Patent Publication Number: US-2020297226-A1

Title: Electronic device with optical heart rate monitor

Description:
RELATED APPLICATIONS 
     The present patent application claims priority benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application Ser. No. 62/819,992, filed Mar. 18, 2019, and titled “Improved Optical Cardiac Monitor,” which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND 
     An electronic fitness device may provide optical cardiac monitoring of a user of the device. The user may wear the electronic device such that a housing of the electronic device is located in contact with the skin of the user—typically being worn on the user&#39;s wrist. The cardiac monitoring may include physiological metrics and information such as a user&#39;s heart rate and pulse oximetry. The electronic fitness device may include optical devices, such as an optical transmitter, which emits an optical signal into the user&#39;s skin, and an optical receiver, which receives transmissions or reflections of the optical signal from the skin and generates a photoplethysmogram (PPG) signal corresponding to the intensity of the received optical signal. The electronic fitness device processes the PPG signal to determine the user&#39;s heart rate and pulse oximetry. Occasionally, while the user is active or exercising, the electronic fitness device may move out of a normal position and become tilted on the user&#39;s wrist. In this situation, the optical transmitter and/or the optical receiver may become separated from the user&#39;s skin—leading to a lower optical signal level and a reduction in a signal to noise ratio of the PPG signal. Under these circumstances, the electronic fitness device may not be able to accurately determine the user&#39;s heart rate and pulse oximetry. 
     SUMMARY 
     Embodiments of the present technology provide an electronic fitness device with more robust operation that is capable of accurately determining a user&#39;s heart rate and pulse oximetry when the device becomes tilted on the user&#39;s wrist. The electronic fitness device broadly comprises a housing, a first optical transmitter array, a first optical receiver, and a second optical receiver. The housing includes a bottom wall configured to contact a user&#39;s wrist. The first optical transmitter array is positioned at a first location on the bottom wall and is operable to output a plurality of optical signals that pass through a user&#39;s skin, with each optical signal having a unique wavelength. The first optical receiver is positioned at a second location on the bottom wall and is operable to receive the optical signals from the first optical transmitter array such that the optical signals travel along a first signal path and a first distance from the first optical transmitter array to the first optical receiver. The second optical receiver is positioned at a third location on the bottom wall and is operable to receive the optical signals from the first optical transmitter array such that the optical signals travel along a second signal path and a second distance from the first optical transmitter array to the second optical receiver, wherein the second signal path is roughly orthogonal to the first signal path and the second distance is different from the first distance. 
     Another embodiment of the present technology provides an electronic fitness device comprising a housing, a first optical transmitter array, a second optical transmitter, a first optical receiver, a second optical receiver, a third optical receiver, and a fourth optical receiver. The housing includes a bottom wall configured to contact a user&#39;s wrist. The first optical transmitter array is positioned at a first location on the bottom wall and includes a first optical transmitter, a second optical transmitter, and a third optical transmitter, with each optical transmitter operable to output a first optical signal that passes through a user&#39;s skin. Each first optical signal has a unique wavelength. The second optical transmitter array is positioned at a second location on the bottom wall and including a first optical transmitter, a second optical transmitter, and a third optical transmitter, with each optical transmitter operable to output a second optical signal that passes through a user&#39;s skin. Each second optical signal has a wavelength equal to a wavelength of a corresponding first optical signal of the first optical transmitter array. The first optical receiver is spaced apart from the third optical receiver with the first optical transmitter array and the second optical transmitter array positioned therebetween. The second optical receiver is spaced apart from the fourth optical receiver with the first optical transmitter array and the second optical transmitter array positioned therebetween. Each optical receiver is operable to receive the first optical signals and the second optical signals. The first optical signals travel along a different signal path from the first optical transmitter array to each of the optical receivers, and the second optical signals travel along a different signal path from the second optical transmitter array to each of the optical receivers. 
     Another embodiment of the present technology provides an electronic fitness device comprising a housing, a memory element, a first optical transmitter array, a second optical transmitter, a first optical receiver, a second optical receiver, a third optical receiver, and a fourth optical receiver. The housing includes a bottom wall configured to contact a user&#39;s wrist. The memory element is configured to store a signal to noise ratio threshold. The first optical transmitter array is positioned at a first location on the bottom wall and is operable to output a plurality of first optical signals that pass through a user&#39;s skin. Each first optical signal has a unique wavelength. The second optical transmitter array is positioned at a second location on the bottom wall and is operable to output a plurality of second optical signals that pass through the user&#39;s skin. Each second optical signal has a wavelength equal to a wavelength of a corresponding first optical signal. Each optical receiver is positioned proximate to the first optical transmitter array and the second optical transmitter array. Each optical receiver is operable to receive the first optical signals and the second optical signals, and is operable to generate a first electronic signal corresponding to the first optical signals and a second electronic signal corresponding to the second optical signals. The processing element is coupled with the memory element and each of the optical receivers. The processing element is configured to: receive the first electronic signal and the second electronic signal from each of the optical receivers, determine a signal to noise ratio of each of the first electronic signals and the second electronic signals, and process the first electronic signals and the second electronic signals if the signal to noise ratio of the first electronic signals and the second electronic signals is above the signal to noise threshold. 
