Registers – Coded record sensors – Particular sensor structure
Reexamination Certificate
2001-02-27
2004-12-28
Frech, Karl D. (Department: 2876)
Registers
Coded record sensors
Particular sensor structure
C235S462160, C235S462080
Reexamination Certificate
active
06834806
ABSTRACT:
BACKGROUND
I. Field of the Invention
The present invention relates to a method for locating the transitions between the elements of a bar code.
II. Prior Art and Other Considerations
In this description and the following claims, the expression “bar code” encompasses also “stacked” codes, that is, with more stacked bar sequences.
As known, a bar code is an optical code, printed or otherwise marked on a support, for example a label, a package, etc., consisting of a sequence of dark, typically black, rectangular elements called bars, separated by light, typically white rectangular elements called spaces. The sequence of elements is bounded at both sides by much larger zones, called “quite zones”.
The desired information is coded in the number of elements present and/or in the width of each of them. More in particular, a system for coding information through bar codes provides for an alphabet whose symbols are bars and spaces having different widths, typically multiple of a fundamental width called the code “module”.
The operations needed to read the “word”—that is, the set of symbols of said alphabet—represented by a specific bar code essentially comprise the code illumination, the detection of the light diffused by it through a suitable photodetector, its conversion into an electrical signal, the processing of the electrical signal and its intepretation, or decoding.
The light source can comprise LEDs or lasers, whereas the photodetecting element can comprise a single photodiode, a linear or matrix CCD (Charge Coupled Device) or C-MOS (Complementary Metal Oxide Semiconductor). Moreover, the bar code can be read both by fixed and portable readers. Furthermore, part of the operations for processing the electrical signal and for interpreting and decoding it can be carried out at a remote station.
In all of the above mentioned types of readers, at the output of the photodetector an analog electrical signal is obtained, referred to as “analog video signal” in the following description, which reproduces the reflectance modulations of the elements forming the code along a scan line, or “scan”, of the same code. In fact, the light with which the code is illuminated is absorbed by the bars and reflected by the spaces. Thus, the analog video signal has an alternate pattern with peaks at the spaces, valleys at the bars and rising and falling edges at the transitions between bars and spaces, and vice versa.
In the prior art, the subsequent processing for decoding the bar code can occur directly on the analog video signal, or prior to said subsequent processing, the analog video signal can be sampled, thus obtaining a “sampled video signal” which, of course, maintains said features. Typically, the processing on the analog video signal is carried out with hardware components, while processing on the sampled video signal is carried out via software.
In the following description and attached claims, the expression “sampled signal” encompasses also its software representation.
Moreover, the not further qualified expression “video signal” is used to indicate an indifferently analog or sampled video signal, whereas—where important—a video signal shall be further qualified.
The ideal alternate pattern of the video signal is however subject to several non-ideal factors.
A first non-ideal factor is due to the fact that the smaller the elements, that is to say, the higher the code resolution, the less marked the respective peaks or valleys of the signal.
An analogous non-ideal factor is present in low contrast codes, that is to say, in which the difference of luminosity between bars and spaces is small, for example due to the nature of the support on which the code is reproduced. The measure of a code contrast is expressed by its “PCS” (Printed Contrast Signal), defined as (Vw−Vb)/Vw, where Vw is the white peak voltage and Vb is the black peak voltage.
Another non-ideal factor is represented by the noise, of electronic nature, due to the reader components, and of ambient nature, due for example to fluctuations in the code illumination.
During the code recognition, said non-ideal factors of the video signal along the scan line may bring to “skipping” one or more narrow elements adjacent wider elements or vice versa, to recognising as elements respectively peaks or valleys that instead are due to noise.
Moreover, the acquisition signal may reflect stains, abrasions, slavering or other damages present in some points of the support bearing the code. To prevent the errors caused thereby, a plurality of scans along different code lines is usually carried out during the reading of a bar code.
Another non-ideal factor may be represented by the uneven envelope, typically bell-wise, of the video signal along the scan line, such that at the edges of the code the peaks or the valleys, respectively, have a lower intensity than at the central portion of the code. An uneven envelope may be caused, for example, by the different illumination of the various code portions, in particular in readers where the entire code width is illuminated simultaneously, typically through a series of LEDs, and/or by the losses at the edges of the photodetector, in particular in the above-cited linear sensors.
In the processing and decoding of the video signal, it is essential to be able to establish with great precision the width of the bar code elements.
Among the known methods for evaluating the width of the bar code elements, some are based on locating the transitions between the elements of the code, from which the width of the same elements is then obtained.
However, said known methods for locating the transitions between the bar code elements, implemented both through hardware and software, are quite complex and such as to require the use of many components, or the use of large-capacity memories, of multipliers and microprocessors having a high computational capability, such as DPSs (Digital Signal Processors).
The technical problem at the basis of the present invention is that of allowing locating the transitions between the elements of a bar code in a quick and reliable manner also in readers having limited resources for processing the video signal.
SUMMARY
In a first aspect thereof, the invention relates to a method for locating the transitions between the elements of a bar code, comprising the steps of:
a) providing a video signal representative of the intensity of light diffused by the code as a function of the position along a scan line of the code,
b) carrying out at least one low-pass filtering of the video signal (S) for obtaining a respective filtered signal (Fa, Fb, Fc),
c) carrying out at least one comparison between a first comparison signal (A) selected between the video signal (S) and a first filtered signal (Fa, Fb), and a second comparison signal (B), being a filtered signal (Fb, Fc), the second comparison signal (B) being more strongly filtered than the first comparison signal (A), recognising the crossings between the compared signals as transitions between elements of the code at the respective positions.
In this description, the expression “more strongly filtered” is used to indicate the signal filtered with a lower cut-off frequency, and in this meaning the video signal is regarded as filtered with an infinite cut-off frequency.
In fact, the Applicant has recognised that the low-pass filtering has the effect of reducing the peak-to-valley dynamics of the video signal—in a way that is inversely related to the filtering cut-off frequency—still maintaining its mean value.
At the same time, the low-pass filtering advantageously has the effect of suppressing high-frequency noise.
Therefore, the video signal and a filtered version thereof, as well as two versions thereof filtered with different cut-off frequencies, thus cross each other exactly in high slope zones around the mean value, that is to say, at the rising and falling edges present in the video signal—which as said accurately represents the waveform of the luminous intensity—at the transitions between bars and spaces.
Locating the transitions is t
Datalogic S.p.A.
Frech Karl D.
Nixon & Vanderhye P.C.
Walsh Daniel
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