Asked by Emerson De Souza
on 18 Jun 2011

Hi,

I would like to extract the values of frequency and amplitude

from a fft and save them in separated tables.

For example, for the wave y = A1*sin(2*pi*w1*t) + A2*sin(2*pi*w2*t);

I would have the tables:

1) F, with the values w1 and w2 and another table

2) A, with the values A1 and A2.

I run the script below to ONLY DISPLAY the frequencies and amplitudes

but I don't know how to extract and save.

I wonder if someone have experience on how to write some lines to perform

this action.

Thank you in advance

Emerson

SCRIPT TO DISPLAY F AND A:

clear all;

close all;

%Wave properties

w1=5; % Frequency1 (Hz)

A1=1; % Amplitude1 (dimensionless)

w2=1; % Frequency2 (Hz)

A2=5; % Amplitude2 (dimensionless)

% Time properties

Datapoints = 1000; % Number of recorded data;

Length=10; % Length (seconds);

Step= Length/Datapoints; % Fraction of seconds

t = (0:Datapoints-1)*Step; % Time vector

% Sum of a w1-Hz sinusoid and a w2-Hz sinusoid

y = A1*sin(2*pi*w1*t) + A2*sin(2*pi*w2*t);

% Fourier Transformation

NFFT = Datapoints; % Next power of 2 from length of y

Y = fft(y,NFFT)/Datapoints;

fs=Datapoints/Length;

f = fs/2*linspace(0,1,NFFT/2+1);

% Plot single-sided amplitude spectrum.

plot(f,2*abs(Y(1:NFFT/2+1)))

axis([0 6 0 6])

title('Single-Sided Amplitude Spectrum of y(t)')

xlabel('Frequency (Hz)')

ylabel('Amplitude - |Y(f)|')

Answer by Paulo Silva
on 18 Jun 2011

Accepted answer

You can use the function findpeaks of the Signal Processing Toolbox™ to find the two peaks and their frequency, if you have that toolbox read the documentation about findpeaks.

Other way of finding the frequencies after you find the f value

[B,IX] = sort(2*abs(Y(1:NFFT/2+1))); %order the amplitudes

A1=B(end); %amplitude of the first peak A2=B(end-1); %amplitude of second peak f1=f(IX(end)); %frequency of first peak f2=f(IX(end-1)); %frequency of second peak

Now to save them in tables please show us one example of saved data.

Maybe this way:

AmpTab=[A1 A2]; FreTab=[f1 f2];

Emerson De Souza
on 18 Jun 2011

Thank you Paulo Silva,

I added your suggested 5 lines at the end of the code

and made a table by collecting the individual values together as

follow:

Frequencies=[f1 f2];

Amplitudes=[AA1 AA2];

The next question is:

How do we generalize your lines to collect multiple frequencies

and amplitudes without limiting to this particular case of two f and A?

Thank you in advance for your help

Emerson

Paulo Silva
on 18 Jun 2011

BFloor=0.1; %BFloor is the minimum amplitude value (ignore small values)

Amplitudes=B(B>=BFloor) %find all amplitudes above the BFloor

Frequencies=f(IX(1+end-numel(Amplitudes):end)) %frequency of the peaks

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