% dsb2.m
% Matlab demonstration script for DSB-AM modulation. The message signal
% is m(t)=sinc(100t). 
echo on 
t0=.2;						% signal duration
ts=0.001;					% sampling interval
fc=250;						% carrier frequency
snr=20;						% SNR in dB (logarithmic)
fs=1/ts; 					% sampling frequency
df=0.3;                                        	% required freq. resolution
t=[-t0/2:ts:t0/2]; 				% time vector
snr_lin=10^(snr/10);				% linear SNR
m=sinc(100*t); 					% the message signal
c=cos(2*pi*fc.*t);				% the carrier signal
u=m.*c;						% the DSB-AM modulated signal
[M,m,df1]=fftseq(m,ts,df);                     	% Fourier transform
M=M/fs;                                        	% scaling
[U,u,df1]=fftseq(u,ts,df);                    	% Fourier transform
U=U/fs;                                       	% scaling
f=[0:df1:df1*(length(m)-1)]-fs/2;           	% frequency vector
signal_power=spower(u(1:length(t)));           	% compute modulated signal power
noise_power=signal_power/snr_lin;		% compute noise power
noise_std=sqrt(noise_power);			% compute noise standard deviation
noise=noise_std*randn(1,length(u));            	% generate noise sequence
r=u+noise;					% add noise to the modulated signal
[R,r,df1]=fftseq(r,ts,df);                     	% Fourier transform
R=R/fs;                                        	% scaling
pause  % Press a key to show the modulated signal power
signal_power
pause  %Press any key to see a plot of the message 
clf
subplot(2,2,1)
plot(t,m(1:length(t)))
xlabel('Time')
title('The message signal')   
pause % Press any key to see a plot of the carrier
subplot(2,2,2)
plot(t,c(1:length(t)))
xlabel('Time')
title('The carrier') 
pause  % Press any key to see a plot of the modulated signal
subplot(2,2,3)
plot(t,u(1:length(t)))
xlabel('Time')
title('The modulated signal') 
pause   % Press any key to see a plot  of the magnitude of the message and the
	% modulated signal in the frequency domain.
subplot(2,1,1)
plot(f,abs(fftshift(M)))
xlabel('Frequency')
title('Spectrum of the message signal')
subplot(2,1,2)
plot(f,abs(fftshift(U)))
title('Spectrum of the modulated signal')
xlabel('Frequency') 
pause  % Press a key to see a noise sample
subplot(2,1,1)
plot(t,noise(1:length(t)))
title('noise sample') 
xlabel('Time')
pause  % Press a key to see the modulated signal and noise
subplot(2,1,2)
plot(t,r(1:length(t)))
title('Signal and noise')
xlabel('Time')
pause  % Press a key to see the modulated signal and noise in freq. domain
subplot(2,1,1)
plot(f,abs(fftshift(U)))
title('Signal spectrum')
xlabel('Frequency')
subplot(2,1,2) 
plot(f,abs(fftshift(R))) 
title('Signal and noise spectrum')
xlabel('Frequency')