Patent Publication Number: US-11651610-B2

Title: Heart rate and respiration rate measurement using a fingerprint sensor

Description:
FIELD 
     The present disclosure relates to the field of user interfaces. In particular, the present disclosure relates to measuring heart rate and respiration rate with an ultrasonic fingerprint sensor. 
     BACKGROUND 
     Fingerprint sensing and matching is a commonly used technique for personal identification or verification. For example, one approach to fingerprint identification involves scanning a sample fingerprint or an image with a biometric reader/sensor and storing the image and/or unique characteristics of the fingerprint image. The characteristics of a sample fingerprint may then be compared to information for reference fingerprints already in a database to determine proper identification of a person, such as for verification purposes. 
     Ultrasonic fingerprint sensors have become increasingly popular in mobile devices. Such sensors detect ridges and valleys of a user&#39;s fingerprint by transmitting ultrasonic signals toward the user&#39;s finger and measuring the signals detected thereby. While useful, such ultrasonic sensors may be limited to detecting fingerprints for authenticating users. It is desirable to have apparatuses and methods for using ultrasonic fingerprint sensors, to perform additional functions, as described below. 
     SUMMARY 
     The present disclosure relates to methods and apparatuses for measuring heart rate and respiration rate by using ultrasonic fingerprint sensors. In one embodiment, an ultrasonic fingerprint sensor in a mobile device is operated to capture an initial snapshot (also called “ultrasound image”) of reflection from a user&#39;s finger&#39;s surface, of acoustic energy transmitted at a first frequency. Additionally, the ultrasonic fingerprint sensor is operated repeatedly to capture over time, a sequence of sets, each set including one or more additional snapshots of reflection from one or more depths within the user&#39;s finger, of acoustic energy transmitted at a second frequency which is significantly lower (e.g. more than 30% lower, or in some embodiments even more than 40% lower) than the first frequency. Measurements in the additional snapshots, which capture movement of subdermal structures within the user&#39;s finger, are processed to determine whether any signal oscillating at a rate in a predetermined range for heart rate (or respiration rate) is present. When a signal is found, its rate may be used in several ways, depending on the embodiment, e.g. to enable functionality that is currently disabled in the device (such as the display of the device) when a fingerprint in the initial snapshot matches a reference fingerprint of an authorized user, or to identify and track (and in some embodiments display) the user&#39;s heart rate (or respiration rate), based on the signal&#39;s rate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The aforementioned features and advantages of the disclosure, as well as additional features and advantages thereof, will be more clearly understandable after reading detailed descriptions of embodiments of the disclosure in conjunction with the non-limiting and non-exhaustive aspects of the following drawings. Like numbers are used throughout the figures. 
         FIG.  1 A  illustrates an exemplary block diagram of a mobile device according to aspects of the present disclosure. 
         FIG.  1 B  illustrates an exemplary implementation of the sensor subsystem of the mobile device of  FIG.  1 A  according to aspects of the present disclosure. 
         FIG.  2 A  illustrates an exemplary implementation of measuring a heart rate or a respiration rate according to aspects of the present disclosure. 
         FIG.  2 B  illustrates a graph used to obtain two frequencies F 1  and F 2  specified to an ultrasonic fingerprint sensor for generation of fingerprint snapshots and subdermal snapshots respectively according to aspects of the present disclosure. 
         FIG.  2 C  illustrates blocks  202 A and  202 B that implement block  202  of  FIG.  2 A  and blocks  204 A and  204 B that implement block  204  of  FIG.  2 A  in an exemplary implementation according to aspects of the present disclosure. 
         FIG.  3    illustrates blocks  310 ,  320  and  330  that together implement block  206  of  FIG.  2 A  according to certain aspects of the present disclosure. 
         FIG.  4    illustrates an example of filtering each snapshot to remove noise according to aspects of the present disclosure. 
         FIG.  5    illustrates an example of identifying most active locations according to aspects of the present disclosure. 
         FIG.  6    illustrates measuring a heart rate or a respiration rate according to aspects of the present disclosure. 
         FIG.  7    illustrates an exemplary block diagram of a device that may be configured to implement measuring heart rate and respiration rate using an ultrasonic fingerprint sensor according to aspects of the present disclosure. 
         FIGS.  8 A- 8 C  illustrate an example of an ultrasonic fingerprint sensor according to aspects of the present disclosure. 
         FIG.  9 A  illustrates an example of a four-by-four array of sensor pixels for an ultrasonic sensor array according to aspects of the present disclosure. 
         FIG.  9 B  illustrates an example of a high-level block diagram of an ultrasonic sensor system according to aspects of the present disclosure. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of methods and apparatuses for measuring heart rate and respiration rate using ultrasonic fingerprint sensors are disclosed. The following descriptions are presented to enable a person skilled in the art to make and use the disclosure. Descriptions of specific embodiments and applications are provided only as examples. Various modifications and combinations of the examples described herein may be readily apparent to those skilled in the art, and the general principles defined herein may be applied to other examples and applications without departing from the scope of the disclosure. Thus, the present disclosure is not intended to be limited to the examples described and shown, but is to be accorded the scope consistent with the principles and features disclosed herein. The word “exemplary” or “example” is used herein to mean “serving as an example, instance, or illustration.” Any aspect or embodiment described herein as “exemplary” or as an “example” is not necessarily to be construed as preferred or advantageous over other aspects or embodiments. 
     When a user places a finger on a platen of a fingerprint sensor, some embodiments of the type described herein operate the fingerprint sensor to generate one or more fingerprints, and also operate the fingerprint sensor while the finger is still on the platen to additionally measure a heart rate and/or a respiration rate. Thus, while the user&#39;s finger is still on the platen, the same fingerprint sensor may measure fingerprints during one window of time, measure heart rate in a second window of time, and measure respiratory rate in a third window of time (or alternatively measure the respiratory rate concurrently with the heart rate). The fingerprint may be used for authentication, and the heart rate (or respiration rate) if detected may be used as an indication of liveness of the finger (i.e. determine it is not a spoof). Moreover, the measured heart rate and/or respiration rate may be shown to the user, on a display of a mobile device that contains the fingerprint sensor. 
       FIG.  1 A  illustrates an exemplary block diagram of a mobile device according to aspects of the present disclosure. As shown in  FIG.  1 A , a mobile device  100  (also referred to as an ultrasonic imaging apparatus) may include wireless connection module  102 , controller  104 , sensor subsystem  106 , memory  110  and applications module  108 . In one embodiment, the mobile device  100  of  FIG.  1 A  is implemented as mobile device  700  shown in  FIG.  7    and described below. The mobile device  100  of  FIG.  1 A  may optionally include multimedia subsystem  112 , speaker(s) and microphone(s)  114 , and display  116 . In some implementations, the wireless connection module  102  may be configured to support WiFi and/or Bluetooth in a wireless local area network (LAN) or wireless personal area network (PAN). The controller  104  may include one or more processors, software, hardware, and/or firmware to implement various functions described herein. For example, the controller  104  may be configured to implement functions of the mobile device  100  as described herein, e.g. in reference to  FIGS.  2 A to  9 B . The sensor subsystem  106  may be configured to sense and process various sensor input data and produce sensor output data to controller  104 . The applications module  108  may include a battery charging circuit and power manager, oscillators, phase lock loops, clock generators and timers. 
