provide an audio output signal V OUT , which may operate a speaker, headphone transducer, a line level signal output, and/or other suitable output. 
       FIG. 2B  is a block diagram of selected components of an example audio IC  9 B of a personal audio device, in accordance with embodiments of the present disclosure. 
     Example audio IC  9 B of  FIG. 2B  may be similar in many respects to example audio IC  9 A of  FIG. 2A , except as otherwise described herein. As shown in  FIG. 2B , a microcontroller core  18  may supply a digital audio input signal DIG_IN to a digital gain element  12  to apply a selectable digital gain x selected by control subsystem  20  to digital input signal DIG_IN. The amplified digital audio input signal may be communicated to a digital-to-analog converter (DAC)  14 , which may convert the digital audio input signal to an analog input signal V IN . Together, digital gain element  12  and DAC  14  may be referred to herein as a digital path portion of the signal path from the input node for digital audio input signal DIG_IN to the output node for audio output signal V OUT  depicted in  FIG. 2B . In the relevant art, digital gain element  12  and DAC  14  may sometimes be referred to as an audio compressor. 
     DAC  14  may supply analog input signal V IN  to an amplifier  16 B which may amplify or attenuate analog input signal V IN  in conformity with a selectable analog gain k/x to provide an audio output signal V OUT , which may operate a speaker, headphone transducer, a line level signal output, and/or other suitable output Amplifier  16 B may be referred to herein as an analog path portion of the signal path from the input node for digital audio input signal DIG_IN to the output node for output voltage signal V OUT  depicted in  FIG. 2B . In the relevant art, amplifier  16 B may sometimes be referred to as an audio expander. 
     As shown in  FIG. 2B , audio IC  9 B may include a control subsystem  20  configured to, based on one or more characteristics of digital audio input signal DIG_IN, control selectable digital gain x of gain element  12  and a selectable analog gain k/x of amplifier  16 B. In embodiments in which a volume control is present, a volume control signal may be provided from a microcontroller or other digital control circuit responsive to a user interface, volume knob encoder or program command, or other suitable mechanism. 
     As an example of the dynamic range enhancement functionality of audio IC  9 B, when digital audio input signal DIG_IN is at or near zero decibels (0 dB) relative to the full-scale voltage of the digital audio input signal, control subsystem  20  may select a first digital gain (e.g., x 1 ) for the selectable digital gain and a first analog gain (e.g., k/x 1 ) for the selectable analog gain. However, if the magnitude of digital audio input signal DIG_IN is below a particular predetermined threshold magnitude relative to the full-scale voltage of digital audio input signal DIG_IN (e.g., −20 dB), control subsystem  20  may select a second digital gain (e.g., x 2 ) greater than the first digital gain (e.g., x 2 &gt;x 1 ) for the selectable digital gain and a second analog gain (e.g., k/x 2 ) lesser than the first analog gain (e.g., k/x 2 &lt;k/x 1 ) for the selectable analog gain. In each case, the cumulative path gain (e.g., k) of the selectable digital gain and the selectable analog gain may be substantially constant (e.g., the same within manufacturing and/or operating tolerances of audio IC  9 B). In some embodiments, k may be approximately equal to 1, such that the cumulative path gain is a unity gain. Such modification of digital gain and analog gain may increase the dynamic range of audio IC  9 B compared to approaches in which the digital gain and analog gain are static, as it may reduce the noise injected into audio output signal V OUT , which noise may be a generally monotonically increasing function of the analog gain of amplifier  16 B. While such noise may be negligible for higher magnitude audio signals (e.g., at or near 0 dB relative to full-scale voltage), the presence of such noise may become noticeable for lower magnitude audio signals (e.g., at or near −20 dB or lower relative to full-scale voltage). By applying a smaller analog gain at amplifier  16 B for smaller signal magnitudes, the amount of noise injected into audio output signal V OUT  may be reduced, while the signal level of audio output signal V OUT  may be maintained in accordance with the digital audio input signal DIG_IN through application of a digital gain to gain element  12  inversely proportional to the analog gain. 
