Patent Publication Number: US-2023137801-A1

Title: Readout architecture for indirect time-of-flight sensing

Description:
BACKGROUND INFORMATION 
     Field of the Disclosure 
     This disclosure relates generally to image sensors, and in particular but not exclusively, relates to time-of-flight sensors. 
     Background 
     Interest in three dimensional (3D) cameras is increasing as the popularity of 3D applications continues to grow in areas such as imaging, movies, games, computers, user interfaces, facial recognition, object recognition, augmented reality, and the like. A typical passive way to create 3D images is to use multiple cameras to capture stereo or multiple images. Using the stereo images, objects in the images can be triangulated to create the 3D image. One disadvantage with this triangulation technique is that it is difficult to create 3D images using small devices because there must be a minimum separation distance between each camera in order to create the 3D images. In addition, this technique is complex and therefore requires significant computer processing power in order to create the 3D images in real time. 
     For applications that require the acquisition of 3D images in real time, active depth imaging systems based on time-of-flight measurements are sometimes utilized. Time-of-flight cameras typically employ a light source that directs light at an object, a sensor that detects the light that is reflected from the object, and a processing unit that calculates the distance to the object based on the round-trip time it takes for the light to travel to and from the object. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Non-limiting and non-exhaustive embodiments of the present invention are described with reference to the following figures, wherein like reference numerals refer to like parts throughout the various views unless otherwise specified. 
         FIG.  1    is a block diagram that shows one example of a time-of-flight light sensing system in accordance with the teachings of the present invention. 
         FIG.  2    is a timing diagram that shows an example of light pulses emitted from a light source relative to the receipt of the reflected light pulses and measurements using various phase shifts in an example time-of-flight sensing system accordance with the teachings of the present invention. 
         FIGS.  3 A- 3 F  are diagrams of various examples of time-of-flight pixel arrays on which the field-of-view for a given exposure is varied or programmed to include one or more fractional portions of the time-of flight pixel arrays, which are scanned by an example time-of-flight light sensing system in accordance with the teachings of the present invention. 
         FIG.  4    is a schematic illustrating one example of a time-of-flight pixel circuit included in a time-of-flight pixel array of a time-of-flight sensing system in accordance with the teachings of the present invention. 
         FIG.  5 A  is a schematic that shows one example of a time-of-flight light sensing system in accordance with the teachings of the present invention. 
         FIG.  5 B  is a schematic that shows another example of a time-of-flight light sensing system in accordance with the teachings of the present invention. 
         FIG.  6    is a timing diagram that shows one example of signals during integration and readout of one example of a time-of-flight light sensing system in accordance with the teachings of the present invention. 
         FIG.  7    is a schematic illustrating another example of a time-of-flight pixel circuit included in a time-of-flight pixel array of a time-of-flight sensing system in accordance with the teachings of the present invention. 
         FIG.  8    is a schematic that shows yet another example of a time-of-flight light sensing system in accordance with the teachings of the present invention. 
     
    
    
     Corresponding reference characters indicate corresponding components throughout the several views of the drawings. Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of various embodiments of the present invention. In addition, common but well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention. 
     DETAILED DESCRIPTION 
     Examples directed to various embodiments of a time-of-flight sensing system with indirect time-of-flight solutions for portions of the field-of-view are described herein. In the following description, numerous specific details are set forth to provide a thorough understanding of the examples. One skilled in the relevant art will recognize, however, that the techniques described herein can be practiced without one or more of the specific details, or with other methods, components, materials, etc. In other instances, well-known structures, materials, or operations are not shown or described in detail in order to avoid obscuring certain aspects. 
     Reference throughout this specification to “one example” or “one embodiment” means that a particular feature, structure, or characteristic described in connection with the example is included in at least one example of the present invention. Thus, the appearances of the phrases “in one example” or “in one embodiment” 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 any suitable manner in one or more examples. 
     Spatially relative terms, such as “beneath,” “below,” “over,” “under,” “above,” “upper,” “top,” “bottom,” “left,” “right,” “center,” “middle,” and the like, may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is rotated or turned over, elements described as “below” or “beneath” or “under” other elements or features would then be oriented “above” the other elements or features. Thus, the exemplary terms “below” and “under” can encompass both an orientation of above and below. The device may be otherwise oriented (rotated ninety degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. In addition, it will also be understood that when an element is referred to as being “between” two other elements, it can be the only element between the two other elements, or one or more intervening elements may also be present. 
     Throughout this specification, several terms of art are used. These terms are to take on their ordinary meaning in the art from which they come, unless specifically defined herein or the context of their use would clearly suggest otherwise. It should be noted that element names and symbols may be used interchangeably through this document (e.g., Si vs. silicon); however, both have identical meaning. 
     As will be discussed, various examples of indirect time-of-flight sensing systems include a light source is configured to emit modulated light to an object. The modulated light is then reflected from the object back to a time-of-flight sensor that includes a time-of-flight pixel array that includes a plurality of time-of-flight pixel circuits. In the various examples, the time-of-flight sensor includes a modulation driver block that synchronizes the modulated light emitted from the light source with the time-of-flight pixel circuits included in the time-of-flight pixel array that are configured to be illuminated by the reflected modulated light from the object and read out. 
     In various examples, a first subset of the time-of-flight pixel circuits included in the time-of-flight pixel array are configured to be enabled, while a second subset of the time-of-flight pixel circuits included in the time-of-flight pixel array are configured to be disabled when sensing the reflected modulated light from the object. In various examples, the first subset of the time-of-flight pixel circuits included in the time-of-flight pixel array are configured be illuminated by the reflected modulated light from the object, while the second subset of the time-of-flight pixel circuits included in the time-of-flight pixel array are configured to be non-illuminated by the reflected modulated light from the object. In various examples, the first subset of the time-of-flight pixel circuits included in the time-of-flight pixel array that are configured be illuminated by the reflected modulated light from the object may be scanned across the time-of-flight pixel array until an entire frame is captured by the time-of-flight pixel array. In other examples, the first subset of the time-of-flight pixel circuits included in the time-of-flight pixel array that are configured be illuminated by the reflected modulated light from the object may be randomly addressed and accessed to monitor or track objects and adjust a region of interest in the time-of-flight pixel array. 
     As will be discussed, in the various examples, the first subset of the pixel circuits may include one or more line shaped regions (e.g., row(s) or column(s) in the time-of-flight pixel array), one or more spot shaped regions (e.g., randomly addressable contiguous region of pixel circuits in the time-of-flight pixel array), and/or one or more non-contiguous regions (e.g., non-neighboring clusters of pixel circuits in the time-of-flight pixel array) of the pixel array included in the time-of-flight sensor. 
     In operation, modulated light is reflected from a portion of the object and impinges onto the pixel circuits included in the first subset of the pixel circuits that are included in the time-of-flight pixel array. Pixel circuits that are not illuminated or non-illuminated by the reflected modulated light may be included in a second subset of the pixel circuits. The pixel circuits included in the second subset of the pixel circuits may be deactivated, which reduces readout speed requirements and helps to save power consumption and cost. 
     The object distance is determined in response to the measured phase of the modulation, which may be used to yield a 3D frame. As will be described in the various examples, a demodulation pixel front-end down-converts and/or mixes this waveform with a differential phase modulation signals that are applied to the transfer gates or transfer transistors of a fractional portion or subset of the indirect time-of-flight pixel circuits included in the time-of-flight pixel array for a given exposure. 
     In the various examples, the differential phase modulation signals have the same frequency as the modulated light to realize homodyne detection by the indirect time-of-flight sensor. Employing different phases in the differential phase modulation signals allows to reconstruction of the encoded distance. In various examples, at least 3 independent measurements (e.g., sub-frames) are employed to decode the 3 unknowns of distance/phase, reflectivity, and ambient light. Typically 4 phases are used (e.g., 0°/180° and 90°/270°). 
     It is noted that phases that are increments of 360° apart cannot be distinguished, which consequently results in ambiguities in the measurements. As a result, the modulation frequency of the differential phase modulation signal is chosen not to exceed a maximum modulation frequency in order to accommodate a desired depth range. However, a tradeoff is that increasing the modulation frequency improves precision. Hence, typically multiple frequencies are incorporated to resolve ambiguities and still yield good precision. At each frequency, all 3/4 phases need to be acquired. 
     To illustrate,  FIG.  1    is a block diagram that shows one example of a time-of-flight light sensing system  100 , in accordance with the teachings of the present invention. As shown in the depicted example, time-of-flight light sensing system  100  includes light source  102  that is synchronized with a time-of-flight sensor that includes a pixel array  110 , which includes a plurality of pixel circuits  112 , and a control circuit  114  that is coupled to the pixel array  110  and light source  102 . 
     As shown in the example, light source  102  and pixel array  110  are positioned at a distance L from object  106 . Light source  102  is configured to emit light  104  towards object  106 . Reflected light  108  is directed back from object  106  to pixel array  110  as shown. It is noted that pixel array  110  and control circuit  114  are represented as separate components in  FIG.  1    for explanation purposes. However, it is appreciated that pixel array  110  and components of control circuit  114  may be integrated onto a same integrated circuit chip or wafer in a non-stacked standard planar sensor. 
     As shown in the depicted example, the light source  102  is configured to illuminate only a portion  107  of object  106  at a time such that portion  109  of object  106  is non-illuminated by the emitted light  104  from light source  102 . In the various examples, the illuminated portion  107  may have various shapes (e.g., one or more line shaped regions, one or more spot shaped regions, etc.) at a time for a given exposure. As such, the reflected light  108  from object  106  illuminates only a corresponding subset  113  of pixel circuits  112  of the pixel array  110  such that another subset  115  of the pixel array  110  is non-illuminated by the reflected light  108  from object  106  in accordance with the teachings of the present invention. In the various examples, the illuminated portion  107  of object  106  may be scanned across object  106  such that the illuminated subset of pixel circuits  113  of pixel array  110  is scanned across the pixel array  110  accordingly in time-of-flight light sensing system  100  in accordance with the teachings of the present invention. 
     In the depicted example, time-of-flight light sensing system  100  is a 3D camera that calculates image depth information of a scene (e.g., object  106 ) based on indirect time-of-flight (e.g., iToF) measurements with an image sensor that includes pixel array  110 . In some examples, it is appreciated that although time-of-flight light sensing system  100  is capable of sensing 3D images, time-of-flight light system  100  may also be utilized to capture 2D images. In various examples, time-of-flight light sensing system  100  may also be utilized to capture high dynamic range (HDR) images. 
     Continuing with the depicted example, each pixel circuit  112  of pixel array  110  determines depth information for a corresponding portion of object  106  such that a 3D image of object  106  can be generated. As will be discussed, depth information is determined by driving the transfer gates of each pixel circuit  112  with differential phase modulation signals to measure the delay/phase difference between emitted light  104  and the received reflected light  108  to indirectly determine a round-trip time for light to propagate from light source  102  to object  106  and back to the pixel array  110  of time-of-flight light sensing system  100 . The depth information may be based on an electric signal or charge that is photogenerated by the photodiode included in each pixel circuit  112 , which is subsequently transferred to a storage node and read out. 
     As illustrated, light source  102  is configured to emit light  104  to the object  106  over a distance L. The emitted light  104  is then reflected from the object  106  as reflected light  108  (e.g., reflected light waves/pulses), some of which propagates towards the pixel array  110  of time-of-flight light sensing system  100  over the distance L and is incident upon the pixel circuits  112  of pixel array  110  as image light. Each pixel circuit  112  included in the pixel array  110  includes a photodetector (e.g., one or more photodiodes, avalanche photodiodes, or single-photon avalanche diodes, or the like) to detect the reflected light  108  and convert the reflected light  108  into an electric signal (e.g., electrons, image charge, etc.). 
     As shown in the depicted example, the round-trip time for emitted light  104  to propagate from light source  102  to object  106  and then be reflected back to pixel array  110  can be used to determine the distance L using the following relationships in Equations (1) and (2) below: 
     