     In some implementations, the sensor subsystem  106  may be configured to sense and detect a swipe motion in low power conditions. For example, the sensor subsystem  106  may be configured to include a sensor having a plurality of sensor pixels, such as an 80 pixels by 180 pixels detector configuration, to determine a swipe motion of a finger or a stylus. In some other implementations, different sensor configurations with different sensor areas may be employed. 
     In certain embodiments, mobile device  100  may include a wireless transceiver that is capable of transmitting and receiving wireless signals via a wireless antenna over a wireless communication network. Some embodiments may include multiple wireless transceivers and wireless antennas to enable transmitting and/or receiving signals according to corresponding multiple wireless communication standards such as, for example, versions of IEEE Std. 802.11, CDMA, WCDMA, LTE, UMTS, GSM, AMPS, Zigbee and Bluetooth, etc. 
     In various embodiments, controller  104  may be configured to execute computer-readable instructions stored in memory  110  such as on a computer-readable storage medium, such as RAM, ROM, FLASH, or disc drive, just to name a few examples. More specifically, the instructions may be executable by one or more processors, specialized processors, or DSPs of controller  104 . Memory  110  may include a non-transitory computer-readable memory and/or a computer-readable memory that stores software code (programming code, instructions, etc.) that are executable by the processors and/or DSPs to perform functions described herein. In some embodiments, memory  110  supports (and is used to implement), means for storing data, including sets of snapshots of reflected acoustic energy and the measurements from the snapshots. Controller  104  may execute instructions in memory  110  to perform one or more aspects of processes/methods discussed below in connection with  FIGS.  2 A,  3 ,  4 ,  5  and  6   . 
     In some implementations, a user interface may include any one of several devices such as, for example, multimedia subsystem  112 , speakers and microphones  114 , display  116 , etc. In a particular implementation, the user interface may enable a user to interact with one or more applications hosted on mobile device  100 . For example, devices may store digital signals in memory  110  to be further processed by controller  104  in response to an action from a user. Similarly, applications hosted on mobile device  100  may store digital signals in memory  110  to present an output signal to a user. 
     Mobile device  100  may also include a camera for capturing still or moving imagery. The camera may include, for example, an imaging sensor (e.g., charge coupled device or CMOS imager), lens, analog to digital circuitry, frame buffers, etc. In some implementations, additional processing, conditioning, encoding or compression of signals representing captured images may be performed by controller  104 . Alternatively, a video processor may perform conditioning, encoding, compression or manipulation of signals representing captured images. Additionally, the video processor may decode/decompress stored image data for presentation on display  116  of mobile device  100 . 
       FIG.  1 B  illustrates an exemplary implementation of the sensor subsystem of the mobile device  100  of  FIG.  1 A  according to aspects of the present disclosure. Sensor subsystem  106  may generate analog signals that may be converted to digital signals using an analog-to-digital converter (ADC). Alternatively, sensor subsystem  106  may generate digital signals. The digital signals are stored in memory  110  and processed by controller  104  in support of one or more applications such as, for example, applications related to activating a device based on detection of a fingerprint image. 
     As shown in  FIG.  1 B , the sensor subsystem  106  may include one or more sensor input devices  122 , sensor processing module  124 , and one or more sensor output devices  126 . The one or more sensor input devices  122  may include an ultrasonic fingerprint sensor for capturing fingerprints and/or measuring heart rate and respiration rate as described herein, e.g. in association with  FIG.  1 A . The one or more sensor input devices  122  may also include one or more other ultrasonic sensors, temperature and moisture sensors, capacitive sensors, microphones, ultrasound microphone arrays, photo detectors, image sensors, touch sensors, pressure sensors, chemical sensors, gyroscopes, accelerometers, magnetometers, GPS and compass. The sensor processing module  124  may be configured to perform one or more of the following functions, including but not limited to: input sensor selection and control, synchronization and timing control, signal processing, sensor platform performance estimation, sensor optimization, sensor fusion, and output sensor/device selection and control. The one or more sensor output devices  126  may produce one or more ultrasonic, voice, visual, biometric, nearness, presence, pressure, stability, vibration, location, orientation, heading, kinetics, electrical and chemical signals. The sensor subsystem  106  may be configured to implement functions of enabling operation of a mobile device, e.g. based on processing one or more two-dimensional ultrasound images of a finger&#39;s surface and subdermal structures within the finger, as described in reference to  FIGS.  2 A,  3 ,  4 ,  5  and  6   . In some implementations, one or more capacitive sensors may be configured to measure capacitance values of a touch of the finger. 
     The sensor processing module  124  may be configured to process sensor input data from the one or more sensor input devices  122 , and produce output commands or signals to the one or more sensor output devices  126 . According to aspects of the present disclosure, direct user inputs may be used to predictably manipulate power control behavior. In some embodiments, a mobile device may be configured to accept user commands (via direct, voice/aural and/or visual inputs) and be configured to sense a multitude of use, use environment and use contexts. In some implementations, an ultrasonic fingerprint sensor can be used to recognize a user&#39;s gestures, e.g. movements of a finger such as left/right/up/down, single or double taps, or press-and-hold motions that can be used to activate certain functions more quickly such as taking pictures. 
     In some implementations, the sensor processing module  124  may include an application-specific integrated circuit (ASIC) that includes circuitry such as a plurality of voltage regulators for generating a plurality of power supply voltages; memory, finite-state machines, level shifters and other associated circuitry for generating control signals to an ultrasonic fingerprint sensor (see  FIG.  9 B ) which includes an ultrasonic transmitter and an ultrasonic receiver (which in turn includes an ultrasonic sensor pixel circuit array); circuitry for generating transmitter excitation signals input to the ultrasonic transmitter to identify a frequency of acoustic energy to be transmitted, range-gate delay signals input to the ultrasonic receiver to determine depth at which reflection of the acoustic energy is to be measured, diode bias signals and receiver bias signals to the ultrasonic fingerprint sensor; circuitry for analog signal conditioning, analog-to-digital conversion and digital processing of the received pixel output signals from the ultrasonic receiver; and interface circuitry for sending digital output signals to an applications processor of a mobile device. The applications processor may execute the methods described in this disclosure. 
     In other implementations, in addition to the ASIC circuitry described in the prior paragraph, the ASIC may also include a microcontroller to autonomously execute one or more initial stages of methods and processes described below in reference to  FIGS.  2 A,  3 ,  4 ,  5  and  6    locally on the ASIC. For low power operations, it may be desirable that the microcontroller make determinations before requesting and enlisting the processing resources of an applications processor and/or data processor and/or other components in mobile device  100 . 
     In yet other implementations, in addition to the microcontroller and ASIC circuitry noted above, the ASIC may also include an ultrasonic fingerprint sensor&#39;s associated circuitry such as row-drivers and column-gate drivers to scan pixel circuits in the ultrasonic sensor pixel circuit array (in the ultrasonic receiver). In these implementations, the ASIC may execute the functions of sensing the output signals of pixel circuits at (x,y) locations in a two-dimensional array, in addition to the functions of finger presence detection and other functions described herein. 
