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From: prk@charm.UUCP (Paul Kolodner)
Newsgroups: net.audio
Subject: Lost bits in digital recording
Message-ID: <670@charm.UUCP>
Date: Sun, 2-Jun-85 10:24:08 EDT
Article-I.D.: charm.670
Posted: Sun Jun  2 10:24:08 1985
Date-Received: Tue, 4-Jun-85 06:04:53 EDT
Organization: Physics Research @ AT&T Bell Labs Murray Hill NJ
Lines: 30

A recent posting asked if it's not possible for bits to be
lost when digital recordings are copied.  This question
brought several thoughts to my mind:
1.  Good digital systems have noise levels which are much
smaller than the least significant bit.  That's why you
can, in principle, copy over and over without ANY degradation.
My bet is that the signals in digital recording satisfy this
condition by a large margin.
2.  Computer memories reserve a significant fraction of their
space for error-correction bits.  They suffer from having
bits shot out at random by ionizing radiation, chiefly due to
cosmic rays and radiactive impurities in IC packages.  With
the right error-correcting code, however, the mean time between
uncorrectable error can be made much longer than the obsolescence
time of the machine.  Scientific American published a nice little
article about this within the last two years.
3.  About digital recordings:  Even though CD-player blurbs talk about
"error-correction", I'll bet there is none -  there's no time
for it.  The real issue is, if there is an incorrect bit now and then,
could you hear it ?  NO!  A single glitch in a digital record
leads to white noise of very low amplitude, as I have verified by
toying with artificial data sets simulating my experimental
data.  You can't hear it.  Think of this another way.  Suppose
Beethoven is playing along, and then one bit gets out of place.
This causes a click of duration 20 microsec.  How sensitive
would you be to that?  Not very sensitive.  These glitches 
would have to be quite frequent to be annoying.  They're not -
we're talking about rare events anyway, or else digital recording
would never get off the ground to begin with (see point #1 above).
Stop worrying and get some sleep.