On Sat, 9 Aug 2008 01:32:06 -0700 (PDT), Chris Bore <chris.bore@gmail.com> wrote:>On Aug 8, 4:24�pm, Jerry Avins <j...@ieee.org> wrote: >> Chris Bore wrote: >> > On Aug 6, 10:21 pm, Jerry Avins <j...@ieee.org> wrote: >> >> Chris Bore wrote: >> >>> But the frequency spectrum exists over all time, not just during the >> >>> impulse. >> >>> So I don't think this can be used to explain why the lightning >> >>> disrupts radio for a short time. >> >> Chris, >> >> >> The lightning isn't really an impulse. Some have durations of >> >> milliseconds, few less than tens of microseconds. That's more than >> >> enough to localize the interference in time. >> >> >> � �... >> >> >> Jerry >> >> -- >> >> Engineering is the art of making what you want from things you can get. >> >> ����������������������������������������������������������������������� >> >> > Jerry, >> >> > I agree but.. >> >> > Frequency, as defined by a Fourier Transform, is a steady state >> > frequency spectrum whose components exist at fixed amplitude over all >> > time. >> >> > So the Prof can't use the broadband nature of the frequency spectrum >> > to explain a time-limited event. He is mixing orthogonal domains. >> >> > He can say the spectrum is broadband. But he can't say the spectrum is >> > somehow broadband for a short time. >> >> > I think the Prof should have accepted that lightning interferes >> > because it is a bloody great spark, and left it at that. >> >> > Chris >> > ================ >> > Chris Bore >> > BORES Signal Processing >> >www.bores.com >> >> An impulse creates all frequencies, and they exist for all time. The >> interference caused by lightning is brief, showing that lightning isn't >> a true impulse. All well and good, but we knew that all along, didn't >> we? If you like, think of a lightning stroke as abrupt keying, producing >> lots of splatter. We've all heard the splatter from an SCR motor >> controller excited at the mains rate extending well up into the AM >> broadcast spectrum. Long ago, listening to the FM tuner I had built with >> less-than-perfect limiters, I heard a noise like !!!!!!! !!!!!!! !!!!!!! >> and ran out to the street shouting, "Dad! It's misfiring on one >> cylinder!" Quasi-impulses make splatter that is only quasi-eternal. :-) >> >> Jerry >> -- >> Engineering is the art of making what you want from things you can get. >> �����������������������������������������������������������������������- Hide quoted text - >> >> - Show quoted text - > >The frequency spectrum of ANY signal exists for all time. > >So you cant logically base a claim on the frequency spectrum, to >explain something (like interference) that only lasts a short time. > >If you work in the frequency domain, then you cannot invoke or involve >time. The domains are orthogonal. > >What the Prof did is wrong. He explained short duration (broadband) >interference based on an eternal, unchanging, frequency spectrum. > >The spectrum is always the same, so he cant claim it somehow comes and >goes. > > >ChrisI don't think you need to argue it that way to be relevant. The question as reported by the OP has to do with 'why lightning causes disruptions to TV and Radio signals?' TVs and radios have frequency-selective inputs and only sense certain bands. Sensitivity to energy outside of those bands, or even the tuned channel, is greatly attenuated. An interference source with a wide spectrum and a lot of power is more likely to disrupt receivers than an interferer of equal power with a narrow spectrum. Since lightning has a short duration in time we know it has a broad spectrum. It therefore interferes with many receivers. Q.E.D.? I don't think it's any more complicated than that, personally. Eric Jacobsen Minister of Algorithms Abineau Communications http://www.ericjacobsen.org Blog: http://www.dsprelated.com/blogs-1/hf/Eric_Jacobsen.php
Lightning and Fourier transform of an impulse
Started by ●August 5, 2008
Reply by ●August 9, 20082008-08-09
Reply by ●August 9, 20082008-08-09
