Patent Publication Number: US-2021161415-A1

Title: Physiological monitoring devices and methods for noise reduction in physiological signals based on subject activity type

Description:
CLAIM OF PRIORITY 
     This application is a continuation of U.S. patent application Ser. No. 15/922,610 filed Mar. 15, 2018, which claims priority from U.S. Provisional Patent Application No. 62/475,746 entitled “PHYSIOLOGICAL MONITORING DEVICES AND METHODS FOR NOISE REDUCTION IN PHYSIOLOGICAL SIGNALS BASED ON SUBJECT ACTIVITY TYPE” filed Mar. 23, 2017, and also claims priority as a continuation-in-part from U.S. patent application Ser. No. 15/299,684 entitled “PHYSIOLOGICAL MONITORING DEVICES AND METHODS THAT IDENTIFY SUBJECT ACTIVITY TYPE,” filed Oct. 21, 2016, which claims priority from U.S. Provisional Patent Application No. 62/245,919 entitled “PHYSIOLOGICAL MONITORING DEVICES AND METHODS THAT IDENTIFY SUBJECT ACTIVITY TYPE,” filed Oct. 23, 2015, in the United States Patent and Trademark Office, the disclosures of which are incorporated by reference herein in their entireties. 
    
    
     FIELD 
     The present invention relates generally to monitoring devices and methods, more particularly, to monitoring devices and methods for measuring physiological information. 
     BACKGROUND 
     Photoplethysmography (PPG) is based upon shining light into the human body and measuring how the scattered light intensity changes with each pulse of blood flow. The scattered light intensity will change in time with respect to changes in blood flow or blood opacity associated with heart beats, breaths, blood oxygen level (SpO 2 ), and the like. Such a sensing methodology may require the magnitude of light energy reaching the volume of flesh being interrogated to be steady and consistent so that small changes in the quantity of scattered photons can be attributed to varying blood flow. If the incidental and scattered photon count magnitude changes due to light coupling variation between the source or detector and the skin or other body tissue, then the signal of interest can be difficult to ascertain due to large photon count variability caused by motion artifacts. Changes in the surface area (and volume) of skin or other body tissue being impacted with photons, or varying skin surface curvature reflecting significant portions of the photons may also significantly impact optical coupling efficiency. Physical activity, such a walking, cycling, running, etc., may cause motion artifacts in the optical scatter signal from the body, and time-varying changes in photon intensity due to motion artifacts may swamp-out time-varying changes in photon intensity due to blood flow changes. Each of these changes in optical coupling can dramatically reduce the signal-to-noise ratio (S/N) of biometric PPG information to total time-varying photonic interrogation count. This can result in a much lower accuracy in metrics derived from PPG data, such as heart rate and breathing rate. 
     An earphone, such as a headset, earbud, etc., may be a good choice for incorporation of a photoplethysmography (PPG) device because it is a form factor that individuals are familiar with, it is a device that is commonly worn for long periods of time, and it is often used during exercise which is a time when individuals may benefit from having accurate heart rate data (or other physiological data). Unfortunately, incorporation of a photoplethysmography device into an earphone poses several challenges. For example, earphones may be uncomfortable to wear for long periods of time, particularly if they deform the ear surface. Moreover, human ear anatomy may vary significantly from person to person, so finding an earbud form that will fit comfortably in many ears may be difficult. Other form-factors for PPG devices, such as wristbands, armbands, clothing, and the like may be problematic for motion artifacts as well. 
     Some previous efforts to reduce motion artifacts from wearable PPG devices have focused on passive filtering, adaptive filtering (U.S. Pat. No. 8,923,941), or removing noise associated with footsteps (U.S. Pat. No. 8,157,730) with harmonics associated with a subject&#39;s cadence (US 2015/0366509). Each of these methods is associated with both strengths and weaknesses, depending on the wearable PPG sensor location or the type of motion noise that is present for the subject wearing the PPG sensor. 
     SUMMARY 
     It should be appreciated that this Summary is provided to introduce a selection of concepts in a simplified form, the concepts being further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of this disclosure, nor is it intended to limit the scope of the invention. 
     According to some embodiments of the present invention, a monitoring device configured to be attached to a body of a subject includes a sensor that is configured to detect and/or measure physiological information from the subject (e.g., a PPG sensor, etc.), at least one motion sensor configured to detect and/or measure subject motion information, and at least one processor that is configured to receive and analyze physiological sensor signals produced by the physiological sensor and motion sensor signals produced by the motion sensor. The monitoring device may be configured to be positioned at or within an ear of the subject, secured to an appendage of the subject, or integrated into a form-factor that is wearable at a location of the body of the subject (for example, on a wrist). The physiological sensor may be an optical sensor that includes at least one optical emitter and at least one optical detector, although various other types of sensors may be utilized. The at least one processor is configured to process motion sensor signals to identify an activity characteristic of the subject. For example, the at least one processor is configured to determine if the subject is engaged in a periodic activity such as exercising, running, walking, etc., or if the subject is engaged in a lifestyle or non-periodic activity such as reading a book, desk work, eating, etc. A motion sensor&#39;s signal during random motion associated with lifestyle activities will be different than the signal during periodic activities. Once an activity characteristic is determined, the at least one processor is configured to select a noise reference in response to identification of the activity characteristic of the subject, and then process physiological sensor signals using the noise reference to generate output signals having reduced noise relative to the physiological sensor signals and produce physiological information about the subject. 
     In some embodiments, the at least one motion sensor may be first and second motion sensors, and selecting the noise reference may include selecting the first motion sensor responsive to identification of the activity characteristic as periodic motion, or selecting the second motion sensor responsive to identification of the activity characteristic as non-periodic motion. 
     In some embodiments, the physiological sensor may be an optical sensor, and a signal response of the second motion sensor may more closely correspond to motion-related optical noise present in the physiological sensor signal than a signal response of the first motion sensor. 
     In some embodiments, processing the physiological sensor signal using the noise reference may include processing the physiological sensor signal using a frequency-domain-based algorithm or circuit responsive to identification of the activity characteristic as periodic motion, or processing the physiological sensor signal using a time-domain-based algorithm or circuit responsive to identification of the activity characteristic as non-periodic motion. 
     In some embodiments, the physiological sensor may be a photoplethysmography (PPG) sensor, the first motion sensor may be an inertial sensor, and wherein the second motion sensor may be an optomechanical sensor. 
     In some embodiments, processing the physiological sensor signal using the noise reference may further include, in response to identification of the activity characteristic as periodic motion, processing the physiological sensor signal using a first algorithm or circuit, or, in response to identification of the activity characteristic as non-periodic motion, processing the physiological sensor signal using a second algorithm or circuit that is different from the first algorithm or circuit. 
