Hello all. I am working on a problem for class that is taking some time data and changing it to frequency data. I am using excel and the fast fourier transform(fft). I am having a problem graphing the results. I am very new to signal processing. I am only given the data which is 1024 points long. I will only ask for help with one because if I figure this one out I can do the rest. The period is .117. Making the sampline rate 1/.117. I have the two columns of data here is an example. The first column is (1*(1/.117)/1024) and the second is the fft values. pres43 abs imag 0.006643282 162509.4667 0.013286565 388.025981 0.019929847 600.4277992 0.026573129 499.1078937 0.033216412 447.4588355 0.039859694 102.7576392 0.046502976 276.7567742 0.053146259 358.0531147 0.059789541 289.5977569 0.066432823 129.8177899 0.073076105 91.85556182 0.079719388 250.3116294 0.08636267 35.77585877 0.093005952 218.271273 0.099649235 216.1229107 0.106292517 215.5078932 0.112935799 110.6895935 0.119579082 153.2837982 0.126222364 194.9400366 0.132865646 96.83955867 My problems is that I don't know how to graph the values correctly. The graph as it stands is not righ. Someone told me that I have to remove the DC zero values? I do not know how to do that and that is why I am asking for help.
FFT graphing help
Started by ●March 26, 2008
Reply by ●March 26, 20082008-03-26
phenommsu1 wrote:> Hello all. I am working on a problem for class that is taking some time > data and changing it to frequency data. I am using excel and the fast > fourier transform(fft). > > I am having a problem graphing the results. I am very new to signal > processing. > > I am only given the data which is 1024 points long. > I will only ask for help with one because if I figure this one out I can > do the rest. > > The period is .117. Making the sampline rate 1/.117. > > I have the two columns of data here is an example. > The first column is (1*(1/.117)/1024) and the second is the fft values. >[SNIP DATA SAMPLE]> > My problems is that I don't know how to graph the values correctly. The > graph as it stands is not righ.Describe what you think is "not right" with the graph.> Someone told me that I have to remove the > DC zero values? I do not know how to do that and that is why I am asking > for help. >Due to how you phrased that sentence, I'm not sure which of two things you were being told to do. But think about what that large value in the first bin indicates. I think you're closer to the answer that you think. You just have to ask yourself the right question ;)
Reply by ●March 26, 20082008-03-26
>phenommsu1 wrote: >>[SNIP DATA SAMPLE] >> >> My problems is that I don't know how to graph the values correctly.The>> graph as it stands is not righ. > >Describe what you think is "not right" with the graph. > > >> Someone told me that I have to remove the >> DC zero values? I do not know how to do that and that is why I amasking>> for help. >> > >Due to how you phrased that sentence, I'm not sure which of two things >you were being told to do. But think about what that large value in the >first bin indicates. > >I think you're closer to the answer that you think. You just have to ask>yourself the right question ;) > > >1. Well I know the fft graph has is not correct because it containsvalues that should not be present. 2. I do not know how to remove those values. 3. I tried to subtract the avg of the entire sample from each data point before fft. Then I did the fft, but I still had the same problem. 4. I do not know what the large value in the first bin indicates. I thought the meaning was that I needed to do number 3. So I am not sure about the question I should be asking. Should I be working with the data before the fft or after the fft is applied. Thanks
Reply by ●March 26, 20082008-03-26
Reply by ●March 26, 20082008-03-26
On Mar 26, 12:15 pm, "phenommsu1" <green...@gmail.com> wrote:> Any help will be greatly appreciated. > > Thanks.Can you clarify the dimensions of the values you gave for sample rate, period, and data index? Is the data periodic? If so, around what index does it appear to start to repeat? What is your formula for "abs imag"?