On Sat, 9 Aug 2008 01:32:06 -0700 (PDT), Chris Bore <chris.bore@gmail.com> wrote:>On Aug 8, 4:24�pm, Jerry Avins <j...@ieee.org> wrote: >> Chris Bore wrote: >> > On Aug 6, 10:21 pm, Jerry Avins <j...@ieee.org> wrote: >> >> Chris Bore wrote: >> >>> But the frequency spectrum exists over all time, not just during the >> >>> impulse. >> >>> So I don't think this can be used to explain why the lightning >> >>> disrupts radio for a short time. >> >> Chris, >> >> >> The lightning isn't really an impulse. Some have durations of >> >> milliseconds, few less than tens of microseconds. That's more than >> >> enough to localize the interference in time. >> >> >> � �... >> >> >> Jerry >> >> -- >> >> Engineering is the art of making what you want from things you can get. >> >> ����������������������������������������������������������������������� >> >> > Jerry, >> >> > I agree but.. >> >> > Frequency, as defined by a Fourier Transform, is a steady state >> > frequency spectrum whose components exist at fixed amplitude over all >> > time. >> >> > So the Prof can't use the broadband nature of the frequency spectrum >> > to explain a time-limited event. He is mixing orthogonal domains. >> >> > He can say the spectrum is broadband. But he can't say the spectrum is >> > somehow broadband for a short time. >> >> > I think the Prof should have accepted that lightning interferes >> > because it is a bloody great spark, and left it at that. >> >> > Chris >> > ================ >> > Chris Bore >> > BORES Signal Processing >> >www.bores.com >> >> An impulse creates all frequencies, and they exist for all time. The >> interference caused by lightning is brief, showing that lightning isn't >> a true impulse. All well and good, but we knew that all along, didn't >> we? If you like, think of a lightning stroke as abrupt keying, producing >> lots of splatter. We've all heard the splatter from an SCR motor >> controller excited at the mains rate extending well up into the AM >> broadcast spectrum. Long ago, listening to the FM tuner I had built with >> less-than-perfect limiters, I heard a noise like !!!!!!! !!!!!!! !!!!!!! >> and ran out to the street shouting, "Dad! It's misfiring on one >> cylinder!" Quasi-impulses make splatter that is only quasi-eternal. :-) >> >> Jerry >> -- >> Engineering is the art of making what you want from things you can get. >> �����������������������������������������������������������������������- Hide quoted text - >> >> - Show quoted text - > >The frequency spectrum of ANY signal exists for all time. > >So you cant logically base a claim on the frequency spectrum, to >explain something (like interference) that only lasts a short time. > >If you work in the frequency domain, then you cannot invoke or involve >time. The domains are orthogonal. > >What the Prof did is wrong. He explained short duration (broadband) >interference based on an eternal, unchanging, frequency spectrum. > >The spectrum is always the same, so he cant claim it somehow comes and >goes. > > >ChrisI don't think you need to argue it that way to be relevant. The question as reported by the OP has to do with 'why lightning causes disruptions to TV and Radio signals?' TVs and radios have frequency-selective inputs and only sense certain bands. Sensitivity to energy outside of those bands, or even the tuned channel, is greatly attenuated. An interference source with a wide spectrum and a lot of power is more likely to disrupt receivers than an interferer of equal power with a narrow spectrum. Since lightning has a short duration in time we know it has a broad spectrum. It therefore interferes with many receivers. Q.E.D.? I don't think it's any more complicated than that, personally. Eric Jacobsen Minister of Algorithms Abineau Communications http://www.ericjacobsen.org Blog: http://www.dsprelated.com/blogs-1/hf/Eric_Jacobsen.php
Reply by ●August 11, 20082008-08-11
>Since lightning has a short duration in time we know it has a broad >spectrum. It therefore interferes with many receivers. Q.E.D.?Accepted, but... The specrum is broad for ever. So why, then, is the interference not also eternal? The answer is, that the common (and your, in this response) interpretation of 'frequency' is of a sinusoid whose amplitude can change with time. Hence, a broad spectrum can be of components whose amplitude is zero outside the 'impulse' of the lightning. But this is not how the Fourier Transform uses frequency. The amplitude of any component of a spectrum derived by Fourier Transform is fixed, constant, for all time. That is not the intuitive, and common, interpretation. My beef with the Prof's explanation is that he invoked 'spectrum' as in Fourier Transform - therefore he cannot then abuse this to introduce components whose amplitude somehow changes (to be large during, and zero outside of, the impulse). Fine, if he abandons Fourier Transforms and talks about frequency spectra in a non-Fourier way. But if he quotes Fourier then he is stuck with unchanging spectra. Chris ======================== Chris Bore BORES Signal Processing www.bores.com
Reply by ●August 11, 20082008-08-11