       FIG.  2 A  illustrates an exemplary implementation of measuring heart rate and respiration rate using an ultrasonic fingerprint sensor, according to aspects of the present disclosure. In this exemplary implementation, a sequence of sets of one or more snapshots (or ultrasound images) at one or more depths within a user&#39;s finger may be captured sequentially in time by the ultrasonic fingerprint sensor, and a controller may be configured to use changes over time in the sequence of sets, due to movements of one or more subdermal structures in the user&#39;s finger, to determine the user&#39;s heart rate and/or respiration rate. 
     In one embodiment, controller  104  (see  FIG.  1 A ) responds to placement of a user&#39;s finger adjacent to an ultrasonic fingerprint sensor in sensor input device(s)  122  (see  FIG.  1 B ), by operating the ultrasonic fingerprint sensor. Specifically, the ultrasonic fingerprint sensor is operated to transmit acoustic energy at a first frequency toward a surface of the finger (e.g. see block  202 A in  FIG.  2 C ) and capture one or more initial snapshots (also called “fingerprint snapshots”) of one or more reflections of the acoustic energy from the finger&#39;s surface (e.g. see block  202 B in  FIG.  2 C ). Depending on the embodiment, the ultrasonic fingerprint sensor may be operated indirectly by controller  104  sending commands to sensor processing module  124  of  FIG.  1 B . In some embodiments, the acoustic energy is transmitted by the ultrasonic fingerprint sensor as an unmodulated pulse having a predetermined number of cycles of a sine wave at the first frequency. The just-described operation, which may be performed by controller  104  in some embodiments of mobile device  100  is illustrated by block  202  in  FIG.  2 A  (which may be implemented as blocks  202 A and  202 B in  FIG.  2 C ). 
     The first frequency specified by controller  104  in block  202  may be determined ahead of time in some embodiments to be high enough for acoustic energy sensed in the ultrasonic fingerprint sensor&#39;s pixel circuits, at (x,y) locations in a two-dimensional array (e.g. 80×180 in size), as measurements that have sufficient spatial resolution to identify lines representing ridges and valleys on the finger&#39;s surface. One embodiment of controller  104  specifies in block  202 , as the ultrasonic transmitter&#39;s frequency, a frequency F 1  (e.g. 18.5 MHz illustrated in  FIG.  2 B ) which is experimentally determined, based on occurrence of a local maxima in a graph of signal strength (measured in reflection of acoustic energy) on the y-axis and frequency of the acoustic energy transmitted on the x-axis (see  FIG.  2 B ). In some implementations, controller  104  may be configured to obtain the ultrasonic transmitter&#39;s frequency specified in block  202 , based on variation of a current temperature from a reference temperature at which an initial value for frequency F 1  is experimentally determined (e.g. 18.5 MHz illustrated in  FIG.  2 B ). 
     Moreover, to capture the initial snapshots in block  202 , controller  104  may specify a time delay (also referred to as range gate delay) between operating the ultrasonic receiver and the ultrasonic transmitter, based on an amount of time needed for sound to travel a first distance from the ultrasonic transmitter to the finger&#39;s surface (which may be placed on a surface of a platen), and a second distance from the finger&#39;s surface to the ultrasonic receiver. 
     On completion of block  202 , measurements by the ultrasonic fingerprint sensor which are included in one or more initial snapshots, may be stored in a memory of mobile device  100 , e.g. in memory  110  in  FIG.  1 A . The just-described storage operation, which may be performed by controller  104  in some embodiments of mobile device  100  is illustrated by block  203  in  FIG.  2 A . Also, controller  104  may be configured to process the one or more initial snapshot(s) in memory  110  in a normal manner, e.g. to extract a fingerprint therefrom, and compare the extracted fingerprint to a reference fingerprint of an authorized user. If the extracted fingerprint matches the reference fingerprint, controller  104  may authenticate the user and enable the user to gain access to mobile device  100 . In addition to authenticating a user based on an extracted fingerprint, certain embodiments of controller  104  may be configured to determine liveness of the user&#39;s finger (e.g. find a signal oscillating at a heart rate or at a respiration rate) as described herein, before enabling access to mobile device  100 . 
     Additionally, controller  104  may be configured to further operate the ultrasonic fingerprint sensor repeatedly (as illustrated by block  204  in  FIG.  2 A ), to capture over a window of time (e.g. of 3 seconds duration), a sequence of sets, each set including one or more additional snapshots (“subdermal snapshots”) of reflection from one or more depths within the user&#39;s finger, of acoustic energy transmitted (e.g. also as an unmodulated pulse having a predetermined number of cycles of a sine wave) at a second frequency by the ultrasonic fingerprint sensor. The just-described operation, which may be performed by controller  104  in some embodiments of mobile device  100  is illustrated by block  204  in  FIG.  2 A  (which may be implemented as blocks  204 A and  204 B in  FIG.  2 C ). 
     In block  204  in  FIG.  2 A  (and more particularly in block  204 A in  FIG.  2 C ), controller  104  is configured to not specify the first frequency (which is specified in block  202 ), because any reflections of acoustic energy at the first frequency from the one or more subdermal structures in the finger have intensities at the ultrasound receiver that are too low to be distinguishable from noise. Specifically, there is a tradeoff between resolution and scan depth: to obtain better resolution, the frequency needs to be increased, however, as the frequency increases the SNR decreases (the signal is attenuated) and the acoustic energy cannot travel deeper into the finger. Hence, controller  104  is configured in some embodiments, to specify in block  204 , a second frequency that is sufficiently low for the reflections of acoustic energy from the one or more subdermal structures to have intensities at the ultrasound receiver sufficiently high to be distinguishable from noise. 
     The second frequency specified by controller  104  in block  204  in  FIG.  2 A  (and more particularly in block  204 A in  FIG.  2 C ), may be determined ahead of time in some embodiments to be significantly lower (e.g. more than 30% lower, or in some embodiments even more than 40% lower) than the first frequency used in block  202 . In several embodiments, the second frequency is predetermined to be low enough for acoustic energy to pass through skin and reach subdermal structures within the finger. One embodiment of controller  104  specifies in block  204 , as the ultrasonic transmitter&#39;s frequency, a frequency F 2  (e.g. 9.25 MHz illustrated in  FIG.  2 B ) which is also experimentally determined, based on occurrence of another local maxima in the graph of signal strength (measured in reflection of acoustic energy) on the y-axis and frequency of the acoustic energy transmitted on the x-axis (see  FIG.  2 B ). In the specific embodiment illustrated in  FIG.  2 B , frequency F 2  (which is used to determine oscillation of an internal organ) is precisely half of the frequency F 1  (which is used to create a fingerprint), although this relationship between these two frequencies is approximate in other embodiments. In a manner similar or identical to block  202 , some implementations of controller  104  may be additionally configured to obtain the ultrasonic transmitter&#39;s frequency specified in block  204 , based on variation of the current temperature from a reference temperature at which the initial value for frequency F 2  is experimentally determined (e.g. 9.25 MHz illustrated in  FIG.  2 B ). 
     Moreover, to capture the additional snapshots in block  204 , controller  104  may specify one or more time delays (or range gate delays), based on an amount of time needed for sound to travel a first distance from the ultrasonic transmitter to one or more depths in the finger, and a second distance from the one or more depths in the finger to the ultrasonic receiver. The one or more depths in the finger may be selected to be, for example, in a range centered at half the thickness of a human finger (e.g. 4 mm), with the range having a width also of half the thickness of the human finger (e.g. 4 mm also). The number of depths used in block  204  may be configured ahead of time, e.g. based on computational power and memory of mobile device  100 . 
     In some embodiments, controller  104  is implemented with a system clock of 128 MHz, and converts each depth to a time delay (or range gate delay, abbreviated as RGD) based on speed of sound at 1500 meters per second as follows:
 
