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From: kr0u+@andrew.cmu.edu (Kevin William Ryan)
Newsgroups: sci.physics,sci.electronics
Subject: Re: A question about the Nyquist theorm
Message-ID: <0bk2mHK00WBN42yop7@andrew.cmu.edu>
Date: 18 Feb 91 19:35:47 GMT
References: <1751@manta.NOSC.MIL> <D04gX2w163w@shark.cs.fau.edu>,
	<1991Feb17.192147.22753@rodan.acs.syr.edu>
Organization: Biology, Carnegie Mellon, Pittsburgh, PA
Lines: 29
In-Reply-To: <1991Feb17.192147.22753@rodan.acs.syr.edu>

amichiel@rodan.acs.syr.edu (Allen J Michielsen)
>In article <D04gX2w163w@> terryb.bbs@shark.cs.fau.edu (terry bohning) writes:
>>north@manta.NOSC.MIL (Mark H. North) writes:
>>> >     i.e. If you have a 60HZ sine wave, and you sample at 120HZ, you're
>>> >only going to get two points per cycle.
>>> And imagine that those two points are phased such that they land at the
>>> zero crossing of the 60Hz signal. All your samples are zero! This is 
>>The catch is that you *know* you're sampling the highest input freq at
>>2 points per cycle.  That is, the input signal is bandlimited.  So if
>>someone gives you a set of all zero samples and you know the sample
>>rate is 120 Hz, the only frequency it can be is 60 Hz.
> 
>And what theory are you using to eliminate all other even multiples of 60
>like 120....

    Then you've violated the Nyquist criterion (shame, shame) which
states that the _maximum_ frequency must be less than one half the
sampling rate. If the signal contains frequencies above this, good luck
reconstructing it. Because you can't. You don't have the information,
and in fact your reconstructed signal will contain frequencies not in
the original - frequencies aliased in from the signal freqencies that
were greater than 1/2 the sampling rate. 

    This is usually enforced with some sort of prefiltering of the input
frequency. The bandlimiting requirement is _very_ important. 

                                                    kwr

Internet: kr0u+@andrew.cmu.edu