CD degradation

[john@tessi.com (John Zimmer)]

neilg@sfu.ca (Neil K. Guy) writes:

> If you want to see this right now, hold an older CD up to a bright
>light. You'll probably see a number of bright points of light on the
>disc. Those pinholes are apparently spots where the aluminium has
>oxidised and is thus transparent. In fact, if you look at one of the
>early CDs, which were made when oxygen contamination was more of a
>problem than now, you'll see quite a peppering of little bright
>pinholes. (for some reason these pinholes always give me the
>creeps...) So one day all our CDs may well be totally clear and thus
>unplayable.

You'll see those same pinholes on many new CD's, as well.  They are the 
(inconsequential) result of imperfections in the aluminizing process (not
oxidation), and should not change over time.  

> Good CD players with a decent tracking mechanism can
>accurately track the laser and compensate for data loss by filling in
>the gaps - extrapolating your favourite Kate number. So when you
>listen to a CD a small percentage of the sound you hear was never made
>by Ms. Bush at all, but is a kind of computer best guess. Sort of.
>However, cheaper CD players often skip and otherwise choke on badly
>pinholed CDs.

I'm not sure about missing/erroneous data being interpolated, I thought 
most systems when exceeding their ability to cope with data errors either 
muted briefly or stopped.  I do understand that most errors are of such a 
scale that the error correction system built into the CD format allows a 
player to *exactly* reconstruct the same information, as if it had been read 
straight off the disc.  Bits is bits.

All this assumes proper manufacturing, of course.  No one can save us from 
garbage.


_____________________________________________________________
John Zimmer                                    john@tessi.com

        "The happy phantom has no right to bitch."
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