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

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?

Answer 1

[Star Strider]

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

[nevin]

This was helpful. Thank you sir!

[Star Strider]

My pleasure!

[Stephen23]

Nice answer.