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From: stebbins@musial.ucr.edu (john stebbins)
Newsgroups: sci.electronics
Subject: Re: A question about the Nyquist theorm
Message-ID: <12122@ucrmath.ucr.edu>
Date: 19 Feb 91 18:29:22 GMT
References: <1758@manta.NOSC.MIL> <1759@manta.NOSC.MIL> <2189@umriscc.isc.umr.edu> <1772@manta.NOSC.MIL>
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Reply-To: stebbins@musial.ucr.edu (john stebbins)
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In article <1772@manta.NOSC.MIL>, north@manta.NOSC.MIL (Mark H. North) writes:
|> In article <2189@umriscc.isc.umr.edu> robf@mcs213f.cs.umr.edu (Rob
Fugina) writes:
|> >In article <1759@manta.NOSC.MIL> north@manta.NOSC.MIL (Mark H.
North) writes:
|> >>Sorry to answer my own post but I take that last paragraph back. I think
|> >>you are wrong after all. Look at it this way -- suppose I tell you I'm
|> >>going to send you one of two signals, either 1 volt 60 Hz or a DC voltage
|> >>between -1 and 1 volt. You may sample at 120 Hz. You get all identical
|> >>samples at 0.5 volts. Which signal did I send?
|> >>Mark
|> >
|> >You sent a DC signal of 0.5 volts.  If it were AC, you the samples would
|> >be alternating positive and negative of the same magnitude.
|> >
|> Yes, thanks for pointing that out. How about all zero samples? Yes, I know,
|> pretty damn likely it was the DC signal. I think I made my point, you must
|> sample at >2nu to reconstruct the signal.
|> 
|> Mark


Here's a different example coupled with a question.

Suppose I was sampleing a 20khz sign wave at 44khz and my first sample
just happened to occur at the positive peak of the sign wave.  My next
sample would occur a little before ( and thus above ) the negative peak.
And the next would occur a little more before ( and a little more below )
the next positive peak.  This continues until zero crossing at which point
my samples start growing instead of decreasing.  Its pretty easy to see why
filtering the sample back down to 20khz will reproduce a 20khz signal, but
how does the filtering recover the original amplitude of my sign wave. 
It appears that what I'll get is an am modulated signal that is some 
combination of the 20khz signal and the 44khz sample rate.

By the way, I chose 20khz and 44khz because they are a combination that
is suppose to work (ie. CD rates).

John Stebbins
stebbins@ucrmath.ucr.edu