Path: utzoo!attcan!uunet!snorkelwacker!usc!cs.utexas.edu!uwm.edu!alice.UUCP
From: jj@alice.UUCP (jj, like it or not)
Newsgroups: rec.audio.high-end
Subject: Re: Armor-Alling CDs
Message-ID: <2626@uwm.edu>
Date: 27 Feb 90 14:19:50 GMT
Sender: news@uwm.edu
Lines: 39
Approved: tjk@csd4.csd.uwm.edu


Tom,
	I'd like to snip this "jitter" discussion off
right now.

	The rate that the D/A convertor uses for de-sampling
is fixed not by any consideration of what the rate off the
CD is, but rather by a crystal clock that is run independantly
of the spinning CD, and to which the CD is, via a complicated
buffer-control strategy, controlled.

	Hence, the only jitter in the output samples is due
to the non-symmetry of the crystal oscillator, and these
days most of the manufacturers know well to divide the clock
by a few powers of two (which has a strong and well-understood
effect on crystal jitter) in order to eliminate the jitter
problems.  Sample/holds and D/A delays are much more significant
in general that the crystal clock problems.

	Repeating:  The rate that comes off the CD
is only marginally the same as the conversion rate
in the short term (taking into account the  14/8 expansion)
but is exactly, within 1 part/length-of-cd, the same over
the long term.  The short-term rate variations are well known,
understood, and buffered completely out of the signal before
it reaches the convertor. Period.

	Any effect (the existance of which I severely doubt)
from Armor-All, green ink, green cheese, disc dampers, or
the like MUST come from something other than digital recovery
issues.  The hypothesis about power-supply decoupling (while
indicative of bad design) is somewhat plausible, given other
instances of bad design (some digital) in early oversampling
chips.
-- 
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