## How to demodulate a sound through MATLAB

### CHUL SHIN (view profile)

on 22 Sep 2016
Latest activity Answered by Zhao Wang

### Zhao Wang (view profile)

on 27 Sep 2016
DSB-SC Demodulation in MATLAB
Hi.
I have to record a sound which was already modulated in a certain carrier frequency (from 4000Hz to 16000Hz), and demodulate the recorded sound with MATLAB so that I can play the demodulated sound, which is close to the original sound. And these procedures have to run in MATLAB.
So far, I made a code which can load the sound data and plot the data in fft(fast fourier transform). Here is my code. --------------------------------------------
N=length(data);
F1=0:fs/N:fs-fs/N;
FFT_data=fft(data);
figure;
plot(F1(1:N),abs(FFT_data(1:N)));
xlabel('Frequency')
ylabel('Amplitude')
axis([0 2000 0 1000])
grid on
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But I can't really develope it into the DSB-SC demodulation code.
Can anyone help me with this?
And this is the demodulation code which I've found in a website.
I hope this code to be helpful.
--------------------------------------------
fc=154000;
fm=fc/10;
fs=100*fc;
t=0:1/fs:4/fm;
xc=cos(2*pi*fc*t);
xm=cos(2*pi*fm*t);
figure(1)
subplot(2,1,1),plot(t,xc);
title('carrier signal of 154 khz');
xlabel('time (sec)');
ylabel('amplitude');
subplot(2,1,2),plot(t,xm);
title('message signal of 15.4 khz');
xlabel('time (sec)');
ylabel('amplitude');
% DSB-SC MODULATION
z1= xm.*xc;
figure(2)
subplot(2,1,1),plot(t,z1);
title('DSB-SC MODULATION IN TIME DOMAIN');
xlabel('time (sec)');
ylabel('amplitude');
l1=length(z1);
f=linspace(-fs/2,fs/2,l1);
Z1=fftshift(fft(z1,l1)/l1);
subplot(2,1,2),plot(f,abs(Z1));
title('DSB SC MODULATION IN FREQUENCY DOMAIN');
xlabel('frequency(hz)');
ylabel('amplitude');
axis([-200000 200000 0 0.3]);
s1=z1.*xc;
S1=fftshift(fft(s1,length(s1))/length(s1));
figure(3)
plot(f,abs(S1));
title(' demodulated signal IN FREQUENCY DOMAIN before filtering');
xlabel('frequency(hz)');
ylabel('amplitude');
axis([-200000 200000 0 0.3]);
hold on
Hlp=1./sqrt(1+(f./fc).^(2*100));
plot(f,Hlp,'g');
title(' frequency response of low pass filter');
xlabel('frequency(hz)');
ylabel('amplitude');
axis([-200000 200000 0 2]);
E1=Hlp.*S1;
figure(4)
subplot(2,1,1),plot(f,E1);
title(' Recover signal IN FREQUENCY DOMAIN after filtring');
xlabel('frequency(hz)');
ylabel('amplitude');
axis([-200000 200000 0 0.3]);
e1=ifft(ifftshift(E1))*length(E1);
subplot(2,1,2),plot(t,(1/0.5)*e1);
title(' Recover signal IN Time DOMAIN after filtring');
xlabel('time(sec)');
ylabel('amplitude');
--------------------------------------------