Depth [mm]=RGD/2/128 [MHz]*1500 [m/s]*1 e 3
 
RGDs=[500,1100]
 
Depth [mm]=[500,1100]/2/128 e 6*1500*1 e 3=[2.9,6.4]
 
The values 500 and 1100 of RGD shown above are number of cycles, of a clock oscillating at 128 MHz. However, it should be recognized that any suitable clock speed may be used to determine the RGD. In an illustrative embodiment, controller  104  uses an RGD of 650-800 corresponding to a depth of ˜4-4.5 mm. The just-described values of 650 and 800 are also expressed in number of clock cycles at 128 MHz. Some embodiments may use RGD of 4.5-8 μsec.
 
     On completion of one or more loops of operation of block  204 , measurements by the ultrasonic fingerprint sensor which are included in one or more additional snapshots, may be stored in a memory of mobile device  100 , e.g. in memory  110  in  FIG.  1 A . The just-described storage operation, which may be performed by controller  104  in some embodiments of mobile device  100  is illustrated by block  205  in  FIG.  2 A . 
     Controller  104  may be configured to process the one or more additional snapshot(s) in memory  110  in block  206  ( FIG.  2 A ), to determine whether any signal oscillating at a rate in a predetermined range for an internal organ of a human is present. In one illustrative example, the internal organ is a heart, and the predetermined range is selected to have a lower limit of 40 beats per minute (or 40 cycles per minute) and an upper limit of 200 beats per minute (or 200 cycles per minute). In another illustrative example, the internal organ is human lungs, and the predetermined range is selected to have a lower limit of 8 breaths per minute (or 8 cycles per minute) and an upper limit of 40 breaths per minute (or 40 cycles per minute). Other examples of predetermined ranges for a human organ as described herein may use lower limits and upper limits that approximate the just-described values, for example to within 10%. Depending on the embodiment, controller  104  may be configured to respond to finding a signal in block  206 , by using the signal&#39;s oscillation rate in several ways as per block  209  in  FIG.  2 A . For example, block  209  may enable functionality that is currently disabled in mobile device  100  (such as turning on power to display  116  in  FIG.  1 A ), when a fingerprint in an initial snapshot (see block  202 ) is found to match a reference fingerprint of an authorized user, or to identify and track (and in some embodiments, show on display  116 ) as a current value of the user&#39;s heart rate (or respiration rate), the signal&#39;s oscillation rate. 
     In some implementations, when controller  104  finds no signal in block  206 , controller  104  is configured to operate block  208  in which duration of the above-described window (see block  204 ) is increased, e.g. by 1 second. When block  206  is again operated, on measurements captured over the increased duration window, controller  104  may find a signal. The just-described loop, between blocks  208 ,  204 ,  205  and  206  may be repeated a predetermined number of times, e.g. 9 times (to reach a final window size of 12 seconds). If controller  104  does not detect a signal (or a predetermined number of signals) during the loop, controller  104  may determine that the object placed proximate to the fingerprint sensor is not a live finger or extremity (i.e., the object is a spoof). However, if controller  104  detects a signal (or a predetermined number of signals) in one or more of the loops, controller  104  may determine that the object placed proximate to the fingerprint sensor is a live finger of the user. In some embodiments, controller  104  which operates one or more of blocks  202 - 209  in  FIG.  2 A  supports (and is used to implement), means for controlling the operations of the mobile device and the ultrasonic fingerprint sensor, as well as means for controlling the operations of the methods described herein. 
     In some embodiments of controller  104 , block  206  which determines whether any signal oscillating at a heart rate (or respiration rate) is present in the additional measurements (captured in block  204 ) may be operated, by operating one or more of blocks  310 - 330  illustrated in  FIG.  3   . Specifically, in block  310 , controller  104  may be configured to filter each snapshot to remove noise (e.g. by signal processing in image domain). Thereafter, in block  320 , controller  104  may be configured to identify most active locations (wherein blood flow is likely) in the finger (e.g. by signal processing in time domain). Finally, in block  330 , controller  104  may be configured to measure heart rate or respiration rate (e.g. by signal processing in frequency domain). 
     Controller  104  may be configured to operate block  310  ( FIG.  3   ) by operating one or more of blocks  411 - 413  ( FIG.  4   ) as follows. In block  411 , controller  104  may be configured to retrieve from memory  110 , all snapshots within a time window (e.g. of 3 second in duration) that have been stored by block  205  ( FIG.  2 A ). In block  412 , controller  104  may be configured apply a low pass filter, such as a 2D median filter, to each snapshot, to obtain spatially filtered measurements. In block  413 , controller  104  may be configured to store in memory  110 , the spatially filtered measurements obtained by block  412 . 
     As noted above in reference to block  204  (see  FIGS.  2 A and  2 C ), the ultrasonic fingerprint sensor is operated repeatedly over a window of time, to capture a sequence of sets, and each set includes one or more snapshots at each of one or more depths (e.g. measured along z axis). Hence, measurements in the sequence may be indexed by time across the snapshots, e.g. at a specific location in the finger whose x,y coordinates are determined by a specific pixel circuit that measures acoustic energy at an (x,y) location in a two-dimensional array in the ultrasound receiver, and whose z coordinate is determined by depth in the finger (in turn determined by a time delay, also referred to as range gate delay). Thus, for each (x, y, z) location which is sampled in the finger (also called “subdermal location”), memory  110  contains a sequence of measurements indexed by time, which is stored by block  205  (e.g. see  FIG.  2 C ). Additionally, at each (x, y, z) location sampled in the finger, there may be a sequence of spatially filtered measurements indexed by time, which is stored in memory  110  by block  413  (if operated, depending on the embodiment). 
     In some embodiments, controller  104  may be configured to operate block  320  ( FIG.  3   ) by operating one or more of blocks  511 - 517  and optional block  518  ( FIG.  5   ) as follows. In block  511 , controller  104  may be configured to apply a bandpass filter to each sequence of measurements indexed by time (which may, in some embodiments, have been spatially filtered to remove noise in block  310 ). The result of operation of block  511  is a bandpass filtered sequence for each location sampled within the finger, and each bandpass filtered sequence is generated at least by removing (or reducing the amplitude of) any signal that oscillates in time below a lower limit or above an upper limit. 
     In one illustrative embodiment, controller  104  operates block  511  to determine whether any signal oscillating at a human heart rate is present, and uses 40 beats per minute as the lower limit of the bandpass filter and 200 beats per minute as the upper limit. In another illustrative embodiment, controller  104  operates block  511  to determine whether any signal oscillating at a human respiration rate is present, and uses 8 breaths per minute as the lower limit of the bandpass filter and 40 breaths per minute as the upper limit. 
     In still another embodiment, block  206  (see  FIG.  2 A ) is implemented to contain therein two parallel branches that are respectively used to determine heart rate and respiration rate simultaneously. These two parallel branches start in a common block  310 , followed by two copies of block  320  to identify two different sets of most active locations in the finger (see  FIG.  3   ) for use in determining heart rate and respiration rate respectively (e.g. by applying two bandpass filters, with corresponding predetermined ranges of 40-200 cycles per minute and 8-40 cycles per minute). The two copies of block  320  are followed by corresponding two copies of block  330  used to find respective peaks in the frequency domain, thereby to identify the heart rate and respiration rate. In one such embodiment, a time window is 8 seconds in duration, and both heart rate and respiration rate are determined simultaneously, from a common set of measurements. In such an embodiment, heart rate is determined as soon as measurements are accumulated for 3 seconds, although determination of respiration rate takes an additional 5 seconds (so that sufficient measurements are accumulated in its time window of 8 seconds). In other embodiments, a different time window may be used (e.g., 16 seconds) to provide sufficient time to measure the respiratory rate. 
     Yet another embodiment may be configured to operate block  206  (and therefore block  320 ) twice, a first time to determine heart rate and a second time to determine respiration rate, and these rates may be determined based on measurements in respective time windows that do not overlap one another. 
     In many such embodiments, wherein signals of both heart rate and respiration rate are determined from subdermal snapshots in a finger, most of the blocks and operations described herein may be operated and/or performed similarly or identically to one another, except for the above-noted difference in upper and lower limits of the predetermined ranges. 
     In block  512 , controller  104  may be configured to compute variance of each bandpass filtered sequence generated in block  511 , by application of the bandpass filter. For example, in one illustrative embodiment, controller  104  computes variance as follows: 
               1   T     ⁢       ∑     t   =   0       T   -   1       ⁢       (       x   ⁡     [   t   ]       -     μ   x       )     2             
where T is number of samples (or measurements) in a sequence along the time axis, t. Thereafter, in block  513 , controller  104  ranks the subdermal locations being sampled (or probed), based on variance of corresponding bandpass filtered sequences, which is computed in block  512 . Next, based on the rank ordering in block  513 , controller  104  selects in block  514 , a predetermined number N of subdermal locations (e.g.  1000  subdermal locations), which have the highest statistical variances (and hence most active).
 
