topic Re: Which FFT to use? in LabVIEW
https://forums.ni.com/t5/LabVIEW/Which-FFT-to-use/m-p/3273171#M955829
<BLOCKQUOTE><HR /><LI-USER uid="32263"></LI-USER> wrote:<BR /><P>Bob,</P><P> </P><P>Try to keep reminding yourself that without students, you would be without a job.</P><P> </P><P>Lynn</P><HR /></BLOCKQUOTE><P>Do you know for how many years I have been <U>begging</U> the Engineering School to allow me to give a class on LabVIEW, or Topics in Data Acquisition and Analysis? It would cost them $0.00, as my salary is covered by another Department. Instead, they <U>require</U> Matlab. I have managed (twice) to give a single lecture to the Senior Design Class during their <U>final</U> semester (when they're all busy trying to finish their required Project).</P><P> </P><P>So I'm trying another trick -- I put my business card in the box with the 2015 Distribution USB Key, saying "Call me if you need help with installation". So far, I've gotten a few calls ...</P><P> </P><P>BS</P>Mon, 28 Mar 2016 00:24:10 GMTBob_Schor2016-03-28T00:24:10ZWhich FFT to use?
https://forums.ni.com/t5/LabVIEW/Which-FFT-to-use/m-p/3273088#M955781
<P>Hi, I know there are several ways to perform fourier transform in LabVIEW but I do not know which to use correctly.</P><P> </P><P>I have a signal that is in array and I want that signal to be analyzed using fourier transform.</P><P> </P><P>I've tried the <STRONG>Spectral Measurement Express VI</STRONG>, <STRONG>Power Spectrum.vi</STRONG>, <STRONG>FFT Power Spectrum and PSD.vi</STRONG>,<STRONG> FFT Spectrum (Mag-Phase).vi</STRONG>, and <STRONG>FFT.vi</STRONG> (don't know the correct FFT size to use here, I just used 128).</P><P> </P><P> </P><P>Which should I use?</P><P> </P><P> </P><P>Thanks!</P>Sun, 27 Mar 2016 03:07:24 GMThttps://forums.ni.com/t5/LabVIEW/Which-FFT-to-use/m-p/3273088#M955781JersonJose2016-03-27T03:07:24ZRe: Which FFT to use?
https://forums.ni.com/t5/LabVIEW/Which-FFT-to-use/m-p/3273090#M955783
<P>If you have a 1D array of DBL, the plain FFT is probably easiest.</P><P>How many points are in your array? Just leave the "size" disconnected!</P><P> </P><P>Typically it is more important what you want to know about the sgnal, not what function to use.</P><P>Are you interested in the complex transform? the magnitude? Something else?</P><P> </P><P>A plain FFT just operates on the data, which is assumed to be equally spaced in time and the transform will be equally spaced in frequency.. The "df" of the transform is determined by the "dt" of the original signal.</P>Sun, 27 Mar 2016 04:20:05 GMThttps://forums.ni.com/t5/LabVIEW/Which-FFT-to-use/m-p/3273090#M955783altenbach2016-03-27T04:20:05ZRe: Which FFT to use?
https://forums.ni.com/t5/LabVIEW/Which-FFT-to-use/m-p/3273092#M955785
<P>Okay I'll just use the plain FFT and leave the "size" untouched. I'll probably look at the real part of the transform. I am going to use the "Complex To Re/Im".</P><P> </P><P>Thank you! <span class="lia-unicode-emoji" title=":slightly_smiling_face:">ðŸ™‚</span></P>Sun, 27 Mar 2016 04:53:05 GMThttps://forums.ni.com/t5/LabVIEW/Which-FFT-to-use/m-p/3273092#M955785JersonJose2016-03-27T04:53:05ZRe: Which FFT to use?
https://forums.ni.com/t5/LabVIEW/Which-FFT-to-use/m-p/3273093#M955786
<BLOCKQUOTE><HR /><LI-USER uid="335421"></LI-USER> wrote:<BR /><P>I am going to use the "Complex To Re/Im".</P><P> </P><HR /></BLOCKQUOTE><P>To do what?</P>Sun, 27 Mar 2016 05:00:10 GMThttps://forums.ni.com/t5/LabVIEW/Which-FFT-to-use/m-p/3273093#M955786altenbach2016-03-27T05:00:10ZRe: Which FFT to use?
