Graph is Blank ? why?

Question

Alia Hicks 2020-8-13

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/578985-graph-is-blank-why

评论： Walter Roberson 2020-8-13

The graph for Magnitude vs Frequency is blank. Why is that? The graph of time vs ich isn't blank. y_fft is a 4001x1 column with each entry being Nan. I do not understand why

load AdaptiveFiltereData
d = AdaptiveFilteredData;
fs =100;
dt = .001;
t = 0:dt:4.0001;
%ich = AdaptiveFilteredData.ichannel;
%qch = AdaptiveFilteredData.qchannel;
%plot(ich,qch);
%%PLOTS
%plot(t,ich);
%plot(t,qch);
figure;
plot(t,ich)
xlabel('time(s)');
ylabel('Amplitude');
grid on
Nsamps = length(ich);                  % Window length
y_fft = abs(fft(ich));                 %Retain Magnitude
%f = fs(1/(dt*n)*(0:n));
f = fs*(0:Nsamps/2-1)/Nsamps;           %Prepare freq data for plot
figure;
plot(f, y_fft(1:length(f))); 
%Magnitude vs Freplot(f, y_fft);                         % FFT plot
xlim([0 1]);
xlabel('Frequency(Hz)');
ylabel('FFT Magnitude'); 
grid on

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hosein Javan 2020-8-13

can you share the image of plot(t,ich) or its data file?

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Answer 1

Walter Roberson 2020-8-13

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此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/578985-graph-is-blank-why#answer_479094

"The graph of time vs ich isn't blank"

At least one entry in ich is nan or infinite, and that is leading to the fft being all nan. fft can only process finite points.

Another possibility is that the individual points are all finite but that they are large enough that the fft calculations are overflowing. For example if the values exceed sqrt(realmax) then multiplication could create infinity

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hosein Javan 2020-8-13

编辑：hosein Javan 2020-8-13

Walter Roberson thank you for replying. I've seen it before. I wonder how someone has managed to do something with matlab with its limitation of realmax.if it says it is not symbolic. is it using strings to compute these large data? or is it below realmax but can show more digits? another thing. what about speed? is it as slow as symbolic toolbox? because however matlabbuiltin functions are written in C language.

Walter Roberson 2020-8-13

If you are talking about John's contributions then the answer is that he defined a class and operations on the class. The class uses blocks of numbers as decimal digits, and standard techniques for extended precision calculations.

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