     Thereafter, in block  515 , controller  104  may be configured to perform a test (e.g. check if variances of the N selected locations exceed a threshold or other such test) related to noise. If the test in block  515  is not met, block  516  may be operated to declare that no signal is found, followed by going to block  208  (see  FIG.  2 A , described above). When the test in block  515  is met, block  517  may be operated, to store in memory  110 , coordinates of the N selected locations. Finally, in an optional block  518 , the N selected locations may be used to identify location of peak variance, and the peak variance&#39;s location may be used to select a central depth and one or more depths around the central depth, for use in operating the ultrasound receiver to capture subdermal snapshots in a next operation of block  204  (see  FIG.  2 A , described above). In such embodiments, an initial set of depths may be predetermined to be in the range [−150,150] around a center RGD of ˜650, in blocks of 50 or less. 
     Controller  104  may be configured to operate block  330  ( FIG.  3   ) by operating one or more of blocks  611 - 616  ( FIG.  6   ) as follows. In block  611 , controller  104  may be configured to apply a Fourier transform, to each time domain sequence (which may, in some embodiments, have been bandpass filtered in block  511  as described above) at each subdermal location in the N subdermal locations selected for having highest variance over time (e.g. as described above in block  514  ( FIG.  5   ). On completion of block  611 , controller  104  may obtain N frequency domain vectors. Thereafter, in block  612 , controller  104  may be configured to combine the N frequency domain vectors, e.g. by aggregation (or by averaging), to obtain a single frequency domain vector. Subsequently, in block  613 , controller  104  may be configured to identify a peak in the single frequency domain vector. The peak is tested in block  613  to have sufficient signal-to-noise ratio, and if not then controller  104  may go to block  614  to wait for a new set of subdermal snapshots to become available (e.g. by sliding the time window forward, when an iteration of sampling is completed). 
     In some embodiments, block  613  may be configured to calculate an estimated signal-to-noise ratio and a quality factor, to determine the presence of a peak (and hence a signal oscillating at a rate in the predetermined range for an internal organ of a human, as per block  206 ) based on the following formulae. 
     
       
         
           
             Peak 
             ⁢ 
             
               = 
               Δ 
             
             ⁢ 
             
               Y 
               ⁡ 
               
                 ( 
                 
                   f 
                   = 
                   
                     f 
                     c 
                   
                 
                 ) 
               
             
           
         
       
       
         
           
             Noise 
             ⁢ 
             
               = 
               Δ 
             
             ⁢ 
             
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                 ( 
                 
                   f 
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               SNR 
               est 
             
             = 
             
               
                 
                   Peak 
                   2 
                 
                 
                   MeanNoise 
                   2 
                 
               
               · 
               
                 π 
                 
                   2 
                   ⁢ 
                   
                     N 
                     fft 
                   
                 
               
             
           
         
       
       
         
           
             
               QF 
               est 
             
             = 
             
               
                 Peak 
                 - 
                 
                   Mean 
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                 Std 
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     When a peak is found, in some embodiments, controller  104  may be configured to operate block  615 , wherein the identified peak is used to track a new peak in the frequency domain. The tracking in block  615  is performed in the frequency domain on measurements in one or more newly captured subdermal snapshots, using an incremental frequency which is an order of magnitude smaller than another incremental frequency used in the Fourier transform, e.g. using Extended Kalman Filter (“EKF”) estimation and/or maximum likelihood (“ML”) estimation. A frequency of the new peak obtained by tracking in block  615  is stored in block  616  by controller  104  in memory  110 , for use as a heart rate of the user (or a respiration rate of the user), e.g. in block  209  ( FIG.  2 A ). 
     The frequency stored in memory  110  by block  616  may be shown by some embodiments, on display  116  of mobile device  100  ( FIG.  1 A ), e.g. as a heart rate or a respiration rate. Depending on the embodiment, the frequency stored in memory  110  by block  616  may be used by controller  104  to determine liveness and based thereon enable power to display  116 , as described above. In some embodiments, a time window of at least 3 seconds in duration is used to determine heart rate as described above. Hence, after a finger is placed on platen  40  of an ultrasonic fingerprint sensor  10  ( FIGS.  8 A- 8 C ) in such embodiments, a pulse of low ultrasound frequency is repeatedly transmitted towards the finger for at least 3 seconds before the display is powered up and heart rate displayed thereon (if the user&#39;s fingerprint is determined to be authentic). 
     In some embodiments, block  615  may be configured to calculate estimates using a quadratic interpolation method, based on a peak and its two nearest samples as follows.
 
 y 1= Xa ( k− 1);
 
 y 2= Xa ( k );
 
 y 3= Xa ( k+ 1);
 
 d =( y 3− y 1)/(2*(2* y 2− y 1− y 3));
 
 f _interp= f ( k )+ d*df;  
 
where:
 