     In some embodiments, the first algorithm or circuit may be based on frequency-domain processing and/or filtering, and wherein the second algorithm or circuit may be based on time-domain processing and/or filtering. 
     In some embodiments, the first and second algorithms or circuits may utilize a same sensor or sensors of the at least one motion sensor. 
     In some embodiments, the first algorithm or circuit may be configured to perform spectral subtraction, and the second algorithm or circuit may be configured to perform adaptive filtering. 
     In some embodiments, the optomechanical sensor may be configured to modulate light generated by an optical emitter in response to body motion, and to direct the light upon a pathway which is substantially unaffected by blood flow. 
     In some embodiments, body motion information contained in the noise reference may be greater than blood flow information contained therein. 
     In some embodiments, the noise reference may include information associated with motion of the subject and may be substantially free of information associated with blood flow of the subject. 
     In some embodiments, the physiological information may include one or more of the following: subject heart rate, subject respiration rate, subject respiratory rate interval (RRi), subject heart rate variability (HRV), subject blood pressure, subject blood analyte levels, subject cardiovascular properties. 
     According to some embodiments of the present invention, a monitoring device configured to be attached to a bodily location of a subject includes a physiological sensor configured to detect and/or measure physiological data from the subject and generate a physiological sensor signal representative thereof, at least one motion sensor configured to detect and/or measure subject motion data and generate a motion sensor signal representative thereof, and at least one processor coupled to the physiological sensor and the motion sensor. The at least one processor is configured to perform operations including processing the motion sensor signal received from the at least one motion sensor to identify an activity characteristic of the subject, selecting a noise reference in response to identification of the activity characteristic, and processing the physiological sensor signal received from the physiological sensor using the noise reference to generate an output signal having reduced noise relative to the physiological sensor signal. 
     According to some embodiments of the present invention, a computer program product includes a non-transitory computer readable storage medium having computer readable program code embodied therein. When executed by at least one processor, the computer readable program code causes the at least one processor to perform operations including processing a motion sensor signal received from at least one motion sensor to identify an activity characteristic of the subject, selecting a noise reference in response to identification of the activity characteristic, and processing a physiological sensor signal received from the physiological sensor using the noise reference to generate an output signal having reduced noise relative to the physiological sensor signal. 
     Monitoring devices according to embodiments of the present invention can be advantageous over some conventional monitoring devices because, by detecting an activity characteristic of a subject, a corresponding noise reference can be selected and/or modified to reduce noise that may be present in a physiological sensor signal so as to provide more accurate physiological information about a subject. For example, for optimal heart rate tracking accuracy, it may be advantageous to have some context about the type of activity being performed by a subject (i.e., whether the subject is engaged in lifestyle activities or periodic activities) to select a noise reference that may more closely correspond to motion-related noise in the signal from which the heart rate signal. 
     It is noted that aspects of the invention described with respect to one embodiment may be incorporated in a different embodiment although not specifically described relative thereto. That is, all embodiments and/or features of any embodiment can be combined in any way and/or combination, even if not specifically described in that manner. These and other aspects of the present invention are explained in detail below. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which form a part of the specification, illustrate various embodiments of the present invention. The drawings and description together serve to fully explain embodiments of the present invention. 
         FIGS. 1A-1B  illustrate a monitoring device that can be positioned within an ear of a subject, according to some embodiments of the present invention. 
         FIG. 2A  illustrates a monitoring device that can be positioned around an appendage (such as an arm, leg, finger, toe, etc.) of the body of a subject, according to some embodiments of the present invention. 
         FIG. 2B  is a cross sectional view of the monitoring device of  FIG. 2A . 
         FIG. 3  is a block diagram of a monitoring device according to some embodiments of the present invention. 
         FIGS. 4-7  are flowcharts of operations for monitoring a subject according to embodiments of the present invention. 
         FIG. 8  is a graph illustrating heart rate data for a subject plotted over time compared to a benchmark for a range of activity mode settings of a monitoring device, according to some embodiments of the present invention. 
         FIG. 9  is a graph illustrating heart rate data for a subject plotted over time compared to a benchmark for a range of activity mode settings of a monitoring device, according to some embodiments of the present invention. 
         FIG. 10  is a spectrogram of accelerometer data showing the subject monitored in  FIGS. 8 and 9  performing non-periodic activity and periodic activity. 
         FIG. 11  is a flowchart of operations for selecting a noise reference responsive to monitoring a subject according to embodiments of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention will now be described more fully hereinafter with reference to the accompanying figures, in which embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Like numbers refer to like elements throughout. In the figures, certain layers, components or features may be exaggerated for clarity, and broken lines illustrate optional features or operations unless specified otherwise. In addition, the sequence of operations (or steps) is not limited to the order presented in the figures and/or claims unless specifically indicated otherwise. Features described with respect to one figure or embodiment can be associated with another embodiment or figure although not specifically described or shown as such. 
     It will be understood that when a feature or element is referred to as being “on” another feature or element, it can be directly on the other feature or element or intervening features and/or elements may also be present. In contrast, when a feature or element is referred to as being “directly on” another feature or element, there are no intervening features or elements present. It will also be understood that, when a feature or element is referred to as being “secured”, “connected”, “attached” or “coupled” to another feature or element, it can be directly secured, directly connected, attached or coupled to the other feature or element or intervening features or elements may be present. In contrast, when a feature or element is referred to as being “directly secured”, “directly connected”, “directly attached” or “directly coupled” to another feature or element, there are no intervening features or elements present. Although described or shown with respect to one embodiment, the features and elements so described or shown can apply to other embodiments. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. As used herein, the singular forms “a”, “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. 
     As used herein, the terms “comprise”, “comprising”, “comprises”, “include”, “including”, “includes”, “have”, “has”, “having”, or variants thereof are open-ended, and include one or more stated features, elements, steps, components or functions but does not preclude the presence or addition of one or more other features, elements, steps, components, functions or groups thereof. Furthermore, as used herein, the common abbreviation “e.g.”, which derives from the Latin phrase “exempli gratia,” may be used to introduce or specify a general example or examples of a previously mentioned item, and is not intended to be limiting of such item. The common abbreviation “i.e.”, which derives from the Latin phrase “id est,” may be used to specify a particular item from a more general recitation. 
     As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items and may be abbreviated as “/”. 
     As used herein, phrases such as “between X and Y” and “between about X and Y” should be interpreted to include X and Y. As used herein, phrases such as “between about X and Y” mean “between about X and about Y.” As used herein, phrases such as “from about X to Y” mean “from about X to about Y.” 
