Xref: utzoo sci.electronics:17816 sci.physics:16833 Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!sdd.hp.com!uakari.primate.wisc.edu!zaphod.mps.ohio-state.edu!casbah.acns.nwu.edu!ucsd!nosc!manta!north From: north@manta.NOSC.MIL (Mark H. North) Newsgroups: sci.electronics,sci.physics Subject: Re: A question about the Nyquist theorm Message-ID: <1758@manta.NOSC.MIL> Date: 17 Feb 91 22:40:33 GMT References: <1751@manta.NOSC.MIL> Organization: Naval Ocean Systems Center, San Diego Lines: 24 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. >The Nyquist theorem is at least, not greater than. Oppenheim & Schafer, >"Digital Signal Processing", Prentice-Hall, 1975, pg. 28 bottom. >In reality, of course, since ideal filters are unavailable for >band-limiting, the rate must be higher. I knew that 8^). Actually, I got to thinking about it since this discussion came up and I believe I emailed someone mentioning the above possibility but this would be as they say a trivial (and useless) case wouldn't you agree? Mark