     k is the peak index 
     Xa(k) is the peak value 
     f(k) is the frequency of the peak 
     df is the frequency spacing 
     f_interp is the final interpolated frequency 
       FIG.  7    illustrates an exemplary block diagram of a device that may be configured to implement the methods and apparatuses for measuring fingerprints, heart rate, and/or respiration rate using an ultrasonic fingerprint sensor, according to aspects of the present disclosure. A device that implements measuring (and in some embodiments showing) heart rate and/or respiration rate using an ultrasonic fingerprint sensor may include one or more features of mobile device  700  shown in  FIG.  7   . In certain embodiments, mobile device  700  may include a wireless transceiver  721  that is capable of transmitting and receiving wireless signals  723  via wireless antenna  722  over a wireless communication network. Wireless transceiver  721  may be connected to bus  701  by a wireless transceiver bus interface  720 . Wireless transceiver bus interface  720  may, in some embodiments be at least partially integrated with wireless transceiver  721 . Some embodiments may include multiple wireless transceivers  721  and wireless antennas  722  to enable transmitting and/or receiving signals according to a corresponding multiple wireless communication standards such as, for example, versions of IEEE Std. 802.11, CDMA, WCDMA, LTE, UMTS, GSM, AMPS, Zigbee and Bluetooth®, etc. Mobile device  700  of some embodiments may be implemented as a smartphone (or similar electronic device). Depending on the embodiment, mobile device  700  may be made sufficiently small in size to be carried in a hand of an adult human. 
     Mobile device  700  may also include GPS receiver  755  capable of receiving and acquiring GPS signals  759  via GPS antenna  758 . GPS receiver  755  may also process, in whole or in part, acquired GPS signals  759  for estimating a location of a mobile device. In some embodiments, processor(s)  711 , memory  740 , DSP(s)  712  and/or specialized processors (not shown) may also be utilized to process acquired GPS signals, in whole or in part, and/or calculate an estimated location of mobile device  700 , in conjunction with GPS receiver  755 . Storage of GPS or other signals may be performed in memory  740  or registers (not shown). 
     Also shown in  FIG.  7   , mobile device  700  may include digital signal processor(s) (DSP(s))  712  connected to the bus  701  by a bus interface  710 , processor(s)  711  connected to the bus  701  by a bus interface  710  and memory  740 . Bus interface  710  may be integrated with the DSP(s)  712 , processor(s)  711  and memory  740 . In various embodiments, functions may be performed in response to execution of one or more computer-readable instructions stored in memory  740  such as on a computer-readable storage medium, such as RAM, ROM, FLASH, or disc drive, just to name a few examples. The one or more instructions may be executable by processor(s)  711 , specialized processors, or DSP(s)  712 . Memory  740  may include a non-transitory computer-readable memory and/or a computer-readable memory that stores software code (programming code, instructions, etc.) that are executable by processor(s)  711  and/or DSP(s)  712  to perform functions described herein. In a particular implementation, wireless transceiver  721  may communicate with processor(s)  711  and/or DSP(s)  712  through bus  701  to enable mobile device  700  to be configured as a wireless station. Processor(s)  711  and/or DSP(s)  712  may perform methods and functions, and execute instructions to execute one or more aspects of processes/methods discussed in connection with  FIGS.  2 A,  3 ,  4 ,  5  and  6   . 
     Also shown in  FIG.  7   , a user interface  735  may include any one of several devices such as, for example, a speaker, microphone, display device, vibration device, keyboard, touch screen, etc. A user interface signal provided to a user may be one or more outputs provided by any of the speaker, microphone, display device, vibration device, keyboard, touch screen, etc. In a particular implementation, user interface  735  may enable a user to interact with one or more applications hosted on mobile device  700 . For example, devices of user interface  735  may store digital signals in memory  740  to be further processed by DSP(s)  712  or processor  711  in response to action from a user. Similarly, applications hosted on mobile device  700  may store digital signals in memory  740  to present an output signal to a user. In another implementation, mobile device  700  may optionally include a dedicated audio input/output (I/O) device  770  comprising, for example, a dedicated speaker, microphone, digital to analog circuitry, analog to digital circuitry, amplifiers and/or gain control. In another implementation, mobile device  700  may include touch sensors  762  responsive to touching, pressure, or ultrasonic signals on a keyboard or touch screen device. 
     Mobile device  700  may also include a dedicated camera device  764  for capturing still or moving imagery. Dedicated camera device  764  may include, for example an imaging sensor (e.g., charge coupled device or CMOS imager), lens, analog to digital circuitry, frame buffers, etc. In one implementation, additional processing, conditioning, encoding or compression of signals representing captured images may be performed at processor  711  or DSP(s)  712 . Alternatively, a dedicated video processor  768  may perform conditioning, encoding, compression or manipulation of signals representing captured images. Additionally, dedicated video processor  768  may decode/decompress stored image data for presentation on a display device (not shown) on mobile device  700 . 
     Mobile device  700  may also include sensors  760  coupled to bus  701  which may include, for example, inertial sensors and environmental sensors. Inertial sensors of sensors  760  may include, for example accelerometers (e.g., collectively responding to acceleration of mobile device  700  in three dimensions), one or more gyroscopes or one or more magnetometers (e.g., to support one or more compass applications). Environmental sensors of mobile device  700  may include, for example, temperature sensors, barometric pressure sensors, ambient light sensors, and camera imagers, microphones, just to name few examples. Sensors  760  may include one or more ultrasonic fingerprint sensors. Sensors  760  may generate analog signals that may be converted to digital signals using an analog-to-digital converter (ADC). Alternatively, sensors  760  may generate digital signals. The digital signals are stored in memory  740  and processed by DPS(s) or processor  711  in support of one or more applications such as, for example, applications directed to measuring heart rate and/or respiration rate and/or applications directed to positioning or navigation operations. 
     In a particular implementation, mobile device  700  may include a dedicated modem processor  766  capable of performing baseband processing of signals received and down-converted at wireless transceiver  721  or GPS receiver  755 . Similarly, dedicated modem processor  766  may perform baseband processing of signals to be up-converted for transmission by wireless transceiver  721 . In alternative implementations, instead of having a dedicated modem processor, baseband processing may be performed by a processor or DSP (e.g., processor  711  or DSP(s)  712 ). 
       FIGS.  8 A- 8 C  illustrate an example of an ultrasonic fingerprint sensor according to aspects of the present disclosure. As shown in  FIG.  8 A , an ultrasonic fingerprint sensor  10  may include an ultrasonic transmitter  20  and an ultrasonic receiver  30  under a platen  40 . The ultrasonic transmitter  20  may be a piezoelectric transmitter that can generate ultrasonic waves  21  (see  FIG.  8 B ). The ultrasonic receiver  30  may include a piezoelectric material and a two dimensional array of pixel circuits (e.g. in an ultrasonic sensor pixel circuit array, shown in  FIG.  9 B ), disposed in or on a substrate. In some implementations, the substrate may be a glass, plastic or semiconductor substrate such as a silicon substrate. In operation, the ultrasonic transmitter  20  may generate one or more ultrasonic waves that travel through the ultrasonic receiver  30  to the exposed surface  42  of the platen  40 . At the exposed surface  42  of the platen  40 , the ultrasonic energy may be transmitted, absorbed or scattered by an object  25  that is in contact with the platen  40 , such as the skin of a fingerprint ridge  28 , or reflected back. In those locations where air contacts the exposed surface  42  of the platen  40 , e.g., valleys  27  between fingerprint ridges  28 , most of the ultrasonic wave will be reflected back toward the ultrasonic receiver  30  for detection (see  FIG.  8 C ). In some embodiments, ultrasonic fingerprint sensor  10  in  FIGS.  8 A- 8 C  supports (and is used to implement), means for sensing ultrasound. 
     Control electronics  50  may be coupled to the ultrasonic transmitter  20  and ultrasonic receiver  30  and may supply timing signals that cause the ultrasonic transmitter  20  to generate one or more ultrasonic waves  21 . The control electronics  50  may then receive signals from the ultrasonic receiver  30  that are indicative of reflected ultrasonic energy (also called acoustic energy)  23 . The control electronics  50  may use output signals received from the ultrasonic receiver  30  to construct a three-dimensional image of the object  25 . In some implementations, the control electronics  50  may also, over time, successively sample the output signals to detect movement of structures within object  25 . In some embodiments, control electronics  50  in  FIGS.  8 A- 8 C  supports (and is used to implement), means for controlling the operations of the ultrasonic fingerprint sensor, as well as means for controlling the operations of the methods described herein. 