     Spatially relative terms, such as “under”, “below”, “lower”, “over”, “upper” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if a device in the figures is inverted, elements described as “under” or “beneath” other elements or features would then be oriented “over” the other elements or features. Thus, the exemplary term “under” can encompass both an orientation of over and under. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. Similarly, the terms “upwardly”, “downwardly”, “vertical”, “horizontal” and the like are used herein for the purpose of explanation only unless specifically indicated otherwise. 
     It will be understood that although the terms first and second are used herein to describe various features or elements, these features or elements should not be limited by these terms. These terms are only used to distinguish one feature or element from another feature or element. Thus, a first feature or element discussed below could be termed a second feature or element, and similarly, a second feature or element discussed below could be termed a first feature or element without departing from the teachings of the present invention. 
     Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the specification and relevant art and should not be interpreted in an idealized or overly formal sense unless expressly so defined herein. Well-known functions or constructions may not be described in detail for brevity and/or clarity. 
     The term “about”, as used herein with respect to a value or number, means that the value or number can vary more or less, for example by +/−20%, +/−10%, +/−5%, +/−1%, +/−0.5%, +/−0.1%, etc. 
     The terms “sensor”, “sensing element”, and “sensor module”, as used herein, are interchangeable and refer to a sensor element or group of sensor elements that may be utilized to sense information, such as information (e.g., physiological information, body motion, etc.) from the body of a subject and/or environmental information in a vicinity of the subject. A sensor/sensing element/sensor module may comprise one or more of the following: a detector element, an emitter element, a processing element, optics, mechanical support, supporting circuitry, and the like. Both a single sensor element and a collection of sensor elements may be considered a sensor, a sensing element, or a sensor module. 
     The term “optical emitter”, as used herein, may include a single optical emitter and/or a plurality of separate optical emitters that are associated with each other. 
     The term “optical detector”, as used herein, may include a single optical detector and/or a plurality of separate optical detectors that are associated with each other. 
     The term “wearable sensor module”, as used herein, refers to a sensor module configured to be worn on or near the body of a subject. 
     The terms “monitoring device” and “biometric monitoring device”, as used herein, are interchangeable and include any type of device, article, or clothing that may be worn by and/or attached to a subject and that includes at least one sensor/sensing element/sensor module. Exemplary monitoring devices may be embodied in an earpiece, a headpiece, a finger clip, a digit (finger or toe) piece, a limb band (such as an arm band or leg band), an ankle band, a wrist band, a nose piece, a sensor patch, eyewear (such as glasses or shades), apparel (such as a shirt, hat, underwear, etc.), a mouthpiece or tooth piece, contact lenses, or the like. 
     The term “monitoring” refers to the act of measuring, quantifying, qualifying, estimating, sensing, calculating, interpolating, extrapolating, inferring, deducing, or any combination of these actions. More generally, “monitoring” refers to a way of getting information via one or more sensing elements. For example, “blood health monitoring” includes monitoring blood gas levels, blood hydration, and metabolite/electrolyte levels. 
     The term “headset”, as used herein, is intended to include any type of device or earpiece that may be attached to or near the ear (or ears) of a user and may have various configurations, without limitation. Headsets incorporating optical sensor modules, as described herein, may include mono headsets (a device having only one earbud, one earpiece, etc.) and stereo headsets (a device having two earbuds, two earpieces, etc.), earbuds, hearing aids, ear jewelry, face masks, headbands, and the like. In some embodiments, the term “headset” may include broadly headset elements that are not located on the head but are associated with the headset. For example, in a “medallion” style wireless headset, where the medallion comprises the wireless electronics and the headphones are plugged into or hard-wired into the medallion, the wearable medallion would be considered part of the headset as a whole. Similarly, in some cases, if a mobile phone or other mobile device is intimately associated with a plugged-in headphone, then the term “headset” may refer to the headphone-mobile device combination. The terms “headset” and “earphone”, as used herein, are interchangeable. 
     The term “physiological” refers to matter or energy of or from the body of a creature (e.g., humans, animals, etc.). In embodiments of the present invention, the term “physiological” is intended to be used broadly, covering both physical and psychological matter and energy of or from the body of a creature. 
     The term “body” refers to the body of a subject (human or animal) that may wear a monitoring device, according to embodiments of the present invention. The term “bodily location” refers to a location of the body on which (or in which) a monitoring device may be worn. 
     The term “processor” is used broadly to refer to a signal processor or computing system or processing or computing method which may be localized or distributed. For example, a localized signal processor may comprise one or more signal processors or processing methods localized to a general location, such as to a wearable device. Examples of such wearable devices may comprise an earpiece, a headpiece, a finger clip, a digit (finger or toe) piece, a limb band (such as an arm band or leg band), an ankle band, a wrist band, a nose piece, a sensor patch, eyewear (such as glasses or shades), apparel (such as a shirt, hat, underwear, etc.), a mouthpiece or tooth piece, contact lenses, or the like. Examples of a distributed processor comprise “the cloud”, the internet, a remote database, a remote processor computer, a plurality of remote processors or computers in communication with each other, or the like, or processing methods distributed amongst one or more of these elements. The key difference is that a distributed processor may include delocalized elements, whereas a localized processor may work independently of a distributed processing system. As a specific example, microprocessors, microcontrollers, ASICs (application specific integrated circuits), analog processing circuitry, or digital signal processors are a few non-limiting examples of physical signal processors that may be found in wearable devices. 
     The term “remote” does not necessarily mean that a remote device is a wireless device or that it is a long distance away from a device in communication therewith. Rather, the term “remote” is intended to reference a device or system that is distinct from another device or system or that is not substantially reliant on another device or system for core functionality. For example, a computer wired to a wearable device may be considered a remote device, as the two devices are distinct and/or not substantially reliant on each other for core functionality. However, any wireless device (such as a portable device, for example) or system (such as a remote database for example) is considered remote to any other wireless device or system. 
     The terms “signal analysis frequency” and “signal sampling rate”, as used herein, are interchangeable and refer to the number of samples per second (or per other unit) taken from a continuous sensor (i.e., physiological sensor and environmental sensor) signal to ultimately make a discrete signal. 