       
         
           
             
               
                 
                   
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                     c 
                   
                 
               
               
                 
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                   1 
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                   L 
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                         T 
                         
                           T 
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                       c 
                     
                     2 
                   
                 
               
               
                 
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     where c is the speed of light, which is approximately equal to 3×10 8  m/s, and T TOF  corresponds to the round-trip time, which is the amount of time that it takes for the light to travel to and from the object  106  as shown in  FIG.  1   . Accordingly, once the round-trip time is known, the distance L may be calculated and subsequently used to determine depth information of object  106 . 
     As shown in the depicted example, a control circuit  114  is coupled to pixel array  110  and light source  102 , and includes logic and memory that when executed causes time-of-flight light sensing system  100  to perform operations for determining the round-trip time. Determining the round-trip time may be based on, at least in part, timing signals generated by control circuit  114 . For indirect time-of-flight (iTOF) measurements, the timing signals are representative of the delay/phase difference between the light waves/pulses of when the light source  102  emits light  104  and when the photodetectors in pixel circuits  112  detect the reflected light  108 . 
     In some examples, time-of-flight light sensing system  100  may be included in a device (e.g., a mobile phone, a tablet, a camera, etc.) that has size and power constraints determined, at least in part, based on the size of the device. Alternatively, or in addition, time-of-flight light sensing system  100  may have specific desired device parameters such as frame rate, depth resolution, lateral resolution, etc. 
       FIG.  2    is a timing diagram that illustrates the timing relationship between example light pulses emitted from a light source relative to the receipt of the reflected light pulses and measurements using various phase shifts in an example time-of-flight imaging system accordance with the teachings of the present invention. Specifically,  FIG.  2    shows emitted light  204 , which represents the modulated light pulses that are emitted from light source  102  to illuminate one or more portions  107  of object  106 , and corresponding pulses reflected light  208 , which represents the reflected light pulses that are back-reflected from the illuminated one or more portions  107  of object  106  and received by corresponding one or more portions  113  of the pixel circuits  112  pixel array  110  of  FIG.  1   . 
     The example depicted in  FIG.  2    also illustrates measurement pulses of the differential phase modulation signals including a 0° phase modulation signal  214 A and a 180° phase modulation signal  214 B, as well as measurement pulses including a 90° phase modulation signal  216 A and a 270° phase modulation signal  216 B, which as shown are all phase-shifted relative to the phase of the pulses of emitted light  204 . In addition,  FIG.  2    shows that the 0° phase modulation signal  214 A and 180° phase modulation signal  214 B, as well as the 90° phase modulation signal  216 A and 270° phase modulation signal  216 B pulses are all modulated at the same frequency as the modulated emitted light  204  and reflected light  208  to realize homodyne detection of the reflected light  208  in accordance with the teachings of the present invention. Utilizing the different phases for the example measurement pulses as shown allows reconstruction of the encoded distance. In the various examples, at least 3 independent measurements (e.g., sub-frames) are utilized to decode  3  unknowns: distance/phase, reflectivity, and ambient. In examples described herein, 4 phases are utilized (e.g., 0°, 180°, 90°, and 270°). 
     As will be discussed, the 0° phase modulation signal  214 A and 180° phase modulation signal  214 B, as well as the 90° phase modulation signal  216 A and 270° phase modulation signal  216 B pulses correspond to the switching of transfer transistors that are included in the pixel circuits  112  of pixel array  110 . In operation, the switching of the transfer transistors in the pixel circuits  112  of pixel array  110  can be used to measure the charge that is photogenerated in the one or more photodiodes that are included the pixel circuits  112  in response to the reflected light  208  to determine the delay or phase difference φ between the pulses of emitted light  204  and the corresponding pulses of reflected light  208 . 
     For instance, the example illustrated in  FIG.  2    shows that charge Q 1  is photogenerated by the pulses of 0° phase modulation signal  214 A and that charge Q 2  is photogenerated by the pulses of 180° phase modulation signal  214 B in response to reflected light  208 . Similarly, charge Q 3  is photogenerated by the pulses of 90° phase  216 A and charge Q 4  is photogenerated by the pulses of 270° phase modulation signal  216 B in response to reflected light  208 . In various examples, the measurements of Q 1 , Q 2 , Q 3 , and Q 4  can then be used to determine the delay or phase difference φ between the emitted light  204  and the reflected light  208 , and therefore the time of flight T TOF  of light from the light source  102  to the object  106  and then back to the pixel array  110  in accordance with the teachings of the present invention. 
       FIGS.  3 A- 3 F  are diagrams of various examples of time-of-flight pixel arrays  310 A- 310 F of a time-of-flight sensor on which the field-of-view for a given exposure is varied or programmed to include one or more fractional portions of the time-of flight pixel arrays, which are scanned by an example time-of-flight light sensing system in accordance with the teachings of the present invention. It is appreciated that the example pixel arrays  310 A- 310 F illustrated in  FIGS.  3 A- 3 F  may be examples of pixel array  110  shown in  FIG.  1   , and that similarly named and numbered elements described above are coupled and function similarly below. 
     In particular,  FIG.  3 A  shows an example in which the illumination field-of-view includes the entire pixel array  310 A. In a flash lidar (light detection and ranging), the entire field-of-view is illuminated by light reflected from an object that is illuminated by the illumination source. As such, the illuminated portion  313  of pixel array  310 A is illustrated in  FIG.  3 A  to cover the entire pixel array  310 A. Consequently, in the example depicted in  FIG.  3 A , the illumination source is typically operated at the strongest intensity for which peak power, electromagnetic interference (EMI), thermal management, etc., can be tolerated. The illumination time is limited by either motion blur or safety requirements. Typically, long exposure times (˜ms) result. During these long exposures times, significant amounts of undesired background signal from ambient light and dark current is accumulated. This presents a tremendous challenge for outdoor operation as well as for non-Silicon devices (SiGe, Ge, InGaAs, InP, GaAs, etc.), which exhibit significantly higher dark current. Another challenge related to the long exposure times is that the demodulation signaling, which is one of the key power contributors of indirect time-of-flight sensing, has to operate for the entire duration on the entire pixel array  310 A. 
     In comparison,  FIG.  3 B  shows an example pixel array  310 B in which only a fractional portion or a subset  313  of the pixel circuits of pixel array  310 B is illuminated by light reflected from an object that is illuminated by the illumination source in accordance with the teachings of the present invention. As such, another subset  315  of pixel circuits of pixel array  310 B is non-illuminated by the light reflected from the object. In the example shown in  FIG.  3 B , a horizontal line shaped portion or subset  313  of one or more rows of pixel array  310 B is illuminated by the reflected light. In various examples, the subset  313  of pixel circuits of pixel array  310 B that is illuminated by the reflected light may be scanned across the pixel array  310 B until the entire frame captured. 
       FIG.  3 C  shows another example pixel array  310 C in which only a fractional portion or a subset  313  of the pixel circuits of pixel array  310 C is illuminated by light reflected from an object that is illuminated by the illumination source. As such, another subset  315  of pixel circuits of pixel array  310 C is non-illuminated by the light reflected from the object. In the example shown in  FIG.  3 C , a vertical line shaped portion or subset  313  of one or more columns of pixel array  310 C is illuminated by the reflected light. In various examples, the subset  313  of pixel circuits of pixel array  310 C that is illuminated by the reflected light may be scanned across the pixel array  310 C until the entire frame captured. 
       FIG.  3 D  shows yet another example pixel array  310 D in which only a fractional portion or a subset  313  of the pixel circuits of pixel array  310 D is illuminated by light reflected from an object that is illuminated by the illumination source. As such, another subset  315  of pixel circuits of pixel array  310 D is non-illuminated by the light reflected from the object. In the example shown in  FIG.  3 D , horizontal line shaped portions or subset  313  of one or more rows of pixel array  310 D are illuminated by the reflected light. The example depicted in  FIG.  3 D  shows that the illuminated portions or subset  313  may be non-contiguous portions of the pixel array  310 D. In various examples, the portions of pixel array  310 D that are illuminated by the reflected light may be scanned across the pixel array  310 D until the entire frame captured. 
       FIG.  3 E  shows still another example pixel array  310 E in which only a fractional portion or a subset  313  of the pixel circuits of pixel array  310 E is illuminated by light reflected from an object that is illuminated by the illumination source. As such, another subset  315  of pixel circuits of pixel array  310 E is non-illuminated by the light reflected from the object. In the example shown in  FIG.  3 E , vertical line shaped portions of one or more columns of pixel array  310 E are illuminated by the reflected light. The example depicted in  FIG.  3 E  shows that the illuminated portions or subset  313  may be non-contiguous portions of the pixel array  310 E. In various examples, the portion of pixel array  310 E that is illuminated by the reflected light may be scanned across the pixel array  310 E until the entire frame captured. 
