Xref: utzoo sci.electronics:17809 sci.physics:16828 Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!swrinde!zaphod.mps.ohio-state.edu!caen!uflorida!sphere!ruck From: ruck@sphere.UUCP (John R Ruckstuhl Jr) Newsgroups: sci.electronics,sci.physics Subject: Re: A question about the Nyquist theorm Message-ID: <319@sphere.UUCP> Date: 18 Feb 91 06:10:11 GMT References: <1751@manta.NOSC.MIL> Followup-To: sci.electronics Organization: Private; Gainesville, FL Lines: 31 In article , 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 > > why you must sample at greater than 2nu. > 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. Or 0 Hz. And supposing the signal you sampled *was* 60 Hz. You have no magnitude information. You cannot reconstruct. > The Nyquist theorem is at least, not greater than. Oppenheim & Schafer, > "Digital Signal Processing", Prentice-Hall, 1975, pg. 28 bottom. Yes. They say "at least twice the highest frequency". But the equation they give is not ambiguous: Wmax < pi/Tsample (or, 2*Wmax < Wsample) (same page). Terry, please be very careful of misinformation. Best regards, ruck. -- John R Ruckstuhl, Jr ruck%sphere@cis.ufl.edu, sphere!ruck University of Florida ruck@cis.ufl.edu, uflorida!ruck