     This summary is provided to introduce a selection of concepts in a simplified form that are further described below in the detailed description. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Other aspects and advantages of the present technology will be apparent from the following detailed description of the embodiments and the accompanying drawing figures. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING FIGURES 
       Embodiments of the present technology are described in detail below with reference to the attached drawing figures, wherein: 
         FIG. 1  is a top view of an electronic fitness device, constructed in accordance with various embodiments of the present technology, worn on a user&#39;s wrist; 
         FIG. 2  is a schematic block diagram of various electronic components of the electronic fitness device; 
         FIG. 3  is a bottom view of the electronic fitness device, illustrating a plurality of optical transmitter arrays and a plurality of optical receivers; 
         FIG. 4A  is a schematic block diagram of a first embodiment of the optical transmitter arrays and the optical receivers; 
         FIG. 4B  is a schematic block diagram of a second embodiment of the optical transmitter arrays and the optical receivers; 
         FIG. 5A  is a schematic block diagram of a first optical transmitter array and the optical receivers illustrating signal paths for a plurality of optical signals; 
         FIG. 5B  is a schematic block diagram of a second optical transmitter array and the optical receivers illustrating signal paths for a plurality of optical signals; 
         FIG. 5C  is a schematic block diagram of a third optical transmitter array and the optical receivers illustrating signal paths for a plurality of optical signals; 
         FIG. 6  is a schematic block diagram of a third embodiment of the optical transmitter arrays and the optical receivers; 
         FIG. 7  is a schematic cross section of the electronic fitness device while being worn flush against the user&#39;s wrist; and 
         FIG. 8  is a schematic cross section of the electronic fitness device while being worn and tilted on the user&#39;s wrist. 
     
    
    
     The drawing figures do not limit the present technology to the specific embodiments disclosed and described herein. While the drawings do not necessarily provide exact dimensions or tolerances for the illustrated components or structures, the drawings are to scale as examples of certain embodiments with respect to the relationships between the components of the structures illustrated in the drawings. 
     DETAILED DESCRIPTION 
     The following detailed description of the technology references the accompanying drawings that illustrate specific embodiments in which the technology can be practiced. The embodiments are intended to describe aspects of the technology in sufficient detail to enable those skilled in the art to practice the technology. Other embodiments can be utilized and changes can be made without departing from the scope of the present technology. The following detailed description is, therefore, not to be taken in a limiting sense. The scope of the present technology is defined only by the appended claims, along with the full scope of equivalents to which such claims are entitled. 
     In this description, references to “one embodiment”, “an embodiment”, or “embodiments” mean that the feature or features being referred to are included in at least one embodiment of the technology. Separate references to “one embodiment”, “an embodiment”, or “embodiments” in this description do not necessarily refer to the same embodiment and are also not mutually exclusive unless so stated and/or except as will be readily apparent to those skilled in the art from the description. For example, a feature, structure, act, etc. described in one embodiment may also be included in other embodiments, but is not necessarily included. Thus, the present technology can include a variety of combinations and/or integrations of the embodiments described herein. 
     Embodiments of the present technology provide an electronic fitness device that may be worn on a user&#39;s wrist, such as the electronic fitness device shown in  FIG. 1 , and provides optical cardiac monitoring by generating and utilizing photoplethysmogram (PPG) signals. The use of PPG signals for optical cardiac monitoring, such as measuring a user&#39;s pulse or heart rate, a pulse oximetry (“Pulse Ox”) level (also known as a level of blood oxygen saturation, or SpO2), an estimated stress level, a maximum rate of oxygen consumption (VO2 max), or the like. The electronic fitness device includes a first optical transmitter array and a second optical transmitter array, each of which is configured to output optical signals having a plurality of wavelengths. The optical signals pass through the user&#39;s skin and are received once they exit the user&#39;s skin by a plurality of optical receivers. Each optical receiver generates an electronic PPG signal for each optical signal received. The PPG signals are communicated to a processing element which processes the PPG signals to determine the user&#39;s heart rate or pulse oximetry. 
     In exemplary embodiments, the electronic fitness device includes four optical receivers positioned on a bottom wall of the electronic fitness device and oriented to form a quadrilateral shape with a space in the center of the quadrilateral. The first optical transmitter array is positioned in the space adjacent, or proximate, to two of the optical receivers. The second optical transmitter array is positioned in the space adjacent, or proximate, to the other two optical receivers. Hence, the first optical transmitter array is spaced apart from the second optical transmitter array along first and second orthogonal axes. This configuration of the optical transmitter arrays and the optical receivers provides signal diversity for the optical signals passing through the user&#39;s skin from the optical transmitter arrays to the optical receivers. Signal diversity includes having the optical signals travel different distances, through different angles, in different directions, and along different paths when passing through the user&#39;s skin. Signal diversity is beneficial because it allows for more accurate determination of the user&#39;s heart rate and pulse oximetry. 
     The configuration of the optical transmitter arrays and the optical receivers also provides improved performance when the electronic fitness device becomes tilted on the user&#39;s wrist, as may be likely when the user is exercising or working out. Prior art electronic fitness devices may have just one optical transmitter. When the electronic fitness device becomes tilted on the user&#39;s wrist, the optical transmitter becomes separated from the user&#39;s skin. Separation of the optical transmitter from the user&#39;s skin results in the optical signal level being greatly reduced when it is received by an optical receiver, which in turn generates a PPG signal with a relatively low signal to noise ratio (SNR). Consequently, the processing element may not be able to determine cardiac information from a PPG signal with a low SNR. Having the configuration of the present technology, with the first and second optical transmitter arrays spaced apart from one another along first and second orthogonal axes, results in one of the optical transmitter arrays making good contact with the user&#39;s skin if the other optical transmitter array gets separated from the user&#39;s skin when the electronic fitness device is tilted. And with good skin contact, the optical signal level will be relatively high, which results in the optical receivers generating PPG signals with a relatively high SNR. Given PPG signals with a high SNR, the processing element is able to determine the user&#39;s cardiac information. 