       FIG. 3  illustrates a block diagram of selected components of an example amplifier  18 , in accordance with embodiments of the present disclosure. In some embodiments, amplifier  18  may be used to implement all or a portion of amplifier  16 A of  FIG. 2A . In these and other embodiments, amplifier  18  may be used to implement all or a portion of amplifier  16 B of  FIG. 2B . 
     As shown in  FIG. 3 , amplifier  18  may include a first stage  22  (e.g., an analog front end) configured to receive analog input signal V IN  at an amplifier input of amplifier  18  and generate an intermediate signal V INT  which is a function of analog input signal V IN , a final output stage  24  configured to generate audio output signal V OUT  at an amplifier output of amplifier  18  as a function of intermediate signal V INT , a signal feedback network  26  coupled between the amplifier output and the amplifier input, and a control subsystem  28  for controlling the operation of certain components of amplifier  18 , as described in greater detail below. 
     First stage  22  may include any suitable analog front end circuit for conditioning analog input signal V IN  for use by final output stage  24 . For example, first stage  22  may include one or more analog integrators  32  cascaded in series, as shown in  FIG. 3 . 
     Final output stage  24  may include any suitable driving circuit for driving audio output signal V OUT  as a function of intermediate signal V INT  (thus, also making audio output signal V OUT  a function of analog input signal V IN ) wherein final output stage  24  is switchable among a plurality of modes including at least a first mode in which final output stage  24  generates audio output signal V OUT  as a modulated output signal which is a function of intermediate signal V INT  and a second mode in which final output stage  24  generates audio output signal V OUT  as an unmodulated output signal which is a function of intermediate signal V INT . To carry out this functionality, final output stage  24  may include a class-D audio output stage  42  which may be enabled in the first mode (and disabled in the second mode) to generate audio output signal V OUT  as a modulated output signal which is a function of intermediate signal V INT  and a class-AB audio output stage  44  which may be enabled in the second mode (and disabled in the first mode) to generate audio output signal V OUT  as an unmodulated output signal which is a function of intermediate signal V INT . 
     Class-D audio amplifier  42  may comprise any suitable system, device, or apparatus configured to amplify intermediate signal V INT  and convert intermediate signal V INT  into a series of pulses by pulse width modulation, pulse density modulation, or another method of modulation, such that intermediate signal V INT  is converted into a modulated signal in which a characteristic of the pulses of the modulated signal (e.g., pulse widths, pulse density, etc.) is a function of the magnitude of intermediate signal V INT . After amplification by class-D audio amplifier  42 , its output pulse train may be converted back to an unmodulated analog signal by passing through a passive low-pass filter, wherein such low-pass filter may be inherent in output circuitry of class-D audio amplifier  42  or a load driven by final output stage  24 . As shown in  FIG. 3 , class-D audio amplifier  42  may include a control input for receiving a control input from control subsystem  28  in order to selectively enable class-D audio amplifier  42  during the first mode and disable class-D audio amplifier  42  during the second mode (e.g., prevent class-D audio amplifier  42  from driving the amplifier output of amplifier  18  by disabling or decoupling a supply voltage from class-D audio amplifier  42  or by disabling or decoupling driving devices of the amplifier output of amplifier  18 ). 
     Class-AB audio amplifier  44  may comprise any suitable system, device, or apparatus configured to amplify intermediate signal V INT  with a linear gain and convert intermediate signal V INT  into an unmodulated audio output signal V OUT . As shown in  FIG. 3 , class-AB audio amplifier  44  may include a control input for receiving a control input from control subsystem  28  in order to selectively enable class-AB audio amplifier  44  during the second mode and disable class-AB audio amplifier  44  during the first mode (e.g., prevent class-AB audio amplifier  44  from driving the amplifier output of amplifier  18  by disabling or decoupling a supply voltage from class-AB audio amplifier  44  or by disabling or decoupling driving devices of the amplifier output of amplifier  18 ). 
