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From: hsu@cvl.UUCP (Dave Hsu)
Newsgroups: net.audio
Subject: Re: Noise due to digital errors, error correction, etc
Message-ID: <519@cvl.UUCP>
Date: Wed, 5-Jun-85 23:46:42 EDT
Article-I.D.: cvl.519
Posted: Wed Jun  5 23:46:42 1985
Date-Received: Sun, 9-Jun-85 03:53:13 EDT
References: <3811@alice.UUCP>
Organization: Computer Vision Lab, U. of Maryland, College Park
Lines: 65

> > From allegra!ulysses!mhuxr!mhuxn!charm!prk Wed Dec 31 19:00:00 1969
> > 
> > ...
> > 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.
>
> Precisely.
>
> > 3.  About digital recordings:  Even though CD-player blurbs talk about
> > "error-correction", I'll bet there is none -  there's no time
>
> You're very, very wrong.  There is significant error correction power
> in the simplest of the CD players.  In the most fancy ones, they
> use ALL of the 80% redundancy in the stored data to fix things.
>
> > 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
>
> YES!  A single MSB inverted shows up as a gargantuan, earsplitting
> !!!CLICK!!!   The fact that you never hear any CLICK's shows just
> how well the error correction works. (In fact, if things are that
> bad, it zeros thee sample, rather than CLICK!ing at you.)  In fact,
> the MSB's are a bit more protected for exactly that reason.
> ...
> > 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 
> Quite sensitive.  Sorry to be so directly contradictory,
> but your article exactly contradicts my experience.
> > would have to be quite frequent to be annoying.  They're not -
> > ...
> > 
> Granted that one hit of an LSB wouldn't make any difference,
> still, one hit of an MSB causes quite a ruckus.  If you meant
> to say that the effect after error correction was essentially
> nill, that's a different story.
> -- 
> 
> (ihnp4/allegra)!alice!jj

The bottom line is...that's not the reason ECC was ABSOLUTELY necessary.
When 'soft fails' (i.e. cosmic radiation etc. -caused errors) first came
under recent scrutiny, it was estimated that the average home computer
at the time suffered perhaps 2 soft-fails per week.  At this rate, the 
error become almost insignificant, but ECC would be desirable.

The obvious reason for ECC is that famous culprit...the long blob of scum.
Whether from a defect or from surface contamination, you will get much
more than 20 usec of noise.  In fact, since track number, running time, and
other information are evidently encoded on each track, ECC becomes absolutely
necessary or your player might go berserk trying to find where it was.  You
don't believe the effects of minor errors? There's one eluding my correction
scheme on a Chopin Etudes disk; it always bounces back to the same track.

Thank heavens (and technology) for the CD.  Goodbye to the Rice Krispies
guys hiding out in my records.

-dave