       FIG.  3 F  shows yet another example pixel array  310 F in which only a fractional portion or a subset  313  of the pixel circuits of pixel array  310 F is illuminated by light reflected from an object that is illuminated by the illumination source. As such, another subset  315  of pixel circuits of pixel array  310 F is non-illuminated by the light reflected from the object. In the example shown in  FIG.  3 F , a spot shaped portion of one or more pixel circuits of pixel array  310 F are illuminated by the reflected light. In various examples, the subset  313  of pixel circuits of pixel array  310 F that is illuminated by the reflected light may be randomly accessible or addressable. As such, by providing a random access illumination/exposure, a region-of-interest (ROI) may be monitored by the time-of-flight sensing system. In the various example, the ROI may be updated quickly by the host based on changes in the observed data stream/scenery. In various examples, the host may track objects and adjust the ROI accordingly in accordance with the teachings of the present invention. 
     Therefore, with regard to all of the examples depicted in  FIGS.  3 B- 3 F , instead of distributing a certain radiant flux Φ over a given solid angle Ω corresponding to an entire defined field-of-view (FOV) for a given exposure T INT , the entire radiant flux Φ may instead be focused into an angle ΔΩ=Ω/N, which illuminates only a 1/N portion of the object instead of the entire object. As such, there are one or more portions of the object in the field-of-view that are non-illuminated by the modulated light emitted by the light source in accordance with the teachings of the present invention. Accordingly, the reflected modulated light from the object illuminates only a subset  313  of the time-of-flight pixel circuits in the pixel arrays  310 B- 310 F as shown in  FIGS.  3 B- 3 F . As such, another subset  315  of the time-of-flight pixel circuits in the pixel arrays  310 B- 310 F shown in  FIGS.  3 B- 3 F  is non-illuminated. This increases the irradiance to E×N, which reduces the exposure to T INT /N resulting in the same signal carrier count. However, since the exposure is reduced to T INT /N, so is the parasitically generated background signal. 
     In operation, after the first ΔΩ 1  is illuminated for T INT /N and the corresponding subset of pixel circuits of the pixel array is read out, the emitted modulated light from the light source may then be scanned across the object, which therefore results in the reflected modulated light from the object to be scanned across the pixel arrays  310 B- 310 F as shown. In other words, the radiant flux Φ may then be scanned or focused on ΔΩ 1 , ΔΩ 2 , . . . ΔΩN across the object for given exposures until the entire frame is captured in the same total exposure T INT  in accordance with the teachings of the present invention. It is appreciated therefore that example time-of-flight sensing systems in which only fractional portions of the objects are illuminated at a time, and therefore the corresponding illuminated fractional portions of pixel arrays  310 B- 310 F are scanned have the advantage that modulation power is efficiently utilized, with a power consumption reduction by factor N in accordance with the teachings of the present invention. 
       FIG.  4    is a schematic illustrating one example of a time-of-flight pixel circuit  412  included in a pixel array of a time-of-flight sensor in accordance with the teachings of the present invention. It is appreciated that the pixel circuit  412  of  FIG.  4    may be an example of one of the pixel circuits  112  included in pixel array  110  shown in  FIG.  1   , and that similarly named and numbered elements described above are coupled and function similarly below. 
     As shown in the example depicted in  FIG.  4   , pixel circuit  412  includes a photodiode  418  configured to photogenerate charge in response to incident light. In one example, the light that is incident on photodiode  418  is the reflected modulated light  108  that is reflected from an object  106  as described in  FIG.  1   . A first floating diffusion FD  422 A is configured to store a first portion of charge photogenerated in the photodiode  418 , such as for example charge Q 1  or Q 3  described in  FIG.  2   . A second floating diffusion FD  422 B is configured to store a second portion of charge photogenerated in the photodiode  418 , such as for example charge Q 2  or Q 4  described in  FIG.  2   . 
     A first transfer transistor  420 A is configured to transfer the first portion of charge from the photodiode  418  to the first floating diffusion FD  422 A in response to a first phase modulation signal TXA. In one example, the first phase modulation signal TXA may be an example of one of the phase modulation signals described in  FIG.  2   , such as for example 0° phase modulation signal  214 A or 90° phase modulation signal  214 C. A second transfer transistor  420 B is configured to transfer the second portion of charge from the photodiode  418  to the second floating diffusion FD  422 B in response to a second phase modulation signal TXB. In one example, the second phase modulation signal TXB may be an example of another one of the phase modulation signals described in  FIG.  2   , such as for example 180° phase modulation signal  214 B or 270° phase modulation signal  214 D. In the various examples, the first phase modulation signal TXA and the second phase modulation signal TXB are out of phase with each other, such as for example 180° out of phase with each other. In the example, a first storage node MEM  434 A is configured to store the first portion of charge from the first floating diffusion FD  422 A through a first sample and hold transistor  426 A, and a second storage node MEM  434 B is configured to store the second portion of charge from the second floating diffusion FD  422 B through a second sample and hold transistor  426 B. In the various examples, the first and second sample and hold transistors  426 A and  426 B are coupled to be responsive to a sample and hold signal SH. 
     Continuing with the example depicted in  FIG.  4   , the first storage node MEM  434 A is coupled to a first capacitor  428 A and a gate of a first source follower transistor  430 A. A first row select transistor  432 A is coupled to a source of the first source follower transistor  430 A. In the various examples, the first row select transistor  432 A is also coupled to a first bitline BL 1 , through which first output signal information may be read out from pixel circuit  412 . Similarly, the second storage node MEM  434 B is coupled to a second capacitor  428 B and a gate of a second source follower transistor  430 B. A second row select transistor  432 B is coupled to a source of the second source follower transistor  430 B. In the various examples, the second row select transistor  432 B is also coupled to a second bitline BL 2 , through which second output signal information may be read out from pixel circuit  412 . In the various examples, the first and second row select transistors  432 A and  432 B are coupled to be responsive to a row select signal RS. 
     In the various examples, pixel circuit  412  also includes a first reset transistor  424 A coupled between a supply rail and the first floating diffusion FD  422 A. In various examples, first reset transistor  424 A is configured to reset the first floating diffusion FD  422 A as well the first storage node MEM  434 A in response to a reset signal RST. In the example depicted in  FIG.  4   , the first reset transistor  424 A is configured to reset the first storage node MEM  434 A through the first sample and hold transistor  426 A. In various examples, it is appreciated that first reset transistor  424 A may be operated in a way that excess carriers generated by photodiode  418  may be guided to the power supply by first reset transistor  424 A or in a way that photosensitivity of photodiode  418  is disabled. 
     Similarly, pixel circuit  412  also includes a second reset transistor  424 B coupled between the supply rail and the second floating diffusion FD  422 B. In various examples, second reset transistor  424 B is configured to reset the second floating diffusion FD  422 B as well the second storage node MEM  434 B in response to the reset signal RST. In the example depicted in  FIG.  4   , the second reset transistor  424 B is configured to reset the second storage node MEM  434 B through the second sample and hold transistor  426 B. In various examples, it is appreciated that second reset transistor  424 B may be operated in a way that excess carriers generated by photodiode  418  may be guided to the power supply by second reset transistor  424 B or in a way that photosensitivity of photodiode  418  is disabled. 
       FIG.  5 A  is a schematic that shows one example of a time-of-flight light sensing system  500 A in accordance with the teachings of the present invention. It is appreciated that the time-of-flight light sensing system  500 A of  FIG.  5 A  may be an example of the time-of-flight light sensing system  100  shown in  FIG.  1   , and that similarly named and numbered elements described above are coupled and function similarly below. 
     As shown in the depicted example, time-of-flight light sensing system  500 A includes a light source  502  that is synchronized with a time-of-flight sensor including a pixel array  510 . In the various examples, the light source  502  is configured to emit modulated light to only a portion of an object at a time, such as for example portion  107  of object  106  as illustrated for example in  FIG.  1   . In various examples, a laser may be employed as the light source  502 , and the field-of-view of the laser may be controllable by various examples of scanning mechanisms. For instance, in various examples, the light source  502  may be implemented electronically (e.g., via an addressable laser array), or mechanically (e.g., via a bulk or a MEMS mirror), or optically/photonically (e.g., via a phased array, liquid crystal, etc.), or with any other suitable type of light source or technique to emit the modulated light to the object in accordance with the teachings of the present invention. 
