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From: jon@boulder.UUCP (Jon Corbet)
Newsgroups: net.audio
Subject: Re: CD reflections
Message-ID: <264@boulder.UUCP>
Date: Sat, 12-Jan-85 15:43:10 EST
Article-I.D.: boulder.264
Posted: Sat Jan 12 15:43:10 1985
Date-Received: Tue, 15-Jan-85 00:53:19 EST
Organization: National Center for Atmospheric Research
Lines: 24

[Finally! a use for my EE degree!]

>From: rjn@hpfcmp.UUCP (rjn)
>HIGH END FIDELITY - If we assume (and perhaps we shouldn't)  that all we
>need to capture are complex signals composed entirely of symetrical sine
>waves whose  highest  overtone is 20KHz, a (2x)  digitizing  rate in the
>vicinity  of 40KHz just won't do.  For  example,  suppose we  digitize a
>pure  20KHz   signal  at  40KHz,   and  happen  to   capture   only  the
>zero-crossings.  How  much  information  does  that  get us?  We need at
>least 3x  digitizing  (60 KHz) to  reconstruct  a pure sine  wave, and I
>suspect that actual music demands that we use at least 4x (80 KHz).

	Wrong.  In an ideal system, one needs to sample at no more than twice
the highest frequency component.  This is known as the "Nyquist criterion."
In reality, one needs to go a little faster, since we have not invented the
perfect low pass filter yet...this is why CD's use something closer to 44K.
This is theoretically enough to reconstruct PERFECTLY any signal that does
not have components greater than 22KHz; by limiting themselves to 20KHz,
the CD makers are actually giving themselves some slop.

--
Jonathan Corbet
National Center for Atmospheric Research, Field Observing Facility
{seismo|hplabs}!hao!boulder!jon