>Since lightning has a short duration in time we know it has a broad >spectrum. It therefore interferes with many receivers. Q.E.D.?Accepted, but... The specrum is broad for ever. So why, then, is the interference not also eternal? The answer is, that the common (and your, in this response) interpretation of 'frequency' is of a sinusoid whose amplitude can change with time. Hence, a broad spectrum can be of components whose amplitude is zero outside the 'impulse' of the lightning. But this is not how the Fourier Transform uses frequency. The amplitude of any component of a spectrum derived by Fourier Transform is fixed, constant, for all time. That is not the intuitive, and common, interpretation. My beef with the Prof's explanation is that he invoked 'spectrum' as in Fourier Transform - therefore he cannot then abuse this to introduce components whose amplitude somehow changes (to be large during, and zero outside of, the impulse). Fine, if he abandons Fourier Transforms and talks about frequency spectra in a non-Fourier way. But if he quotes Fourier then he is stuck with unchanging spectra. Chris ======================== Chris Bore BORES Signal Processing www.bores.com
Reply by ●August 11, 20082008-08-11
Chris Bore wrote:> > >Since lightning has a short duration in time we know it has a broad > >spectrum. It therefore interferes with many receivers. Q.E.D.? > > Accepted, but... > > The specrum is broad for ever. So why, then, is the interference not > also eternal?The story is the frequencies cancel out for most of eternity. If you take all the possible cosine functions and sum them they should sum coherently at t=0 and cancel at t not 0. Another way to look at it is the Fourier pair for a gaussian pulse. A very narrow pulse in one domain will produce a very wide pulse in the other. So a pulse that is vanishingly small in width in the time domain will be so wide it will be virtually flat in the frequency domain. This story of course implies that the lightening pulse does exist for all time. We perceive it as a short duration event because it just rises to a magnitude that is measurable for a very short amount of time. -jim> > The answer is, that the common (and your, in this response) > interpretation of 'frequency' is of a sinusoid whose amplitude can > change with time. Hence, a broad spectrum can be of components whose > amplitude is zero outside the 'impulse' of the lightning. But this is > not how the Fourier Transform uses frequency. The amplitude of any > component of a spectrum derived by Fourier Transform is fixed, > constant, for all time. That is not the intuitive, and common, > interpretation. My beef with the Prof's explanation is that he invoked > 'spectrum' as in Fourier Transform - therefore he cannot then abuse > this to introduce components whose amplitude somehow changes (to be > large during, and zero outside of, the impulse). > > Fine, if he abandons Fourier Transforms and talks about frequency > spectra in a non-Fourier way. But if he quotes Fourier then he is > stuck with unchanging spectra. > > Chris > ======================== > Chris Bore > BORES Signal Processing > www.bores.com----== Posted via Pronews.Com - Unlimited-Unrestricted-Secure Usenet News==---- http://www.pronews.com The #1 Newsgroup Service in the World! >100,000 Newsgroups ---= - Total Privacy via Encryption =---
Reply by ●August 11, 20082008-08-11
Chris Bore wrote:> > >Since lightning has a short duration in time we know it has a broad > >spectrum. It therefore interferes with many receivers. Q.E.D.? > > Accepted, but... > > The specrum is broad for ever. So why, then, is the interference not > also eternal?The story is the frequencies cancel out for most of eternity. If you take all the possible cosine functions and sum them they should sum coherently at t=0 and cancel at t not 0. Another way to look at it is the Fourier pair for a gaussian pulse. A very narrow pulse in one domain will produce a very wide pulse in the other. So a pulse that is vanishingly small in width in the time domain will be so wide it will be virtually flat in the frequency domain. This story of course implies that the lightening pulse does exist for all time. We perceive it as a short duration event because it just rises to a magnitude that is measurable for a very short amount of time. -jim> > The answer is, that the common (and your, in this response) > interpretation of 'frequency' is of a sinusoid whose amplitude can > change with time. Hence, a broad spectrum can be of components whose > amplitude is zero outside the 'impulse' of