     In some embodiments, a signal gain (e.g., V OUT /V INT ) of final output stage  24  in the first mode may be approximately equal to the signal gain of final output stage  24  in the second mode. In these and other embodiments, an offset (e.g., direct current offset) of final output stage  24  in the first mode may be approximately equal to the offset of final output stage  24  in the second mode. 
     Signal feedback network  26  may include any suitable feedback network for feeding back a signal indicative of audio output signal V OUT  to the amplifier input of amplifier  18 . For example, as shown in  FIG. 3 , signal feedback network  26  may include variable feedback resistors  48 , wherein resistances of variable feedback resistors  48  are controlled by control signals received from control subsystem  28 , as described in greater detail below. 
     Control subsystem  28  may include any suitable system, device, or apparatus configured to receive information indicative of audio output signal V OUT , intermediate signal V INT , and/or other operational characteristics of amplifier  18 , and based at least thereon, control operation of one or more components of amplifier  18 . For example, control subsystem  28  may be configured to, based on a characteristic of analog input signal V IN  (e.g., which may be determined from receiving and analyzing intermediate signal V INT  and/or audio output signal V OUT ), switch between the first mode and the second mode of final output stage  24 . Such characteristic may include one or more of a frequency of analog input signal V IN , an amplitude of analog input signal V IN , a signal-to-noise ratio of analog input signal V IN , a noise floor of analog input signal V IN , or another noise characteristic of analog input signal V IN . For example, in some embodiments, control subsystem  28  may be configured to switch final output stage  24  from the first mode to the second mode when an amplitude of analog input signal V IN  decreases below a threshold amplitude, and may be configured to switch final output stage  24  from the second mode to the first mode when an amplitude of analog input signal V IN  increases above the same threshold amplitude or another threshold amplitude. In some embodiments, to reduce audio artifacts associated with switching between modes, control subsystem  28  may also be configured to switch between modes only when the amplitude of audio output signal V OUT  is approximately zero (e.g., when a modulated signal generated by class-D audio amplifier  42  is at its minimum voltage in its generated pulse train). 
     In addition, control subsystem  28  may also be configured to perform calibration of final output stage  24 . For example, control subsystem  28  may receive and analyze intermediate signal V INT  and audio output signal V OUT  to determine a gain of class-D audio amplifier  42  (e.g., the signal gain of final output stage  24  in the first mode) and a gain of class-AB audio amplifier  44  (e.g., the signal gain of final output stage  24  in the second mode), and based thereon, modify the gain of class-D audio amplifier  42  and/or the gain of class-AB audio amplifier  44  in order to calibrate the signal gain of final output stage  24  in the second mode to match the signal gain of final output stage  24  in the first mode. As another example, control subsystem  28  may receive and analyze intermediate signal V INT  and/or audio output signal V OUT  to determine an offset (e.g., direct current offset) of class-D audio amplifier  42  (e.g., the offset of final output stage  24  in the first mode) and an offset of class-AB audio amplifier  44  (e.g., the offset of final output stage  24  in the second mode), and based thereon, modify the offset of class-D audio amplifier  42  and/or the offset of class-AB audio amplifier  44  in order to calibrate the offset of final output stage  24  in the second mode to match the offset of final output stage  24  in the first mode. 
     In these and other embodiments, control subsystem  28  may also be configured to control characteristics of first stage  22  (e.g., integrators  32 ) and/or signal feedback network  26 . Control subsystem  28  may maintain such characteristics and structure of first stage  22  and signal feedback network  26  as static when switching between the first mode and the second mode of final output stage  24  and when switching between the second mode and the first mode. Maintaining the characteristics and structure of first stage  22  and signal feedback network  26  as static when switching between modes allows the modes to share the same analog front end and feedback network, thus reducing or minimizing the likelihood of mismatched signal gain and offset between the modes, and thus reducing or minimizing audio artifacts caused by switching between modes. However, after control subsystem  28  has switched final output stage  24  to the second mode (e.g., amplifier output driven by class-AB amplifier  44 ), control subsystem  28  may modify characteristics of first stage  22  and/or signal feedback network  26  in order to decrease a noise floor of amplifier  18 . For example, in some embodiments, control subsystem  28  may modify characteristics of integrators  32  (e.g., resistances and/or capacitances of filters internal to integrators  32 ) and/or other components of first stage  22  in order to decrease a noise floor of amplifier  18  when final output stage  24  operates in the second mode. As another example, in these and other embodiments, control subsystem  28  may modify characteristics of signal feedback network  26  (e.g., resistances of variable feedback resistors  48 ) in order to decrease a noise floor of amplifier  18  when final output stage  24  operates in the second mode. When making such modification, control subsystem  28  may, before switching final output stage  24  from the second mode to the first mode, return such characteristics to their unmodified states. 
