Question

MATLAB CODE that do DSBSC Modulator/Demodulator system to transmit an audio signal of your choice using a carrier to signal frequency ratio of fc / fm = 60. The channel bandwidth is 10 KHz.

Answer 1

DSBSC=double side band supressed carrier. It is a type of Amplitude modulated signal that includes both the sidebands without the carrier, thus named so.

Given: f_c=60f_m; channel bandwidth=10KHz, therefore, maximum bandwidth allowed of DSBSC signal bandwidth should be 10KHz.

Now, DSBSC bandwidth= twice the modulating signal bandwidth=2*f_m

This gives us f_m=5KHz. Also, f_c=60fm=60*5=300KHz.

Note that time period t=1/f that implies t_c=1/f_c=1/300 ms & t_m=1/f_m=1/5 ms.

MATLAB CODE:

tmin=0;

tmax=10^-3;

step=(tmax-tmin)/1000;

t=tmin:step:tmax;

Am=1; %Amplitude of modulating signal not given, so assumed to be 1 volt.

fm=5*10^3; %modulating signal frequency=5KHz.

m=Am*sin(2*pi*fm*t); %modulating signal

figure(1);

subplot(3,1,1);

plot(t,m);

xlabel('time');

ylabel('amplitude')';

title('modulating signal');

grid on;

tmin=0;

tmax=10^-3;

step=(tmax-tmin)/1000;

t=tmin:step:tmax;

Ac=1; %Amplitude of carrier signal not given, so assumed to be 1 volt.

fc=3*10^5; %carrier signal frequency=300 KHz.

c=Ac*sin(2*pi*fc*t); %carrier signal

subplot(3,1,2);

plot(t,c);

xlabel('time');

ylabel('amplitude');

title('carrier signal');

grid on;

s=m.*c; %modulated signal DSBSC.

subplot(3,1,3);

plot(t,s);

xlabel('time');

ylabel('amplitude');

title('DSBSC signal');

grid on;

hold on;

plot(t,Ac*Am*m,'r');

hold on;

plot(t,-Ac*Am*m,'r');

hold off;

Any doubts, please ask as thumbs down won't solve the doubt. Thanks!