     In the depicted example, the time-of-flight sensor also includes a modulation driver block  534 , which is coupled to the light source  502  and the pixel array  510 . In operation, the control and readout of the enabled and disabled time-of-flight pixel circuits included in time-of-flight pixel array  510  is synchronized with the modulated light that is emitted by light source  502  to the object. As shown in the depicted example, time-of-flight pixel array  510  includes a plurality of time-of-flight pixel circuits  512 A- 512 I. In the example, it is noted that each of the time-of-flight pixel circuits  512 A- 512 I may be an example of the time-of-flight pixel circuit  412  described in detail in  FIG.  4   . Therefore, it is appreciated that each of the time-of-flight pixel circuits  512 A- 512 I is not described again in detail in  FIG.  5 A  for the sake of brevity. 
     In the example depicted in  FIG.  5 A , the time-of-flight pixel circuits  512 A- 512 I are arranged into rows and columns in time-of-flight pixel array  510 . It is noted that time-of-flight pixel array  510  is illustrated with time-of-flight pixel circuits  512 A- 512 I arranged into 3 rows and 3 columns for explanation purposes. In other examples, it is appreciated of course that the time-of-flight pixel array  510  may include a greater or fewer number of rows and/or a greater or fewer number of columns. 
       FIG.  5 A  depicts an example in which the reflected modulated light from the object is configured to illuminate one or more rows of time-of-flight pixel array  510 , but not all of the rows of the time-of-flight pixel array  510  at a time. For instance, a first subset of the plurality of time-of-flight pixel circuits (e.g., the row including time-of-flight pixel circuits  512 A- 512 C) is configured to be illuminated by the reflected modulated light from the object while a second subset of the plurality of time-of-flight pixel circuits (e.g., the rows including time-of-flight pixel circuits  512 D- 512 I) is configured to be non-illuminated by the reflected modulated light from the object. 
     As shown in the depicted example, modulation driver block  534  includes a phase lock loop circuit  540 , which is configured to generate the first and second phase modulation signals TXA and TXB. For instance, as described in the example of  FIG.  4   , the first and second phase modulation signals TXA and TXB are coupled to be received by the transfer transistors of time-of-flight pixel circuits  512 A- 512 I. A light source driver  544  coupled between the light source  502  and the phase lock loop circuit  540 . In operation, the light source driver circuit  544  is configured to synchronize the modulated light emitted from the light source  502  to the object with the first and second phase modulation signals TXA and TXB in response to the phase lock loop circuit  540 . 
     Continuing with the depicted example, a plurality of driver circuits  536 A- 536 F is coupled to the phase lock loop circuit  540 . In the example, the driver circuit  536 A is configured to generate the first phase modulation signal TXA and the driver circuit  536 B is configured to generate the second phase modulation signal TXB for the row of time-of-flight pixel array  510  that includes time-of-flight pixels  512 A- 512 C. The driver circuit  536 C is configured to generate the first phase modulation signal TXA and the driver circuit  536 D is configured to generate the second phase modulation signal TXB for the row of time-of-flight pixel array  510  that includes time-of-flight pixels  512 D- 512 F. The driver circuit  536 E is configured to generate the first phase modulation signal TXA and the driver circuit  536 F is configured to generate the second phase modulation signal TXB for the row of time-of-flight pixel array  510  that includes time-of-flight pixels  512 G- 512 I. 
     In the example, modulation driver block  534  also includes a plurality of driver switches  538 A- 538 F. Each one of the plurality of driver switches  538 A- 538 F is coupled to an output of a respective one of the plurality of driver circuits  536 A- 536 F as shown in  FIG.  5 A . As such, driver switch  538 A is coupled to an output of driver circuit  536 A, driver switch  538 B is coupled to an output of driver circuit  536 B, driver switch  538 C is coupled to an output of driver circuit  536 C, driver switch  538 D is coupled to an output of driver circuit  536 D, driver switch  538 E is coupled to an output of driver circuit  536 E, and driver switch  538 F is coupled to an output of driver circuit  536 F. 
     As shown in the example, modulation driver block  534  also includes a modulation control circuit  535  that is coupled the plurality of driver switches  538 A- 538 F. In operation, the modulation control circuit  535  is configured to turn off or disable the driver switches that are coupled to non-illuminated time-of-flight pixel circuits while the modulation control circuit  535  is configured to turn on or enable the driver switches that are coupled to illuminated time-of-flight pixel circuits. 
     To illustrate, in the example above in which the first subset of the plurality of time-of-flight pixel circuits (e.g., the row including time-of-flight pixel circuits  512 A- 512 C) is configured to be illuminated by the reflected modulated light from the object while a second subset of the plurality of time-of-flight pixel circuits (e.g., the rows including time-of-flight pixel circuits  512 D- 512 I) is configured to be non-illuminated by the reflected modulated light from the object, the modulation control circuit  535  is configured to turn on or enable driver switches  538 A and  538 B and turn off or disable driver switches  536 C- 536 F. As such, the transfer transistors of the illuminated, and therefore enabled, time-of-flight pixel circuits  512 A- 512 C are coupled to receive and be responsive to the first and second phase modulation signals TXA and TXB, while the the transfer transistors of the non-illuminated, and therefore disabled, time-of-flight pixel circuits  512 D- 512 F are not coupled to receive and therefore not be responsive to the first and second phase modulation signals TXA and TXB. 
     Continuing with the depicted example, modulation driver block  534  further includes a scan synchronize circuit  542  coupled to the modulation control circuit  535  as shown. In operation, the scan synchronize circuit  542  is configured to synchronize the scanning of the modulated light emitted by the light source across the object with the scanning of the first subset of the plurality of time-of-flight pixel circuits that are illuminated by the reflected modulated light across the time-of-flight pixel array  510 . In other words, the scan synchronize circuit  542  is configured to synchronize the scanning of the modulated light emitted by the light source  502  across the object with the activation of the appropriate driver switches  538 A- 538 F that are coupled to the corresponding time-of-flight pixel circuits  512 A- 512 I that are illuminated by the modulated light that is reflected from the object in accordance with the teachings of the present invention. 
     As shown in the depicted example, a row control circuit  548  is coupled to the plurality of time-of-flight pixel circuits  512 A- 512 I of time-of-flight pixel array  510 . In the example, the row control circuit  548  is configured to generate a reset signal RST that is coupled to control the first reset transistor and the second reset transistor of each one of the plurality of time-of-flight pixel circuits  512 A- 512 I as shown. In the example, the row control circuit  548  is further configured to generate a sample and hold signal SH that is coupled to control the first sample and hold transistor and the second sample and hold transistor of each one of the plurality of time-of-flight pixel circuits  512 A- 512 I as shown. In the example, the row control circuit  548  is further configured to generate a row select signal RS coupled to control the first row select transistor and the second row select transistor of each one of the plurality of time-of-flight pixel circuits  512 A- 512 I as shown. In the depicted example, a column readout circuit  550  is coupled to the first bitline BL 1  and the second bitline BL 2  of each one of the plurality of time-of-flight pixel circuits  512 A- 512 I to read out each one of the plurality of time-of-flight pixel circuits  512 A- 512 I of time-of-flight pixel array  510  as shown. 
       FIG.  5 B  is a schematic that shows another example of a time-of-flight light sensing system  500 B in accordance with the teachings of the present invention. It is appreciated that the example time-of-flight light sensing system  500 B of  FIG.  5 B  may be another example of the time-of-flight light sensing system  500 A of  FIG.  5 A , or another example of the time-of-flight light sensing system  100  shown in  FIG.  1   , and that similarly named and numbered elements described above are coupled and function similarly below. It also appreciated that the example time-of-flight light sensing system  500 B of  FIG.  5 B  shares many similarities with the example time-of-flight light sensing system  500 A of  FIG.  5 A . 
     For instance, as shown in the example depicted in  FIG.  5 B , time-of-flight light sensing system  500 B also includes a light source  502  that is synchronized with a time-of-flight sensor including a pixel array  510 . In the various examples, the light source  502  is configured to emit modulated light to only a portion of an object at a time, such as for example portion  107  of object  106  as illustrated for example in  FIG.  1   . In various examples, a laser may be employed as the light source  502 , and the field-of-view of the laser may be controllable by various examples of scanning mechanisms. For instance, in various examples, the light source  502  may also be implemented electronically (e.g., via an addressable laser array), or mechanically (e.g., via a bulk or a MEMS mirror), or optically/photonically (e.g., via a phased array, liquid crystal, etc.), or with any other suitable type of light source or technique to emit the modulated light to the object in accordance with the teachings of the present invention. 
     In the depicted example, the time-of-flight sensor also includes a modulation driver block  534 , which is coupled to the light source  502  and the pixel array  510 . In operation, the control and readout of the enabled and disabled time-of-flight pixel circuits included in time-of-flight pixel array  510  is synchronized with the modulated light that is emitted by light source  502  to the object. As shown in the depicted example, time-of-flight pixel array  510  also includes a plurality of time-of-flight pixel circuits  512 A- 512 I. In the example, it is noted that each of the time-of-flight pixel circuits  512 A- 512 I may also be an example of the time-of-flight pixel circuit  412  described in detail in  FIG.  4   . Therefore, it is appreciated that each of the time-of-flight pixel circuits  512 A- 512 I is also not described again in detail in  FIG.  5 B  for the sake of brevity. 
