Extract signal values from a data file

Question

Hussein Kokash 2022-12-19

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1881407-extract-signal-values-from-a-data-file

评论： Star Strider 2022-12-23

data_30.txt

Hello all,

I am trying to extract some basic signal values from a data file that I have which is a plot of a sinosodial wave vs length:

data = dlmread('data_30.txt');

z = data(:,1);

y = data(:,2);

figure(1)

plot(z,y,'b')

xlabel('Spanwise length')

ylabel('Velocity')

I want to identify and match the following:

% Fs=800;

% Tf=2;

% t=0:1/Fs:Tf;

% f=[40 75];

% Amp=[4.5 9.22];

% sigma=1.33;

% y=Amp(1)*exp(j*2*pi*t*f(1))+Amp(2)*exp(j*2*pi*t*f(2));

% N=(sigma/sqrt(2))*(randn(size(t))+j*randn(size(t)));

% y=y+N;

% fy=FFT(y,Fs);

The above code is part of the code:

PSD (Power Spectral Density), and Amplitude Spectrum with adjusted FFT

Version 1.3.0.0 (63.8 KB) by Youssef Khmou

I know that the length in my case is z, Can you guide me on how to identify the above values (Fs, Tf, f, Amp, N, etc..)?

The data file is attacehd, I appreciate your input.

Thanks!

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Answer 1

Star Strider 2022-12-19

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链接

此回答的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/1881407-extract-signal-values-from-a-data-file#answer_1131747

在 MATLAB Online 中打开

I am not certain what ‘identify and match’ means. It is certainly possible to filter the 40 and 75 Hz frequencies individually, and their both defining a bandwidth, with a bandpass filter (introduced in R2018a, so you should have it) —

data = readmatrix('https://www.mathworks.com/matlabcentral/answers/uploaded_files/1236212/data_30.txt');

z = data(:,1);

y = data(:,2);

Fs = 1/mean(diff(z))

Fs = 500

f=[40 75];

figure(1)

plot(z,y,'b')

xlabel('Spanwise length')

ylabel('Velocity')

Hz40 = bandpass(y, [-1 1]+f(1), Fs, 'ImpulseResponse','iir');

Hz75 = bandpass(y, [-1 1]+f(2), Fs, 'ImpulseResponse','iir');

Hz4075 = bandpass(y, f, Fs, 'ImpulseResponse','iir');

figure

subplot(3,1,1)

plot(z, Hz40)

grid

ylabel('Amplitude')

title('40 Hz')

subplot(3,1,2)

plot(z, Hz75)

grid

ylabel('Amplitude')

title('75 Hz')

subplot(3,1,3)

plot(z, Hz4075)

grid

xlabel('z')

ylabel('Amplitude')

title('40-75 Hz')

Experiment to get different results.

.

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Star Strider 2022-12-19

在 MATLAB Online 中打开

My pleasure!

I generally use the pwelch function to calculate the power spectral density (PSD). I have no idea what the code snippet you quoted does, so I have no idea how to incorporate it into my code or this code. If you want to filter it before computing the PSD, that is certainly an option.

You can also get the appropriate values by indexing into them in the output of pwelch. I do that here using ismembertol to get the indices and then present the results in two tables, the first with the absolute values and the second with decibels —

data = readmatrix('https://www.mathworks.com/matlabcentral/answers/uploaded_files/1236212/data_30.txt');

z = data(:,1);

y = data(:,2);

Fs = 1/mean(diff(z))

Fs = 500

figure

pwelch(y)

[pxx,f,pxxc] = pwelch(y,[],[],[],Fs,'ConfidenceLevel',0.95);

fidx = ismembertol(f, [40 75], 0.01);

freqs = f(fidx);

PSDs = pxx(fidx);

PSDci = pxxc(fidx,:);

PSDTable = table(freqs,PSDs,PSDci)

PSDTable = 4×3 table

freqs PSDs PSDci ______ __________ ________________________ 39.062 1.2498e-10 6.9326e-11 2.895e-10 41.016 1.3974e-10 7.7511e-11 3.2368e-10 74.219 2.7447e-11 1.5224e-11 6.3575e-11 76.172 1.9644e-11 1.0896e-11 4.55e-11

PSDdBTable = table(freqs,pow2db(PSDs),pow2db(PSDci),'VariableNames',{'freqs','PSDs (dB)','PSDci (dB)'})

PSDdBTable = 4×3 table

freqs PSDs (dB) PSDci (dB) ______ _________ __________________ 39.062 -99.031 -101.59 -95.384 41.016 -98.547 -101.11 -94.899 74.219 -105.62 -108.17 -101.97 76.172 -107.07 -109.63 -103.42

figure

plot(f, pow2db(pxx), 'DisplayName','PSD')

hold on

plot(f, pow2db(pxxc), '--r', 'DisplayName','95% CI')

hold off

grid

xlabel('Frequency (Hz)')

ylabel('PSD (dB/Hz)')

legend('Location','best')

Experiment to get different results.

.

Hussein Kokash 2022-12-23

Hello , sorry for the trouble,

I have used this code but extended it so that it covers a 2D plane of coordinates (x, y) with their velocities (U, V)

clear all; clc;%close all;

path = pwd; % put current path here

S = dir(fullfile(path,'data_*.txt'));

addpath('/media/hf9098/easystore/MATLAB/POD-umich')

R = dir(fullfile(path,'data_*.txt'));

S = natsortfiles(R);

if iscell(S) == 0;

S = (S);

end

for k = 1:numel(S)

folder = S(k).folder;

filename = S(k).name;

F = fullfile(S(k).folder,S(k).name);

data = dlmread(F);

x = data(:,1); % x coordinate

y = data(:,2); % y coordinate

U(:,k) = data(:,4); % u velocity

V(:,k) = data(:,5); % v velocity

Fs(k) = 1/mean(diff(x));

figure(1)

pwelch(V)

[pxx,f,pxxc] = pwelch(V,[],[],[],Fs,'ConfidenceLevel',0.95);

fidx = ismembertol(f, [40 75], 0.01);

freqs = f(fidx);

PSDs = pxx(fidx);

PSDci = pxxc(fidx,:);

PSDTable = table(freqs,PSDs,PSDci)

PSDdBTable = table(freqs,pow2db(PSDs),pow2db(PSDci),'VariableNames',{'freqs','PSDs (dB)','PSDci (dB)'})

figure(2)

plot(f, pow2db(pxx), 'DisplayName','PSD')

hold on

plot(f, pow2db(pxxc), '--r', 'DisplayName','95% CI')

hold off

grid

xlabel('Frequency (Hz)')

ylabel('PSD (dB/Hz)')

legend('Location','best')

end

I am getting this error:

Error using table (line 307)

'PSDs (dB)' is not a valid variable name.

Error in testpwelch (line 36)

PSDdBTable = table(freqs,pow2db(PSDs),pow2db(PSDci),'VariableNames',{'freqs','PSDs (dB)','PSDci (dB)'})

Any thoughts?

Thanks!

Star Strider 2022-12-23

As always, my pleasure!

The ability to use table variables that were not valid MATLAB variable names was introduced after R2018b. I forgot about that. Use: 'PSDs_dB' and such instead (or create your own variable names, since there is nothing particularly sacred about the ones I chose). Anything that is a valid MATLAB variable name should work.

Try this instead —

PSDdBTable = table(freqs,pow2db(PSDs),pow2db(PSDci),'VariableNames',{'freqs','PSDs_dB','PSDci_dB'})

That should run without error.

.

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