Patent Publication Number: US-10760959-B2

Title: Apparatus for and method of electro-optically reading a target in the presence of ambient light by detecting and suppressing the ambient light

Description:
FIELD OF THE DISCLOSURE 
     The present disclosure relates generally to an apparatus for, and a method of, electro-optically reading a target in the presence of ambient light and, more particularly, to detecting and suppressing the ambient light, especially when emitted from fluorescent lamps and light emitting diodes (LEDs) operated at kilohertz frequencies. 
     BACKGROUND 
     Moving laser beam readers or laser scanners have long been used as data capture devices to electro-optically read targets, such as one-dimensional bar code symbols, particularly of the Universal Product Code (UPC) type, printed on labels associated with products in many venues, such as supermarkets, warehouse clubs, department stores, and other kinds of retailers, as well as many other venues, such as libraries and factories. The moving laser beam reader generally includes a housing, a laser for emitting a laser beam, a focusing lens assembly for focusing the laser beam to form a beam spot having a certain size at a focal plane in a range of working distances relative to the housing, a scan component for repetitively scanning the beam spot across a target in a scan pattern, for example, a scan line or a series of scan lines, across the target multiple times per second, and a photodetector for detecting return laser light reflected and/or scattered from the target and for converting the detected return laser light into an output analog electrical information signal bearing information related to the target. This analog electrical information signal varies in amplitude as a function of time due to the time-varying return laser light along each scan line, and varies in frequency as a function of the density of the symbol, as well as the distance at which the symbol is being read. The moving laser beam reader also includes signal processing receiver circuitry including a digitizer for digitizing the variable analog information signal, and a microprocessor for decoding the digitized signal based upon a specific symbology used for the target. The decoded signal identifies the product and is transmitted to a host, e.g., a cash register in a retail venue, for further processing, e.g., product price look-up or product inventorying. 
     In one advantageous embodiment, during operation of the moving laser beam reader in a venue having one or more external light sources that emit ambient light, an operator holds the housing in his or her hand, and aims the housing at the target, and then initiates the data capture and the reading of the target by manual actuation of a trigger on the housing. The ambient light is also concomitantly detected by the photodetector, which generates an analog electrical ambient light signal. In the event that the external source is sunlight, then the ambient light is substantially constant in magnitude, and therefore, the analog electrical ambient light signal has a constant illumination DC component. In the event that the external source is an incandescent bulb or a fluorescent lamp energized at 50 Hz or 60 Hz, then the analog electrical ambient light signal has a constant illumination DC component and a relatively small time-varying AC frequency component at 50 Hz or 60 Hz. In the event that the fluorescent lamp is operated at higher frequencies for greater luminous efficiency, or in the event that the external source includes light emitting diodes (LEDs) operated at higher frequencies, then the analog electrical ambient light signal has a constant illumination DC component and a relatively larger time-varying AC frequency component at kilohertz frequencies, typically anywhere from 30 kHz to 300 kHz. 
     In some circumstances, the presence of the ambient light signal interferes with, and weakens, the information signal. To prevent such interference, the constant illumination DC component of the ambient light signal can generally be filtered out from the information signal. Also, filters can be used to suppress the ambient light signal when its time-varying frequency component is very far in frequency away from the frequency of the information signal. However, if the time-varying frequency component of the ambient light signal is too close in frequency to the frequency of the information signal, then the ambient light signal can interfere and impede the decoding of the information signal, thus degrading the performance of the reader. By way of non-limiting example, an information signal of about 50 kHz can be generated during reading of a low density symbol located relatively close to the reader, e.g., about 10 inches away. If the ambient light source includes LEDs operated at about 50 kHz, then the respective frequencies of the ambient light signal and the information signal are too close and will cause an interference, and perhaps cause the symbol not to be successfully decoded and read. 
     To prevent such interference, it is also known to reduce the ambient light signal by placing a detector lens in front of the photodetector. The detector lens can be implemented either as a transmissive or a reflective component. The detector lens limits the field of view (FOV) of the photodetector. The FOV is minimized and limited to desirably include substantially only the laser beam spot. Still, even within this limited FOV, the ambient light signal can still be too high for successful decoding. 