the lightning. But this is > not how the Fourier Transform uses frequency. The amplitude of any > component of a spectrum derived by Fourier Transform is fixed, > constant, for all time. That is not the intuitive, and common, > interpretation. My beef with the Prof's explanation is that he invoked > 'spectrum' as in Fourier Transform - therefore he cannot then abuse > this to introduce components whose amplitude somehow changes (to be > large during, and zero outside of, the impulse). > > Fine, if he abandons Fourier Transforms and talks about frequency > spectra in a non-Fourier way. But if he quotes Fourier then he is > stuck with unchanging spectra. > > Chris > ======================== > Chris Bore > BORES Signal Processing > www.bores.com----== Posted via Pronews.Com - Unlimited-Unrestricted-Secure Usenet News==---- http://www.pronews.com The #1 Newsgroup Service in the World! >100,000 Newsgroups ---= - Total Privacy via Encryption =---
Reply by ●August 11, 20082008-08-11
Chris Bore wrote:> > >Since lightning has a short duration in time we know it has a broad > >spectrum. It therefore interferes with many receivers. Q.E.D.? > > Accepted, but... > > The specrum is broad for ever. So why, then, is the interference not > also eternal?The story is the frequencies cancel out for most of eternity. If you take all the possible cosine functions and sum them they should sum coherently at t=0 and cancel at t not 0. Another way to look at it is the Fourier pair for a gaussian pulse. A very narrow pulse in one domain will produce a very wide pulse in the other. So a pulse that is vanishingly small in width in the time domain will be so wide it will be virtually flat in the frequency domain. This story of course implies that the lightening pulse does exist for all time. We perceive it as a short duration event because it just rises to a magnitude that is measurable for a very short amount of time. -jim> > The answer is, that the common (and your, in this response) > interpretation of 'frequency' is of a sinusoid whose amplitude can > change with time. Hence, a broad spectrum can be of components whose > amplitude is zero outside the 'impulse' of the lightning. But this is > not how the Fourier Transform uses frequency. The amplitude of any > component of a spectrum derived by Fourier Transform is fixed, > constant, for all time. That is not the intuitive, and common, > interpretation. My beef with the Prof's explanation is that he invoked > 'spectrum' as in Fourier Transform - therefore he cannot then abuse > this to introduce components whose amplitude somehow changes (to be > large during, and zero outside of, the impulse). > > Fine, if he abandons Fourier Transforms and talks about frequency > spectra in a non-Fourier way. But if he quotes Fourier then he is > stuck with unchanging spectra. > > Chris > ======================== > Chris Bore > BORES Signal Processing > www.bores.com----== Posted via Pronews.Com - Unlimited-Unrestricted-Secure Usenet News==---- http://www.pronews.com The #1 Newsgroup Service in the World! >100,000 Newsgroups ---= - Total Privacy via Encryption =---
Reply by ●August 11, 20082008-08-11
On Aug 8, 1:08 pm, "dvsarw...@yahoo.com" <dvsarw...@gmail.com> wrote:> On Aug 7, 11:44 am, cs_post...@hotmail.com wrote > (among other things) that > > > Or to put it bluntly, the frequency domain is practically speaking a > > bad way to represent one-shot or random (non-periodic) events. > > ALL real-life signals are one-shot and nonperiodic. NO human being > has observed an alleged periodic signal, say sin(wt), for ALL t to > verifyI think you ignored the presence of "practically speaking" in the passage you quoted ;-) You might also note that I was saying the frequency domain is a bad way to localize one-shot events. It's a somewhat less bad way to represent things that appear periodic on the order of timescales of interest.> The Fourier transform of the finite-duration (i.e. one-shot) pulse > rect(t) is sinc(f).Yes, this is theoretically true, but it's still a lousy way to represent, in practical terms, when an impulse or a rectangular pulse occurred. Using the pure frequency domain is largely impractical for representing the time-varying component of interesting non-periodic signals. What is practically possible is to use a series in time of snapshots of the frequency domain. More often, when we look at time domain events in the frequency domain, we are not trying to localize them in time, but to see the frequency domain cost of having an abrupt transition - ie, the bandwidth.