     Embodiments of the technology will now be described in more detail with reference to the drawing figures. Referring initially to  FIGS. 1-8 , an electronic fitness device  10  is illustrated. An exemplary electronic fitness device  10  may be embodied by a smart watch or a fitness band that is typically worn on a user&#39;s wrist, but may also be embodied by bands or belts worn on the user&#39;s arm, leg, head or torso. Other examples of the electronic fitness device  10  may include smartphones, personal data assistants, or the like which include a surface, operable to retain optical devices, that can be pressed against the user&#39;s skin. The electronic fitness device  10  may broadly comprise a housing  12 , a wrist band  14 , a display  16 , a user interface  18 , a communication element  20 , a location determining element  22 , a first optical transmitter array  24 , a second optical transmitter array  26 , a third optical transmitter array  27 , a first optical receiver  28 A, a second optical receiver  28 B, a third optical receiver  28 C, a fourth optical receiver  28 D, a fifth optical receiver  28 E, a sixth optical receiver  28 F, a seventh optical receiver  28 G, an eighth optical receiver  28 H, a plurality of lenses  30 , a memory element  32 , and a processing element  34 . 
     The housing  12  generally houses or retains other components of the electronic fitness device  10  and may include or be coupled to the wrist band  14 . As seen in  FIG. 3 , the housing  12  may include a bottom wall  36 , an upper surface  38 , and at least one side wall  40  that bound an internal cavity (not shown in the figures). The bottom wall  36  includes a lower, outer surface that contacts the user&#39;s wrist while the user is wearing the electronic fitness device  10 . The bottom wall  36  may be substantially flat with a slight curvature that enables the bottom wall  36  to contact a substantial portion of the user&#39;s wrist. The upper surface  38  opposes the bottom wall  36 . In various embodiments, the upper surface  38  may further include an opening that extends from the upper surface to the internal cavity. In some embodiments, such as the exemplary embodiments shown in the figures, the bottom wall  36  of the housing  12  may have a round, circular, or oval shape, with a single circumferential side wall  40 . In other embodiments, the bottom wall  36  may have a four-sided shape, such as a square or rectangle, or other polygonal shape, with the housing  12  including four or more sidewalls. The bottom wall  36  may include one or more openings in which the optical transmitter arrays  24 ,  26  and the optical receivers  28  are placed, positioned, or located. The one or more openings within the bottom wall  36  may be covered by one or more lenses  30  through which the optical signal may be transmitted and received. 
     The display  16  generally presents the information mentioned above, such as time of day, current location, and the like. The display  16  may be implemented in one of the following technologies: light-emitting diode (LED), organic LED (OLED), Light Emitting Polymer (LEP) or Polymer LED (PLED), liquid crystal display (LCD), thin film transistor (TFT) LCD, LED side-lit or back-lit LCD, or the like, or combinations thereof. In some embodiments, the display  16  may have a round, circular, or oval shape. In other embodiments, the display  16  may possess a square or a rectangular aspect ratio which may be viewed in either a landscape or a portrait orientation. 
     The user interface  18  generally allows the user to directly interact with the electronic fitness device  10  and may include pushbuttons, rotating knobs, or the like. In various embodiments, the display  16  may also include a touch screen occupying the entire display  16  or a portion thereof so that the display  16  functions as at least a portion of the user interface  18 . The touch screen may allow the user to interact with the electronic fitness device  10  by physically touching, swiping, or gesturing on areas of the display  16 . 
     The communication element  20  generally allows communication with external systems or devices. The communication element  20  may include signal and/or data transmitting and receiving circuits, such as antennas, amplifiers, filters, mixers, oscillators, digital signal processors (DSPs), and the like. The communication element  20  may establish communication wirelessly by utilizing radio frequency (RF) signals and/or data that comply with communication standards such as cellular 2G, 3G, 4G, LTE, or 5G, Institute of Electrical and Electronics Engineers (IEEE) 802.11 standard such as Wi-Fi, IEEE 802.16 standard such as WiMAX, Bluetooth™, or combinations thereof. In addition, the communication element  20  may utilize communication standards such as ANT, ANT+, Bluetooth™ low energy (BLE), the industrial, scientific, and medical (ISM) band at 2.4 gigahertz (GHz), or the like. Alternatively, or in addition, the communication element  20  may establish communication through connectors or couplers that receive metal conductor wires or cables which are compatible with networking technologies such as Ethernet. In certain embodiments, the communication element  20  may also couple with optical fiber cables. The communication element  20  may be in electronic communication with the memory element  32  and the processing element  34 . 
     The location determining element  22  generally determines a current geolocation of the electronic fitness device  10  and may receive and process radio frequency (RF) signals from a multi-constellation global navigation satellite system (GNSS) such as the global positioning system (GPS) utilized in the United States, the Galileo system utilized in Europe, the GLONASS system utilized in Russia, or the like. The location determining element  22  may accompany or include an antenna to assist in receiving the satellite signals. The antenna may be a patch antenna, a linear antenna, or any other type of antenna that can be used with location or navigation devices. The location determining element  22  may include satellite navigation receivers, processors, controllers, other computing devices, or combinations thereof, and memory. The location determining element  22  may process a signal, referred to herein as a “location signal”, from one or more satellites that includes data from which geographic information such as the current geolocation is derived. The current geolocation may include coordinates, such as the latitude and longitude, of the current location of the electronic fitness device  10 . The location determining element  22  may communicate the current geolocation to the processing element  34 , the memory element  32 , or both. 
     Although embodiments of the location determining element  22  may include a satellite navigation receiver, it will be appreciated that other location-determining technology may be used. For example, cellular towers or any customized transmitting radio frequency towers can be used instead of satellites may be used to determine the location of the electronic fitness device  10  by receiving data from at least three transmitting locations and then performing basic triangulation calculations to determine the relative position of the device with respect to the transmitting locations. With such a configuration, any standard geometric triangulation algorithm can be used to determine the location of the electronic fitness device  10 . The location determining element  22  may also include or be coupled with a pedometer, accelerometer, compass, or other dead-reckoning components which allow it to determine the location of the device  10 . The location determining element  22  may determine the current geographic location through a communications network, such as by using Assisted GPS (A-GPS), or from another electronic device, such as a fitness device or a mobile device (e.g., smartphone). The location determining element  22  may even receive location data directly from a user. 