     To detect and measure the magnitude of the ambient light signal, it is known to analyze an output analog electrical signal of the photodetector when the laser is deenergized, i.e., when there is no information signal. This typically occurs at the start of a reading session, e.g., during a start of scan (SOS) frame, which, for example, advantageously lasts for about 16 milliseconds. When the laser is deenergized, however, the target cannot be read, and the reader must wait for the laser to be energized. If the ambient light signal is, for some reason, not detected during this initial deenergization of the laser, then the reader must wait for the next time that the laser is deenergized to attempt the detection of the ambient light signal again. All this waiting negatively impacts the reader&#39;s aggressiveness and renders its performance sluggish. 
     Accordingly, there is a need to suppress such interference caused by such ambient light and to enhance reader performance without having to deenergize the laser. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
       The accompanying figures, where like reference numerals refer to identical or functionally similar elements throughout the separate views, together with the detailed description below, are incorporated in and form part of the specification, and serve to further illustrate embodiments of concepts that include the claimed invention, and explain various principles and advantages of those embodiments. 
         FIG. 1  is a schematic view of a handheld moving laser beam reader apparatus operative for detecting and suppressing an ambient light signal in accordance with the present disclosure. 
         FIG. 2  is an electrical circuit schematic of one embodiment of a signal processing receiver circuit in one mode of operation for use in the apparatus of  FIG. 1 . 
         FIG. 3  is analogous to  FIG. 2 , but in another mode of operation. 
         FIG. 4  is an electrical circuit schematic of part of another embodiment of a signal processing receiver circuit for use in the apparatus of  FIG. 1 . 
         FIG. 5  is a front view of a set of photodetectors of a detector assembly employed in the circuit of  FIGS. 2-4 . 
         FIG. 6  is a front view of the set of photodetectors of  FIG. 5  depicting return laser light thereon from a target, as well as ambient light. 
         FIG. 7  is a diagram of light rays passing through a detector lens to the set of photodetectors of  FIG. 5 . 
         FIG. 8  is a flow chart depicting steps performed in accordance with the method of the present disclosure. 
     
    
    
     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 and locations of some of the elements in the figures may be exaggerated relative to other elements to help to improve understanding of embodiments of the present invention. 
     The apparatus and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. 
     DETAILED DESCRIPTION 
     One aspect of this disclosure relates to an apparatus or reader for electro-optically reading a target, e.g., a bar code symbol, in the presence of ambient light to be suppressed. The reader includes a laser, typically a semiconductor laser diode, for emitting a laser beam. The reader also includes a scan component for scanning the laser beam across the target for reflection and scattering therefrom as return laser light. The reader also includes a detector assembly for detecting the return laser light from the target and for converting the detected return laser light into an analog electrical information signal bearing information related to the target, and for concomitantly detecting the ambient light and for converting the detected ambient light into an analog electrical ambient light signal. The detector assembly includes a first photodetector having a first field of view and a first light collection area, a second photodetector having a second field of view and a second light collection area in close proximity to the first light collection area, and a detector lens spaced from the first and second photodetectors and operative for focusing and directing the return laser light substantially onto the first light collection area over the first field of view. The first light collection area is operative for generating a first analog electrical output signal comprised of the information signal and the ambient light signal, and the second light collection area is operative for generating a second analog electrical output signal substantially comprised of the ambient light signal. The reader further includes signal processing circuitry for processing the first and second output signals to determine a magnitude of the ambient light signal, and for suppressing the ambient light signal when the determined magnitude of the ambient light signal at least equals, and preferably, exceeds, a threshold. 
     More particularly, the signal processing circuitry includes a controller, e.g., a programmed microprocessor, for comparing the ambient light signal from the second photodetector with the threshold. The signal processing circuitry is operative for processing the first output signal in one mode of operation when the ambient light signal from the second photodetector is below the threshold, and for suppressing the ambient light signal in the first output signal and for processing substantially only the information signal in the first output signal in another mode of operation when the ambient light signal from the second photodetector at least equals the threshold. 
     In one embodiment, the signal processing circuitry includes a correlated double sampling (CDS) circuit for suppressing the ambient light signal in the first output signal, and a control switch having two switching states. The controller is operative for switching the control switch to one of the switching states in which the CDS circuit is bypassed during the one mode of operation, and for switching the control switch to the other of the switching states in which both the information signal and the ambient light signal in the first output signal are conducted to the CDS circuit during the other mode of operation. 
