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Laser scanner having analog digitizer with increased noise immunityRelated Patent Categories: Registers, Coded Record Sensors, Particular Sensor Structure, Optical, Bar Code, Reader Processing Circuitry, Analog To Square Wave Converter (e.g., Digitizer, Binarizer, Slicer)Laser scanner having analog digitizer with increased noise immunity description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20060192012, Laser scanner having analog digitizer with increased noise immunity. Brief Patent Description - Full Patent Description - Patent Application Claims
FIELD OF THE INVENTION [0001] The present invention relates to a laser scanner for reading bar codes and, more particularly, to a laser scanner having an analog digitizer with increased noise immunity. BACKGROUND OF THE INVENTION [0002] Laser scanners are widely used for reading bar codes, including one dimensional and two dimensional bar codes. In a laser scanner, a laser generates a beam, the beam is scanned or rapidly moved across a bar code or a portion of a bar code. Typically, the laser beam is focused by a lens and repetitively scanned by means of an oscillating or rotating mirror. Essentially, the beam generates a beam spot that moves across a target bar code. [0003] The space elements of the bar code reflect the laser beam illumination and the dark or black bar elements of the bar code absorb the laser beam. The reflected light from the bar code is focused by scanner light-receiving optics through a bandpass filter and onto a photodetector circuitry, such as a photodiode. The pattern of reflected light, as received by the photodiode of the laser scanner, is a representation of the pattern of the bar code. That is, a sequence of time when the photodiode is receiving reflected light represents the laser beam spot moving across a space of the bar code, while a sequence of time when the photodiode is not receiving reflected light represents the laser moving across a dark bar. Since the scanning speed or velocity of the laser is known, the elapsed time of the photodiode receiving reflected light can be converted into a width of a bar code element corresponding to a space, while the elapsed time of the photodiode not receiving reflected light can be converted into a width of a bar code element corresponding to a bar. [0004] The photodiode is part of photodiode circuitry which converts the reflected light into an analog signal. The laser scanner includes a digitizer to digitize the analog signal generated by the photodiode. The digitizer outputs a digital bar code pattern (DPB) signal representative of the bar code pattern. A decoder of the laser scanner inputs the DPB signal and decodes the bar code. The decoded bar code typically includes payload information about the product that the bar code is affixed to. Upon successful decoding of the scanned bar code, the scanner may provide an audio and/or visual signal to an operator of the scanner to indicate a successful read and decode of the bar code. The scanner typically includes a display to display payload information to the operator and a memory to store information decoded from the bar code. [0005] To successfully read and decode a bar code, the digitizer must accurately interpret the analog signal output by the photodiode circuitry and determine where the edges, that is, the transition points of successive bar code elements are. Noise makes the digitization process problematic. Noise can include optical noise such as ambient light, paper grain or speckle noise, printing defects. Noise may also include electrical sources of noise such as radiated (EMI) or conducted (scanner circuitry induced noise). A digitizer must differentiate the signal representative of the bar code pattern from various sources of noise. Typically, digitizers use an edge detection process wherein an edge transition (black to white (bar to space) or white to black (space to bar)) between bar code elements is deemed to have been detected only if the level of the differentiated signal is above a specified or predetermined threshold. Additional criteria that may be used include amount of signal drop from its pick value or changing of direction of the differentiated signal. Such features give the digitizer a degree of noise immunity, that is, reducing the possibility that edge detection was triggered by noise rather than the bar code element edge transition. [0006] The edge detection process of the digitizer also requires that the edge polarities have to alternate. Edge polarity tells whether the edge marks a transition from space to a bar (positive-going edge or positive edge) or a transition from bar to a space (negative-going edge or negative edge). By requiring alternating edges, the edge detection process ensures that the resulting DBP signal represent a sequence of bar code elements that are properly ordered as: bar-space-bar-space-bar-space, etc. [0007] Alternating polarity edge detection is suitable when the analog bar code signal from the photodiode is not noisy. However, noise and the convolution effect of the laser beam may cause a distortion of the photodiode analog signal such that two or more consecutive edges of the signal may have the same polarity. Empirical evidence suggests that when the signal-to-noise ratio (SNR) of the analog signal drops below a certain value, the probability of such a situation increases significantly and at SNR <=8 dB consecutive edges having the same polarity becomes very likely. [0008] FIG. 1 illustrates the problem with alternating edge polarity edge detection. FIG. 1 is a plot of analog voltage output of photodiode circuitry (including voltage control circuitry and a differentiator) versus time. Typically, the analog voltage output 30 is the first derivative of the photodiode current. A negative polarity edge labeled A is below a predetermined edge detection threshold -T. (The edge detection threshold -T may be a static value or a dynamic value which changes based on various scanning parameters.) Thus, a digitizer utilizing alternating edge polarity will toggle the DBP line low at the location of point A, signaling the beginning of a space of the bar code pattern. The next edge B is above a positive edge threshold +T and would cause the digitizer to toggle the DBP line high. However, edge B is caused by noise and, in fact, edge C should be the proper transition point between bar code elements E1 and E2 rather than point B. However, since the transition of the DBP signal has already occurred at edge B, it can not be reversed at edge C since edges B and C are of the same polarity. The digitizer will toggle the