     According to aspects of the present disclosure, the ultrasonic transmitter  20  may be a plane wave generator including a substantially planar piezoelectric transmitter layer. Ultrasonic waves may be generated by applying a voltage to the piezoelectric layer to expand or contract the layer, depending upon the signal applied, thereby generating a plane wave. The voltage may be applied to the piezoelectric transmitter layer via a first transmitter electrode and a second transmitter electrode. In this fashion, an ultrasonic wave may be made by changing the thickness of the layer via a piezoelectric effect. This ultrasonic wave travels toward a finger (or other object), passing through the platen  40 . A portion of the wave not absorbed or transmitted into a finger may be reflected, so as to pass back through the platen  40  and be received by the ultrasonic receiver  30 . The first and second transmitter electrodes may be metallized electrodes, for example, metal layers that coat opposing sides of the piezoelectric transmitter layer. 
     The ultrasonic receiver  30  may include a two dimensional array of pixel circuits disposed in or on a substrate, which also may be referred to as a wafer or a backplane, and a piezoelectric receiver layer. In some implementations, each pixel circuit may include one or more silicon or thin-film transistor (TFT) elements, electrical interconnect traces and, in some implementations, one or more additional circuit elements such as diodes, capacitors, and the like. Each pixel circuit may be configured to convert an electric charge generated in the piezoelectric receiver layer proximate to the pixel circuit into an electrical signal. Each pixel circuit may include a pixel input electrode that electrically couples the piezoelectric receiver layer to the pixel circuit. 
     In the illustrated implementation, a receiver bias electrode is disposed on a side of the piezoelectric receiver layer proximal to platen  40 . The receiver bias electrode may be a metallized electrode and may be grounded or biased to control which signals are passed to the silicon or TFT sensor array. Ultrasonic energy that is reflected from the exposed (top) surface  42  of the platen  40  is converted into localized electrical charges by the piezoelectric receiver layer. These localized charges are collected by the pixel input electrodes and are passed on to the underlying pixel circuits. The charges may be amplified by the pixel circuits and provided to the control electronics, which processes the output signals. A simplified schematic of an example pixel circuit is shown in  FIG.  9 A , however one of ordinary skill in the art will appreciate that many variations of and modifications to the example pixel circuit shown in the simplified schematic may be contemplated. 
     Control electronics  50  may be electrically connected to the first transmitter electrode and the second transmitter electrode, as well as to the receiver bias electrode and the pixel circuits in or on the substrate. The control electronics  50  may operate substantially as discussed previously with respect to  FIGS.  8 A- 8 C . 
     The platen  40  may be any appropriate material that can be acoustically coupled to the receiver, with examples including plastic, ceramic, glass, sapphire, stainless steel, aluminum, a metal, a metal alloy, polycarbonate, a polymeric material, or a metal-filled plastic. In some implementations, the platen  40  may be a cover plate, e.g., a cover glass or a lens glass for a display device or an ultrasonic fingerprint sensor. Detection and imaging may be performed through relatively thick platens if desired, e.g., 3 mm and above. 
     Examples of piezoelectric materials that may be employed according to various implementations include piezoelectric polymers having appropriate acoustic properties, for example, acoustic impedance between about 2.5 MRayls and 5 MRayls. Specific examples of piezoelectric materials that may be employed include ferroelectric polymers such as polyvinylidene fluoride (PVDF) and polyvinylidene fluoride-trifluoroethylene (PVDF-TrFE) copolymers. Examples of PVDF copolymers include 60:40 (molar percent) PVDF-TrFE, 70:30 PVDF-TrFE, 80:20 PVDF-TrFE, and 90:10 PVDR-TrFE. Other examples of piezoelectric materials that may be employed include polyvinylidene chloride (PVDC) homopolymers and copolymers, polytetrafluoroethylene (PTFE) homopolymers and copolymers, and diisopropylammonium bromide (DIPAB). 
     The thickness of each of the piezoelectric transmitter layer and the piezoelectric receiver layer may be selected so as to be suitable for generating and receiving ultrasonic waves. In one example, a PVDF piezoelectric transmitter layer may be approximately 28 μm thick and a PVDF-TrFE receiver layer may be approximately 12 μm thick. Example frequencies of the ultrasonic waves are in the range of 5 MHz to 30 MHz, with wavelengths on the order of a quarter of a millimeter or less. 
       FIGS.  8 A- 8 C  show example arrangements of ultrasonic transmitters and ultrasonic receivers included in an ultrasonic fingerprint sensor, with other arrangements possible. For example, in some implementations, the ultrasonic transmitter  20  may be above the ultrasonic receiver  30 , i.e., closer to the object of detection. In some implementations, the piezoelectric receiver layer may serve as both an ultrasonic transmitter and an ultrasonic receiver. A piezoelectric layer that may serve as either an ultrasonic transmitter or an ultrasonic receiver may be referred to as a piezoelectric transceiver layer or as a single-layer transmitter/receiver layer. In some implementations, the ultrasonic fingerprint sensor may include an acoustic delay layer. For example, an acoustic delay layer may be incorporated into the ultrasonic fingerprint sensor  10  between the ultrasonic transmitter  20  and the ultrasonic receiver  30 . An acoustic delay layer may be employed to adjust the ultrasonic pulse timing, and at the same time electrically insulate the ultrasonic receiver  30  from the ultrasonic transmitter  20 . The delay layer may have a substantially uniform thickness, with the material used for the delay layer and/or the thickness of the delay layer selected to provide a desired delay in the time for reflected ultrasonic energy to reach the ultrasonic receiver  30 . In doing so, the range of time during which an energy pulse that carries information about the object by virtue of having been reflected by the object may be made to arrive at the ultrasonic receiver  30  during a time range when it is unlikely that energy reflected from other parts of the ultrasonic fingerprint sensor  10  is arriving at the ultrasonic receiver  30 . In some implementations, the silicon or TFT substrate and/or the platen  40  may serve as an acoustic delay layer. 
       FIG.  9 A  depicts a 4×4 subarray of pixel circuits  31 A . . .  31 J . . .  31 N in an ultrasonic sensor pixel circuit array  31  included in an ultrasonic receiver  30  ( FIGS.  8 A- 8 C ) which in turn is included within an ultrasonic fingerprint sensor  10  ( FIGS.  8 A- 8 C ). Each pixel circuit  31 J ( FIG.  9 A ) may, for example, be associated with a local region of piezoelectric sensor material  35 J, a receive bias electrode  36 J, a diode bias voltage electrode  37 J, a pixel input electrode  34 J, a peak detection diode  32 J and a readout transistor  33 J; many or all of these elements may be formed on or in the backplane to form pixel circuit  31 J. In practice, the local region of piezoelectric sensor material  35 J of each pixel circuit  31 J may transduce received ultrasonic energy into electrical charges. The peak detection diode  32 J may register the maximum amount of charge detected by the local region of piezoelectric sensor material  35 J. Each row of the ultrasonic sensor pixel circuit array  31  ( FIGS.  8 A- 8 C ) may be scanned, e.g., through a row select mechanism  38 , a gate driver, or a shift register, and the readout transistor  33 J for each column may be triggered to allow the magnitude of the peak charge for each pixel circuit  31 J to be read by pixel readout circuitry  39  which may include, e.g., a multiplexer and an A/D converter. The ultrasonic sensor pixel circuit array  31  ( FIGS.  8 A- 8 C ) may include one or more silicon transistors or TFTs to allow gating, addressing, and resetting of the pixel circuits  31 A . . .  31 J . . .  31 N. 
     Each pixel circuit  31 J may provide information about a small portion of the object (such as finger of a user) detected by the ultrasonic fingerprint sensor  10 . While, for convenience of illustration, the example shown in  FIG.  9 A  is of a relatively coarse resolution, ultrasonic fingerprint sensors having a resolution on the order of 500 pixels per inch or higher may be configured with a layered structure. The detection area of the ultrasonic fingerprint sensor  10  may be selected depending on the intended object of detection. For example, the detection area (e.g., active area) may range from about 5 mm×5 mm for a single finger to about 3 inches×3 inches for four fingers. Smaller and larger areas, including square, rectangular and non-rectangular geometries, may be used as appropriate for the object. 
       FIG.  9 B  shows an example of a high-level block diagram of an ultrasonic sensor system  90 . All elements shown in  FIG.  9 B , except for ultrasonic sensor pixel circuit array  31  and ultrasonic transmitter  20 , may form part of control electronics  50 . Thus, control electronics  50  may include a sensor controller  51  (e.g. in a microcontroller of an ASIC described above) that in turn may include a control unit  51 C configured to control various aspects of sensor system  90 , e.g., ultrasonic transmitter  20 &#39;s timing and excitation waveforms, bias voltages for the ultrasonic receiver  30  and pixel circuits  31 A . . .  31 J . . .  31 N, pixel addressing, signal filtering and conversion, readout frame rates, and so forth. The sensor controller  51  may also include a data processor  51 D that receives data from the ultrasonic sensor pixel circuit array  31 . The data processor  51 D may translate the digitized data into image data of a fingerprint or format the data for further processing. 