     In another embodiment, the signal processing circuitry includes a bandpass filter for suppressing the ambient light signal in the first output signal, and a control switch having two switching states. The controller is operative for switching the control switch to one of the switching states in which the bandpass filter is bypassed during the one mode of operation, and for switching the control switch to the other of the switching states in which both the information signal and the ambient light signal in the first output signal are conducted to the bandpass filter during the other mode of operation. 
     Another aspect of this disclosure resides in the above-described detector assembly, which comprises a first photodetector having a first field of view and a first light collection area, a second photodetector having a second field of view and a second light collection area in close proximity to the first light collection area, and a detector lens spaced from the first and second photodetectors and operative for focusing and directing the return laser light substantially onto the first light collection area over the first field of view. The first light collection area is operative for generating a first analog electrical output signal comprised of the information signal and the ambient light signal, and the second light collection area is operative for generating a second analog electrical output signal substantially comprised of the ambient light signal. Preferably, each light collection area is elongated, and the light collection areas lie on a common substrate. Advantageously, the second light collection area lies exteriorly of, and preferably, surrounds, the first light collection area. 
     A method, in accordance with still another aspect of this disclosure, of electro-optically reading a target in the presence of ambient light to be suppressed, is performed by emitting a laser beam; by scanning the laser beam across the target for reflection and scattering therefrom as return laser light; by detecting the return laser light from the target; by converting the detected return laser light into an analog electrical information signal bearing information related to the target; by concomitantly detecting the ambient light; by converting the detected ambient light into an analog electrical ambient light signal; by generating a first analog electrical output signal comprised of the information signal and the ambient light signal from a first photodetector having a first field of view and a first light collection area; by generating a second analog electrical output signal substantially comprised of the ambient light signal from a second photodetector having a second field of view and a second light collection area in close proximity to the first light collection area; by focusing and directing the return laser light substantially onto the first light collection area over the first field of view; by processing the first and second output signals to determine a magnitude of the ambient light signal; and by suppressing the ambient light signal when the determined magnitude of the ambient light signal at least equals a threshold. 
     Turning now to the drawings,  FIG. 1  depicts a handheld, moving laser beam reader  10  implemented in a gun-shaped housing  55  having a pistol-grip type of handle  53 . The housing  55  contains a laser light source  46 , preferably a semiconductor laser diode, for emitting an outgoing laser beam  51  to a target, such as a bar code symbol  70 , for reflection and scattering therefrom; a detector assembly  58 , preferably a pair of photodiodes PD 1  and PD 2 , and a detector lens L, as described more fully below in connection with  FIGS. 5-7 , for detecting incoming light  52 ; a focusing optical assembly  57 , preferably one or more focusing lenses, for focusing the outgoing laser beam  51  as a laser beam spot on the symbol  70 ; an application specific integrated circuit (ASIC)  20  mounted on a printed circuit board (PCB)  61 ; a programmed microprocessor or controller  40 , also preferably mounted on the PCB  61 ; and a power source or battery  62 , preferably mounted in the handle  53 . A light-transmissive window  56  at a front end of the housing  55  allows the outgoing laser light beam  51  to exit the housing  55 , and the incoming light  52  to enter the housing  55 . A user holds the reader  10  by the handle  53 , and aims the reader  10  at the symbol  70 , preferably at a distance away from the symbol  70 . To initiate reading, the user pulls a trigger  54  on the handle  53 . The reader  10  may optionally include a keyboard  48  and a display  49  readily accessible to the user. 
     As further depicted in  FIG. 1 , the outgoing laser beam  51  emitted by the laser light source  46  passes through a partially-silvered mirror  47  to a scan component or oscillating scan mirror  59 , which is coupled to a drive motor  60 , preferably energized when the trigger  54  is manually pulled. The oscillation of the mirror  59  causes the outgoing laser beam  51  to sweep back and forth in a desired scan pattern, e.g., a scan line, across the symbol  70 . A variety of mirror and motor configurations can be used to move the laser beam in the desired scan pattern. For example, the mirror  59  need not be a concave mirror as illustrated, but could be a planar mirror that is repetitively and reciprocally driven in alternate circumferential directions about a drive shaft on which the planar mirror is mounted. 
