Subject:
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Re: Barcodes & error detection
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Newsgroups:
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lugnet.robotics
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Date:
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Thu, 8 Sep 2005 03:34:15 GMT
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Viewed:
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923 times
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In lugnet.robotics, Steve Baker <sjbaker1@airmail.net> wrote:
> there is a sharp distinction between error
> DETECTION and error CORRECTION.
Agreed, and, for this application, the bit overhead to allow on-board
correction is probably just not worth it. After all, if an error is detected,
the forklift can always back up and try to read the barcode again.
> It's also important to characterize the errors you are getting.
Almost exclusively single bit errors of a specific 'polarity' (to borrow
Kevin's term; a '1' is read as a '0'). In detail, I set Gus to running around an
oval reading barcode representing the numbers from 0 to 11. In more than 1470
barcodes, there were 40 errors, 39 of which were single bit events where a 1 was
read as a 0 (that last error was a "bit doubling", where a single bit was read
twice due to "wiggle" in the motion of the forklift & a barcode that had shifted
to an angle *and* the light sensor was near the edge... yes, I've spent a fair
bit of time troubleshooting ;-).
> For single bit error detection, your parity scheme
> is pretty much the best you can do.
So I feared. Error correcting codes amaze me, but everything I'd read or
studied tends to apply to long words, not the minimalist ones I'm trying to use.
[snip very interesting theoretical discussion]
> Sometimes, adding single bit error correction can
> actually worsen your overall error rate.
Very interesting! I never thought of it that way. I was starting to think in
terms of number of errors in certain words relative to others ('1'-poor words,
in my current situation, are more robust), but I certainly didn't take that
thought as far as you have.
--
Brian Davis
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Message is in Reply To:
 | | Re: Barcodes & error detection
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| (...) Well, there is a sharp distinction between error DETECTION and error CORRECTION. It's also important to characterize the errors you are getting. Are you getting single bit errors or multi-bit errors? You will certainly need to use more bits in (...) (19 years ago, 8-Sep-05, to lugnet.robotics)
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