Laser scanner having analog digitizer with increased noise immunity

Abstract

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 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 successive edges of the same polarity are sensed wherein both of the edges are above a threshold level and further wherein the second edge is of greater magnitude than the first edge. When a correction impulse is received on the DBP signal by the decoder, the decoder corrects the DBP signal by removing the first edge and toggling on the second edge.

Description

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. T...

AI Technical Summary

Benefits of technology

[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.

[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.

Problems solved by technology

Noise makes the digitization process problematic.

Noise can include optical noise such as ambient light, paper grain or speckle noise, printing defects.

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.

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.

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