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.comAn 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. �����������������������������������������������������������������������
Lightning and Fourier transform of an impulse
Started by ●August 5, 2008
Reply by ●August 8, 20082008-08-08
Reply by ●August 8, 20082008-08-08
Jerry Avins 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. :-) > > JerryJerry, you really should have used a coherer as you were detecting a spark gap transmission. (how many youngsters will this confuse ;)
Reply by ●August 8, 20082008-08-08
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 verify that it indeed satisfies the conditions that a periodic signal must satisfy. For all you know, the signal is going to disappear at 5 pm when someone turns off the oscillator and goes home. So let's just trash the frequency domain and forget the notion entirely. The Fourier transform of the finite-duration (i.e. one-shot) pulse rect(t) is sinc(f). What this says in terms of the Fourier integral (or inverse Fourier transform) is that if we have (uncountably many) oscillators (at all possible frequencies) with infinitesimally small amplitudes (given by sinc(f)) and all in phase (peaking at t = 0), then these complex exponential signals, lasting for all time, have the curious property that their "sum" cancels out and results in 0 at all time instants t such that |t| > 1/2, while for |t| < 1/2, these signals "add up" to 1 exactly. We accept this notion (or I like to think that many denizens of comp.dsp do) and use it all the time. The pulse rect (t) delivers energy to a load only during the interval (-1/2, 1/2). The complex oscillators are running for all t, but since each has infinitesimal amplitude, each individual oscillator delivers zero energy. To get any energy from them, we have to use an uncountable number. If we use all the oscillators, then since the "sum" disappears for |t| > 1/2, there is no energy delivered for |t| > 1/2. If we use a subset of them (say all the oscillators whose frequencies are less than 10 Hz) by passing rect(t) through a lowpass filter, then energy *will* be delivered to the filter load even after t = 1/2, and indeed even before t = -1/2 if one thinks of ideal LPFs which are noncausal filters. Just to muddy the waters even further....
Reply by ●August 8, 20082008-08-08
Jerry Avins wrote: (snip)> 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. :-)SCR controllers usually come through on AM radio, but not on FM. I always believed it was a combination of the higher frequency for FM, and the FM advantage (as you say, the limiter) but I don't know how much of each. In Seattle, we have an electric bus system fed by overhead DC power lines. When driving under the wires, especially when a bus is starting up, there is an approximately 360Hz splatter on the AM radio. It follows the wires, even if the bus isn't so close. -- glen
Reply by ●August 8, 20082008-08-08
Richard Owlett wrote: (snip)> Jerry, you really should have used a coherer as you were detecting a > spark gap transmission. (how many youngsters will this confuse ;)There was an article in Popular Electronics some years ago about the coherer. -- glen
Reply by ●August 8, 20082008-08-08
glen herrmannsfeldt wrote:> Richard Owlett wrote: > (snip) > >> Jerry, you really should have used a coherer as you were detecting a >> spark gap transmission. (how many youngsters will this confuse ;) > > There was an article in Popular Electronics some years ago > about the coherer.Coherers are not hard to make. Back when U.S. money was real, the filings from a dime and a nickel worked quite well. The filings were mixed and sealed into a glass tube with tiny corks or globs of hard wax. Wires passed through the ends without touching. The device begaved like an open circuit until RF energy made the filings cohere, establishing continuity. Flowing current maintains the continuity, but shaking the tube disrupts it. Mounting the coherer on the arm of a buzzer and wiring it in series with the buzzer and a battery sounded a buzz whenever RF energy was strong enough and provided self quenching. Jerry -- Engineering is the art of making what you want from things you can get. �����������������������������������������������������������������������
Reply by ●August 9, 20082008-08-09
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. Chris
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 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 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
