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Re: CDs, was DATs, Was Re: arsclist Duplicating casette tapes
George Brock-Nannestad wrote:
> From: Patent Tactics, George Brock-Nannestad
> Mike Richter wrote:
> > Note that as Jerry has pointed out, BLER is not a sufficient measure
> > of quality. However, it may be adequate for purposes such as
> > identifying the faulty disc in an archived pair.
> ----- the usefulness of BLER in this context is that its increase over
> time (cleaned of inter-apparatus variability!) will indicate the
> progress of decay. If it does not increase, there is no need to
> recopy yet. Do not make a BLER determination based on just one
> run, but average e.g. three consecutive runs and repeat after a
> month. If there is no change, repeat after two months. Still no
> change, repeat after 6 months, and if you are lucky you may go
> annual. Once an increase is noted reduce the time interval again.
> For a large collection you will need a log file and staff.
> The expensive testers are able to test the media before recording,
> but the cheap (but time-consuming) BLER test needs a recording,
> the erroneous reading of which activates the error correction. The
> output signal is perfect up to the time where non-correctible errors
Although BLER is one of many useful quality indicators, it is a poor
end-of-life indicator. See http://www.mscience.com/longev.html for test
results instead of opinions. A disc may degrade to the point of read
failure without a perceptable increase in BLER.
Our FAQs at http://www.mscience.com/faq.html point out the problems of
using BLER as a sole indicator of quality. Drill one 2 mm diameter hole
in the information area of a CD, making it unreadable in most drives.
BLER will increase slightly but will remain well within spec.
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