Reply by ●August 11, 20082008-08-11
On Aug 9, 4:32 am, Chris Bore <chris.b...@gmail.com> wrote:> If you work in the frequency domain, then you cannot invoke or involve > time. The domains are orthogonal.The time and frequency domains theoretically represent the same information. Practically, humanly speaking, some things are a lot easier to visualize or describe in one rather than the other. So you may be looking at the frequency domain, and wish to consider what the impact of 'something' is. Human experience may make it easier to describe the 'something' in terms of the time domain. So we do that, and then we ask what this things that we are very familiar with in the time domain looks like in the frequency domain.
Reply by ●August 11, 20082008-08-11
jim wrote:> > Chris Bore wrote: >>> Since lightning has a short duration in time we know it has a broad >>> spectrum. It therefore interferes with many receivers. Q.E.D.? >> Accepted, but... >> >> The specrum is broad for ever. So why, then, is the interference not >> also eternal? > > The story is the frequencies cancel out for most of eternity. If you > take all the possible cosine functions and sum them they should sum > coherently at t=0 and cancel at t not 0. > Another way to look at it is the Fourier pair for a gaussian pulse. > A very narrow pulse in one domain will produce a very wide pulse in > the other. So a pulse that is vanishingly small in width in the time > domain will be so wide it will be virtually flat in the frequency > domain. This story of course implies that the lightening pulse does > exist for all time. We perceive it as a short duration event because > it just rises to a magnitude that is measurable for a very short > amount of time.Chris has a valid objection. The sum of the eternal frequencies present is indeed zero at times well removed from the generating impulse. However, it is not obvious that a narrow-band subset of them is also self canceling. Nevertheless, if the band is wide enough, the cancellation is in fact largely complete. Consider a variable-bandwidth filter excited by an impulse. As the bandwidth decreases, the cancellation becomes poorer, and the duration of the filtered output increases. When the bandwidth becomes very narrow -- the Q is made very high -- the output can persist for a long time. We say that the filter "rings". It actually does ring in the time domain, but in frequency, it is merely selective. As usual, two sides of the same coin.>> The answer is, that the common (and your, in this response) >> interpretation of 'frequency' is of a sinusoid whose amplitude can >> change with time. Hence, a broad spectrum can be of components whose >> amplitude is zero outside the 'impulse' of the lightning. But this is >> not how the Fourier Transform uses frequency. The amplitude of any >> component of a spectrum derived by Fourier Transform is fixed, >> constant, for all time. That is not the intuitive, and common, >> interpretation. My beef with the Prof's explanation is that he invoked >> 'spectrum' as in Fourier Transform - therefore he cannot then abuse >> this to introduce components whose amplitude somehow changes (to be >> large during, and zero outside of, the impulse). >> >> Fine, if he abandons Fourier Transforms and talks about frequency >> spectra in a non-Fourier way. But if he quotes Fourier then he is >> stuck with unchanging spectra.I agree that the professor was too glib and oblivious of the subtleties. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
