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Path: utzoo!watmath!clyde!burl!ulysses!allegra!bellcore!petrus!karn
From: karn@petrus.UUCP
Newsgroups: net.audio
Subject: Re: Lost bits in digital recording
Message-ID: <365@petrus.UUCP>
Date: Mon, 3-Jun-85 15:02:01 EDT
Article-I.D.: petrus.365
Posted: Mon Jun  3 15:02:01 1985
Date-Received: Wed, 5-Jun-85 00:12:56 EDT
References: <670@charm.UUCP>
Organization: Bell Communications Research, Inc
Lines: 47

> 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.

I believe this is incorrect. Error correction IS provided in CD players,
and it IS essential. The "raw" error rate on CDs is something like 1
bit in 10,000, although they generally occur in bursts. At the net data
rate of 1.4 megabits/sec, this would be about 140 bad bits/second.
In PCM, the audibility of a bad bit depends on its significance within the
sample. If the low order bit were bad, you would not likely hear it. But
it is entirely possible that the high order bit is corrupted, and this you
would DEFINTELY hear. Try listening to one of the test CDs with an
impulse-response test signal to see what this would sound like. I suspect
that listening to Beethoven at the proper volume would not only be
intolerable without error correction, it would probably also fry your
tweeters.

On a related topic (digital vs analog generation degradation), the advantage
of digital is not that bits cannot EVER be read in error; as somebody
pointed out, this can happen as long as there's noise in the raw analog
signal coming off the media.  The real advantage of existing systems is that
through error correction, occasional errors in raw data bits can be fully
corrected before they're re-written to the copy.  The over all effect is
that the bit-error-rate vs signal-to-noise ratio on the tape or disk can be
made even more strongly nonlinear. At moderate-to-high S/N ratios,
the probability of getting a bad bit after the error correction circuitry is
virtually nil, despite occasional errors in the "raw" data. The raw data
errors simply don't occur close enough to destroy enough redundancy to
keep the error correction circuit from doing its job.

However, at some lower value of S/N (the threshhold) the output error rate
skyrockets as the correction circuit is suddenly overwhelmed by the
number of input errors. As long as you operate with enough margin (just a few
dB is often enough), however, the copying process is virtually error-free.

Phil