     The term “sensor module interrogation power”, as used herein, refers to the amount of electrical power required to operate one or more sensors (i.e., physiological sensors and environmental sensors) of a sensor module and/or any processing electronics or circuitry (such as microprocessors and/or analog processing circuitry) associated therewith. Examples of decreasing the sensor interrogation power may include lowering the voltage or current through a sensor element (such as lowering the voltage or current applied to a pair of electrodes), lowering the polling (or polling rate) of a sensor element (such as lowering the frequency at which an optical emitter is flashed on/off in a PPG sensor), lowering the sampling frequency of a stream of data (such as lowering the sampling frequency of the output of an optical detector in a PPG sensor), selecting a lower-power algorithm (such as selecting a power-efficient time-domain processing method for measuring heart rate vs. a more power-hungry frequency-domain processing method), or the like. Lowering the interrogation power may also include powering only one electrode, or powering fewer electrodes, in a sensor module or sensor element such that less total interrogation power is exposed to the body of a subject. For example, lowering the interrogation power of a PPG sensor may comprise illuminating only one light-emitting diode rather than a plurality of light-emitting diodes that may be present in the sensor module, and lowering the interrogation power of a bioimpedance sensor may comprise powering only one electrode pair rather than a plurality of electrodes that may be present in the bioimpedance sensor module. 
     The term “polling” typically refers to controlling the intensity of an energy emitter of a sensor or to the “polling rate” and/or duty cycle of an energy emitter element in a sensor, such as an optical emitter in a PPG sensor or an ultrasonic driver in an ultrasonic sensor. Polling may also refer to the process of collecting and not collecting sensor data at certain periods of time. For example, a PPG sensor may be “polled” by controlling the intensity of one or more optical emitters, i.e. by pulsing the optical emitter over time. Similarly, the detector of a PPG sensor may be polled by reading data from that sensor only at a certain point in time or at certain intervals, i.e., as in collecting data from the detector of a PPG sensor for a brief period during each optical emitter pulse. A sensor may also be polled by turning on or off one or more elements of that sensor in time, such as when a PPG sensor is polled to alternate between multiple LED wavelengths over time or when an ultrasonic sensor is polled to alternate between mechanical vibration frequencies over time. 
     The terms “sampling frequency”, “signal analysis frequency”, and “signal sampling rate”, as used herein, are interchangeable and refer to the number of samples per second (or per other unit) taken from a continuous sensor or sensing element (for example, the sampling rate of the thermopile output in a tympanic temperature sensor). 
     The term “noise reference” refers to a reference information source for an undesired, dynamic noise signal that may be present within a sensor signal. A noise reference signal may be generated by a noise reference source, and this noise reference signal may be processed (i.e., via digital and/or analog electronic processing) to attenuate noise from the sensor signal, yielding a filtered signal that contains cleaner sensor information (i.e., such that the desired sensor information remains with reduced noise). As a specific example, for a PPG sensor worn by a subject, a motion noise reference may include an accelerometer, a pressure sensor, a piezoelectric sensor, a capacitive sensor, an inductive sensor, an optical sensor, or the like. Ideally, the noise reference signal may contain information associated with the motion of the subject, but may contain little (if any) information associated with the blood flow of the subject. Stated another way, the ratio of body motion information to arterial blood flow information contained in the noise reference signal may be relatively high. 
     Motion noise information generated by the motion noise reference can be processed with PPG sensor data to attenuate dynamically changing motion noise from the PPG sensor, such that the resulting signal contains cleaner information about blood flow (and less motion noise information). In accordance with some embodiments described herein, one motion noise reference for a wearable PPG sensor is an optomechanical sensor also known as a “blocked channel” sensor. This optomechanical sensor can be designed or configured to modulate light generated by an optical emitter in response to body motion, and to direct that light upon a pathway that does not interact with blood flow; in this manner, the optomechanical sensor can be configured to measure body motion and not blood flow motion, so that the noise reference signal may be used to filter out motion noise from the PPG signal with analog or digital signal processing (i.e. filtering). Nonlimiting examples of such noise references are presented in commonly-owned U.S. Pat. No. 9,289,175 entitled “LIGHT-GUIDING DEVICES AND MONITORING DEVICES INCORPORATING SAME,” and International Patent Application No. PCT/US2016/046273 entitled “METHODS AND APPARATUS FOR DETECTING MOTION VIA OPTOMECHANICS,” the disclosures of which are incorporated by reference herein. 
     It should be noted that “algorithm” and “circuit” are referred to herein. An algorithm refers to an instruction set, such as an instruction set with sequential steps and logic, that may be in memory whereas a circuit refers to electronic components and/or traces that may implement such logic operations. 
     It should be noted that processes for managing hysteresis are implied herein. Namely, several embodiments herein for controlling sensors (and other wearable hardware) may involve a processor sending commands to a sensor element depending on the sensor readings. Thus, in some embodiments, a sensor reading (such as a reading from an optical detector or a sensing electrode) above X may result in a processor sending a command to electrically bias another sensor element (such as an optical emitter or a biasing electrode) above Y. Similarly, as soon as the sensor reading drops below X, a processor may send a command to bias another sensor element below Y. However, in borderline situations this may cause unwanted hysteresis in the biasing command, as sensor readings may rapidly toggle above/below X resulting in the toggling of the biasing of another sensor element above/below Y. As such, hysteresis management may be integrated within the algorithm(s) for controlling the execution of a processor. For example, the processor may be configured by the algorithm to delay a biasing command by a period of time Z following the timing of a prior biasing command, thereby preventing or reducing the aforementioned toggling. 
     In the following figures, various monitoring devices will be illustrated and described for attachment to the ear or an appendage of the human body. However, it is to be understood that embodiments of the present invention are not limited to those worn by humans. 
     The ear is an ideal location for wearable health and environmental monitors. The ear is a relatively immobile platform that does not obstruct a person&#39;s movement or vision. Monitoring devices located at an ear have, for example, access to the inner-ear canal and tympanic membrane (for measuring core body temperature), muscle tissue (for monitoring muscle tension), the pinna, earlobe, and elsewhere (for monitoring blood gas levels), the region behind the ear (for measuring skin temperature and galvanic skin response), and the internal carotid artery (for measuring cardiopulmonary functioning), etc. Virtually all areas of the ear support enough blood perfusion to enable an assessment of blood flow and blood-flow-derived metrics (such as breathing rate, heart rate, blood pressure, RR-interval, and the like) via photoplethysmography (PPG). A particularly good location for PPG may be the region covering the anti-tragus and concha regions of the ear, as the region has high perfusion, is more resistant to motion artifacts, and is in a region away from the ear canal. The ear is also at or near the point of exposure to: environmental breathable toxicants of interest (volatile organic compounds, pollution, etc.); noise pollution experienced by the ear; and lighting conditions for the eye. Furthermore, as the ear canal is naturally designed for transmitting acoustical energy, the ear provides a good location for monitoring internal sounds, such as heartbeat, breathing rate, and mouth motion. 