     As further depicted in  FIG. 1 , the incoming light  52  may have two light components that come from two different sources. The first light component is return laser light derived from the laser light source  46  and is generated by reflection and/or scattering of the laser light beam  51  back by the symbol  70  through the window  56 . The second light component is ambient light  82  derived from an external ambient light source  80  operative for emitting the ambient light  82 , which passes through the window  56 . As described above, the external light source  80  at a venue can be sunlight, one or more incandescent bulbs, one or more fluorescent lamps, one or more light emitting diodes (LEDs), and the like. In the exemplary reader  10  shown in  FIG. 1 , the incoming light  52  reflects off of the scan mirror  59  and the partially-silvered mirror  47  and impinges on the detector assembly  58 . The detector assembly  58  produces an analog electrical output signal proportional to the intensity of the received return light  52 . A first component signal of the output signal, which is produced from the return laser light derived from the laser light source  46 , is hereinafter described as an “information” signal (V ABP ) bearing information related to the symbol  70 . The subscript ABP is an abbreviation for analog bar pattern. A second component signal of the output signal, which is produced from the ambient light derived from the ambient light source  80 , is hereinafter described as an “ambient light” signal (V AMP ). 
     As also described above, when fluorescent lamps and LEDs are operated at kilohertz frequencies, there are circumstances where the time-varying frequency component of the ambient light signal V AMB  is too close in frequency to the frequency of the information signal V ABP , in which event the ambient light signal V AMB  can interfere and impede the decoding of the information signal V ABP , thus degrading the performance of the reader  10 . By way of non-limiting example, an information signal V ABP  of about 50 kHz can be generated during reading of a low density symbol  70  located relatively close to the reader  10 , e.g., about 10 inches away. If the LEDs are operated at about 50 kHz, then the respective frequencies of the ambient light signal V AMB  and the information signal V ABP  are too close and will cause an interference, and perhaps cause the symbol  70  not to be successfully read. One aspect of this disclosure is to detect and suppress such interference, without denergizing the laser light source  46 , and without negatively impacting the reader&#39;s aggressiveness and without rendering its performance as sluggish. 
     In accordance with this disclosure, the detector assembly  58 , as best shown in  FIGS. 5-7 , includes a first photodetector PD 1  having a first field of view (FOV 1 ) and a first light collection area  12 , a second photodetector PD 2  having a second field of view (FOV 2 ) and a second light collection area  14 , and a detector lens L spaced from the first and second photodetectors PD 1 , PD 2  and operative for focusing and directing the return laser light substantially onto the first light collection area  12  over the first field of view FOV 1 . The first light collection area  12  is in close proximity to, or at least partly, and, as shown, may be entirely, surrounded by, the second collection area  14 . The detector lens L may be a stand-alone component as shown in  FIG. 7 , or may be integrally implemented as curved surfaces of the mirrors  47  and  59  of  FIG. 1 . The detector lens L optically modifies and configures the first field of view FOV 1  and the second field of view FOV 2 . FOV 1  may be smaller than FOV 2  as shown in  FIG. 7 , or FOV 2  may be divided into subfields, each of which is smaller than FOV 1 . 
     The first photodetector PD 1  can thus, for example, be termed an interior or central photodetector, while the second photodetector PD 2  can be termed an exterior photodetector. The first and the second collection areas  12 ,  14  are generally planar and are situated, typically by wafer-scale processing, on a common, generally planar substrate  90 , preferably made of silicon. The first and the second light collection areas  12 ,  14  have centers with optical axes that pass therethrough and that are generally perpendicular to the substrate  90 . These optical axes are substantially collinear, or are parallel to each other. Although PD 1  and PD 2  have been illustrated with light collection areas  12 ,  14  that are shaped as rectangles, it will be understood that many other shapes, e.g., ovals, may be used. Although the first light collection area  12  of PD 1  has been illustrated as being centrally and symmetrically located within the second light collection area  14  of PD 2 , it will be understood that this need not be; for example, the first light collection area  12  can be offset from, or shifted to an asymmetrical position relative to, the second light collection area  14  such that the second light collection area  14  bounds the first light collection area  12  on only three of its four sides. 
