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    % MATLAB script for Illustrative Problem 3.2.
% 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')