     Optical coupling into the blood vessels of the ear may vary between individuals. As used herein, the term “coupling” refers to the interaction or communication between excitation energy (such as light) entering a region and the region itself. For example, one form of optical coupling may be the interaction between excitation light generated from within an optical sensor of a wearable device—such as a wristband, armband, legband, skin-worn patch, neckband, ring, earbud, other device positioned at or within an ear, a headband, or the like—and the blood vessels. In one embodiment, this interaction may involve excitation light entering the skin and scattering from a blood vessel such that the temporal change in intensity of scattered light is proportional to a temporal change in blood flow within the blood vessel. Another form of optical coupling may be the interaction between excitation light generated by an optical emitter within a wearable device and a light-guiding region of the wearable device. Thus, a wearable device with integrated light-guiding capabilities, wherein light can be guided to multiple and/or select regions along the wearable device, can assure that each individual wearing the device will generate an optical signal related to blood flow through the blood vessels. Optical coupling of light to a particular skin region of one person may not yield photoplethysmographic signals for each person. Therefore, coupling light to multiple regions may assure that at least one blood-vessel-rich region will be interrogated for each person wearing the device. Coupling multiple regions of the body to light may also be accomplished by diffusing light from a light source in the device. 
     According to some embodiments of the present invention, “smart” monitoring devices including, but not limited to, wrist-worn devices, armbands, earbuds, and the like, are provided that can modify biometric signal extraction algorithms based upon a recognition of an activity level of a subject. 
       FIGS. 1A-1B  illustrate a monitoring apparatus  20  configured to be positioned within an ear of a subject, according to some embodiments of the present invention. The illustrated apparatus  20  includes an earpiece body or housing  22 , a sensor module  24 , a stabilizer  25 , and a sound port  26 . When positioned within the ear of a subject, the sensor module  24  has a region  24   a  configured to contact a selected area of the ear. The illustrated sensor region  24   a  is contoured (i.e., is “form-fitted”) to matingly engage a portion of the ear between the anti tragus and acoustic meatus, and the stabilizer is configured to engage the anti-helix. However, monitoring devices in accordance with embodiments of the present invention can have sensor modules with one or more regions configured to engage various portions of the ear. Various types of device configured to be worn at or near the ear may be utilized in conjunction with embodiments of the present invention. 
       FIGS. 2A-2B  illustrate a monitoring apparatus  30  in the form of a sensor band  32  configured to be secured to an appendage (e.g., an arm, wrist, hand, finger, toe, leg, foot, neck, etc.) of a subject. The band  32  includes a sensor module  34  on or extending from the inside surface  32   a  of the band  32 . The sensor module  34  is configured to detect and/or measure physiological information from the subject and includes a sensor region  34   a  that is contoured to contact the skin of a subject wearing the apparatus  30 . 
     Embodiments of the present invention may be utilized in various devices and articles including, but not limited to, patches, clothing, etc. Embodiments of the present invention can be utilized wherever PPG and blood flow signals can be obtained and at any location on the body of a subject. Embodiments of the present invention are not limited to the illustrated monitoring devices  20 ,  30  of  FIGS. 1A-1B and 2A-2B . 
     The sensor modules  24 ,  34  for the illustrated monitoring devices  20 ,  30  of  FIGS. 1A-1B and 2A-2B  are configured to detect and/or measure physiological information from a subject wearing the monitoring devices  20 ,  30 . In some embodiments, the sensor modules  24 ,  34  may be configured to detect and/or measure one or more environmental conditions in a vicinity of the subject wearing the monitoring devices  20 ,  30 . 
     A sensor module utilized in accordance with embodiments of the present invention may be an optical sensor module that includes at least one optical emitter and at least one optical detector. Exemplary optical emitters include, but are not limited to light-emitting diodes (LEDs), laser diodes (LDs), compact incandescent bulbs, micro-plasma emitters, IR blackbody sources, or the like. In addition, a sensor module may include various types of sensors including and/or in addition to optical sensors. For example, a sensor module may include one or more inertial sensors (e.g., an accelerometer, piezoelectric sensor, vibration sensor, photoreflector sensor, etc.) for detecting changes in motion, one or more thermal sensors (e.g., a thermopile, thermistor, resistor, etc.) for measuring temperature of a part of the body, one or more electrical sensors for measuring changes in electrical conduction, one or more skin humidity sensors, and/or one or more acoustical sensors. Sensor modules may include physiological sensors for monitoring vital signs, blood flow properties, body fluids, muscle motion, body motion, and the like, Sensor modules may include environmental sensors for monitoring ambient light, ambient temperature, ambient humidity, airborne pollutants, wind, and the like. It should be understood that a wide variety of sensors may be incorporated into a wearable sensor module. 
     Referring to  FIG. 3 , a monitoring device (e.g., monitoring devices  20 ,  30 ), according to embodiments of the present invention, is illustrated schematically. The illustrated monitoring device includes a sensor module  24 ,  34  having one or more physiological sensors configured to detect and/or measure physiological information from the subject, and a motion sensor  50  (e.g., an accelerometer) configured to detect and/or measure subject motion information. The monitoring device  20 ,  30  also includes at least one processor  40  that is coupled to the sensor(s) of a sensor module  24 ,  34 , to the motion sensor  50 , and that is configured to receive and analyze signals produced by the sensor(s) and motion sensor  50 . 
     According to some embodiments of the present invention, the processor  40  is configured to process motion sensor signals to identify an activity characteristic (e.g., periodic or non-periodic activity) of the subject, select or modify a biometric signal extraction algorithm (implemented by computer-readable memory and/or circuit  60 ) in response to identification of the type of subject activity, and then process physiological sensor signals via the modified biometric signal extraction algorithm or circuit  60  to produce physiological information about the subject, such as subject heart rate, subject respiration rate, subject RRi (i.e., the time-difference between consecutive R-peaks in the ECG or PPG waveform), subject HRV, subject blood pressure, subject blood analyte levels, subject cardiovascular properties, and the like. Although many aspects of this invention are most related to PPG sensors, a variety of other physiological sensors may be used within the scope of this invention. 
     In some embodiments, the motion sensor  50  is an accelerometer and a filter is applied to each axis of the accelerometer data to remove the DC component therefrom. A spectral transform is performed on each axis of the DC-removed signal over a given time span. An accelerometer spectrum is then generated by taking the root of the sum of the squares of each axis of the accelerometer that is associated with that time span. Other techniques may also be used. 
     The 1st, 2nd, and 3rd, most intense frequencies and magnitudes in the accelerometer spectrum are identified. The relative proportion of energy within the peaks is compared to all other frequencies to determine if the criteria for periodicity is met. If so, the given segment of time is determined to represent periodic activity and an Automatic Mode Switching feature of the monitoring device is set to inform an algorithm  60  (e.g., a heart rate algorithm, etc.) of the periodic context. If not, an Automatic Mode Switching feature of the monitoring device is set to inform the HR algorithm of the non-periodic context. 
