%call it like question_8('sound1.wav')
function question_8(url)
[x,fs]=wavread(url);
new_x=x(1:512);
subplot(2,1,1);
stem(new_x);
title('The sampled sound signal');
xlabel('time'), ylabel('amplitude');
grid on;
hold on
 
 
subplot(2,1,2)
N=(fs/2)*linspace(-1,1,length(x));
stem(N,fftshift(abs(fft(new_x))));
 
title('Sampled signal at frequency-domain');
xlabel('frequency'), ylabel('amplitude');
grid on;
end

function am_modulation
 
N=2000;
F=(-N/2:N/2-1)/N;
 
user_entry1 = input('Please enter carrier freq [Press enter for default fc=0.25] :');
if isempty(user_entry1)
    fc=0.25;
else
    fc=user_entry1;
end
 
user_entry4 = input('Please enter amplitude of carrier wave [Press enter for default Ac=1] :');
if isempty(user_entry4)
    Ac=1;
else
    Ac=user_entry4;
end
 
n=0:1:256;
 
c=Ac*cos(2*pi*n*fc);
subplot(2,2,1)
plot(F, fftshift(abs(fft(c,N))));
title('Carrier Wave at frequency-domain');
xlabel('frequency'), ylabel('amplitude');
grid on;
 
 
 
user_entry2 = input('Please enter message freq [Press enter for default fm=0.01] :');
if isempty(user_entry2)
    fm=0.01;
else
    fm=user_entry2;
end
 
user_entry5 = input('Please enter message amplitude [Press enter for default Am=1] :');
if isempty(user_entry5)
    Am=1;
else
    Am=user_entry5;
end
 
m=Am*cos(2*pi*fm*n);
subplot(2,2,2)
plot(F, fftshift(abs(fft(m,N))));
title('Message at frequency-domain');
xlabel('frequency'), ylabel('amplitude');
grid on;
 
 
user_entry3 = input('Please enter amplitude sensitivity cnst. [Press enter for default ka=0.5] :');
if isempty(user_entry3)
    ka=0.5;
else
    ka=user_entry3;
end
 
s=(1+m.*ka).*c;
 
subplot(2,2,[3 4])
plot(F,fftshift(abs(fft(s,N))));
title('AM waveform at frequency-domain');
xlabel('frequency'), ylabel('amplitude');
grid on;

%Amplitude Modulation with a periodic sqare wave and its spectrum analysis
%Show the time domain and frquency domain representation of DSB-AM and
%DSB-SC modulations
%By : John Roach – 6 March 2009
%visit my site at http://johnroach.info
N = 1024; %N point FFT N>fc to avoid freq domain aliasing
fs = 4096; % Sample frequency
t = (0:N-1)/fs;
fc = 600; %Carrier Frequency
fm2 = 80; %Message Frequency
Ec = 20; %Carrier Amplitude
Em2 = 5; %Messagae Amplitude
% Try changing the message and carrier amplitudes to see the effect in
% DSB-AM modulation
%———Double SideBand Full Carrier Modulation (DSB-FC(AM))
A = Ec + Em2*square(2*pi*fm2*t);%Envelope/eliminate the carrier amplitude
m = A.*sin(2*pi*fc*t); %to convert DSB-AM to DSB-SC
Mf = 2/N*abs(fft(m,N));
f = fs * (0 : N/2) / N;%Since the fft result is symmetrical, only the
%positive half is sufficient for spectral representation
close all;
figure(’Name’,'Time/Fequency domain representations of DSB-AM signals’);
subplot(2,1,1); %Time domain plot
plot(t(1:N/2),m(1:N/2),t(1:N/2),A(1:N/2),’r',t(1:N/2),-A(1:N/2),’r');
title(’Time Domain Representation’);
xlabel(’Time’); ylabel(’Modulated signal’);
subplot(2,1,2); %Frequency Domain Plot
plot(f(1:256),Mf(1:256));
title(’Frequency Domain Representation’);
xlabel(’Frequency (Hz)’); ylabel(’Spectral Magnitude’);
%———-Double SideBand Suppressed Carrier DSB-SC———-
A = Em2*square(2*pi*fm2*t) ; %Envelope/eliminate the carrier amplitude
m = A.*sin(2*pi*fc*t); %to convert DSB-AM to DSB-SC
Mf = 2/N*abs(fft(m,N));
figure(’Name’,'Time/Fequency domain representations of DSB-SC signals’);
subplot(2,1,1); %Time domain plot
plot(t(1:N/2),m(1:N/2),t(1:N/2),A(1:N/2),’r',t(1:N/2),-A(1:N/2),’r');
title(’Time Domain Representation’);
xlabel(’Time’); ylabel(’Modulated signal’);
subplot(2,1,2); %Frequency Domain Plot
plot(f(1:256),Mf(1:256));
title(’Frequency Domain Representation’);
xlabel(’Frequency (Hz)’); ylabel(’Spectral Magnitude’);
text(15,60,’Carrier’);
%——————————————————————–