     In the example depicted in  FIG.  5 B , the time-of-flight pixel circuits  512 A- 512 I are arranged into rows and columns in time-of-flight pixel array  510 . It is noted that time-of-flight pixel array  510  is illustrated with time-of-flight pixel circuits  512 A- 512 I arranged into 3 rows and 3 columns for explanation purposes. In other examples, it is appreciated of course that the time-of-flight pixel array  510  may include a greater or fewer number of rows and/or a greater or fewer number of columns. 
       FIG.  5 B  depicts an example in which the reflected modulated light from the object is configured to illuminate one or more rows of time-of-flight pixel array  510 , but not all of the rows of the time-of-flight pixel array  510  at a time. For instance, a first subset of the plurality of time-of-flight pixel circuits (e.g., the row including time-of-flight pixel circuits  512 A- 512 C) is configured to be illuminated by the reflected modulated light from the object while a second subset of the plurality of time-of-flight pixel circuits (e.g., the rows including time-of-flight pixel circuits  512 D- 512 I) is configured to be non-illuminated by the reflected modulated light from the object. 
     As shown in the depicted example, modulation driver block  534  also includes a phase lock loop (PLL) circuit  540 , which is configured to generate the first and second phase modulation signals TXA and TXB. One of the difference between time-of-flight pixel light sensing system  500 B of  FIG.  5 B  and time-of-flight pixel light sensing system  500 A of  FIG.  5 A  is that time-of-flight pixel light sensing system  500 B of  FIG.  5 B  also includes delay lock loop (DLL) circuits  541 A,  541 B, . . . ,  541 C, which are coupled in series to the output of the phase lock loop circuit  540 . In the example, each of the delay lock loop circuits  541 A,  541 B, . . . ,  541 C is configured to generate the first and second phase modulation signals TXA and TXB with a respective phase shift or delay for the rows or groups of rows of the time-of-flight pixel array  510  that are coupled to the respective delay lock loop circuit  541 A,  541 B, . . . ,  541 C. It is appreciated that in another example, the delay lock loop circuits  541 A,  541 B, . . . ,  541 C could also be implemented to introduce the phase delay between rows or groups of columns of the time-of-flight pixel array depending on the configuration. As such, it is appreciated that deliberate phase-shift added with the delay lock loop circuits  541 A,  541 B, . . . ,  541 C which spreads out peak currents and hence improves the first and second phase modulation signals TXA and TXB and reduces electromagnetic interference (EMI) by avoiding excess current or power spikes when modulating the transfer transistors included in the time-of-flight pixel circuits  512 A- 512 I of the time-of-flight pixel array  510  in accordance with the teachings of the present invention. As shown in the depicted example, the first and second phase modulation signals TXA and TXB generated by the phase lock loop (PLL) circuit  540  are generated by the respective series coupled delay lock loop (DLL) circuits  541 A,  541 B, . . . ,  541 C with the corresponding phase shift. 
     Another difference between time-of-flight pixel light sensing system  500 B of  FIG.  5 B  and time-of-flight pixel light sensing system  500 A of  FIG.  5 A  is that in time-of-flight pixel light sensing system  500 B of  FIG.  5 B , each of the first and second phase modulation signals TXA and TXB generated by the respective delay lock loop (DLL) circuits  541 A,  541 B, . . . ,  541 C is received at a second input (e.g., bottom input) of a respective logic circuits  537 A,  537 B,  537 C,  537 D, . . . ,  537 E,  537 F as shown. In the depicted example, the logic circuits  537 A,  537 B,  537 C,  537 D, . . . ,  537 E,  537 F are illustrated as logical AND gates, which are configured to gate the first and second phase modulation signals TXA and TXB in response to a respective enable signal received at a first input (e.g., top input) of each of the logic circuits  537 A,  537 B,  537 C,  537 D, . . . ,  537 E,  537 F. In other examples, it is appreciated that other suitable types of logic circuits or combinations of logic circuits could be utilized, such as for example NAND/OR/NOR gates, etc., depending on the polarity of the signals included in time-of-flight light sensing system  500 B. In the example, the modulation control circuit  535  included in modulation driver block  534  is configured to generate the enable signal received at the first input of each of the logic circuits  537 A,  537 B,  537 C,  537 D, . . . ,  537 E,  537 F. In operation, the modulation control circuit  535  is therefore configured to disable the respective logic circuits  537 A,  537 B,  537 C,  537 D, . . . ,  537 E,  537 F that are coupled to non-illuminated time-of-flight pixel circuits while the modulation control circuit  535  is configured to enable the respective logic circuits  537 A,  537 B,  537 C,  537 D, . . . ,  537 E,  537 F that are coupled to illuminated time-of-flight pixel circuits. Thus, the first and second phase modulation signals TXA and TXB are therefore generated at the respective outputs of each of the logic circuits  537 A,  537 B,  537 C,  537 D, . . . ,  537 E,  537 F when a corresponding enable signal is received from the modulation control circuit  535 . 
     Continuing with the example depicted in  FIG.  5 B , the first and second phase modulation signals TXA and TXB from the logic circuits  537 A,  537 B,  537 C,  537 D, . . . ,  537 E,  537 F are coupled to be received by the transfer transistors of time-of-flight pixel circuits  512 A- 512 I. For instance, in the depicted example, the logic circuit  537 A is configured to generate the first phase modulation signal TXA and the logic circuit  537 B is configured to generate the second phase modulation signal TXB for the row of time-of-flight pixel array  510  that includes time-of-flight pixels  512 A- 512 C. The logic circuit  537 C is configured to generate the first phase modulation signal TXA and the logic circuit  537 D is configured to generate the second phase modulation signal TXB for the row of time-of-flight pixel array  510  that includes time-of-flight pixels  512 D- 512 F. The logic circuit  537 E is configured to generate the first phase modulation signal TXA and the logic circuit  537 F is configured to generate the second phase modulation signal TXB for the row of time-of-flight pixel array  510  that includes time-of-flight pixels  512 G- 512 I. 
     In operation, the first subset of the plurality of time-of-flight pixel circuits (e.g., the row including time-of-flight pixel circuits  512 A- 512 C) is configured to be illuminated by the reflected modulated light from the object while a second subset of the plurality of time-of-flight pixel circuits (e.g., the rows including time-of-flight pixel circuits  512 D- 512 I) is configured to be non-illuminated by the reflected modulated light from the object. As such, the modulation control circuit  535  is configured to enable logic circuits  537 A and  537 B and disable logic circuits  537 C- 537 F. As such, the transfer transistors of the illuminated, and therefore enabled, time-of-flight pixel circuits  512 A- 512 C are coupled to receive and be responsive to the first and second phase modulation signals TXA and TXB, while the transfer transistors of the non-illuminated, and therefore disabled, time-of-flight pixel circuits  512 D- 512 F are not coupled to receive and therefore not be responsive to the first and second phase modulation signals TXA and TXB. Operation is similar for when the modulation control circuit  535  is configured to enable logic circuits  537 C and  537 D and disable logic circuits  537 A- 537 B and  537 E- 537 F, or when the modulation control circuit  535  is configured to enable logic circuits  537 E and  537 F and disable logic circuits  537 A- 537 D, and so on. 
     A light source driver  544  coupled between the light source  502  and the phase lock loop circuit  540 . In operation, the light source driver circuit  544  is configured to synchronize the modulated light emitted from the light source  502  to the object with the first and second phase modulation signals TXA and TXB in response to the phase lock loop circuit  540 . 
     Continuing with the depicted example, modulation driver block  534  further includes a scan synchronize circuit  542  coupled to the modulation control circuit  535  as shown. In operation, the scan synchronize circuit  542  is configured to synchronize the scanning of the modulated light emitted by the light source across the object with the scanning of the first subset of the plurality of time-of-flight pixel circuits that are illuminated by the reflected modulated light across the time-of-flight pixel array  510 . In other words, the scan synchronize circuit  542  is configured to synchronize the scanning of the modulated light emitted by the light source  502  across the object with the enabling of the appropriate logic circuits  537 A- 537 F that are coupled to the corresponding time-of-flight pixel circuits  512 A- 512 I that are illuminated by the modulated light that is reflected from the object in accordance with the teachings of the present invention. 
     As shown in the depicted example, a row control circuit  548  is coupled to the plurality of time-of-flight pixel circuits  512 A- 512 I of time-of-flight pixel array  510 . In the example, the row control circuit  548  is configured to generate a reset signal RST that is coupled to control the first reset transistor and the second reset transistor of each one of the plurality of time-of-flight pixel circuits  512 A- 512 I as shown. In the example, the row control circuit  548  is further configured to generate a sample and hold signal SH that is coupled to control the first sample and hold transistor and the second sample and hold transistor of each one of the plurality of time-of-flight pixel circuits  512 A- 512 I as shown. In the example, the row control circuit  548  is further configured to generate a row select signal RS coupled to control the first row select transistor and the second row select transistor of each one of the plurality of time-of-flight pixel circuits  512 A- 512 I as shown. In the depicted example, a column readout circuit  550  is coupled to the first bitline BL 1  and the second bitline BL 2  of each one of the plurality of time-of-flight pixel circuits  512 A- 512 I to read out each one of the plurality of time-of-flight pixel circuits  512 A- 512 I of time-of-flight pixel array  510  as shown. 