     Alternatively or in addition, a determination is made whether the ratio of peak frequencies fit a pattern associated with periodicity (e.g., whether peak  2  is twice the frequency of peak  1 , etc.). If so, the given segment of time is determined to represent periodic activity and an Automatic Mode Switching feature of the monitoring device is set to inform an algorithm  60  (e.g., a heart rate algorithm, etc.) of the periodic context. If not, an Automatic Mode Switching feature is set to inform the HR algorithm of the non-periodic context. 
     Referring now to  FIG. 4 , a method of monitoring a subject via a monitoring device, such as monitoring devices  20 ,  30 , according to some embodiments of the present invention, will be described. The monitoring device includes a physiological sensor configured to detect and/or measure physiological information from the subject and a motion sensor. The physiological sensor and the motion sensor are in communication with a processor that is configured to receive and analyze signals produced by the physiological sensor and motion sensor. The processor may be part of the monitoring device or may be a remotely located processor. 
     The subject is monitored for a change in physical activity level or type (Block  100 ). If a change is detected (Block  110 ), the processor  40  identifies an activity characteristic (e.g., determines if the activity is periodic or non-periodic) (Block  120 ). For example, via an algorithm stored in or implemented by memory or circuit  60 , the processor determines if the subject is engaged in a non-periodic or lifestyle activity such as reading a book, desk work, eating, etc., or a periodic activity such as exercising, running, walking, etc. In response to determining the type of activity, the processing of the biometric signal extraction algorithm may be modified in one or more ways (Block  130 ). For example, in response to determining the type of activity, at least one parameter of the biometric signal extraction algorithm may be modified, and this modified biometric signal extraction algorithm may be selected for processing. Some specific examples of parameters that may be modified are described in US 2015/0366509 entitled “CADENCE DETECTION BASED ON INERTIAL HARMONICS” (published Dec. 24, 2015), US 2016/0029964 entitled “PHYSIOLOLOGICAL MONITORING DEVICES WITH ADJUSTABLE SIGNAL ANALYSIS AND INTERROGATION POWER AND MONITORING METHODS USING THE SAME” (published Feb. 4, 2016), and US 2015/0011898 entitled “PHYSIOLOLOGICAL METRIC ESTIMATION RISE AND FALL LIMITING” (published Jan. 8, 2015), the disclosures of which are incorporated by reference herein. For example, parameters affecting the rise or fall time of biometric tracking (such as the tracking of heart rate, breathing rate, RRi, etc.), parameters affecting the subtraction or redaction of unwanted frequencies in the PPG signal (i.e., which frequency bands to subtract or redact (set to zero) based for example on amplitude and/or power), and/or parameters affecting which types of filters to execute (highpass, lowpass, bandpass, active, subtractive or redactive filters, or the like) may be modified. In response to determining the type of activity, the processor may thus select a biometric signal extraction algorithm (which may be all or part of algorithm stored in or implemented by memory or circuit  60 ) so as to be best suited for processing sensor data during such activity (Block  130 ) (i.e., that is best suited for removing motion or environmental noise from that particular activity). 
     It should be noted that, in addition to or alternatively than modifying an algorithm or selecting an algorithm, a circuit may be modified or selected. For example, the determination of periodicity in Block  120  may send a signal to one or more transistors (Block  130 ) to switch the analog filtering circuits to “notch out” the periodic signal; or, the determination of non-periodicity in Block  120  may send a signal to one or more transistors (Block  130 ) to switch the analog filtering circuits to a bandpass filter that includes the biometric signals of interest. It should also be noted that both algorithms and circuits may simultaneously be modified or selected in the embodiments described herein. Thus, as another exemplary embodiment, the determination of periodicity in Block  120 , using a low-power time-domain algorithm (such as an autocorrelation function, or the like), may initiate an algorithm in Block  130  that determines which frequencies should be removed. The output of the algorithm (i.e., the output from a processor) may send a signal to a bank of analog filters in Block  130  to remove these frequencies without the need for digital removal. This may be advantageous because executing a spectral transform may be overly power-hungry in a wearable device. Physiological sensor signals are then processed via the selected biometric signal extraction algorithm to produce physiological information about the subject, such as subject heart rate, subject respiration rate, subject RRi, subject HRV, subject blood pressure, subject blood analyte levels, subject cardiovascular properties, and the like. 
     Identification of an activity characteristic (e.g., determining if the activity is periodic or non-periodic) may also be associated with particular metrics to be assessed. For example, detecting that periodic motion exists may be a criteria for determining that a cardiac efficiency metric is being assessed. In contrast, if the subject&#39;s motion is aperiodic, then a cardiac efficiency score (Average Cadence/Average HR) may be meaningless. Also, one or more processors or functions may be deactivated or run in power-saving mode based on the detection of type of motion (e.g., a pedometer may be deactivated responsive to detection of aperiodic motion). Additionally, an energy expenditure calculation which factors both HR and footsteps into an equation for the estimation of calories burned may be tuned to detect “fidgeting” (subtle motions) rather than footsteps and update the energy expenditure model to incorporate fidgets and HR, rather than footsteps and HR. The weighting factors for fidgets and HR, for example, may be different than the weighting factors for footsteps and HR when estimating energy expenditure. 
     As illustrated in  FIG. 5 , identifying an activity characteristic of a subject (Block  120 ) may include filtering a signal from each axis of the accelerometer over a given time period to remove a DC component therefrom (Block  122 ), producing an accelerometer spectrum using the filtered signals (Block  124 ), identifying one or more frequencies in the spectrum having a magnitude above a threshold level (Block  126 ), and/or determining if the one or more frequencies in the spectrum match a pattern associated with a known subject activity mode (Block  128 ). It should be noted that the operations of  FIG. 5  may be controlled by algorithms, circuitry, or a combination of both. 
     Referring now to  FIG. 6 , a method of monitoring a subject via a monitoring device, such as monitoring devices  20 ,  30 , according to some embodiments of the present invention, will be described. The monitoring device includes a PPG sensor and a motion sensor which are in communication with a processor that is configured to receive and analyze signals produced by the sensor module and motion sensor. The processor may be part of the monitoring device or may be a remotely located processor. It should be noted that the operations of  FIG. 6  may be controlled by algorithms, circuitry, or a combination of both. 
