Plotting the frequency spectrum / didnt understand the code

 X_mags = abs(fftshift(fft(signal)));
 bin_vals = [0 : N-1];
 N_2 = ceil(N/2);
 fax_Hz = (bin_vals-N_2)*fs/N;
 plot(fax_Hz, X_mags)

Answer 1

[Geoff Hayes]

Qusay - let us assume the following concerning your signal, sampling rate (Fs), and FFT block size (N)

 Fs = 4096;                    % sampling rate in Hertz
 t  = linspace(0,1-1/Fs,Fs);   % one second time vector given Fs
 N  = 4096;                    % FFT block size is same as sampling rate (true for your ex)
 signal = 2.3*sin(2*pi*t*227); % signal with amplitude of 2.3 and frequency of 227 Hertz

Now for your code

 % transforms the signal from the time domain to the frequency domain with fft
 % note that y is complex
 Y = fft(signal);

 % shifts the frequency components so that the zero frequency is at the centre
 % of spectrum
 Y = fftshift(Y);

 % determines the magnitude of the complex data
 Y = abs(Y);

The following lines of code just sets the frequencies for each bin and can be reduced to

 f = [0:N-1] * Fs/N;

 % must ensure that zero is the centre frequency due to our fftshift from above
 f = f - ceil(N/2);

 % plot the data
 plot(f,Y);

The result of the plot is

Note the frequency at 227 Hz, as expected. The symmetry is due to the input signal being real.

[qusay hawari]

Thank you, appreciated your help ^_^