     The first optical transmitter array  24  outputs, transmits, or emits a plurality of optical signals that are to pass, or travel, through the user&#39;s skin and exit before being received by the optical receivers  28 . The first optical transmitter array  24  includes a plurality of optical transmitters  42  (each optical transmitter  42  indicated in  FIGS. 4A, 4B, 5A, and 6  with a “TX A” prefix). In some embodiments, each optical transmitter  42  may include a photonic generator, such as a light-emitting diode (LED), a modulator, a top emitter, an edge emitter, or the like. The photonic generator receives an electrical input signal from the processing element  34  that may be a control signal, such as an electric voltage or electric current that is analog or digital, or data, either of which is indicative of activating or energizing the optical transmitter  42  to output an optical signal having a desired amplitude, frequency, and duration. The photonic generator of each optical transmitter  42  transmits or outputs electromagnetic radiation having a particular wavelength (the optical signal) in the visible light spectrum, which is typically between approximately 400 nanometers (nm) to 700 nm, or in the infrared spectrum, which is typically between approximately 700 nm to 1 millimeter (mm). However, in some embodiments, the photonic generator transmits electromagnetic radiation in wavelength range of 1000 nm to 1500 nm. The wavelength of the optical signal is generally determined by, or varies according to, the material from which the photonic generator of each optical transmitter  42  is formed. The optical signal may comprise a sequence of pulses, a periodic or non-periodic waveform, a constant level for a given period of time, or the like, or combinations thereof. In other embodiments, each optical transmitter  42  may include a driver circuit, with electronic circuitry such as amplifier and an optional filter, electrically coupled to the photonic generator. The driver circuit may receive the electrical input signal (control signal) from the processing element  34  and the driver circuit may generate an electric voltage or electric current to the photonic generator, which in turn, outputs the optical signal. 
     An exemplary first optical transmitter array  24  includes a first optical transmitter  42 A 1  configured or operable to output an optical signal having a first wavelength (λ 1 ), a second optical transmitter  42 A 2  configured or operable to output an optical signal having a second wavelength (λ 2 ), and a third optical transmitter  42 A 3  configured or operable to output an optical signal having a third wavelength (λ 3 ). In other embodiments, the first optical transmitter array  24  may include a larger number of optical transmitters  42  or a smaller number of optical transmitters  42 . The first wavelength λ 1  may range from approximately 540 nm to approximately 580 nm. The second wavelength λ 2  may range from approximately 620 nm to approximately 700 nm. The third wavelength λ 3  may range from approximately 850 nm to approximately 950 nm. The first wavelength λ 1  may be utilized to determine the user&#39;s heartrate. The second wavelength λ 2  and the third wavelength λ 3  may be utilized in combination to determine the user&#39;s pulse oximetry. 
     The optical transmitters  42  of first optical transmitter array  24  may be positioned relative to one another in any arrangement. For instance, in some embodiments (as shown in the figures), the optical transmitters  42  are positioned within the first optical transmitter array  24  with their centers equally spaced from one another in an equilateral triangle formation. In other embodiments, the optical transmitters  42  may be positioned with their centers aligned along a linear axis, such as along a horizontal axis or along a vertical axis, or in any arrangement in which the optical transmitters  42  are sufficiently spaced apart. 
     The second optical transmitter array  26  includes one or more optical transmitters  44  (each optical transmitter  44  indicated in  FIGS. 4A, 4B, 5B and 6  with a “TX B” prefix) and is similar in structure, function, and operation to the first optical transmitter array  24  and transmits or emits a plurality of optical signals that are to pass, or travel, through the user&#39;s skin and exit before being received by the optical receivers  28 . An exemplary second optical transmitter array  26  includes a first optical transmitter  44 B 1  configured or operable to output an optical signal having the first wavelength λ 1 , a second optical transmitter  44 B 2  configured or operable to output an optical signal having the second wavelength λ 2 , and a third optical transmitter  44 B 3  configured or operable to output an optical signal having the third wavelength λ 3 . As with the first optical transmitter array  24 , the wavelengths output by the second optical transmitter array  26  may be utilized to perform particular functions. That is, the first wavelength λ 1  may be utilized to determine the user&#39;s heartrate, while the second wavelength λ 2  and the third wavelength λ 3  may be utilized in combination to determine the user&#39;s pulse oximetry. 
     The first optical transmitter array  24  is positioned at a first location, or in a first opening, on the bottom wall  36  of the housing  12 , and the second optical transmitter array  26  is positioned at a second location, or in a second opening, on the bottom wall  36 , as shown in  FIGS. 3 and 4A . The first optical transmitter array  24  and the second optical transmitter array  26  are spaced apart from one another along a first axis  46  and along a second axis  48 , orthogonal to the first axis  46 , on the bottom wall  36 . As shown in  FIG. 4A , the first optical transmitter array  24  and the second optical transmitter array  26  are spaced apart from one another along the first axis  46 , e.g., an X axis, and along the second axis  48 , e.g., a Y axis, when viewing the bottom wall  36  as an XY plane. Accordingly, the first optical transmitter array  24  and the second optical transmitter array  26  may be considered to be positioned diagonal from one another. In some embodiments, additional transmitter arrays may be included (resulting in a total of four or more transmitter arrays) in a bottom wall  36  of housing  12 . For example, the additional transmitter arrays may be positioned proximate to transmitter arrays  24  and  26 . 