     The method includes filtering a motion signal generated by the motion sensor to attenuate DC information in the motion signal and generate a DC-filtered motion signal (Block  200 ). A spectral representation of the DC-filtered motion signal is generated (Block  210 ) and a plurality of frequencies in the spectral representation are identified having a magnitude above a threshold level (Block  220 ). In other words, the most intense frequencies in the spectral representation are identified. Additional operations may include filtering a step rate component from a measured heart rate component based on a function of a difference therebetween as described in US 2015/0018636 entitled “REDUCTION OF PHYSIOLOGICAL METRIC ERROR DUE TO INERTIAL CADENCE” (published Jan. 15, 2015), comparing frequency measurements from an inertial sensor with one or more thresholds as described in US 2015/0366509 entitled “CADENCE DETECTION BASED ON INERTIAL HARMONICS” (published Dec. 24, 2015), and measuring signal quality to determine how a user should adjust a monitoring device to improve biometric fit, as described in US 2016/0094899 entitled “METHODS AND APPARATUS FOR IMPROVING SIGNAL QUALITY IN WEARABLE BIOMETRIC MONITORING DEVICES” (published Mar. 31, 2016), individually or together in combination, the disclosures of which are incorporated by reference herein. Information associated with the plurality of frequencies is then processed to determine a type of motion of the subject, for example, whether the subject motion is periodic or non-periodic motion (Block  230 ). Information on the subject motion type is sent to a biometric tracking algorithm or circuit or is otherwise used by the processor to select a biometric tracking algorithm or circuit (Block  240 ), and a PPG sensor signal is processed using the biometric tracking algorithm or circuit corresponding to the subject motion type information to generate biometric information about the subject (Block  250 ). Exemplary biometric information includes, but is not limited to, subject heart rate, subject respiration rate, subject RRi, subject HRV, subject blood pressure, subject blood analyte levels, subject cardiovascular properties, etc. 
     Referring now to  FIG. 7 , a method of monitoring a subject via a monitoring device, such as monitoring devices  20 ,  30 , according to some embodiments of the present invention, will be described. The monitoring device includes a PPG sensor and a motion sensor which are in communication with a processor that is configured to receive and analyze signals produced by the sensor module and motion sensor. The processor may be part of the monitoring device or may be a remotely located processor. It should be noted that the operations of  FIG. 7  may be controlled by algorithms, circuitry, or a combination of both. 
     The method includes processing PPG sensor signals via the at least one processor to identify a statistical relationship between RR-intervals in the PPG sensor signals (Block  300 ) and processing the statistical relationship between RR-intervals to determine a type of motion of the subject (e.g., periodic or non-periodic) or a physiological status of the subject (Block  310 ). A biometric signal extraction algorithm or circuit is modified or selected via the processor in response to an identified statistical relationship between RR-intervals (Block  320 ). PPG sensor signals are then processed via the selected biometric signal extraction algorithm to produce physiological information about the subject (Block  320 ), such as subject heart rate, subject respiration rate, subject RR-interval (RRi), subject HRV, subject blood pressure, subject blood analyte levels, subject cardiovascular properties, etc. A particular example of processing RR-intervals to determine a type of motion of the subject may include processing a series of RR-intervals to identify the existence of stress, which may be associated with aperiodic (non-periodic) motion. 
     It should be noted that it is known to those skilled in the art that a series of RR-intervals may be processed to generate an assessment of fatigue, stress (particularly psychosocial stress), autonomic nervous functioning, hemodynamics, cardiac disease—such as atrial fibrillation or cardiac arrhythmia—or the like. However, the inventors have discovered that the statistical analysis alone may not be sufficient to generate sufficient physiological information for a diagnosis of a health condition or other physiological state. Thus, in one particular embodiment of  FIG. 7 , the RRi-intervals may be statistically processed (Block  310 ) to generate an assessment that a subject status of atrial fibrillation is likely. Once this status is known to be likely, the signal extraction algorithm may be modified or selected to more concretely determine a status of atrial fibrillation or to diagnose another condition (such as poor or abnormal hemodynamics or cardiac output, which may be associated with atrial fibrillation). As a specific example, Block  320  may switch to an algorithm that allows more frequencies to pass through during signal extraction, such that the extracted signal from Block  320  may be further processed by Block  330  to generate sufficient physiological information to diagnose atrial fibrillation. It should be understood that this describes a particular embodiment, and other embodiments that rely on  FIG. 7  may be used with the present invention. As another specific example, Block  310  may determine that a subject may likely be in a status of fatigue. Block  320  may then modify or select a signal extraction algorithm that increases the signal resolution (i.e., the sampling rate and/or dynamic range), such that the extracted signal may be processed by Block  330  to generate a more definitive assessment of fatigue. In yet another example, Block  310  may determine that a subject may likely be in a status of stress, Block  320  may then modify or select a signal extraction algorithm that switches from a screening spectral-based (frequency domain-based) RRi calculator to a higher resolution time-domain-based RRi calculator, the extracted RRi signal may then be processed by Block  330  to generate a more definitive assessment of stress. 
       FIG. 8  is a graph illustrating heart rate data  400 ,  402 ,  404  for a subject plotted over time compared to the actual heart rate ( 406 ), the heart rate determined by a validated benchmark device, for a range of activity mode settings for a monitoring device according to some embodiments of the present invention. For the illustrated duration, the subject was not walking/jogging/running (i.e., the subject was not engaged in a periodic activity), but was doing deskwork (i.e., the subject was engaged in a non-periodic activity), where arms and hands were moving as is typical for desk work. As seen, the heart rate in Mode  3  (the “lifestyle mode” indicated by  404 ) most closely agrees with that of the benchmark device heart rate ( 406 ), and the monitoring device correctly uses the output for Mode  3 . 
       FIG. 9  is a graph illustrating heart rate data  400 ,  402 ,  404  for a subject plotted over time compared to the actual heart rate ( 406 ), the heart rate determined by a validated benchmark device, for a range of activity mode settings for a monitoring device according to some embodiments of the present invention. For the illustrated duration, the subject is jogging. As seen, the heart rate in Mode  1  ( 400 ) most closely agrees with that of the benchmark device heart rate ( 406 ), and the monitoring device correctly uses the output for Mode  1 . Note that the heart rate reported using Lifestyle mode ( 404 ) does not match the benchmark ( 406 ) as well as Mode  1  output ( 400 ). 
       FIG. 10  is a spectrogram for exemplary combined accelerometer data showing the subject monitored in  FIGS. 8 and 9  performing non-periodic activity from time (t)=0 to 150 seconds, and performing periodic activity (walking) after t=150 seconds. For accurate heart rate tracking, the monitoring device uses settings associated with Lifestyle Mode (Mode  3 ) prior to t=150 sec and Periodic Mode after t=120 seconds. 
