Is it possible to find the frequency specture from a signal imported from data points (excel)

Hi

I measured a signal with a lot of disturbance. The data was exported to excel and i had imported the data in Matlab and made a plot. Now i want to create a frequency plot. (where i can see the power of each frequency, to see which frequencies are wrong in the signal. I tried to use fft and ifft but with no results.

Do you have suggestions?

Thanks in advance

Lucas

Answer 1

[Mathieu NOE]

hello

for your help see my little demo below for audio wav file analysis. Just adapt the load to your data file format

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% signal
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

% [signal,Fs]=wavread('audio_in_noise1.wav');  %(older matlab)
% or 
[data,Fs]=audioread('audio_in_noise1.wav'); %(newer matlab)

channel = 1;

signal = data(:,channel);

samples = length(signal);
dt = 1/Fs;

t = (0:dt:(samples-1)*dt);

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% FFT parameters
samples = length(signal);
NFFT = 512*4;    % 
NOVERLAP = round(0.75*NFFT);
w = hanning(NFFT);

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% display 1 : averaged FFT spectrum
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

[sensor_spectrum, freq] = pwelch(signal,w,NOVERLAP,NFFT,Fs);

figure(1),semilogx(freq,20*log10(sensor_spectrum));grid
title(['Averaged FFT Spectrum  / Fs = ' num2str(Fs) ' Hz / Delta f = ' num2str(freq(2)-freq(1)) ' Hz ']);
xlabel('Frequency (Hz)');ylabel('Amplitude (dB)');

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% display 2 : time / frequency analysis : spectrogram demo
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

saturation_dB = 80;  % dB range scale (means , the lowest displayed level is 80 dB below the max level)
[sg,fsg,tsg] = specgram(signal,NFFT,Fs,w,NOVERLAP);  

% FFT normalisation and conversion amplitude from linear to dB peak
sg_dBpeak = 20*log10(abs(sg))+20*log10(2/length(fsg));     % NB : X=fft(x.*hanning(N))*4/N; % hanning only

% saturation to given dB range
mini_dB = round(max(max(sg_dBpeak))) - saturation_dB;
sg_dBpeak(sg_dBpeak<mini_dB) = mini_dB;

% plots spectrogram
figure(2);
imagesc(tsg,fsg,sg_dBpeak);axis('xy');colorbar('vert');grid
title(['Spectrogram  / Fs = ' num2str(Fs) ' Hz / Delta f = ' num2str(fsg(2)-fsg(1)) ' Hz ']);
xlabel('Time (s)');ylabel('Frequency (Hz)');

[Cavalle]

Thanks a lot!

I wondered if there is a function simular like pwelch to create the FFT Spectrum graph (Display 1) (out of interest).

Greetings

Lucas Cornette

[Mathieu NOE]

hello

sure, you could make your own code based on fft, but as pwelch is already available....

have fun !