Reply by ●March 27, 20082008-03-27
Let's start over again ;) [which is why I top post rather than bottom posting] *** What does the value in the first bin of a FFT (ANY FFT) represent? When plotting the FFT you say "The graph as it stands is not right." When ask ho it is not right, you reply "Well I know the fft graph has is not correct because it contains values that should not be present." *** Why shouldn't those values be present? Two background questions: 1. What is course you taking? [will help group to phrase responses to thought pattern you are used to] 2. Why are you using Excel? P.S. As it is only 1024 points, send me an email with your input data attached. Then I can ask you specific leading questions. phenommsu1 wrote:> Hello all. I am working on a problem for class that is taking some time > data and changing it to frequency data. I am using excel and the fast > fourier transform(fft). > > I am having a problem graphing the results. I am very new to signal > processing. > > I am only given the data which is 1024 points long. > I will only ask for help with one because if I figure this one out I can > do the rest. > > The period is .117. Making the sampline rate 1/.117. > > I have the two columns of data here is an example. > The first column is (1*(1/.117)/1024) and the second is the fft values. > > pres43 abs imag > 0.006643282 162509.4667 > 0.013286565 388.025981 > 0.019929847 600.4277992 > 0.026573129 499.1078937 > 0.033216412 447.4588355 > 0.039859694 102.7576392 > 0.046502976 276.7567742 > 0.053146259 358.0531147 > 0.059789541 289.5977569 > 0.066432823 129.8177899 > 0.073076105 91.85556182 > 0.079719388 250.3116294 > 0.08636267 35.77585877 > 0.093005952 218.271273 > 0.099649235 216.1229107 > 0.106292517 215.5078932 > 0.112935799 110.6895935 > 0.119579082 153.2837982 > 0.126222364 194.9400366 > 0.132865646 96.83955867 > > My problems is that I don't know how to graph the values correctly. The > graph as it stands is not righ. Someone told me that I have to remove the > DC zero values? I do not know how to do that and that is why I am asking > for help. > >
Reply by ●March 27, 20082008-03-27
The first bin is the DC component. Correct? The values should not be present because they should be zero values, because they represent gaps in the frequency reading. I am using excel because that is what we were assigned to use. It is for intro to biomedical signal processing. I dont know how to send you an attachment, the site only gives your name if for me to send and email not an attachment. But I can post a snip of the data here. 25 points. OriginalSig fft abs imag 162.5666667 162509.466666667 162509.4667 161 -140.639883835191-361.641514463957i 388.025981 159.9666667 -587.357541210861+124.597997153187i 600.4277992 160.4333333 112.744714041656+486.207074200488i 499.1078937 160 193.456405302228+403.477420379003i 447.4588355 159.2666667 32.6729605560732+97.4248945004916i 102.7576392 160.3333333 172.213820584924+216.64882199081i 276.7567742 159.5666667 357.313870260752+22.9963279562987i 358.0531147 159.8333333 105.925098369729+269.530581400113i 289.5977569 159.0666667 65.9363307465512-111.82602050226i 129.8177899 159.2666667 -54.7662602027889+73.7434809308128i 91.85556182 159.1666667 88.055966666471+234.311883025187i 250.3116294 158.0666667 30.7528486227951+18.28043686791i 35.77585877 156.9 211.105579685004+55.4687555965784i 218.271273 158.4 -204.1981590865-70.7970646786261i 216.1229107 158.3 -188.027542697669-105.305722630798i 215.5078932 158.2666667 105.5446891243+33.3542306635607i 110.6895935 158.3666667 78.3394011973472-131.753030358915i 153.2837982 157.4666667 56.547484874658-186.558301359245i 194.9400366 157.2 -84.5708197139807+47.1770768187416i 96.83955867 156.9666667 -49.5335241663969-89.6174641256261i 102.3956049 157.2666667 -83.8826534062392+30.7721614046116i 89.34889736 156.3333333 22.4687435767936+90.7586387084729i 93.49852907 156.0333333 113.448294114525-65.8047049245391i 131.1517237 157.7666667 69.5965604037754-180.19293709252i 193.166187 The first column is the original signal in the time domain from 1-25. The second column is the fft of the first. The third column is the abs value of the fft values. I feel like I am close to understanding, but I am missing something, so thank you very much for your help.