https://forums.ni.com/t5/LabVIEW/Which-FFT-to-use/m-p/3273094#M955787
<P>So I can just look at real portion of the FFT result. Then get the highest magnitude from that portion.</P>Sun, 27 Mar 2016 05:04:53 GMThttps://forums.ni.com/t5/LabVIEW/Which-FFT-to-use/m-p/3273094#M955787JersonJose2016-03-27T05:04:53ZRe: Which FFT to use?
https://forums.ni.com/t5/LabVIEW/Which-FFT-to-use/m-p/3273095#M955788
<P>The real portion does not give you the highest magnitude in general. It could all be in the imaginary part or anywhere in-between.</P><P>To get the magnitude, take the eabsolute value of the complex FFT result instead.</P><P> </P><P>Are you also interested in the phase?</P>Sun, 27 Mar 2016 05:11:33 GMThttps://forums.ni.com/t5/LabVIEW/Which-FFT-to-use/m-p/3273095#M955788altenbach2016-03-27T05:11:33ZRe: Which FFT to use?
https://forums.ni.com/t5/LabVIEW/Which-FFT-to-use/m-p/3273098#M955790
<P>Oh I see didn't know that. I'll do what you said. Thank you.</P><P> </P><P>No. I am not interested in the phase. Actually, what I am really trying to get is the frequency with the highest magnitude. Any idea how I can get that?</P>Sun, 27 Mar 2016 05:31:47 GMThttps://forums.ni.com/t5/LabVIEW/Which-FFT-to-use/m-p/3273098#M955790JersonJose2016-03-27T05:31:47ZRe: Which FFT to use?
https://forums.ni.com/t5/LabVIEW/Which-FFT-to-use/m-p/3273138#M955807
<P>You should probably spend some time to make sure you understand the mathematics behind spectral analysis. Without that understanding it may be very easy to use one or several of the FFT-based tools to get completely meaninless or misleading results.</P><P> </P><P>Since it appears that you are not very aware of the mathematics, it might be better to convert your array of data to a waveform (both Y and dt) and then use FFT Spectrum (Mag-Phase).vi. One of its outputs is the magnitude cluster which includes the array of magnitudes by frequency bin, df (the frequency resolution0 and f0 (the initial frequency). It takes care of all the things you would need to know how to do yourself. The phase information is also available if you should need it in the future. You can ignore inputs like window and averaging. The defaults will be fine to get you started.</P><P> </P><P>Lynn</P>Sun, 27 Mar 2016 13:54:00 GMThttps://forums.ni.com/t5/LabVIEW/Which-FFT-to-use/m-p/3273138#M955807johnsold2016-03-27T13:54:00ZRe: Which FFT to use?
https://forums.ni.com/t5/LabVIEW/Which-FFT-to-use/m-p/3273147#M955812
<BLOCKQUOTE><HR /><LI-USER uid="32263"></LI-USER> wrote:<BR /><P>You should probably spend some time to make sure you understand the mathematics behind spectral analysis. Without that understanding it may be very easy to use one or several of the FFT-based tools to get completely meaninless or misleading results.</P><P> </P><P>Since it appears that you are not very aware of the mathematics ...</P><HR /></BLOCKQUOTE><P>And therein lies the real problem. How many times have we seen questions on the Forum about fairly complex topics that require some understanding of What is Going On (either Signal Theory, Image Analysis techniques, how to connect the wires from a piece of equipment to an A/D converter to get meaningful data) and find neophytes who are seduced by "how Easy it is to use LabVIEW, with all its Express VIs to do the Thinking For You, powerful Spectral Techniques I don't need to understand, etc." and come here to have us explain the underlying principles. Sigh. </P><P> </P><P>Bob (Curmudgeon) Schor</P>Sun, 27 Mar 2016 15:06:18 GMThttps://forums.ni.com/t5/LabVIEW/Which-FFT-to-use/m-p/3273147#M955812Bob_Schor2016-03-27T15:06:18ZRe: Which FFT to use?