     Returning to  FIG. 4 , a method of monitoring a subject via a monitoring device, block  130  may be modified in accordance with additional embodiments based on the detection of periodic or non-periodic motion. For example, in one embodiment shown as Block  144  in  FIG. 11 , if the processor  40  identifies an activity characteristic (e.g., determines if the activity is periodic or non-periodic) (Block  120 ) responsive to monitoring a subject (Block  100 ) and detecting a change in activity (Block  110 ), then a particular noise reference or set of noise references may be selected for improved (or optimal) motion noise reduction (or removal) from a physiological signal (Block  144 ). As a specific example for a PPG sensor (such as that similar to that of  FIG. 2  and/or  FIG. 3 ) worn by a subject, the PPG sensor having both optical sensors and a plurality of motion sensors, it may be desirable to select a particular type of motion sensor (or a particular set of motion sensors) to remove periodic noise and to select a different type of motion sensor (or a set of different motion sensors) to remove aperiodic noise. For example, upon determining at Block  120  that the activity characteristic is periodic, in Block  144  an accelerometer may be selected as the noise reference. As with descriptions of  FIG. 4 , this selection may be controlled by one or more algorithms on a microprocessor and/or electronic circuitry. In contrast, upon detecting that the activity characteristic is aperiodic, an optomechanical sensor, such as “blocked channel sensor”, may be selected as the noise reference. 
     One reason why an accelerometer may be a more suitable noise reference for periodic motion over an optomechanical sensor is because an accelerometer may have a better dynamic range or may have a stronger signal response at intense motions associated with periodic motion (such as running, sprinting, cycling, or the like). In contrast, an optomechanical sensor may have a better sensitivity and/or resolution at less intense motion levels (such as resting, walking, typing, grabbing, or the like) or may have a stronger signal response at less intense motions associated with aperiodic motion. Moreover, for a PPG sensor, because the signal response of an optomechanical noise reference may more closely resemble or otherwise correspond to the unwanted motion-related optical noise from the PPG sensor signal, the PPG and optomechanical sensors are likely to be more linear with each other than would be the paring of a PPG sensor and inertial noise reference (such as an accelerometer). Thus, time-domain-based PPG motion noise removal methods or algorithms (such as adaptive filtering and the like), often well-suited for aperiodic motion, may be more effective with this high linearity. In contrast, frequency-domain-based PPG motion noise removal methods or algorithms (such as spectral subtraction and the like), often well-suited for periodic motion, may not be as sensitive to the linearity between the PPG sensor and the noise reference and may work more effectively with an inertial noise reference. 
     Considering a pressure sensor as an example of a “motion sensor” in  FIG. 4  and  FIG. 11 , in one embodiment a detection of aperiodic motion may result in an accelerometer being deselected as the noise reference and a pressure sensor being selected as a noise reference (or a combination of a pressure sensor and an accelerometer). This is because (as with an optomechanical sensor) a pressure sensor may be more sensitive to aperiodic motion than an inertial sensor but may also be less effective at sensing periodic motion. In some embodiments, Block  144  may not change the noise reference in response to Block  120  but rather may change the algorithm or circuit for noise reduction or removal using the same noise reference. As a specific example, the detection of periodic motion (Block  120 ) may trigger a spectral noise (frequency domain) reduction or removal algorithm using an accelerometer as a noise reference (Block  130 ), but the detection of aperiodic motion may trigger a time-domain noise reduction or removal algorithm using the same accelerometer as a noise reference. This may be beneficial because spectral noise reduction or removal algorithms may be more suited for periodic activities (especially where “cross-overs” between step rate and heart rate are observed) and time-domain noise reduction or removal algorithms may be more suited for aperiodic activities (where “cross-overs” are much less likely). 
     Example embodiments are described herein with reference to block diagrams and flowchart illustrations. It is understood that a block of the block diagrams and flowchart illustrations, and combinations of blocks in the block diagrams and flowchart illustrations, can be implemented by computer program instructions that are performed by one or more computer circuits. These computer program instructions may be provided to a processor circuit of a general purpose computer circuit, special purpose computer circuit, and/or other programmable data processing circuit to produce a machine, such that the instructions, which execute via the processor of the computer and/or other programmable data processing apparatus, transform and control transistors, values stored in memory locations, and other hardware components within such circuitry to implement the functions/acts specified in the block diagrams and flowchart block or blocks, and thereby create means (functionality) and/or structure for implementing the functions/acts specified in the block diagrams and flowchart blocks. 
     These computer program instructions may also be stored in a tangible computer-readable medium that can direct a computer or other programmable data processing apparatus to function in a particular manner, such that the instructions stored in the computer-readable medium produce an article of manufacture including instructions which implement the functions/acts specified in the block diagrams and flowchart blocks. 
     A tangible, non-transitory computer-readable medium may include an electronic, magnetic, optical, electromagnetic, or semiconductor data storage system, apparatus, or device. More specific examples of the computer-readable medium would include the following: a portable computer diskette, a random access memory (RAM) circuit, a read-only memory (ROM) circuit, an erasable programmable read-only memory (EPROM or Flash memory) circuit, a portable compact disc read-only memory (CD-ROM), and a portable digital video disc read-only memory (DVD/Blu-Ray). 
     The computer program instructions may also be loaded onto a computer and/or other programmable data processing apparatus to cause a series of operational steps to be performed on the computer and/or other programmable apparatus to produce a computer-implemented process such that the instructions which execute on the computer or other programmable apparatus provide steps for implementing the functions/acts specified in the block diagrams and flowchart blocks. Accordingly, embodiments of the present invention may be embodied in hardware and/or in software (including firmware, resident software, micro-code, etc.) that runs on a processor such as a digital signal processor, which may collectively be referred to as “circuitry,” “a module” or variants thereof. 
     It should also be noted that in some alternate implementations, the functions/acts noted in the blocks may occur out of the order noted in the flowcharts. For example, two blocks shown in succession may in fact be executed substantially concurrently or the blocks may sometimes be executed in the reverse order, depending upon the functionality/acts involved. Moreover, the functionality of a given block of the flowcharts and block diagrams may be separated into multiple blocks and/or the functionality of two or more blocks of the flowcharts and block diagrams may be at least partially integrated. Finally, other blocks may be added/inserted between the blocks that are illustrated. Moreover, although some of the diagrams include arrows on communication paths to show a primary direction of communication, it is to be understood that communication may occur in the opposite direction to the depicted arrows. 
     The foregoing is illustrative of the present invention and is not to be construed as limiting thereof. Although a few exemplary embodiments of this invention have been described, those skilled in the art will readily appreciate that many modifications are possible in the exemplary embodiments without materially departing from the teachings and advantages of this invention. Accordingly, all such modifications are intended to be included within the scope of this invention as defined in the claims. The invention is defined by the following claims, with equivalents of the claims to be included therein.