>Let's start over again ;) >[which is why I top post rather than bottom posting] > >*** What does the value in the first bin of a FFT (ANY FFT) represent? > >When plotting the FFT you say "The graph as it stands is not right." >When ask ho it is not right, you reply "Well I know the fft graph has is>not correct because it contains values that should not be present." > > >*** Why shouldn't those values be present? > > >Two background questions: >1. What is course you taking? [will help group to phrase responses to >thought pattern you are used to] >2. Why are you using Excel? > > >P.S. >As it is only 1024 points, send me an email with your input data >attached. Then I can ask you specific leading questions. > > > > > >phenommsu1 wrote: >> Hello all. I am working on a problem for class that is taking sometime>> data and changing it to frequency data. I am using excel and the fast >> fourier transform(fft). >> >> I am having a problem graphing the results. I am very new to signal >> processing. >> >> I am only given the data which is 1024 points long. >> I will only ask for help with one because if I figure this one out Ican>> do the rest. >> >> The period is .117. Making the sampline rate 1/.117. >> >> I have the two columns of data here is an example. >> The first column is (1*(1/.117)/1024) and the second is the fft values.>> >> pres43 abs imag >> 0.006643282 162509.4667 >> 0.013286565 388.025981 >> 0.019929847 600.4277992 >> 0.026573129 499.1078937 >> 0.033216412 447.4588355 >> 0.039859694 102.7576392 >> 0.046502976 276.7567742 >> 0.053146259 358.0531147 >> 0.059789541 289.5977569 >> 0.066432823 129.8177899 >> 0.073076105 91.85556182 >> 0.079719388 250.3116294 >> 0.08636267 35.77585877 >> 0.093005952 218.271273 >> 0.099649235 216.1229107 >> 0.106292517 215.5078932 >> 0.112935799 110.6895935 >> 0.119579082 153.2837982 >> 0.126222364 194.9400366 >> 0.132865646 96.83955867 >> >> My problems is that I don't know how to graph the values correctly.The>> graph as it stands is not righ. Someone told me that I have to removethe>> DC zero values? I do not know how to do that and that is why I amasking>> for help. >> >> >
Reply by ●March 27, 20082008-03-27
I am going to try to post the data again without adjusting it in the text box. Maybe that will allow it to be seen better. :) PRES43 fft abs imag 162.5666667 162509.466666667 162509.4667 161 -140.639883835191-361.641514463957i 388.025981 159.9666667 -587.357541210861+124.597997153187i 600.4277992 160.4333333 112.744714041656+486.207074200488i 499.1078937 160 193.456405302228+403.477420379003i 447.4588355 159.2666667 32.6729605560732+97.4248945004916i 102.7576392 160.3333333 172.213820584924+216.64882199081i 276.7567742 159.5666667 357.313870260752+22.9963279562987i 358.0531147 159.8333333 105.925098369729+269.530581400113i 289.5977569 159.0666667 65.9363307465512-111.82602050226i 129.8177899 159.2666667 -54.7662602027889+73.7434809308128i 91.85556182 159.1666667 88.055966666471+234.311883025187i 250.3116294 158.0666667 30.7528486227951+18.28043686791i 35.77585877 156.9 211.105579685004+55.4687555965784i 218.271273 158.4 -204.1981590865-70.7970646786261i 216.1229107 158.3 -188.027542697669-105.305722630798i 215.5078932 158.2666667 105.5446891243+33.3542306635607i 110.6895935 158.3666667 78.3394011973472-131.753030358915i 153.2837982 157.4666667 56.547484874658-186.558301359245i 194.9400366 157.2 -84.5708197139807+47.1770768187416i 96.83955867 156.9666667 -49.5335241663969-89.6174641256261i 102.3956049 157.2666667 -83.8826534062392+30.7721614046116i 89.34889736 156.3333333 22.4687435767936+90.7586387084729i 93.49852907 156.0333333 113.448294114525-65.8047049245391i 131.1517237 157.7666667 69.5965604037754-180.19293709252i 193.166187
Reply by ●March 27, 20082008-03-27
phenommsu1 wrote: [but he top posted following my example which he shouldn't have ;] [I reorganized and did judicious snipping so we can be conventional and bottom post for the rest of the thread] > Richard Owlett wrote: >>Let's start over again ;) >>[which is why I top post rather than bottom posting] >> >>*** What does the value in the first bin of a FFT (ANY FFT) represent? >> >>When plotting the FFT you say "The graph as it stands is not right." >>When ask ho it is not right, you reply "Well I know the fft graph is> >>not correct because it contains values that should not be present." >> >> >>*** Why shouldn't those values be present? >> >> >>Two background questions: >>1. What is course you taking? [will help group to phrase responses to >>thought pattern you are used to] >>2. Why are you using Excel? >> >>> The first bin is the DC component. Correct?Yes. And that answer tells you what first thing you MIGHT want to do to your data. I emphasize *MIGHT* at this point because its not strictly necessary but I suspect it will give you more of what you _expect_ to see. [Raises a side question. Was your input data given to you as a problem set or were you doing a laboratory assignment?] In any case, what you should do is FFT {signal - average of signal}.