     In various embodiments, the electronic fitness device  10  comprises a third optical transmitter array  27  including a plurality of optical transmitters  45  (each optical transmitter  45  indicated in  FIGS. 4B, 5C, and 6  with a “TX C” prefix). The third optical transmitter array  27  is similar in structure, function, and operation to the first optical transmitter array  24  and transmits or emits a plurality of optical signals that are to pass, or travel, through the user&#39;s skin and exit before being received by the optical receivers  28 . An exemplary third optical transmitter array  27  includes a first optical transmitter  45 C 1  configured or operable to output an optical signal having the first wavelength λ 1 , a second optical transmitter  45 C 2  configured or operable to output an optical signal having the second wavelength λ 2 , and a third optical transmitter  45 C 3  configured or operable to output an optical signal having the third wavelength λ 3 . 
     Referring to  FIG. 4B , the third optical transmitter array  27  is positioned at a third location, or in a third opening, on the bottom wall  36  of the housing  12  between the first optical transmitter array  24  and the second optical transmitter array  26  so that the center of each array  24 ,  26 ,  27  lies along a diagonal line. Thus, the third optical transmitter array  27  is spaced apart from the first optical transmitter array  24  and the second optical transmitter array  26  along the first axis  46  and along the second axis  48 . The third optical transmitter array  27  may be positioned approximately midway between the first optical transmitter array  24  and the second optical transmitter array  26  along the first axis  46  and along the second axis  48 . In some embodiments, the third optical transmitter array  27  may be positioned roughly in the center of the bottom wall  36 . 
     The first optical receiver  28 A, the second optical receiver  28 B, the third optical receiver  28 C, and the fourth optical receiver  28 D (indicated in  FIGS. 4A, 4B, 5A, 5B, and 5C  with an “RX” prefix) each receive optical signals that have passed, or traveled, through the user&#39;s skin. In some embodiments, each optical receiver  28  may include a photodetector, such as a photodiode, a phototransistor, a photoresistor, a phototube, or the like. The photodetector receives electromagnetic radiation having multiple wavelengths (typically any of the wavelengths generated by the photonic generators) and in response, generates an electronic PPG signal, comprising an electric current, an electric voltage, or other electrical parameter, that corresponds to the intensity of the modulated optical signal in amplitude and frequency that is transmitted by one of the optical transmitters  42  and reflected from (after passing or traveling through) the user&#39;s skin. Given that the optical receivers  28  may receive multiple optical signals, each having a particular wavelength, each PPG signal generated by any optical receiver  28  may be a particular wavelength-related PPG signal because it includes characteristics or components resulting from, or related to, the particular wavelength of the optical signal output by an optical transmitter  42  of the first or second optical transmitter arrays  24 ,  26 . In other embodiments, each optical receiver  28  may include the photodetector electrically coupled to an amplifier circuit followed by an analog-to-digital converter (ADC). The photodetector may receive electromagnetic radiation having multiple wavelengths and in response, may generate an output signal, comprising an electric current, an electric voltage, or other electrical parameter that corresponds to the intensity of the modulated optical signal in amplitude and frequency that is transmitted by an optical transmitter  42  and reflected from the user&#39;s skin. The amplifier circuit may receive the output signal from the photodetector and amplify it to produce an amplified output signal that is analog and communicated to the ADC. The ADC may sample the amplified output signal and output a PPG signal, which is converted into a corresponding stream of digital data. 
     Each optical receiver  28  may have a rectangular shape with an elongated aspect ratio wherein a length of each optical receiver  28  is much greater than a width of each optical receiver  28 . Alternatively, each optical receiver  28  may have a square shape, a circular shape, and oval shape, or any one of a plurality of other geometric shapes. In embodiments, receivers  28  form a circle, ring or polygon around transmitter arrays  24  and  26 , with the ring being partitioned into 4 or more separate receivers  28 . 
     Referring to  FIGS. 3,5A, 5B, and 5C , the first optical receiver  28 A is positioned at a third location, or in a third opening, on the bottom wall  36  of the housing  12 . The second optical receiver  28 B is positioned at a fourth location, or in a fourth opening, on the bottom wall  36 . The third optical receiver  28 C is positioned at a fifth location, or in a fifth opening, on the bottom wall  36 . The fourth optical receiver  28 D is positioned at a sixth location, or in a sixth opening, on the bottom wall  36 . If the third optical transmitter array  27  is included with the electronic fitness device  10 , then the first optical receiver  28 A is positioned at a fourth location, or in a fourth opening, the second optical receiver  28 B is positioned at a fifth location, or in a fifth opening, the third optical receiver  28 C is positioned at a sixth location, or in a sixth opening, and the fourth optical receiver  28 D is positioned at a seventh location, or in a seventh opening. The first optical receiver  28 A is aligned with, and spaced apart from, the third optical receiver  28 C along the first axis  46  of the bottom wall  36 . The second optical receiver  28 B is aligned with, and spaced apart from, the fourth optical receiver  28 D along the second axis  48 . The optical receivers  28  may be positioned and oriented such that they form a quadrilateral (roughly square) area between them centered roughly in a center of the bottom wall  36 , wherein the first optical receiver  28 A may be oriented at a 180-degree angle relative to the center of the bottom wall  36 , the second optical receiver  28 B may be oriented at a 90-degree angle, the third optical receiver  28 C may be oriented at a 0-degree angle, and the fourth optical receiver  28 D may be oriented at a 270-degree angle. However, it is to be understanding the arrangement depicted in at least  FIG. 3  may be rotated 90-degrees to form a diamond pattern, where the first optical transmitter array  24  is positioned above the second optical transmitter array  26  and the first optical receiver  28 A is positioned approximately 45 degrees of rotation from a center point from an axis extending to the top of the bottom wall  36 , the second optical receiver  28 B is positioned approximately 135 degrees of rotation from said point, the third optical receiver  28 C is positioned approximately 225 degrees of rotation from said point, and the fourth optical receiver  28 D is positioned approximately 315 degrees of rotation from said point. Furthermore, the optical receivers  28  are positioned such that they surround the first optical transmitter array  24  and the second optical transmitter array  26 . That is, the first optical receiver  28 A faces a first side of the first optical transmitter array  24  and the second optical transmitter array  26 . The second optical receiver  28 B faces a second side of the first optical transmitter array  24  and the second optical transmitter array  26 . The third optical receiver  28 C faces a third side of the first optical transmitter array  24  and the second optical transmitter array  26 . The fourth optical receiver  28 D faces a fourth side of the first optical transmitter array  24  and the second optical transmitter array  26 . 
