[X, FS, NBITS]=wavread('Spring');

N= length(X);

sound (X, FS) ;

W=2\N*[0:N-1]; Digital angular frequency of continuous spectrum

FY=fft(X, N);

figure

subplot(211); plot(X); title('Original music signal waveform')

subplot(212); plot(W, abs(FY)); title('Original music signal spectrum')

Subsampling

D=12; j=0;

for i=1: D: length(X /20),

j=j 1;

X1(j)=X(i);

end

sound( X1, FS/12);

N1=1024;

W1=2\N1*[0:N1-1];

F1Y=fft( X1, N1);

figure

subplot(211); plot(X1); title('Signal waveform after subsampling')

subplot(212 ); plot(W1, abs(F1Y)); title('Signal spectrum after subsampling')

AM modulation

n=0: N-1;

x=cos(n*pi*0.8); modulated signal

Nt=length(x);

FtY=fft(x, Nt);

Wt=2/Nt*[0:Nt-1];

y=X.*x'; Modulate the signal

N2=length(y);

F2Y=fft(y, N2);

W2=2/N2*[0: N2-1];

sound(y, FS) ;

figure

plot(Wt, abs(FtY));

grid on

title ('Spectrum of modulated signal cos Figure');

figure

subplot(2, 1, 1); plot(W, abs(FY));

grid on

title ('Spectrum of the original music signal')

subplot(2, 1, 2); plot(W2, abs(F2Y));

grid on

title ('Modulated Signal Spectrum')

I am studying this recently, hope it helps