       FIG.  6    is a timing diagram that shows one example of signals during integration and readout of one example of a time-of-flight light sensing system in accordance with the teachings of the present invention. It is appreciated that the signals illustrated in the timing diagram of  FIG.  6    may examples of the signals found during operation of the time-of-flight sensing system  500 A illustrated in  FIG.  5 A , and/or examples of signals found during operation of the time-of-flight pixel circuit  412  illustrated in  FIG.  4   , and/or examples of signals found during operation of the time-of-flight sensing system  100  illustrated in  FIG.  1   , and that similarly named and numbered elements described above are coupled and function similarly below. 
     In the example depicted  FIG.  6   , it is assumed for illustration purposes that a time-of-flight sensing system includes a time-of-flight pixel array with 480 rows of time-of-flight pixel circuits. As shown in the depicted example, at time T 0 , the reset signal RST&lt; 1 : 48 &gt; 624 A, which is coupled to the rows including the subset of time-of-flight pixel circuits included in rows 1-48, the reset signal RST&lt; 49 : 96 &gt; 624 B, which is coupled to the rows including the subset of time-of-flight pixel circuits included in rows 49-96, . . . , and the reset signal RST&lt; 433 : 480 &gt; 624 C, which is coupled to the rows including the subset of time-of-flight pixel circuits included in rows 433-480, are all activated which turns on the first and second reset transistors in the respective time-of-flight pixel circuits to initialize or reset the respective time-of-flight pixel circuits according. 
     At time T 1 , the reset signal RST&lt; 1 : 48 &gt; 624 A, which is coupled to the rows including the subset of time-of-flight pixel circuits included in rows 1-48, is deactivated and the first phase modulation signal TXA&lt; 1 : 48 &gt; 620 A and the second phase modulation signal TXB&lt; 1 : 48 &gt; 621 A are modulated to modulate the respective transfer transistors (e.g.,  420 A and  420 B). As such, integration of the reflected modulated light incident upon the illuminated subset of time-of-flight pixel circuits included in rows 1-48 of the time-of-flight pixel array occurs between time T 1  and T 2 . In the example, it is assumed that the subset of time-of-flight pixel circuits included in remaining the rows 49-480 are non-illuminated, and therefore remain deactivated between time T 1  and T 2 . 
     At time T 2 , rows 1-48 are no longer illuminated and the reset signal RST&lt; 1 : 48 &gt; 624 A is reactivated and the sample and hold signal SH&lt; 1 : 48 &gt; 626 A, which is coupled to the rows included in the now non-illuminated subset of time-of-flight pixel circuits included in rows 1-48, is deactivated. In addition, the row select signal RS&lt; 1 : 48 &gt; 632 A is now activated. Thus, the respective charges sampled into the memory nodes (e.g.,  434 A and  434 B) during the time interval between T 1  and T 2  of the respective time-of-flight pixel circuits are now held in the time interval between time T 2  and T 3 . Therefore, the held charges in the memory nodes in the previously illuminated but now non-illuminated subset of time-of-flight pixel circuits included in rows 1-48 are now read out through the respective row select transistors (e.g.,  4332 A and  432 B) during the time interval between time T 2  and T 3 , as indicated with the assertion with the row select signal RS&lt; 1 : 48 &gt; 632 A. 
     In addition, it is also appreciated that at time T 2 , the previously non-illuminated subset of time-of-flight pixel circuits included in rows 49-96 during the time interval between time T 1  and T 2  is now illuminated during the time interval between time T 2  and T 3 . As such at time T 2 , the reset signal RST&lt; 49 : 96 &gt; 624 B, which is coupled to the rows including the subset of time-of-flight pixel circuits included in rows 49-96, is now deactivated and the first phase modulation signal TXA&lt; 49 : 96 &gt; 620 B and the second phase modulation signal TXB&lt; 49 : 96 &gt; 621 B are modulated to modulate the respective transfer transistors (e.g.,  420 A and  420 B) of the time-of-flight pixel circuits included in the now illuminated subset of time-of-flight pixel circuits included in rows 49-96 during the time interval between time T 2  and T 3 . As such, integration of the reflected modulated light incident upon the now illuminated subset of time-of-flight pixel circuits included in rows 49-96 of the time-of-flight pixel array occurs between time T 2  and T 3 . In the example, it is assumed that the subset of time-of-flight pixel circuits included in remaining the rows 1-48 and 97-480 are not illuminated, and therefore remain deactivated between time T 2  and T 3 . 
     It is therefore appreciated that the simultaneous readout between time T 2  and T 3  of the subset of time-of-flight pixel circuits included in rows 1-48 at the same time as the integration of the subset of time-of-flight pixel circuits included in rows 49-96 illustrates pipelined integration and readout operations in the time-of-flight pixel array in accordance with the teachings of the present invention. As shown, the first and second storage nodes (e.g., first and second storage nodes MEM  434 A and MEM  434 B) of one of the plurality of time-of-flight pixel circuits included in the second subset (e.g., non-illuminated rows 1-48 during the time interval between T 2  and T 3 ) are configured to be read out through respective row select transistors (e.g., first and second row select transistors  432 A and  432 B) at the same time that the first and second transfer transistors (e.g., first and second transfer transistors  420 A and  420 B) of one of the plurality of time-of-flight pixel circuits included in the first subset (e.g., illuminated rows 49-96 during the time interval between T 2  and T 3 ) are configured to be modulated in response to the first and second phase modulation signals  620 B,  621 B. 
     In the next time period after time T 3 , the held charges in the memory nodes in the subset of time-of-flight pixel circuits included in rows 48-96 are read out when the subset of time-of-flight pixel circuits included in rows 48-96 are no longer illuminated, as indicated with the assertion with the row select signal RS&lt; 48 : 96 &gt; 632 B and the deactivation of sample and hold signal SH&lt; 49 - 96 &gt; 626 B at time T 3 . Furthermore, integration of the subsequently illuminated rows after rows 48-96 are no longer illuminated may now occur while the held charges in the memory nodes in the now non-illuminated subset of time-of-flight pixel circuits included in rows 48-96 are read out to continue pipelined integration and readout operations in the time-of-flight pixel array in accordance with the teachings of the present invention. 
     The pipelined integration and readout operations illustrated above are scanned across the time-of-flight pixel array and continue through the rows of the time-of-flight pixel array. As such, at the time period just prior to time TN, the reset signal RST&lt; 433 : 480 &gt; 624 C is deactivated and the first phase modulation signal TXA&lt; 433 : 480 &gt; 620 C and the second phase modulation signal TXB&lt; 433 : 480 &gt; 621 C are modulated to modulate the respective transfer transistors (e.g.,  420 A and  420 B). As such, integration of the reflected modulated light incident upon the subset of time-of-flight pixel circuits included in rows 433-480 of the time-of-flight pixel array occurs in the time period just prior to time TN. In the example, it is assumed that the subset of time-of-flight pixel circuits included in remaining the rows 1-432 are not illuminated, and therefore remain deactivated in the time period just prior to time TN. 
     At time TN, the reset signal RST&lt; 433 : 480 &gt; 624 C is reactivated and the sample and hold signal SH&lt; 433 : 480 &gt; 626 C, which is coupled to the rows including the subset of time-of-flight pixel circuits included in rows 433-480 is deactivated. Thus, the respective charges sampled into the memory nodes (e.g.,  434 A and  434 B) of the respective time-of-flight pixel circuits are held between time TN and TN+1. Therefore, the held charges in the memory nodes in the subset of time-of-flight pixel circuits included in rows 433-480 are readout between time TN and TN+1. Therefore, the held charges in the memory nodes in the subset of time-of-flight pixel circuits included in rows 433-480 are readout between time TN and TN+1, as indicated with the assertion with the row select signal RS&lt; 433 : 480 &gt; 632 C. 
     As shown in the example, the pipelined integration and readout operations of the time-of-flight pixel array then repeat and loop back to the subset of time-of-flight pixel circuits included in rows 1-48 of the time-of-flight pixel array. In particular, at time TN, like at time T 1 , the reset signal RST&lt; 1 : 48 &gt; 624 A, which is coupled to the rows including the subset of time-of-flight pixel circuits included in rows 1-48, is deactivated and the first phase modulation signal TXA&lt; 1 : 48 &gt; 620 A and the second phase modulation signal TXB&lt; 1 : 48 &gt; 621 A are modulated to modulate the respective transfer transistors (e.g.,  420 A and  420 B). As such, integration of the reflected modulated light incident upon the subset of time-of-flight pixel circuits included in rows 1-48 of the time-of-flight pixel array occurs between time TN and TN+1, like at the time period between time T 1  and T 2 . Similarly, the processing at time TN+1 corresponds to the process that occurs at time T 2 , and so on. 
       FIG.  7    is a schematic illustrating another example of a time-of-flight pixel circuit  712  included in a time-of-flight pixel array of a time-of-flight sensing system in accordance with the teachings of the present invention. It is appreciated that the pixel circuit  712  of  FIG.  7    may be another example of pixel circuit  412  of  FIG.  4   , or another example of one of the pixel circuits  112  included in pixel array  110  shown in  FIG.  1   , and that similarly named and numbered elements described above are coupled and function similarly below. It also appreciated that the example pixel circuit  712  of  FIG.  7    shares many similarities with the example pixel circuit  412  of  FIG.  4   . 