     Referring to  FIG. 6 , in various embodiments, the electronic fitness device  10  comprises a fifth optical receiver  28 E, a sixth optical receiver  28 F, a seventh optical receiver  28 G, and an eighth optical receiver  28 H (indicated as “RX5”, “RX6”, RX7”, and “RX8”) similar in function and operation to the optical receivers  28 A- 28 D described above. In such embodiments, the optical receivers  28 A- 28 D are repositioned from the locations described above and shown in  FIGS. 4A, 4B, 5A, 5B, and 5C . The optical receivers  28 A- 28 H are positioned in openings, or at locations, on the bottom wall  36  along a circumference of a circular formation at angular intervals of approximately 45 degrees. The optical transmitter arrays  24 ,  26 ,  27  are positioned in the area on the bottom wall  36  bounded by the optical receivers  28 . Each optical receiver  28  has a roughly annular sector shape, with side walls that are arcuate in shape or shaped like a portion of a circumference of one of two circles, and end walls shaped roughly like a portion of a radius of the circular formation of the optical receivers  28 . Each receiver  28  is also configured to receive optical signals from each of the optical transmitter arrays  24 ,  26 ,  27  and generate a corresponding PPG signal. 
     The lenses  30 , as shown in  FIGS. 4A, 4B, 5A, 5B, and 5C , generally provide cover for the optical transmitter arrays  24 ,  26  and the optical receivers  28 . In addition, the lenses  30  may be configured, operable, shaped, or formed to provide focusing, collimation, refraction, diffraction, and so forth. Furthermore, some lenses  30 , such as the lenses  30  that cover the optical transmitter arrays  24 ,  26 , may provide some functions, while other lenses  30 , such as the lenses  30  that cover the optical receivers  28 , may provide other functions. The lenses  30  that cover the optical transmitter arrays  24 ,  26  may direct optical signals transmitted by the optical transmitters  42 ,  44  to the skin of the user. The lenses  30  that cover the optical receivers  28  may direct the optical signals exiting from the skin to the optical receivers  28 . 
     The memory element  32  may be embodied by devices or components that store data in general, and digital or binary data in particular, and may include exemplary electronic hardware data storage devices or components such as read-only memory (ROM), programmable ROM, erasable programmable ROM, random-access memory (RAM) such as static RAM (SRAM) or dynamic RAM (DRAM), cache memory, hard disks, floppy disks, optical disks, flash memory, thumb drives, universal serial bus (USB) drives, or the like, or combinations thereof. In some embodiments, the memory element  32  may be embedded in, or packaged in the same package as, the processing element  34 . The memory element  32  may include, or may constitute, a non-transitory “computer-readable medium”. The memory element  32  may store the instructions, code, code statements, code segments, software, firmware, programs, applications, apps, services, daemons, or the like that are executed by the processing element  34 . The memory element  32  may also store data that is received by the processing element  34  or the device in which the processing element  34  is implemented. The processing element  34  may further store data or intermediate results generated during processing, calculations, and/or computations as well as data or final results after processing, calculations, and/or computations. In addition, the memory element  32  may store settings, data, documents, sound files, photographs, movies, images, databases, and the like. In various embodiments, the memory element  32  may store parameters such as threshold values and the like, which are retrieved during the operation of the electronic fitness device  10 . 
     The processing element  34  may comprise one or more processors. The processing element  34  may include electronic hardware components such as microprocessors (single-core or multi-core), microcontrollers, digital signal processors (DSPs), field-programmable gate arrays (FPGAs), analog and/or digital application-specific integrated circuits (ASICs), or the like, or combinations thereof. The processing element  34  may generally execute, process, or run instructions, code, code segments, code statements, software, firmware, programs, applications, apps, processes, services, daemons, or the like. The processing element  34  may also include hardware components such as registers, finite-state machines, sequential and combinational logic, and other electronic circuits that can perform the functions necessary for the operation of the current invention. In certain embodiments, the processing element  34  may include multiple computational components and functional blocks that are packaged separately but function as a single unit. The processing element  34  may be in electronic communication with the other electronic components through serial or parallel links that include universal busses, address busses, data busses, control lines, and the like. 
     The processing element  34  may be operable, configured, or programmed to perform the following functions by utilizing hardware, software, firmware, or combinations thereof. The processing element  34  generates the electrical input signal or control signal, which may include an electric voltage or electric current that is constant or variable, analog or digital, or data, as a single number or a stream of numbers, and communicates the signal to one of either the first optical transmitter array  24  or the second optical transmitter array  26 , in a normal operating mode. However, in other modes of operation, such as a test mode, the processing element  34  is operable to communicate the electrical input signal or control signal to each optical transmitter  42 ,  44  individually at different times, to one or more groups of the optical transmitters  42 ,  44  simultaneously, or to all of the optical transmitters  42 ,  44  simultaneously. Thus, the processing element  34  may generate and transmit six electrical input signals or control signals, one for each optical transmitter  42 ,  44 . 