       FIG. 4  illustrates a block diagram of selected components of a digital microphone IC  52 , in accordance with embodiments of the present disclosure. As shown in  FIG. 4 , digital microphone IC  52  may include two or more audio processing paths  54   a  and  54   b  (which may be referred to herein individually as an audio processing path  54  and collectively as AFE/ADC paths  54 ), each AFE/ADC path  54  including a respective AFE  56  (e.g., AFE  56   a , AFE  56   b ) and a respective ADC (e.g., ADC  58   a , ADC  58   b ). An AFE  56  may receive an analog audio input signal ANALOG_IN via one or more input lines which may allow for receipt of a single-ended signal, differential signal, or any other suitable analog audio signal format and may comprise any suitable system, device, or apparatus configured to condition analog audio input signal ANALOG_IN for processing by ADC  58 . In some embodiments, analog audio input signal ANALOG_IN may comprise an electronic signal generated by a microphone as a function of sound pressure incident upon the microphone. The output of each AFE  56  may be communicated to a respective ADC  58  on one or more output lines. 
     An ADC  58  may comprise any suitable system, device, or apparatus configured to convert an analog audio signal received at its input, to a digital signal representative of analog audio input signal ANALOG_IN. ADC  58  may itself include one or more components (e.g., delta-sigma modulator, decimator, etc.) for carrying out the functionality of ADC  58 . 
     A multiplexer  60  may receive a respective digital audio signal from each of audio processing paths  54 , and may select one of the digital audio signals as the digital audio output signal DIGITAL_OUT based on a control signal generated by and communicated from a control subsystem  64 . 
     Driver  62  may receive the digital audio output signal DIGITAL_OUT output by ADC  58  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)), in the process generating digital audio output signal DIGITAL_OUT for transmission over a bus to a digital audio processor. In  FIG. 4 , the bus receiving digital audio output signal DIGITAL_OUT is shown as single-ended. In some embodiments, driver  62  may generate digital audio output signal DIGITAL_OUT as a differential digital audio output signal. 
     Control subsystem  64  may comprise any suitable system, device, or apparatus for selecting one of the digital audio signals output by the various audio processing paths  54  as digital audio output signal DIGITAL_OUT. In some embodiments, control subsystem  64  may make such selection based on a magnitude of analog audio input signal ANALOG_IN or a signal derivative thereof. For example, control subsystem  64  may include an overload detector that may determine whether or not a signal derivative of analog audio input signal ANALOG_IN (e.g., an analog signal output by AFE  56   a ) is likely to cause clipping or other distortion of digital audio output signal DIGITAL_OUT if a particular audio processing path (e.g., audio processing path  54   a ) is selected. If clipping or other distortion of digital audio output signal DIGITAL_OUT is likely if the particular audio processing path (e.g., audio processing path  54   a ) is selected, control subsystem  64  may generate a control signal so that another audio processing path (e.g., audio processing path  54   b ) is selected. To further illustrate, in some embodiments, audio processing path  54   a  may be a path adapted for low amplitudes of analog audio input signal ANALOG_IN and may thus have a high signal gain, while audio processing path  54   b  may be a path adapted for higher amplitudes of analog audio input signal ANALOG_IN and may thus have a lower signal gain. Thus, if analog audio input signal ANALOG_IN or a derivative thereof is greater than a threshold value indicative of a condition whereby digital audio output signal DIGITAL_OUT may experience clipping or other distortion if audio processing path  54   a  is selected, control subsystem  64  may detect such condition and generate a control signal to select the digital audio signal generated by audio processing path  54   b  as digital audio output signal DIGITAL_OUT. 