     For instance, as shown in the example depicted in  FIG.  7   , pixel circuit  712  includes a photodiode  718  configured to photogenerate charge in response to incident light. In one example, the light that is incident on photodiode  718  is the reflected modulated light  108  that is reflected from an object  106  as described in  FIG.  1   . A first floating diffusion FD  722 A is configured to store a first portion of charge photogenerated in the photodiode  718 , such as for example charge Q 1  or Q 3  described in  FIG.  2   . A second floating diffusion FD  722 B is configured to store a second portion of charge photogenerated in the photodiode  718 , such as for example charge Q 2  or Q 4  described in  FIG.  2   . 
     A first transfer transistor  720 A is configured to transfer the first portion of charge from the photodiode  718  to the first floating diffusion FD  722 A in response to a first phase modulation signal TXA. In one example, the first phase modulation signal TXA may be an example of one of the phase modulation signals described in  FIG.  2   , such as for example 0° phase modulation signal  214 A or 90° phase modulation signal  214 C. A second transfer transistor  720 B is configured to transfer the second portion of charge from the photodiode  718  to the second floating diffusion FD  722 B in response to a second phase modulation signal TXB. In one example, the second phase modulation signal TXB may be an example of another one of the phase modulation signals described in  FIG.  2   , such as for example 180° phase modulation signal  214 B or 270° phase modulation signal  214 D. In the various examples, the first phase modulation signal TXA and the second phase modulation signal TXB are out of phase with each other, such as for example 180° out of phase with each other. 
     Similar to the example pixel circuit  412  of  FIG.  4   , pixel circuit  712  of  FIG.  7    also includes a first storage node MEM  734 A that configured to store the first portion of charge from the first floating diffusion FD  722 A through a first sample and hold transistor  726 A, and a second storage node MEM  734 B that is configured to store the second portion of charge from the second floating diffusion FD  722 B through a second sample and hold transistor  726 B. 
     One of the differences between pixel circuit  712  of  FIG.  7    and pixel circuit  412  of  FIG.  4    is that the pixel circuit  712  of  FIG.  7    also includes a first column sample and hold control transistor  744 A coupled to a gate of the first sample and hold transistor  726 A. In the example, the first column sample and hold control transistor  744 A is coupled to be responsive to a first column sample and hold control signal CSH CTRL . In addition, a first row sample and hold control transistor  742 A is coupled to the first column sample and hold control transistor  744 A as shown. The first row sample and hold control transistor  742 A is coupled to be responsive to a row sample and hold control signal RSH CTRL . In addition, a first sample and hold enable/disable transistor  746 A is also coupled to the gate of the first sample and hold transistor  726 A. The first sample and hold enable/disable transistor  746 A is coupled to be responsive to a sample and hold enable/disable control signal B 1 . 
     Similarly, pixel circuit  712  of  FIG.  7    also includes a second column sample and hold control transistor  744 B coupled to a gate of the second sample and hold transistor  726 B. The second column sample and hold control transistor  744 B is coupled to be responsive to a second column sample and hold control signal CSH CTRL . In one example, the first and second column sample and hold control signals CSH CTRL  may be the same signal. In addition, a second row sample and hold control transistor  742 B is coupled to the second column sample and hold control transistor  744 B as shown. The second row sample and hold control transistor  742 B is coupled to be responsive to the row sample and hold control signal RSH CTRL . In addition, a second sample and hold enable/disable transistor  746 B is coupled to the gate of the second sample and hold transistor  726 B. The second sample and hold enable/disable transistor  746 B is coupled to be responsive to the sample and hold enable/disable control signal B 1 . 
     Therefore, in the various examples, the first sample and hold transistor  726 A is coupled to be responsive to the first column sample and hold control signal CSH CTRL , the row sample and hold control signal RSH CTRL , and the sample and hold enable/disable control signal B 1 . Similarly, the second sample and hold transistor  726 B is coupled to be responsive to the second column sample and hold control signal CSH CTRL , the row sample and hold control signal RSH CTRL , and the sample and hold enable/disable control signal B 1 . In various examples, the first and second column sample and hold control signals CSH CTRL  may be the same signal. 
     Continuing with the example depicted in  FIG.  7   , the first storage node MEM  734 A is coupled to a first capacitor  728 A and a gate of a first source follower transistor  730 A. A first row select transistor  732 A is coupled to a source of the first source follower transistor  730 A. In the various examples, the first row select transistor  732 A is also coupled to a first bitline BL 1 , through which first output signal information may be read out from pixel circuit  712 . Similarly, the second storage node MEM  734 B is coupled to a second capacitor  728 B and a gate of a second source follower transistor  730 B. A second row select transistor  732 B is coupled to a source of the second source follower transistor  730 B. In the various examples, the second row select transistor  732 B is also coupled to a second bitline BL 2 , through which second output signal information may be read out from pixel circuit  712 . In the various examples, the first and second row select transistors  732 A and  732 B are coupled to be responsive to a row select signal RS. 
     Similar to the example pixel circuit  412  of  FIG.  4   , pixel circuit  712  of  FIG.  7    also includes a first reset transistor  724 A coupled between a supply rail and the first floating diffusion FD  722 A and a second reset transistor  724 B coupled between the supply rail and the second floating diffusion FD  722 B. 
     Another one of the differences between pixel circuit  712  of  FIG.  7    and pixel circuit  412  of  FIG.  4    is that the pixel circuit  712  of  FIG.  7    also includes a column reset control transistor  738  coupled to a gate of the first reset transistor  724 A and a gate of the second reset transistor  724 B. The column reset control transistor is coupled to be responsive to a column reset control signal CRST CTRL . In addition, a row reset control transistor  740  is coupled to the column reset control transistor  738 . The row reset control transistor  740  is coupled to be responsive to a row reset control signal RRST CTRL . In addition, a reset enable/disable transistor  736  is also coupled to the gate of the first reset transistor  724 A and the gate of the second reset transistor  724 B as shown. The reset enable/disable transistor is coupled to be responsive to a reset enable/disable control signal B 2 . 
     Therefore, in the various examples, the first and second reset transistors  724 A and  724 B are coupled to be responsive to column reset control signal CRST CTRL , row reset control signal RRST CTRL , and the reset enable/disable control signal B 2 . 
     In various examples, first reset transistor  724 A is configured to reset the first floating diffusion FD  722 A as well the first storage node MEM  734 A. In the example depicted in  FIG.  7   , the first reset transistor  724 A is configured to reset the first storage node MEM  734 A through the first sample and hold transistor  726 A. In various examples, it is appreciated that first reset transistor  724 A may be operated in a way that excess carriers generated by photodiode  718  may be guided to the power supply by first reset transistor  724 A or in a way that photosensitivity of photodiode  718  is disabled. Similarly, second reset transistor  724 B is configured to reset the second floating diffusion FD  722 B as well the second storage node MEM  734 B. In the example depicted in  FIG.  7   , the second reset transistor  724 B is configured to reset the second storage node MEM  734 B through the second sample and hold transistor  726 B. In various examples, it is appreciated that second reset transistor  724 B may be operated in a way that excess carriers generated by photodiode  718  may be guided to the power supply by second reset transistor  724 B or in a way that photosensitivity of photodiode  718  is disabled. 
       FIG.  8    is a schematic that shows yet another example of a time-of-flight light sensing system  800  in accordance with the teachings of the present invention. It is appreciated that the example time-of-flight light sensing system  800  of  FIG.  8    may be another example of the time-of-flight light sensing system  500 A of  FIG.  5 A , or another example of the time-of-flight light sensing system  100  shown in  FIG.  1   , and that similarly named and numbered elements described above are coupled and function similarly below. It also appreciated that the example time-of-flight light sensing system  800  of  FIG.  8    shares many similarities with the example time-of-flight light sensing system  500 A of  FIG.  5 A . 
     For instance, as shown in the example depicted in  FIG.  8   , the example time-of-flight light sensing system  800  includes a light source  802  that is synchronized with a time-of-flight sensor including a pixel array  810 . In the various examples, the light source  802  is configured to emit modulated light to only a portion of an object at a time, such as for example portion  107  of object  106  as illustrated for example in  FIG.  1   . In various examples, a laser may be employed as the light source  802 , and the field-of-view of the laser may be controllable by various examples of scanning mechanisms. For instance, in various examples, the light source  802  may be implemented electronically (e.g., via an addressable laser array), or mechanically (e.g., via a bulk or a MEMS mirror), or optically/photonically (e.g., via a phased array, liquid crystal, etc.), or with any other suitable type of light source or technique to emit the modulated light to the object in accordance with the teachings of the present invention. 