     As shown in  FIG. 6 , each light collection area  12 ,  14  receives the return laser light derived from the laser light source  46 , and receives the ambient light from the ambient light source  80 , but to different extents, as described below. Each light collection area  12 ,  14  converts the received return laser light derived from the laser light source  46  into the information signal V ABP , and also converts the received ambient light from the ambient light source  80  into the ambient light signal V AMB . 
     The first light collection area  12  of PD 1  generates a first analog electrical output signal comprised of the information signal V ABP  and the ambient light signal V AMB  for both close-in and far-out targets, while the second light collection area  14  of PD 2  generates a second analog electrical output signal substantially comprised of the ambient light signal V ABP , especially for far-out targets, because the return laser light derived from the laser light source  46  is focused by the detector lens L onto the first light collection area  12  of PD 1 . Hence, as described below, the second light collection area  14  of PD 2  may be employed to detect the ambient light signal V AMB . It is not necessary to deenergize the laser light source  46  in order for the second light collection area  14  of PD 2  to detect the ambient light signal V AMB . 
     The ASIC  20 , as shown in the embodiment of  FIGS. 2-3 , includes a signal processing receiver circuit having a first channel connected to the first photodetector PD 1 , and a second channel connected to the second photodetector PD 2 . As described above, the first photodetector PD 1  outputs a combined or composite output signal (V ABP +V AMB ) that is the sum of the information signal V ABP  and the ambient light signal V AMB . The first channel of the signal processing receiver circuit includes a front-end receiver section  100  having at least one amplifier (AMP 1 ), advantageously configured as a transimpedance amplifier, to increase the gain of the analog electrical combined output signal (V ABP +V AMB ) received from the first photodetector PD 1 . The gain of the amplifier AMP 1  can be adjusted by varying a resistor  102  via a control line  104  connected to the controller  40 . The combined output signal (V ABP +V AMB ) is either directly conducted to a back-end receiver section  120  in a first mode of operation by having the controller  40  switch a bypass switch S 3  via a control line  106  to a first switching state as depicted in  FIG. 2 , or is directly connected to an ambient light suppression circuit  140  in a second mode of operation by having the controller  40  switch the bypass switch S 3  via the control line  106  to a second switching state as depicted in  FIG. 3 . 
     The switching of the bypass switch S 3  is determined by the signal processing receiver circuit by measuring the ambient light signal V AMB  detected by the second photodetector PD 2 , as described below. If the measured ambient light signal V AMB  is below a threshold value, then the presence of the ambient light signal V AMB  can be tolerated and, hence, the combined output signal (V ABP +V AMB ) is sent to the back-end receiver section  120  in the first mode of operation depicted in  FIG. 2 . If the measured ambient light signal V AMB  equals or exceeds the threshold value, then the presence of the ambient light signal V AMB  will interfere with the reading of the symbol  70  and, hence, the ambient light signal V AMB  must be suppressed by the suppression circuit  140 , and only the information signal V ABP  is sent to the back-end receiver section  120  in the second mode of operation depicted in  FIG. 3 . The threshold value can be stored in advance in a memory of the controller  40 . 
     The controller  40  controls the laser light source  46  with a laser drive circuit  108  via a control line  110 . The laser drive circuit  108  includes a laser power regulator, which is a closed loop feedback system that maintains a constant optical output power by varying the applied forward continuous current I C  ( FIG. 2 ) or a modulated current I MOD  ( FIG. 3 ) to the laser light source  46 . The laser light source  46  includes a laser diode  112  and a monitor photodiode  114 . A small fraction of the output laser light is coupled into the monitor photodiode  114  within the enclosed laser light source  46 . This induces a photocurrent in the monitor photodiode  114  that is proportional to the laser output power. This photocurrent is a negative feedback signal that is used to regulate the laser&#39;s output power. Thus, the controller  40  can either continuously run the laser light source  46  in a continuous mode, or can modulate the laser light source  46  in a modulated mode. 
     Thus, in  FIG. 2 , the combined output signal (V ABP +V AMB ) is conducted to the back-end receiver section  120 , which includes an automatic gain controller (AGC) having an on/off control, at least one active low-pass filter stage having an adjustable cutoff frequency in its bandwidth to filter noise from the combined output signal, and at least one amplifier (AMP 2 ) having an adjustable gain to increase the gain of the combined output signal. Each of the AGC, low-pass filter, and the AMP 2  has a control input connected to control lines  116 ,  118 ,  122 , which can be adjusted under control of the microprocessor  40 . 
