What is the x-axis in fft() command?

y=audioread('speech.wav');

subplot(2,3,1);

plot(y);

xlabel('Samples');

ylabel('Magnitude');

title('Speech signal');

%Taking FFT

fft_sig=fft(y,256);

abs_val=abs(fft_sig);

subplot(2,3,2);

plot(abs_val);

When I give the command plot(abs_val), what will the x-axis of my graph represent according to my code?

Answer 1

[Star Strider]

The x-axis will be frequency, that being defined as cycles/(time unit of the original independent varialble). If that is in seconds, the frequency will be in Hz.

The best way to do that is:

[y,Fs] = audioread('speech.wav'); % Return Sampling Frequency As The Second Output

L = size(y,1);

Fn = Fs/2; % Nyquist Frequency

tv = linspace(0, L-1, L)/Fs; % Time Vector

One way to compute the frequency vector:

Fv = linspace(0, 1, fix(length(fft)/2)+1)*Fn; % One-Sided 'fft'

Iv = 1:numel(Fv); % Index Vector

The fft result to be plotted would then use ‘Fv’ and ‘Iv’ to create an equal-length fft vector.

Use the fftshift function to create a two-sided fft result, then:

Fv = linspace(-Fs/2, Fs/2-Fs/length(s), length(s)); % Two-Sided 'fft': EVEN 'length(s)' (Asymmetric)

Fv = linspace(-Fs/2, Fs/2, length(s)); % Two-Sided 'fft': ODD 'length(s)' (Symmetric)

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