Xref: utzoo sci.electronics:17817 sci.physics:16834 Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!wuarchive!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: <1759@manta.NOSC.MIL> Date: 17 Feb 91 22:53:25 GMT References: <1751@manta.NOSC.MIL> <1758@manta.NOSC.MIL> Organization: Naval Ocean Systems Center, San Diego Lines: 34 In article <1758@manta.NOSC.MIL> north@manta.NOSC.MIL (Mark H. North) writes: >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 > 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