https://forums.ni.com/t5/LabVIEW/Which-FFT-to-use/m-p/3273149#M955814
<P>Bob,</P><P> </P><P>Try to keep reminding yourself that without students, you would be without a job. Interesting that all of the responses so far have been from people with extensive experience in academia.</P><P> </P><P>JersonJose,</P><P> </P><P>After looking at your latest post, it occurs to me that possibly Extract Single Tone Information.vi might work for you also. To use that you need to create a waveform from your data as I described above.</P><P> </P><P>Lynn</P>Sun, 27 Mar 2016 15:20:24 GMThttps://forums.ni.com/t5/LabVIEW/Which-FFT-to-use/m-p/3273149#M955814johnsold2016-03-27T15:20:24ZRe: Which FFT to use?
https://forums.ni.com/t5/LabVIEW/Which-FFT-to-use/m-p/3273171#M955829
<BLOCKQUOTE><HR /><LI-USER uid="32263"></LI-USER> wrote:<BR /><P>Bob,</P><P> </P><P>Try to keep reminding yourself that without students, you would be without a job.</P><P> </P><P>Lynn</P><HR /></BLOCKQUOTE><P>Do you know for how many years I have been <U>begging</U> the Engineering School to allow me to give a class on LabVIEW, or Topics in Data Acquisition and Analysis? It would cost them $0.00, as my salary is covered by another Department. Instead, they <U>require</U> Matlab. I have managed (twice) to give a single lecture to the Senior Design Class during their <U>final</U> semester (when they're all busy trying to finish their required Project).</P><P> </P><P>So I'm trying another trick -- I put my business card in the box with the 2015 Distribution USB Key, saying "Call me if you need help with installation". So far, I've gotten a few calls ...</P><P> </P><P>BS</P>Mon, 28 Mar 2016 00:24:10 GMThttps://forums.ni.com/t5/LabVIEW/Which-FFT-to-use/m-p/3273171#M955829Bob_Schor2016-03-28T00:24:10ZRe: Which FFT to use?
https://forums.ni.com/t5/LabVIEW/Which-FFT-to-use/m-p/3273174#M955831
<P>Bob,</P><P> </P><P>Is the Computer Engineering Department only teaching Fortran? </P><P> </P><P>I firmly believe that an engineering education should emphasize the fundamentals. First, because they are necessary, and second, because they do not change. When it comes to tools, such as programming languages, some standardazation is useful but it is far more important for students to learn that they must learn how to learn and to learn that they must continue to learn throughout their careers. What I learned about Algol in school has been of no use. What I learned about how to program (in the same class) I have used on many different programming languages, including LabVIEW.</P><P> </P><P>Keep trying.</P><P> </P><P>Lynn</P>Mon, 28 Mar 2016 00:38:12 GMThttps://forums.ni.com/t5/LabVIEW/Which-FFT-to-use/m-p/3273174#M955831johnsold2016-03-28T00:38:12ZRe: Which FFT to use?
https://forums.ni.com/t5/LabVIEW/Which-FFT-to-use/m-p/3273202#M955839
<P>Hey Bob,</P><P> </P><P>sorry if that's how you feel about me asking a question here. I just want you to know that LabVIEW is not being taught here (not even the basics of LV). So in order for me to know if I'm doing it right or wrong is to ask question and I think the best place in asking question is here. Regardless if my question is basic or complex I'd rather have it checked (by someone who really knows it) if it is correct or not. I'm just glad that someone is willing to answer my question (basic or complex) I hope I made my point. Thank you.</P><P> </P><P>Hey John (Lynn),</P><P> </P><P>thank you for the response and for the suggestion. I used the Extract Single Tone Information.vi and did the conversion from array to a waveform like you said. It provides me the frequency and the equivalent amplitude. Thank you very much. <span class="lia-unicode-emoji" title=":slightly_smiling_face:">ðŸ™‚</span></P>Mon, 28 Mar 2016 05:12:39 GMThttps://forums.ni.com/t5/LabVIEW/Which-FFT-to-use/m-p/3273202#M955839JersonJose2016-03-28T05:12:39Z