     In the depicted example, the time-of-flight sensor also includes a modulation driver block  834 , which is coupled to the light source  802  and the pixel array  810 . In operation, the control and readout of the enabled and disabled time-of-flight pixel circuits included in time-of-flight pixel array  810  is synchronized with the modulated light that is emitted by light source  802  to the object. As shown in the depicted example, time-of-flight pixel array  810  includes a plurality of time-of-flight pixel circuits  812 A- 812 I. In the example, it is noted that each of the time-of-flight pixel circuits  812 A- 812 I may be an example of the time-of-flight pixel circuit  712  described in detail in  FIG.  7   . Therefore, it is appreciated that each of the time-of-flight pixel circuits  812 A- 812 I is not described again in detail in  FIG.  8    for the sake of brevity. 
     In the example depicted in  FIG.  8   , the time-of-flight pixel circuits  812 A- 812 I are arranged into rows and columns in time-of-flight pixel array  810 . It is noted that time-of-flight pixel array  810  is illustrated with time-of-flight pixel circuits  812 A- 812 I arranged into 3 rows and 3 columns for explanation purposes. In other examples, it is appreciated of course that the time-of-flight pixel array  810  may include a greater or fewer number of rows and/or a greater or fewer number of columns. 
       FIG.  8    depicts an example in which the reflected modulated light from the object is configured to illuminate one or more rows of time-of-flight pixel array  810 , but not all of the rows of the time-of-flight pixel array  810  at a time. For instance, a first subset of the plurality of time-of-flight pixel circuits (e.g., the row including time-of-flight pixel circuits  812 A- 812 C) is configured to be illuminated by the reflected modulated light from the object while a second subset of the plurality of time-of-flight pixel circuits (e.g., the rows including time-of-flight pixel circuits  812 D- 812 I) is configured to be non-illuminated by the reflected modulated light from the object. 
     As shown in the depicted example, modulation driver block  834  includes a phase lock loop circuit  840 , which is configured to generate the first and second phase modulation signals TXA and TXB. For instance, as described in the example of  FIG.  7   , the first and second phase modulation signals TXA and TXB are coupled to be received by the transfer transistors of time-of-flight pixel circuits  812 A- 812 I. A light source driver  844  coupled between the light source  802  and the phase lock loop circuit  840 . In operation, the light source driver circuit  844  is configured to synchronize the modulated light emitted from the light source  802  to the object with the first and second phase modulation signals TXA and TXB in response to the phase lock loop circuit  840 . 
     Continuing with the depicted example, a plurality of driver circuits  836 A- 836 F is coupled to the phase lock loop circuit  840 . In the example, the driver circuit  836 A is configured to generate the first phase modulation signal TXA and the driver circuit  536 B is configured to generate the second phase modulation signal TXB for the row of time-of-flight pixel array  810  that includes time-of-flight pixels  812 A- 812 C. The driver circuit  836 C is configured to generate the first phase modulation signal TXA and the driver circuit  836 D is configured to generate the second phase modulation signal TXB for the row of time-of-flight pixel array  810  that includes time-of-flight pixels  812 D- 812 F. The driver circuit  836 E is configured to generate the first phase modulation signal TXA and the driver circuit  836 F is configured to generate the second phase modulation signal TXB for the row of time-of-flight pixel array  810  that includes time-of-flight pixels  812 G- 812 I. 
     In the example, modulation driver block  834  also includes a plurality of driver switches  838 A- 838 F. Each one of the plurality of driver switches  838 A- 838 F is coupled to an output of a respective one of the plurality of driver circuits  836 A- 836 F as shown in  FIG.  8   . As such, driver switch  838 A is coupled to an output of driver circuit  836 A, driver switch  838 B is coupled to an output of driver circuit  836 B, driver switch  838 C is coupled to an output of driver circuit  836 C, driver switch  838 D is coupled to an output of driver circuit  836 D, driver switch  838 E is coupled to an output of driver circuit  836 E, and driver switch  838 F is coupled to an output of driver circuit  836 F. 
     As shown in the example, modulation driver block  834  also includes a modulation control circuit  835  that is coupled the plurality of driver switches  838 A- 838 F. In operation, the modulation control circuit  835  is configured to turn off or disable the driver switches that are coupled to non-illuminated time-of-flight pixel circuits while the modulation control circuit  835  is configured to turn on or enable the driver switches that are coupled to illuminated time-of-flight pixel circuits. 
     To illustrate, in the example above in which the first subset of the plurality of time-of-flight pixel circuits (e.g., the row including time-of-flight pixel circuits  812 A- 812 C) is configured to be illuminated by the reflected modulated light from the object while a second subset of the plurality of time-of-flight pixel circuits (e.g., the rows including time-of-flight pixel circuits  812 D- 812 I) is configured to be non-illuminated by the reflected modulated light from the object, the modulation control circuit  835  is configured to turn on or enable driver switches  838 A and  838 B and turn off or disable driver switches  836 C- 836 F. As such, the transfer transistors of the illuminated, and therefore enabled, time-of-flight pixel circuits  812 A- 812 C are coupled to receive and be responsive to the first and second phase modulation signals TXA and TXB, while the the transfer transistors of the non-illuminated, and therefore disabled, time-of-flight pixel circuits  812 D- 812 F are not coupled to receive and therefore not be responsive to the first and second phase modulation signals TXA and TXB. 
     Continuing with the depicted example, modulation driver block  834  further includes a scan synchronize circuit  842  coupled to the modulation control circuit  835  as shown. In operation, the scan synchronize circuit  842  is configured to synchronize the scanning of the modulated light emitted by the light source across the object with the scanning of the first subset of the plurality of time-of-flight pixel circuits that are illuminated by the reflected modulated light across the time-of-flight pixel array  810 . In other words, the scan synchronize circuit  842  is configured to synchronize the scanning of the modulated light emitted by the light source  802  across the object with the activation of the appropriate driver switches  838 A- 838 F that are coupled to the corresponding time-of-flight pixel circuits  812 A- 812 I that are illuminated by the modulated light that is reflected from the object in accordance with the teachings of the present invention. 
     One of the differences between the example time-of-flight light sensing system  800  of  FIG.  8    and the example time-of-flight light sensing system  500 A of  FIG.  5 A  is that the time-of-flight light sensing system  800  of  FIG.  8    includes a column control circuit  852  as well as a row control circuit  848 . In the depicted example, column control circuit  852  is configured to generate the column reset control signal (see e.g., CRST CTRL  in  FIG.  7   ) coupled to control the column reset control transistor (see e.g.,  738  in  FIG.  7   ) of each of the time-of-flight pixel circuits  812 A- 812 I. The column control circuit  852  is further configured to generate the first column sample and hold control signal (see e.g., CSH CTRL  in  FIG.  7   ) coupled to control the first column sample and hold control transistor (see e.g.,  744 A in  FIG.  7   ) of each of the time-of-flight pixel circuits  812 A- 812 I. The column control circuit  852  is also configured to generate the second column sample and hold control signal (see e.g., CSH CTRL  in  FIG.  7   ) coupled to control the second column sample and hold control transistor (see e.g.,  744 B in  FIG.  7   ) of each of the time-of-flight pixel circuits  812 A- 812 I. 
     Continuing with the depicted example, row control circuit  848  is configured to generate the row reset control signal (see e.g., RRST CTRL  in  FIG.  7   ) coupled to control the row reset control transistor (see e.g.,  740  in  FIG.  7   ) of each of the time-of-flight pixel circuits  812 A- 812 I. In addition, row control circuit  848  is further configured to generate the row sample and hold control signal (see e.g., RSH CTRL  in  FIG.  7   ) coupled to control the first row sample and hold control transistor and the second row sample and hold control transistor (see e.g.,  742 A and  742 B in  FIG.  7   ) of each of the time-of-flight pixel circuits  812 A- 812 I. Furthermore, the row control circuit  848  is also configured to generate a row select signal RS coupled to control the first row select transistor and the second row select transistor (see e.g.,  732 A and  732 B in  FIG.  7   ) of each of the time-of-flight pixel circuits  812 A- 812 I. 
     In the depicted example, a column readout circuit  850  is coupled to the first bitline BL 1  and the second bitline BL 2  of each one of the plurality of time-of-flight pixel circuits  812 A- 812 I to read out each one of the plurality of time-of-flight pixel circuits  812 A- 812 I of time-of-flight pixel array  810  as shown. 
     In operation, it is appreciated that with the column reset control signals CRST CTRL , the row reset control signals RRST CTRL , the reset enable/disable control signals B 2 , the column sample and hold control signals CSH CTRL , the row sample and hold signals RSH CTRL , and the sample and hold enable/disable control signals B 1  supported by time-of-flight light sensing system  800  as shown in  FIG.  8   , individual time-of-flight pixel circuits  812 A- 812 I may be randomly addressed and accessed to enable spot shaped portions of time-of-flight light sensing system  800  to be illuminated and read out as shown above for example in  FIG.  3 F  in accordance with the teachings of the present invention. 
     The above description of illustrated examples of the invention, including what is described in the Abstract, is not intended to be exhaustive or to limit the invention to the precise forms disclosed. While specific examples of the invention are described herein for illustrative purposes, various modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. 
     These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific examples disclosed in the specification. Rather, the scope of the invention is to be determined entirely by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.