     As another example, control subsystem  64  may include a level detector that may detect an amplitude of analog audio input signal ANALOG_IN or a signal derivative thereof (e.g., a signal generated within ADC  58   b ). Responsive to the amplitude level detected by the level detector, control subsystem  64  may generate the control signal communicated to multiplexer  60 . To illustrate, as analog audio input signal ANALOG_IN decreases from a relatively high amplitude to a lower amplitude, it may cross a threshold amplitude level whereby control subsystem  64  may change the selection of digital audio output signal DIGITAL_OUT from the digital audio signal generated by audio processing path  54   b  (which may be adapted for higher amplitudes of analog audio input signal ANALOG_IN) to the digital audio signal generated by audio processing path  54   a  (which may be adapted for lower amplitudes of analog audio input signal ANALOG_IN). In some embodiments, a threshold amplitude level whereby control subsystem  64  may change the selection of digital audio output signal DIGITAL_OUT from the digital audio signal generated by audio processing path  54   b  to the digital audio signal generated by audio processing path  54   a  may be lower than another threshold amplitude level whereby control subsystem  64  may change the selection of digital audio output signal DIGITAL_OUT from the digital audio signal generated by audio processing path  54   a  to the digital audio signal generated by audio processing path  54   b , in order to provide for hysteresis so that multiplexer  60  does not repeatedly switch between the paths. 
     Each of the various systems described above with respect to  FIGS. 2A-4  (e.g., audio IC  9 A, audio IC  9 B, amplifier  18 , digital microphone IC  52 ) may comprise a polymorphic playback system or an integral portion thereof, in that in each of such systems, one or more parameters of a signal path of such polymorphic playback system may be varied based on one or more characteristics of a playback signal processed by the signal path. Although  FIGS. 2A-4  set forth specific examples of a polymorphic playback system, a polymorphic playback system may include any other type of system in which one or more parameters of a signal path of such system is varied based on one or more characteristics of a playback signal processed by the signal path. 
       FIG. 5A  illustrates a block diagram of selected components of a generalized polymorphic playback system  70 A, in accordance with embodiments of the present disclosure. As shown in  FIG. 5A , polymorphic playback system  70 A may filter an input signal with a low-latency filter  72 , which may then be processed by a polymorphic plant  82  to generate an output signal. Polymorphic plant  82  may comprise any system, device, or apparatus in which one or more parameters of a signal path of the polymorphic playback system are varied based on one or more characteristics of a playback signal processed by the signal path. For example, polymorphic plant  82  may select between polymorphic modes based on comparisons of one or more characteristics of the playback signal processed by the signal path to one or more thresholds set by threshold control subsystem  73 A and received by polymorphic plant  82 . 
     Threshold control subsystem  73 A may be configured to detect an out-of-band noise profile of the input signal by using a noise characterization subsystem  74 A which may filter the input signal using high-latency filter  76  that performs more robust filtering than filter  72 , and then, using combiner  78 , subtracting the input signal as filtered by filter  76  from the input signal as filtered by filter  72  to generate a resulting signal which is an indication of the amount of out-of-band noise present in the filtered signal received by polymorphic plant  82 . Based on this amount of out-of-band noise, a threshold calculation block  80 A may calculate thresholds. For example, threshold calculation block  80 A may increase or decrease relevant thresholds based on the out-of-band noise present. As another example, a range of hysteresis between a first threshold for switching from a first polymorphic mode to a second polymorphic mode, and a second threshold for switching from the second polymorphic mode to the first polymorphic mode, may increase for higher levels of out-of-band noise and decrease for lower levels of out-of-band noise. 