     For example, the control unit  51 C may send a transmitter (Tx) excitation signal to a Tx driver at regular intervals to cause the Tx driver  57  to excite the ultrasonic transmitter  20  and produce planar ultrasonic waves. The control unit  51 C may send level select input signals through a receiver (Rx) bias driver  58  to bias the receive bias electrode  36 J and allow gating of acoustic signal detection by the pixel circuits  31 A . . .  31 J . . .  31 N. A demultiplexer  52  may be used to turn on and off gate drivers  56  that cause a particular row or column of pixel circuits  31 A . . .  31 J . . .  31 N to provide sensor output signals. Output signals from the pixel circuits  31 A . . .  31 J . . .  31 N may be sent through a charge amplifier  53 , a filter  54  such as an RC filter or an anti-aliasing filter, and a digitizer  55  to the data processor  51 D. Note that portions of the system  90  may be included on the silicon or TFT substrate and other portions may be included in an associated integrated circuit (e.g., an ASIC). 
     According to aspects of the present disclosure, an ultrasonic fingerprint sensor may be configured to produce high-resolution fingerprint images for user verification and authentication. In some implementations, the ultrasonic fingerprint sensor may be configured to detect reflected signals proportional to the differential acoustic impedance between an outer surface of a platen and a finger ridge (tissue) and valley (air). For example, a portion of the ultrasonic wave energy of an ultrasonic wave may be transmitted from the sensor into finger tissue in the ridge areas (and used, for example, in measuring heart rate and/or respiration rate as described herein), while the remaining portion of the ultrasonic wave energy is reflected back towards the sensor, whereas a smaller portion of the wave may be transmitted into the air in the valley regions of the finger while the remaining portion of the ultrasonic wave energy is reflected back to the sensor. 
     As described herein, memory  110  and/or memory  740  may provide means for storing data associated with the operation of the mobile devices and/or ultrasonic fingerprint sensors described herein. Sensor subsystem  106 , sensors  760  and/or ultrasonic fingerprint sensor  10  may provide means for sensing ultrasound as well as means for transmitting acoustic energy toward a finger of a user and for receiving one or more reflections of ultrasonic energy from the finger. Controller  104 , applications module  108 , control electronics  50 , sensor controller  51 , processor  711 , and/or DSP  712  may provide means for controlling the operation of the mobile device, the ultrasonic fingerprint sensor, and/or the blocks of the methods described herein. For example, controller  104 , applications module  108 , control electronics  50 , sensor controller  51 , processor  711 , and/or DSP  712  may provide means for controlling or operating the means for sensing ultrasound to transmit acoustic energy at a first frequency toward a surface of a finger (e.g. see block  202 A in  FIG.  2 C ), and for controlling or operating the means for sensing ultrasound to capture a first snapshot of one or more reflections of the acoustic energy at the first frequency from the surface of the finger (e.g. see block  202 B in  FIG.  2 C ). Controller  104 , applications module  108 , control electronics  50 , sensor controller  51 , processor  711 , and/or DSP  712  may also store in the means for storing, a plurality of first measurements in the first snapshot (e.g. see block  203  in  FIG.  2 C ). Controller  104 , applications module  108 , control electronics  50 , sensor controller  51 , processor  711 , and/or DSP  712  may repeatedly operate the means for sensing ultrasound over a window of time to transmit acoustic energy toward the surface of the finger, the acoustic energy being transmitted at a second frequency that is lower than the first frequency (e.g. see block  204 A in  FIG.  2 C ), and capture a sequence of sets, each set comprising one or more second snapshots of reflection of the acoustic energy at the second frequency from one or more depths within the finger (e.g. see block  204 B in  FIG.  2 C ). Controller  104 , applications module  108 , control electronics  50 , sensor controller  51 , processor  711 , and/or DSP  712  may furthermore store in the means for storing, each plurality of second measurements in each second snapshot in said each set in the sequence (e.g. see block  205  in  FIG.  2 C ). It should be recognized that controller  104 , applications module  108 , control electronics  50 , sensor controller  51 , processor  711 , and/or DSP  712  may be programmed using computer-readable instructions stored in memory  110  and/or memory  740  to perform the functions described herein, including but not limited to, the algorithms embodied in the blocks of the methods described in  FIGS.  2 A and  3 - 6   . 
     The methodologies described herein may be implemented by various means depending upon applications according to particular examples. For example, such methodologies may be implemented in hardware, firmware, software, or combinations thereof. In a hardware implementation, for example, a processing unit may be implemented within one or more application specific integrated circuits (“ASICs”), digital signal processors (“DSPs”), digital signal processing devices (“DSPDs”), programmable logic devices (“PLDs”), field programmable gate arrays (“FPGAs”), processors, controllers, micro-controllers, microprocessors, electronic devices, other devices units designed to perform the functions described herein, or combinations thereof. 
     Some portions of the detailed description included herein are presented in terms of algorithms or symbolic representations of operations on binary digital signals stored within a memory of a specific apparatus or special purpose computing device or platform. In the context of this particular specification, the term specific apparatus or the like includes a general purpose computer once it is programmed to perform particular operations pursuant to instructions from program software. Algorithmic descriptions or symbolic representations are examples of techniques used by those of ordinary skill in the signal processing or related arts to convey the substance of their work to others skilled in the art. An algorithm is here, and generally, is considered to be a self-consistent sequence of operations or similar signal processing leading to a desired result. In this context, operations or processing involve physical manipulation of physical quantities. Typically, although not necessarily, such quantities may take the form of electrical or magnetic signals capable of being stored, transferred, combined, compared or otherwise manipulated. It has proven convenient at times, principally for reasons of common usage, to refer to such signals as bits, data, values, elements, symbols, characters, terms, numbers, numerals, or the like. It should be understood, however, that all of these or similar terms are to be associated with appropriate physical quantities and are merely convenient labels. Unless specifically stated otherwise, as apparent from the discussion herein, it is appreciated that throughout this specification discussions utilizing terms such as “processing,” “computing,” “calculating,” “determining” or the like refer to actions or processes of a specific apparatus, such as a special purpose computer, special purpose computing apparatus or a similar special purpose electronic computing device. In the context of this specification, therefore, a special purpose computer or a similar special purpose electronic computing device is capable of manipulating or transforming signals, typically represented as physical electronic or magnetic quantities within memories, registers, or other information storage devices, transmission devices, or display devices of the special purpose computer or similar special purpose electronic computing device. 
     The terms, “and,” and “or” as used herein may include a variety of meanings that will depend at least in part upon the context in which it is used. Typically, “or” if used to associate a list, such as A, B or C, is intended to mean A, B, and C, here used in the inclusive sense, as well as A, B or C, here used in the exclusive sense. Reference throughout this specification to “one example” or “an example” means that a particular feature, structure, or characteristic described in connection with the example is included in at least one example of claimed subject matter. Thus, the appearances of the phrase “in one example” or “an example” in various places throughout this specification are not necessarily all referring to the same example. Furthermore, the particular features, structures, or characteristics may be combined in one or more examples. Examples described herein may include machines, devices, engines, or apparatuses that operate using digital signals. Such signals may include electronic signals, optical signals, electromagnetic signals, or any form of energy that provides information between locations. 
     While there has been illustrated and described what are presently considered to be example features, it will be understood by those skilled in the art that various other modifications may be made, and equivalents may be substituted, without departing from claimed subject matter. Additionally, many modifications may be made to adapt a particular situation to the teachings of claimed subject matter without departing from the central concept described herein. Therefore, it is intended that claimed subject matter not be limited to the particular examples disclosed, but that such claimed subject matter may also include all aspects falling within the scope of the appended claims, and equivalents thereof.