     The signal processing receiver circuit also includes a digitizer  124  which digitizes the combined output signal by processing the same with differentiating circuits, peak detectors, multiplexers, logic elements, and comparators. The digitizer  124  processes the combined output signal from the back-end section  120  to produce a pulse signal where the widths and spacings between the pulses correspond to the widths of the bars and the spacings between the bars of the symbol  70 . The digitizer  124  serves as an edge detector or wave shaper circuit, and threshold points set by the digitizer  124  determines what points of the combined output signal represent bar edges. The pulse signal from the digitizer  124  is applied to a decoder  128 , typically incorporated as software in the programmed controller  40 , which will also have associated program memory and random access data memory. The controller  40  also has an analog-to-digital converter (ADC)  126  connected to the decoder  128  and to the output of the back-end section  120 . The decoder  128  first determines the pulse widths and spacings of the combined output signal from the digitizer  124 . The decoder  128  then analyzes the widths and spacings to find and decode a legitimate bar code symbol. This includes analysis to recognize legitimate characters and sequences, as defined by the appropriate code standard. The controller  40  then communicates with an external host over an interface. 
     The measuring of a magnitude and/or a frequency of the ambient light signal V AMB  is performed by the second photodetector PD 2  and the controller  40  in various ways. As described above, the second photodetector PD 2  outputs an output signal that is substantially comprised of the ambient light signal V AMB . The second channel of the signal processing receiver circuit includes at least one amplifier (AMP 4 )  22 , advantageously configured as a transimpedance amplifier, to increase the gain of the ambient light signal V AMB  received from the second photodetector PD 2 . The gain of the amplifier  22  can be adjusted by varying a resistor  24  via a control line  38  connected to the controller  40 . The amplified ambient light signal V AMB  is conducted to an automatic gain controller (AGC)  26  having an on/off control, at least one active low-pass filter stage  28  having an adjustable cutoff frequency in its bandwidth to filter noise from the amplified ambient light signal V AMB , at least one amplifier (AMP 5 )  30  having an adjustable gain to increase the gain of the amplified ambient light signal V AMB , and, thereupon, to the controller  40 . Each of the AGC  26 , low-pass filter  28 , and the amplifier  30  has a control input connected to control lines  32 ,  34 ,  36 , which can be adjusted under control of the controller  40 . The controller  40  can measure the amplitude of the received ambient light signal V AMB , and compare that amplitude to the threshold. 
     In addition, the controller  40  can detect the ambient light signal V AMB  by executing a fast Fourier transform (FFT) on the output signals on the first and second channels. Assuming that the gains on the first and second channels are kept substantially constant, the FFT signal processing will result in a first set of peaks at a first frequency on the first and second channels, and a second set of peaks at a second frequency on the first and second channels. The controller  40  can then compare each set of peaks at each frequency. If the peaks are unequal in magnitude, then this indicates that the information signal V ABP  is present. If the peaks are about equal in magnitude, then this indicates that the ambient signal V AMB  is present. Once the magnitude of the ambient signal V AMB  is known, then the controller  40  can act to suppress it. 
     In  FIG. 2 , the presence of the ambient light signal V AMB  is tolerated due to its low measurement or magnitude, i.e., below the threshold. However, in  FIG. 3 , the presence of the ambient light signal V AMB  cannot be tolerated due to its high measurement or magnitude, i.e., equal to or above the threshold, which is why the suppression circuit  140  is employed to remove the ambient light signal V AMB . The suppression circuit  140  in  FIG. 3  is configured as a correlated double sampling (CDS) circuit having a pair of switches S 1  and S 2  which are pulsed out of phase with respect to each other. To use the CDS circuit  140 , the controller  40  must pulse or modulate the laser light source  46  with the laser drive circuit  108  via the control line  110  so that a modulated current I MOD  is conducted to the laser light source  46 . The laser light source  46  is pulsed at a rate or frequency sufficiently fast to accurately resolve the narrowest bar or space used in the type of bar code symbol  70  to be read. Typically, this amounts to several pulses during the period of time required for the scanning laser beam to scan across the narrowest bar or space of the bar code symbol  70 . 