       FIG. 5B  illustrates a block diagram of selected components of another generalized polymorphic playback system  70 B, in accordance with embodiments of the present disclosure. As shown in  FIG. 5B , polymorphic playback system  70 B may filter an input signal with a low-latency filter  72 , which may then be processed by a polymorphic plant  82  to generate an output signal. Polymorphic plant  82  may comprise any system, device, or apparatus in which one or more parameters of a signal path of the polymorphic playback system are varied based on one or more characteristics of a playback signal processed by the signal path. For example, polymorphic plant  82  may select between polymorphic modes based on comparisons of one or more characteristics of the playback signal processed by the signal path to one or more thresholds set by threshold control subsystem  73 B and received by polymorphic plant  82 . 
     Threshold control subsystem  73 B may be configured to detect an out-of-band noise profile of the input signal by using a noise characterization subsystem  74 B which may detect an out-of-band noise profile by analyzing statistics associated with the playback signal. In some embodiments, such statistics may be indicative of a frequency of occurrence of the playback signal crossing the one or more playback signal magnitude thresholds. To illustrate, increment block  84  and decrement block  86  may cause a numeric value maintained by a counter  88  to increase each time polymorphic plant  82  switches between polymorphic modes, and compare with comparator  90  the output of counter  88  to a limit to determine if the frequency of switching between polymorphic modes is greater than the relevant limit. Frequent switching between polymorphic modes may indicate a high level of out-of-bound noise, and thus, the output of comparator  90  may be an indication of the amount of out-of-band noise present in the filtered signal received by polymorphic plant  82 . Based on the signal generated by comparator  90 , a threshold calculation block  80 B may calculate thresholds. For example, threshold calculation block  80 B may increase or decrease relevant thresholds based on the out-of-band noise present. As another example, a range of hysteresis between a first threshold for switching from a first polymorphic mode to a second polymorphic mode, and a second threshold for switching from the second polymorphic mode to the first polymorphic mode, may increase for higher levels of out-of-band noise and decrease for lower levels of out-of-band noise. 
     For the purposes of clarity and exposition,  FIG. 5A  shows a system using filtering to extract an out-of-band noise characterization from the playback signal and  FIG. 5B  shows a system that analyzes statistics associated with the playback signal to extract an out-of-band noise characterization. However, in some embodiments, a polymorphic playback system may utilize both filtering to extract an out-of-band noise characterization from the playback signal and statistical analysis of the playback signal to extract an out-of-band noise characterization. 
     Using the systems and methods described above, the setting of the one or more playback signal magnitude thresholds may minimize switching (e.g., oscillation) between the polymorphic modes due to out-of-band noise. 
     As used herein, when two or more elements are referred to as “coupled” to one another, such term indicates that such two or more elements are in electronic communication or mechanical communication, as applicable, whether connected indirectly or directly, with or without intervening elements. 
     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. Accordingly, modifications, additions, or omissions may be made to the systems, apparatuses, and methods described herein without departing from the scope of the disclosure. For example, the components of the systems and apparatuses may be integrated or separated. Moreover, the operations of the systems and apparatuses disclosed herein may be performed by more, fewer, or other components and the methods described may include more, fewer, or other steps. Additionally, steps may be performed in any suitable order. As used in this document, “each” refers to each member of a set or each member of a subset of a set. 
     Although exemplary embodiments are illustrated in the figures and described below, the principles of the present disclosure may be implemented using any number of techniques, whether currently known or not. The present disclosure should in no way be limited to the exemplary implementations and techniques illustrated in the drawings and described above. 
     Unless otherwise specifically noted, articles depicted in the drawings are not necessarily drawn to scale. 
     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. 
     Although specific advantages have been enumerated above, various embodiments may include some, none, or all of the enumerated advantages. Additionally, other technical advantages may become readily apparent to one of ordinary skill in the art after review of the foregoing figures and description. 
     To aid the Patent Office and any readers of any patent issued on this application in interpreting the claims appended hereto, applicants wish to note that they do not intend any of the appended claims or claim elements to invoke 35 U.S.C. § 112(f) unless the words “means for” or “step for” are explicitly used in the particular claim.