     The processing element  34  generates and communicates the electrical input signal or control signal to the first optical transmitter array  24  or the second optical transmitter array  26  according to, or depending on, a signal to noise ratio (SNR) of the PPG signals generated by the optical receivers  28 . Each optical receiver  28  generates the PPG signal resulting from the receipt of the optical signal from either the first optical transmitter array  24  or the second optical transmitter array  26 . And, the PPG signal has an SNR which varies according to numerous factors, such as the distance between the bottom wall  36  and the user&#39;s skin. The SNR is generally inversely proportional to the distance between the bottom wall  36  and the user&#39;s skin. For example, a greater distance, or separation, between the bottom wall  36  and the user&#39;s skin results in a lower SNR. If the SNR of the PPG signals from both the first optical transmitter array  24  and the second optical transmitter array  26  are above an SNR threshold, then the processing element  34  may generate and communicate the electrical input signal or control signal to the first optical transmitter array  24  by default. If the SNR of the PPG signals of one of the first optical transmitter array  24  or the second optical transmitter array  26  is above the SNR threshold and the SNR of the PPG signals of the other one is not, then the processing element  34  may generate and communicate the electrical input signal or control signal to the optical transmitter array  42 ,  44  whose SNR of the PPG signals is above the SNR threshold. If the SNR of the PPG signals of neither the first optical transmitter array  24  nor the second optical transmitter array  26  is above the SNR threshold (perhaps as a result of the electronic fitness device  10  not being worn), then the processing element  34  may not generate and communicate the electrical input signal or control signal to either optical transmitter array  42 ,  44 . 
     The processing element  34  generates and communicates the electrical input signal or control signal to a particular transmitter within either the first optical transmitter array  24  or the second optical transmitter array  26  according to, or based on, the function or operation that is requested to be performed. If a heart rate determination is requested, then the processing element  34  generates and communicates the electrical input signal or control signal for a first time period to the first optical transmitter  42 A 1  of the first optical transmitter array  24  or the first optical transmitter  44 B 1  of the second optical transmitter array  26 , depending on conditions discussed above. The processing element  34  may pause for a second time period. The processing element  34  may then repeat the generation and communication of the electrical input signal or control signal followed by a pause. 
     If a pulse oximetry determination is requested, then the processing element  34  generates and communicates the electrical input signal or control signal for the first time period to the second optical transmitter  42 A 2  of the first optical transmitter array  24  or the second optical transmitter  44 B 2  of the second optical transmitter array  26 . The processing element  34  may pause for a second time period. Then, the processing element  34  generates and communicates the electrical input signal or control signal for a third time period to the third optical transmitter  42 A 3  of the first optical transmitter array  24  or the third optical transmitter  44 B 3  of the second optical transmitter array  26 . The processing element  34  may pause for a fourth time period. The processing element  34  may then repeat the sequence of generating and communicating of the electrical input signal or control signal to the second optical transmitter  42 A 2 ,  44 B 2 , pausing, generating and communicating of the electrical input signal or control signal to the third optical transmitter  42 A 3 ,  44 B 3 , and pausing in a time division multiplexing (TDM) fashion. 
     The processing element  34  receives the PPG signal from each of the optical receivers  28 . As indicated in  FIGS. 5A, 5B, 5C, and 7 , each optical transmitter  42 ,  44  outputs, generates, transmits, or emits electromagnetic radiation, that is, the optical signal, omnidirectionally. Alternatively, a focused optical signal is scattered omnidirectionally by the user&#39;s skin. Thus, whenever any single optical transmitter  42 ,  44  outputs the optical signal, each of the optical receivers  28  receives the optical signal, assuming the bottom wall  36  is in good contact with the user&#39;s skin and the optical signal is passing through the user&#39;s skin. Accordingly, each optical receiver  28  generates and communicates the PPG signal whenever any optical transmitter  42 ,  44  outputs the optical signal. As a result, the processing element  34  receives a first PPG signal from the first optical receiver  28 A, a second PPG signal from the second optical receiver  28 B, a third PPG signal from the third optical receiver  28 C, and a fourth PPG signal from the fourth optical receiver  28 D. In embodiments that include the fifth through the eighth optical receivers  28 E- 28 H, the processing element  34  receives a fifth PPG signal from the fifth optical receiver  28 E, a sixth PPG signal from the sixth optical receiver  28 F, a seventh PPG signal from the seventh optical receiver  28 G, and an eighth PPG signal from the eighth optical receiver  28 H. 
     The processing element  34  may condition the PPG signals by performing actions such as amplifying, filtering, averaging, or the like, or combinations thereof. Then, the processing element  34  may extract data from the PPG signals and determine or calculate the user&#39;s heart rate or pulse oximetry, depending on which cardiac monitoring value was requested. Furthermore, the processing element  34  may determine the SNR of each PPG signal individually or may determine the SNR of the average of two or more PPG signals. The processing element  34  may compare the SNR to the SNR threshold in order to determine to which optical transmitter array  24 ,  26  to communicate the electrical input signal or control signal. 
     In embodiments that include the third optical transmitter array  27 , the processing element  34  may generate and communicate the electrical input signal or control signal to the third optical transmitter array  27  if the SNR of the PPG signals from the optical receivers  28  which resulted from the first optical transmitter array  24  or the second optical transmitter array  26  is below the SNR threshold. Additionally, or alternatively, the processing element  34  may generate and communicate the electrical input signal or control signal to the third optical transmitter array  27  as part of a sequence of generating and communicating the electrical input signal or control signal in a TDM fashion, or based on other criteria. 
