ÐÓ°ÉÔ­´´

Q: There is a sequentially updated bar code appearing on the front
cover of copies of New ÐÓ°ÉÔ­´´. I have tried without success to decipher
these. How does the coding system (or any bar-coding system) work and how
do bar code readers work?

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A: Bar codes consist of patterns of light and dark areas. When a beam
of light is shone at the bar code, the light areas reflect the light back
to the reader while the dark areas absorb the light. Most bar code readers
consist of a low-powered laser as the light source, and a sensor to detect
when the light is reflected back. When the laser beam is passed across a
code, the sensor detects the presence of light spaces because they reflect
well, and black bars because they reflect badly. By recording the pattern
of reflections, the reader can construct an image of the bar code and tries
to match the pattern with one it recognises. Although there seems to be
a variety of different bar code readers the only difference is the way they
move the beam about. Neil Attard Wokingham, Berkshire

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A: The bar coding of New ÐÓ°ÉÔ­´´ and other publications forms part
of the EAN (international article numbering) system that is used to identify
and bar code items that appear in open trade. The bar code simply represents
the number shown below in a machine readable form. The then European article
numbering system was founded in 1976, and incorporated the numbering system
for periodicals already established worldwide, the ISSN (international standard
serial numbering) system. The EAN system prefixes all ISSN numbers (less
their eighth digit – the ISSN check digit) with the digits 977, adds two
digits normally used to indicate changes in price and a final EAN check
digit. This 13 digit number identifies the publication uniquely, but not
the week in which it appears. The two-digit supplementary code identifies
each particular issue. John Pearce Article Number Association(UK) London

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A: The reason why you can’t decipher bar codes is that the encoding
is not as you might expect. All of the digits are encoded as four stripes,
two white and two black in alternating sequence, so there is no correlation
between digit value and stripe width. All digits are encoded within seven
units of width, although the widths of the black and white bits vary. Furthermore,
bar-code scanners have to know which order the digits are in, so they have
to know which way up the bar code is. To do this, the code is split. Bars
on the left half of the code have an odd number of black elements (odd parity)
and bars on the right half have an even number (even parity). The longer
bars which project down into the human-readable part of the bar code are
guard bars. These allow scanning equipment to work out where the code starts
and finishes and where the middle is. D. R. Wilkins Camberley, Surrey

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A: I work part-time on a checkout and so spend several hours a week
contemplating bar codes. The bars of a bar code are a digital version of
the numbers underneath. If the sensor fails to ‘read’ the bar code, I type
in the number. The number matches with one in the store computer database.
This provides the price and a description of the product, which my till
computer prints onto the receipt. The main computer logs the sale and re-orders
the product. Rachel Coleman Wiltshire