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From: greenba@gambia.crd.ge.com (ben a green)
Newsgroups: sci.electronics
Subject: Re: A question about the Nyquist theorm
Message-ID: <GREENBA.91Feb21095446@gambia.crd.ge.com>
Date: 21 Feb 91 14:54:46 GMT
References: <6607@healey> <883@idacrd.UUCP> <4402@eastapps.East.Sun.COM>
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In-reply-to: gsteckel@vergil.East.Sun.COM's message of 20 Feb 91 23:56:43 GMT

In article <4402@eastapps.East.Sun.COM> gsteckel@vergil.East.Sun.COM (Geoff Steckel - Sun BOS Hardware CONTRACTOR) writes:

   In article <883@idacrd.UUCP> mac@idacrd.UUCP (Robert McGwier) writes:
    From article <6607@healey>, by grayt@Software.Mitel.COM (Tom Gray):
    > The shape of the wave is preserved within the sampling pulse. This
    > information allows representation of a signal at exactly 1/2
    > the Nyquist freqency.

    I pose the following question.  Suppose you are sampling at rate N samples
    per second, and you see a constant value V for your A/D sample.  Is the
    frequency of the signal which produced those samples 0 or N/2?  Since

   AARRGGGHHH!  The Nyquist criterion requires that sampling be GREATER
   THAN the highest frequency of interest.  Note also that the amplitude
   response near Fs/2 rolls off towards 0  (sin X / X response).

Furthermore, the claim that "the shape of the wave is preserved within the
smpling pulse" would imply that an analytic signal could be reproduced
entirely from ONE sample, since all the derivatives of the signal are
available in the sample. Can't believe Nyquist said that. 
--
Ben A. Green, Jr.              
greenba@crd.ge.com
  Speaking only for myself, of course.


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