hello,please could anyone tell me how to graph magnitude and phase spectra for given a .mat file (fs=8 kHz)

    clc
    clear all
    close all
    load a_sound.mat a
    x=a;
    y = fft(x);
    m = abs(y);
    p = angle(y);
    f = (0:length(y)-1)*50/length(y);
    subplot(2,1,1)
    plot(f,m)
    title('Magnitude')
    subplot(2,1,2)
    plot(f,rad2deg(p))
    title('Phase')

[Rena Berman]

(Answers Dev) Restored Question.

Answer 1

[Star Strider]

You have a few errors that are easily corrected. I suspect you were getting a ‘Vectors must be the same length’ error in your plot call.

Try this:

y = fft(x)/length(x);
m = abs(y);
p = angle(y);
f = linspace(0, 1, fix(length(y)/2)+1)*Fs/2                 % ‘Fs’ Is Your Sampling Frequency
Iv = 1:length(f);                                           % Index Vector

subplot(2,1,1)
plot(f,m(Iv)*2)
title('Magnitude')
grid
subplot(2,1,2)
plot(f,rad2deg(p(Iv)))
title('Phase')
grid

I don’t know what ‘sound.mat’ is, but remember that sound files are typically (Nx2), so bear in mind that there could be two columns, both of which would be processed together in your code. You may have to separate them in your code in that situation.

Please review the documentation for the fft (link) function, especially the code between the first (top) two plot figures.

[Star Strider]

Your original code with my necessary changes should work.

Have you run my code?

The complete code is:

load a_sound.mat a
x=a;
y = fft(x);
m = abs(y);
p = angle(y);
y = fft(x)/length(x);
m = abs(y);
p = angle(y);
f = linspace(0, 1, fix(length(y)/2)+1)*Fs/2                 % ‘Fs’ Is Your Sampling Frequency
Iv = 1:length(f);                                           % Index Vector

subplot(2,1,1)
plot(f,m(Iv)*2)
title('Magnitude')
subplot(2,1,2)
plot(f,rad2deg(p(Iv)))
title('Phase')

No changes are necessary with respect to the size of ‘a’.

Your code with my changes will work.

[Star Strider]

This works for me:

D = load('mehmet baki a_sound.mat');
x = D.a;
Fs = 8E+3;
y = fft(x);
m = abs(y);
p = angle(y);
y = fft(x)/length(x);
m = abs(y);
p = angle(y);
f = linspace(0, 1, fix(length(y)/2)+1)*Fs/2;                % ‘Fs’ Is Your Sampling Frequency
Iv = 1:length(f);                                           % Index Vector

subplot(2,1,1)
plot(f,m(Iv)*2)
title('Magnitude')
subplot(2,1,2)
plot(f,rad2deg(p(Iv)))
title('Phase')

Absolutely straightforward, with my slight changes to your code.

What is the problem?

[Star Strider]

You do not need to change anything with my code.

If my Answer solves your problem, please Accept it!

[Star Strider]

My pleasure!