am modulation and demodulation matlab

Thread Starter

Jon_Snow

Joined Jan 15, 2013
16
Hey guys I'm trying to build a script to modulate and demodulate an am frequency.

I have my information frequency, and carrier frequency. I modulate the signal and add noise. After I do that I run it through a halfwave rectifier and low pass and high filter. I then use a convolution to remove the noise. I then write the file to a wave file and I should be able to replay file. At the end of all this I should get something similar to the original frequency, but all I'm getting is static.

Here's my code.


close all;
clear all;
fs = 50000;
t = [0:1/fs:5];

f_cut_off1 = 10000;
f_cut_off2 = 10;
f = 2000; %input frequency
fc = 940e3; %carrier frequency

k = length(t)


input_sig = (0.2)*cos(2 * pi * f * t); %input signal

wavwrite(input_sig,fs,32,'input_sig');

carrier_sig = (0.5)*cos( 2 * pi * fc * t); %carrier signal

wavwrite(carrier_sig,fs,32,'carrier_sig')

mod_sig = input_sig .* carrier_sig + carrier_sig; %modulated signal

wavwrite(mod_sig,fs,32,'mod_sig')
%interval to randonize noise
noise = rand(1, k); %random nosie
scale = 2;
new_mod = mod_sig + noise; % mod signal + noise

wavwrite(new_mod,fs,32,'new_mod')

subplot(321), plot(t, mod_sig)
subplot(322), plot(t, new_mod)

%Apply HW Rectifier
for i = 1:k
if new_mod(i) <= 0
x(i) = 0;
else
x(i) = new_mod(i);
end
end

wavwrite(x,fs,32,'hw_rect')

subplot(323), plot(t, x)
rc1 = 1 / (2 * pi * f_cut_off1);
rc2 = 1 / (2 * pi * f_cut_off2);
%Apply Low Pass Filter
num2 = [0 1];
den2 = [rc1 1];
subplot(324), bode(num2, den2)

sys2 = tf(num2, den2);

lpf = lsim(sys2, x, t);




%Apply High Pass Filter

num1 = [rc2 0];
den1 = [rc2 1];

subplot(325), bode(num1, den1)
sys1 = tf(num1, den1);

hpf = lsim(sys1, lpf, t);

vout = hpf;

g = 20;
h = (1 / g) * ones(1, g);

y = conv(h ,vout);



wavwrite(y,fs,32,'vout')



Any ideas? I'm completely stuck.

Thanks!
 

tshuck

Joined Oct 18, 2012
3,535
Hey guys [...]
Any ideas? I'm completely stuck.
Have you tried plotting your signal at different points?
raw audio - with noise - after rectification - after filters - after convolution

That way, you can see what is going on with the waveform...
 

Thread Starter

Jon_Snow

Joined Jan 15, 2013
16
I've plotted the original signal, the modulated signal, the signal plotted with noise, and the signal after the halfwave rectifier. Those all seem to be correct.
 

MrChips

Joined Oct 2, 2009
19,408
Couple of problems:

1) Your carrier frequency is 940kHz, yet your sampling frequency is only 50kHz.
Your sampling frequency must be at least twice the carrier frequency.

2) When you added the noise, the noise value is between 0 and 1. Subtract 0.5 from the noise to make it -0.5 to +0.5
 

Thread Starter

Jon_Snow

Joined Jan 15, 2013
16
The size of the sampling frequency was something I was confused by so I'm going to change that, but what would changing the noise from -.5 to .5 do exactly?
 

MrChips

Joined Oct 2, 2009
19,408
Adding noise from 0 to 1 is like adding a DC offset of 0.5.
When you do the half-wave rectification you are not going to get the signal chopped in half.
 

Thread Starter

Jon_Snow

Joined Jan 15, 2013
16
Ok well I did those 2 things and that definitely helped. I'm still getting static, but at least I can recognize my input signal now.
 

Thread Starter

Jon_Snow

Joined Jan 15, 2013
16
When I remove the noise the static goes away. I'm supposed to put the noise in there however and remove the noise with the convolution. Maybe I should use a higher number than 20 in the convolution?
 

Thread Starter

Jon_Snow

Joined Jan 15, 2013
16
Here's my up to date code.

close all;
clear all;
fs = 2e6;
t = [0:1/fs:5];

f_cut_off1 = 10000;
f_cut_off2 = 10;
f = 2000; %input frequency
fc = 940e3; %carrier frequency

k = length(t);


input_sig = (0.2)*cos(2 * pi * f * t); %input signal

wavwrite(input_sig,fs,32,'input_sig');

carrier_sig = (0.5)*cos( 2 * pi * fc * t); %carrier signal



mod_sig = input_sig .* carrier_sig + carrier_sig; %modulated signal


%interval to randonize noise
noise = rand(1, k) - .5; %random nosie

new_mod = mod_sig + noise; % mod signal + noise


%Apply HW Rectifier
for i = 1:k
if new_mod(i) <= 0
x(i) = 0;
else
x(i) = new_mod(i);
end
end



rc1 = 1 / (2 * pi * f_cut_off1);
rc2 = 1 / (2 * pi * f_cut_off2);

%Apply Low Pass Filter
num2 = [0 1];
den2 = [rc1 1];
subplot(324), bode(num2, den2)

sys2 = tf(num2, den2);

lpf = lsim(sys2, x, t);




%Apply High Pass Filter

num1 = [rc2 0];
den1 = [rc2 1];

subplot(325), bode(num1, den1)
sys1 = tf(num1, den1);

hpf = lsim(sys1, lpf, t);

vout = hpf;

g = 20;
h = (1 / g) * ones(1, g);

y = conv(h ,vout);



wavwrite(y,fs,32,'vout')
 
Top