How do i get the y value for a given x value?

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nevin 2014-10-3

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评论： Stephen23 2014-10-6

I have plotted a frequency spectrum with the command "spect = fft(recording)" and "plot (abs(spect))" Is there a way that I can get matlab to give me the value of Y when the value of X is say 200Hz?

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Answer 1

Star Strider 2014-10-3

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You have to create a frequency vector as well to plot and interpret the fft correctly as a function of frequency, and for that you need the sampling frequency, (Fs) of ‘recording’. Then use interp1 to get the value at any specific frequency.

For example:

Fs = 1000;                                  % Sampling Frequency
Ts = 1/Fs;
Tv = linspace(0,10*pi,505);
recording = sin(290*Tv).*cos(310*Tv)+rand(size(Tv));
spect = fft(recording);
specta = abs(spect);
Fn = Fs/2;                                  % Nyquist Frequency
Fv = linspace(0, 1, length(spect)/2+1)*Fn;
Iv = 1:length(Fv);                          % Index vector
a200 = interp1(Fv, specta(Iv), 200);        % Find Amplitude At 200 Hz
figure(1)
plot(Fv, specta(Iv), 200,a200,'pr')         % Plot fft &  Amplitude at 200 Hz
grid