     As also shown in  FIG. 3 , the signal processing receiver circuit also includes an oscillator  130  that is controlled by the controller  40  via a control line  132 . The oscillator  130  puts out a clock signal (CLK) which has the same frequency as that of the pulsed laser light source  46 . An inverter  134  puts out an inverted clock signal (CLK*) which has the same frequency as the CLK signal, but which is 180 degrees out of phase therewith. The CLK* signal drives the switching of the switch S 1 , and the CLK signal drives the switching of the switch S 2 . 
     In  FIG. 3 , the first photodetector PD 1  outputs a combined output signal (V ABP (MOD) +V AMB ) that is the sum of the modulated information signal V ABP (MOD)  and the ambient light signal VA m. This combined output signal is conducted to each switch S 1  and S 2 . When the switch S 2  is closed, the laser light source  46  is modulated high so that the combined output signal (V ABP(MOD) +V AMB ) passes through the switch S 2  and is stored on capacitor C 2 . When the switch S 1  is closed, the laser light source  46  is modulated low so that only the ambient light signal V AMB  of the combined output signal (V ABP (MOD) +V AMB ) passes through the switch S 1  (V ABP(MOD) =0) and is stored on capacitor C 1 . The resulting stored signals are then buffered by buffers B 1  or B 2  and fed into a differencing amplifier  136  (AMP 3 ), which receives the two buffered signals and subtracts the value of the combined output signal (V ABP(MOD) +V AMB ) from the value of the ambient light signal V AMB , thereby resulting in just the modulated information signal V ABP(MOD) , which is then passed through a low-pass filter  138 . The switching action of the switch S 2  is synchronized with the pulsing of the laser light source  46 , and functions as a demodulator, and moves the modulated information signal V ABP(MOD)  to baseband, thereby resulting in the information signal V ABP , which is then conducted through the back-end section  120 , the digitizer  124 , and the decoder  128 , as described above. The filter  138  has an adjustable cutoff frequency in its bandwidth (BW 1 ) that is adjusted by the controller  40  via a control line  142 . The filter  138  removes any carrier-related residue. 
     The suppression circuit  140  effectively suppresses the ambient light signal, but incurs a noise performance penalty, because receiver bandwidths need to be increased to support the high modulation frequency carrier. During demodulation in the suppression circuit  140 , the noise in baseband is significantly increased. Also, about half of the output power of the laser light source  46  is lost during the modulation (50% duty cycle) to produce the carrier. To avoid this signal-to-noise (S/N) loss (and loss in reader performance), various adjustments can be made to the signal processing circuit. For example, the gain of the amplifier AMP 1  over control line  104  can be increased to improve the signal-to-noise performance. The clock frequency of the oscillator  130  can be lowered over control line  132  to improve fidelity and the signal-to-noise performance. The cutoff frequencies of the filters having bandwidths BW 1 , BW 2  can be adjusted over control lines  142 ,  118  (the lower the bandwidth BW 1 , BW 2 , the lower the noise). 
       FIG. 4  shows another embodiment of a noise suppression circuit  150 . Like reference numerals have been used in  FIG. 4  to identify like parts. Rather than using correlated double sampling as described above in  FIG. 3 , the received information signal is amplitude modulated by modulating the laser light source  46 . The combined output signal (V ABP(MOD) +V AMB ) is conducted to a band-pass filter  152 , which removes the interference signal V AMB . The band-pass filter  152  has an adjustable bandwidth that is adjusted by the controller  40  via a control line  156 . A mixer (M)  154  is then used to demodulate and move the modulated information signal V ABP(MOD)  back to a baseband signal V ABP . The baseband signal V ABP  is then filtered in the low-pass filter  138 , if needed, to remove any carrier-related residue. 
     As depicted in the flow chart of  FIG. 8 , beginning a reading session at start step  200 , the ambient light signal from the second photodetector PD 2 , in accordance with one embodiment, is measured in step  202 . Then, in step  204 , it is determined whether the measured ambient light signal exceeds a threshold. If not, then both the information signal and the ambient light signal from the first photodetector PD 1  are processed in step  206 , and the reading session ends at step  208 . If yes, then the laser light source  46  is modulated in step  210 . Then, the ambient light signal from the first photodetector PD 1  is suppressed in step  210 . Then, only the information signal from the first photodetector PD 1  is processed in step  214 , after which the reading session ends at step  216 . 