     The electronic fitness device  10  may operate as follows. The user may desire to determine his cardiac information, such as heart rate or pulse oximetry. He may utilize the user interface  18  to direct the processing element  34  to begin the process of determining the heart rate and/or the pulse oximetry. Alternatively, or additionally, the processing element  34  may have an operating mode in which it automatically initiates the process of determining the heart rate or pulse oximetry when a predetermined event occurs (e.g., heart-rate variability exceeding a predetermined threshold, body temperature exceeding a predetermined threshold, etc.) or on a periodic basis (e.g., every second, every minute, hourly, daily, etc.). 
     The processing element  34  generates and communicates the electrical input signal or control signal to one of the optical transmitters  42 ,  44  in one of the optical transmitter arrays  24 ,  26 . The particular optical transmitter  42 ,  44  and the particular optical transmitter array  24 ,  26  vary according to, or depend on, which cardiac parameter is requested and the value of the SNR of the PPG signals resulting from the optical signal from each of the optical transmitter arrays  24 ,  26 . When the bottom wall  36  is uniformly flush against the user&#39;s skin as shown in  FIG. 7 , the processing element  34  may generate and communicate the electrical input signal or control signal to one of the optical transmitters  42 ,  44  in either of the optical transmitter arrays  24 ,  26 , although the first optical transmitter array  24  may be chosen by default. The optical transmitter  42  outputs the optical signal which passes or travels through the user&#39;s skin and exits. Upon exit, the optical signal is received by each of the optical receivers  28 , and each optical receiver  28  generates and communicates the PPG signal to the processing element  34 . The processing element  34  receives the PPG signals and conditions them and processes them to determine the requested cardiac parameter. 
     Having the optical signal pass through the user&#39;s skin and travel in different directions, at different angles, along different paths, and over different distances before it is received by the optical receivers  28  helps to satisfy the goal of providing signal diversity of the optical signal—which in turn, leads to a more accurate determination of the user&#39;s heart rate and pulse oximetry. In exemplary embodiments, the optical signal output by each optical transmitter  42 ,  44  travels in four different directions, at four different angles, along four different paths, and over four different distances through the user&#39;s skin before it is received by the optical receivers  28 . 
     During the course of exercising, or from normal activity and movement of the user&#39;s arm, the bottom wall  36  of the housing  12  may become tilted on the user&#39;s wrist so that it is no longer flush, as shown in  FIG. 8 . In this case, one of the optical transmitter arrays  24 ,  26  and/or one or more of the optical receivers  28  may become separated by a small distance, at least, from the user&#39;s skin. In such a case, the separated optical receivers  28  may receive the optical signal that has passed through the skin at a relatively much lower signal level or optical intensity. Perhaps more importantly, the optical signal output by the separated optical transmitter array  24 ,  26  does not launch into the skin properly, and some of the optical signal may be reflected by the surface of the user&#39;s skin—preventing proper penetration of the optical signal into the user&#39;s skin. Thus, the PPG signals generated by the optical receivers  28  which result from the optical signal from the separated optical transmitter array  24  likely have a very low SNR. The processing element  34  receives the PPG signals and determines whether the SNR is above the SNR threshold. If the SNR of the PPG signals is not above the SNR threshold, then the processing element  34  generates and communicates the electrical input signal or control signal to only the optical transmitter array  24 ,  26  which is not separated from the user&#39;s skin. 
     Having the first optical transmitter array  24  and the second optical transmitter array  26  separated from one another along first and second orthogonal axes along the bottom wall  36  of the housing  12  results in at least one of the optical transmitter arrays  24 ,  26  making good contact with the user&#39;s skin when the electronic fitness device  10  becomes tilted on the user&#39;s wrist. One of the optical transmitter arrays  24 ,  26  will make good contact with the user&#39;s skin no matter whether the electronic fitness device  10  becomes tilted along the first axis, such as sideways on the user&#39;s wrist, or along the second axis, such as lengthwise on the user&#39;s wrist. Furthermore, having the optical receivers  28  positioned such that each of the optical transmitter arrays  24 ,  26  is adjacent and/or proximate to two of the optical receivers  28  results in the optical transmitter array  24 ,  26  that makes good contact with the user&#39;s skin being able to communicate with at least two of the optical receivers  28 . This configuration of the optical transmitter arrays  24 ,  26  and the optical receivers  28  ensures signal diversity even when the electronic fitness device  10  is tilted on the user&#39;s wrist and provides robust operation of the electronic fitness device  10  during intense physical activity. 
     In embodiments that include the third optical transmitter array  27 , the electronic fitness device  10  may operate in a similar fashion as described above, except that the third optical transmitter  27  may be utilized if the other two optical transmitter arrays  24 ,  26  are not making good contact with the user&#39;s skin or if otherwise the SNR of the PPG signals originating from the other two optical transmitter arrays  24 ,  26  are below the SNR threshold. Additionally, or alternatively, the third optical transmitter array  27  may be utilized as part of a sequence of utilizing each of the optical transmitter arrays  24 ,  26 ,  27  in a TDM fashion, or as part of other schema. 
     In embodiments that include eight optical receivers  28 , each optical receiver  28  is configured to receive optical signals from each of the optical transmitter arrays  24 ,  26 ,  27  and generate a corresponding PPG signal. Having eight optical receivers  28  positioned in a circular formation at 45-degree increments around the optical transmitter arrays  24 ,  26 ,  27  provides even greater diversity of the optical signal traveling through the user&#39;s skin from the source to the destination. Thus, there is the potential for the optical signal to travel in eight different directions, at eight different angles, along eight different paths, and over eight different distances through the user&#39;s skin. 
     Although the technology has been described with reference to the embodiments illustrated in the attached drawing figures, it is noted that equivalents may be employed and substitutions made herein without departing from the scope of the technology as recited in the claims. 
     Having thus described various embodiments of the technology, what is claimed as new and desired to be protected by Letters Patent includes the following: