DSB-LC modulation

Hi,

I want to modulate a signal (that comes from an audio file) using DSB-LC with a 20kHz carrier. However, I am not sure I am doing it right, as I expected a plot of my modulated signal different from what I've got. Below is my code and plot of the message and modulated signal, both in time domain.

Code:

% message signal from a audio file
[sample,freq] = audioread('male2.wav');
sample = sample(:,1);
[num, c] = size(amostra);
T = 1/freq;
t = 0:T:(num-1)*T;
Mp = max(sample);

% carrier
carrier= cos (20000*pi*t);
Ts = 1/(48*10^3);             % period of sampling of the carrier
tp = 0:Ts:(num-1)*T;

% modulation index (given by μ = Mp/Ac). In this case, μ = 1
Ac = Mp/1;
modulated = (Ac + sample).*carrier';
plot (t, modulated);

Message signal (time domain):

Modulated signal (time domain)

I don't think the modulated signal should be like that. Right? I am supposed to do it for modulation indices of μ = 1, μ = 0.75 and μ = 0.5, but I am facing this problem right in the first case (μ = 1).

Any help will be appreciated. Thank you.

[Star Strider]

It is not easy to understand what you are doing.

The best way to check the result is to do a Fourier transform, fft, or a power spectral density (see the documentation for pwelch) on your modulated signal. Your code intends produce a double-sideband transmitted-carrier signal, so you should have two symmetric sidebands on either side of your 20 kHz carrier.

[Arquelau]

Hi Star Strider,

By looking the spectra, it gets clearer that there is something wrong happening. This is the spectra, using fft:

Looks like what I am getting is only the carrier, with no message at all. Below is the spectra of my message:

So I am expecting this spectra to appear in 20kHz, and not this impulses that I am getting. Is it right?