> The values should not be present because they should be zero values, > because they represent gaps in the frequency reading.Actually they do "belong" there. The keyword is "windowing". Start with Section 9.1 of _The Scientist and Engineer's Guide to Digital Signal Processing_ by Steven W. Smith ( http://www.dspguide.com/ch9/1.htm ) I've got links to more references somewhere, but that will get you started in right direction.> > I am using excel because that is what we were assigned to use.Ok, that tells me something about immediate goals of the course. > It is for intro to biomedical signal processing. Now that takes me back to days before DSP existed. As an undergrad EE student I worked as a tech for Veterinary Physiology and later as Instrumentation Tech for the university's Chemistry Department. The "real world" vs "theory" ;) Is any course specific information posted on the WEB?> > I dont know how to send you an attachment, the site only gives your name > if for me to send and email not an attachment. But I can post a snip of > the data here. 25 points.I thought the attachment would be simple. For future reference "large" pieces of data should be sent as attachments. In this case send me the "OriginalSig" only in the body of an email. You've obviously picked up the idea of not posting large amounts of data on a newsgroup. [large snip - including sample of data]
Reply by ●March 27, 20082008-03-27
On Mar 27, 6:55 am, "phenommsu1" <green...@gmail.com> wrote:> The first bin is the DC component. Correct? > The values should not be present because they should be zero values, > because they represent gaps in the frequency reading. > > I am using excel because that is what we were assigned to use. It is for > intro to biomedical signal processing. > > I dont know how to send you an attachment, the site only gives your name > if for me to send and email not an attachment. But I can post a snip of > the data here. 25 points. > > OriginalSig fft abs imag > 162.5666667 162509.466666667 162509.4667 > 161 -140.639883835191-361.641514463957i 388.025981 > 159.9666667 -587.357541210861+124.597997153187i 600.4277992 > 160.4333333 112.744714041656+486.207074200488i 499.1078937 > 160 193.456405302228+403.477420379003i 447.4588355 > 159.2666667 32.6729605560732+97.4248945004916i 102.7576392 > 160.3333333 172.213820584924+216.64882199081i 276.7567742 > 159.5666667 357.313870260752+22.9963279562987i 358.0531147 > 159.8333333 105.925098369729+269.530581400113i 289.5977569 > 159.0666667 65.9363307465512-111.82602050226i 129.8177899 > 159.2666667 -54.7662602027889+73.7434809308128i 91.85556182 > 159.1666667 88.055966666471+234.311883025187i 250.3116294 > 158.0666667 30.7528486227951+18.28043686791i 35.77585877 > 156.9 211.105579685004+55.4687555965784i 218.271273 > 158.4 -204.1981590865-70.7970646786261i 216.1229107 > 158.3 -188.027542697669-105.305722630798i 215.5078932 > 158.2666667 105.5446891243+33.3542306635607i 110.6895935 > 158.3666667 78.3394011973472-131.753030358915i 153.2837982 > 157.4666667 56.547484874658-186.558301359245i 194.9400366 > 157.2 -84.5708197139807+47.1770768187416i 96.83955867 > 156.9666667 -49.5335241663969-89.6174641256261i 102.3956049 > 157.2666667 -83.8826534062392+30.7721614046116i 89.34889736 > 156.3333333 22.4687435767936+90.7586387084729i 93.49852907 > 156.0333333 113.448294114525-65.8047049245391i 131.1517237 > 157.7666667 69.5965604037754-180.19293709252i 193.166187 > > The first column is the original signal in the time domain from 1-25. The > second column is the fft of the first. The third column is the abs value of > the fft values. I feel like I am close to understanding, but I am missing > something, so thank you very much for your help.162509.466667 / 1024 = 158.700651 , which looks like it could be the average value of the first column. Have you tried subtracting that DC value from every sample in the first column? The values in the gaps will not be zero unless the data in your first column is composed of only of sinusoids which are exactly periodic in the fft aperture (1024 samples) (and there are less sinusoids than half the number of samples so as to leave some gaps). IMHO. YMMV. -- rhn A.T nicholson d.0.t C-o-M