DBP line low next at edge D, an edge with negative polarity marking the end of the bar representing element E2. [0009] What this means is that the decoder receiving the DBP signal as an input will calculate a width of both successive bar code elements E1, E2 improperly. The DBP signal output by the digitizer will result in the decoder calculating the width of bar code element E1 as corresponding to the elapsed time (where elapsed time is the surrogate of distance or element width) between edges A and B, when element E1 should correctly have a width corresponding to the elapsed time between edges A and C. Thus, the calculated width of element E1 will be too short. Similarly, the DBP signal output by the digitizer will result in the decoder calculating the width of bar code element E2 as corresponding to the elapsed time between edges B and D, when element E2 should correctly have a width corresponding to the elapsed time between edges C and D. Thus, the calculated width of element E1 will be too long. If the width error of element E1 exceeds the narrowest element width for the bar code, the decoder will incorrectly read the bar width of point B to point D as including one extra bar code element width. Another similar case is marked as points X, Y, Z. In both situations, the digitizer error leads to an error in bar code element width. This has disastrous consequences for the scanner decoder. In case of symbologies, which use all element combinations, like UPC such situation leads to character misclassification. That results in a failure to decode, however if more than one such error occurs for a single symbol, then that may result in symbol misdecode. The danger of misdecode is increased, if a symbol is decoded using fragments of a bar code coming from separate scans, like it is in case of block decoding or even more often in the case of half block stitching. [0010] What is needed is a digitizer which mitigates DBP distortion resulting from receiving an analog signal having two successive edges of the same polarity. SUMMARY OF THE INVENTION [0011] The present invention is directed to a laser scanner for reading a bar code having a plurality of bar code elements. The laser scanner includes a digitizer which receives as an input an analog signal from a photodetector circuitry and digitizes the analog signal to produce a digital bar code pattern (DBP) signal representative of the bar code. The DBP signal is input to a decoder which reads and decodes the DBP signal thereby decoding the bar code. The digitizer imposes a short duration correction impulse on the DBP signal whenever the digitizer determines successive edges of the same polarity wherein both of the edges are above a threshold level that would cause the DBP signal output to switch and further wherein the second edge is of greater magnitude than the first edge. When a correction impulse is received by the decoder, the decoder appropriately corrects to DBP to ignore the first edge. This mitigates errors in decoding a bar code under high noise conditions. [0012] The laser scanner of the present invention includes: [0013] a) a laser generating a beam scanned over a target bar code; [0014] b) photodetector circuitry receiving light reflected from the target bar code and generating a time-varying analog signal representative of the target bar code; [0015] c) a digitizer coupled to the photodetector circuitry and receiving as an input the analog voltage signal of the photodetector circuitry and generating: [0016] 1) a digital bar code pattern signal toggling between a first state and a second state, the digital bar code pattern signal being switched to the first state when a positive polarity edge of the analog signal is sensed having a magnitude exceeding a predetermined threshold and switching to the second state when a negative polarity edge of the analog signal is sensed having a magnitude exceeding a predetermined threshold; and [0017] 2) a short duration correction impulse generated when a second edge is sensed wherein the second edge has the same polarity as an immediately preceding first edge and further wherein a magnitude of the second edge is greater than a magnitude of the first edge, the impulse signal being imposed on the digital bar code pattern signal and having a state opposite of a present state of digital bar code pattern signal; and [0018] d) a decoder coupled to the digitizer, receiving the digital bar code pattern signal, determining widths of successive bar code elements of the target bar code, and decoding the target bar code, a width of a bar code element corresponding to a duration of an interval when the digital bar code pattern signal remains in a given one of the first and second states, except that when a short duration impulse is sensed on the digital bar code pattern signal, a state of an interval immediately prior to the impulse is interpreted by the decoder to be an opposite state of the state of the interval and a duration of the interval is added to a duration of an interval immediately preceding the interval. [0019] The correction impulse has a duration shorter than a duration of a bar code element having narrowest width of the plurality of bar code elements. The digitizer of the present invention increases digitizer immunity to high noise and improves decoding rates for noisy bar code signals and reduces misdecodes. [0020] These and other objects, advantages, and features of the exemplary embodiment of the invention are described in detail in conjunction with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS Continue reading about Laser scanner having analog digitizer with increased noise immunity... Full patent description for Laser scanner having analog digitizer with increased noise immunity Brief Patent Description - Full Patent Description - Patent Application Claims Click on the above for other options relating to this Laser scanner having analog digitizer with increased noise immunity patent application. ###  How KEYWORD MONITOR works... a FREE service from FreshPatents1. Sign up (takes 30 seconds). 2. Fill in the keywords to be monitored. 3. Each week you receive an email with patent applications related to your keywords. Start now! - Receive info on patent apps like Laser scanner having analog digitizer with increased noise immunity or other areas of interest. ### Previous Patent Application: Bar code signal processing Next Patent Application: Housing for hand held scanner Industry Class: Registers ### FreshPatents.com Support Thank you for viewing the Laser scanner having analog digitizer with increased noise immunity patent info. 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