     It will be understood that the default operating mode is as shown in  FIG. 2 , wherein the front-end receiver section  100  and the back-end receiver section  120  are utilized to process the output signals of the first and second photodetectors. As noted above, the suppression circuits  140 ,  150  add noise. In the case where modulation and a duty cycle are employed, the brightness of the outgoing laser beam is reduced. Hence, the suppression circuits  140 ,  150  are only made operational when they are needed, i.e., when the ambient light level is too high to be ignored. 
     In the foregoing specification, specific embodiments have been described. However, one of ordinary skill in the art appreciates that various modifications and changes can be made without departing from the scope of the invention as set forth in the claims below. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of present teachings. 
     The benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential features or elements of any or all the claims. The invention is defined solely by the appended claims including any amendments made during the pendency of this application and all equivalents of those claims as issued. 
     Moreover in this document, relational terms such as first and second, top and bottom, and the like may be used solely to distinguish one entity or action from another entity or action without necessarily requiring or implying any actual such relationship or order between such entities or actions. The terms “comprises,” “comprising,” “has,” “having,” “includes,” “including,” “contains,” “containing,” or any other variation thereof, are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises, has, includes, contains a list of elements does not include only those elements, but may include other elements not expressly listed or inherent to such process, method, article, or apparatus. An element proceeded by “comprises . . . a,” “has . . . a,” “includes . . . a,” or “contains . . . a,” does not, without more constraints, preclude the existence of additional identical elements in the process, method, article, or apparatus that comprises, has, includes, or contains the element. The terms “a” and “an” are defined as one or more unless explicitly stated otherwise herein. The terms “substantially,” “essentially,” “approximately,” “about,” or any other version thereof, are defined as being close to as understood by one of ordinary skill in the art, and in one non-limiting embodiment the term is defined to be within 10%, in another embodiment within 5%, in another embodiment within 1%, and in another embodiment within 0.5%. The term “coupled” as used herein is defined as connected, although not necessarily directly and not necessarily mechanically. A device or structure that is “configured” in a certain way is configured in at least that way, but may also be configured in ways that are not listed. 
     It will be appreciated that some embodiments may be comprised of one or more generic or specialized processors (or “processing devices”) such as microprocessors, digital signal processors, customized processors, and field programmable gate arrays (FPGAs), and unique stored program instructions (including both software and firmware) that control the one or more processors to implement, in conjunction with certain non-processor circuits, some, most, or all of the functions of the method and/or apparatus described herein. Alternatively, some or all functions could be implemented by a state machine that has no stored program instructions, or in one or more application specific integrated circuits (ASICs), in which each function or some combinations of certain of the functions are implemented as custom logic. Of course, a combination of the two approaches could be used. 
     Moreover, an embodiment can be implemented as a computer-readable storage medium having computer readable code stored thereon for programming a computer (e.g., comprising a processor) to perform a method as described and claimed herein. Examples of such computer-readable storage mediums include, but are not limited to, a hard disk, a CD-ROM, an optical storage device, a magnetic storage device, a ROM (Read Only Memory), a PROM (Programmable Read Only Memory), an EPROM (Erasable Programmable Read Only Memory), an EEPROM (Electrically Erasable Programmable Read Only Memory) and a Flash memory. Further, it is expected that one of ordinary skill, notwithstanding possibly significant effort and many design choices motivated by, for example, available time, current technology, and economic considerations, when guided by the concepts and principles disclosed herein, will be readily capable of generating such software instructions and programs and ICs with minimal experimentation. 
     The Abstract of the Disclosure is provided to allow the reader to quickly ascertain the nature of the technical disclosure. It is submitted with the understanding that it will not be used to interpret or limit the scope or meaning of the claims. In addition, in the foregoing Detailed Description, it can be seen that various features are grouped together in various embodiments for the purpose of streamlining the disclosure. This method of disclosure is not to be interpreted as reflecting an intention that the claimed embodiments require more features than are expressly recited in each claim. Rather, as the following claims reflect, inventive subject matter lies in less than all features of a single disclosed embodiment. Thus, the following claims are hereby incorporated into the Detailed Description, with each claim standing on its own as a separately claimed subject matter.