AM modulation and demodulation

Hello, I have this signal m(t) = cos(2*pi*t), and the AM signal of m : y = (Ac + m).*cos(2*fc*pi*t).

The first plot is the original signal, the second plot is the AM signal and the last is the envelope of the modulated signal.

I want to get the original signal so I try to do demodulation to AM signal, using z = amdemod(y,fc,fs) and next get the envelope of the demolated singal

but I get again the AM's signal envelope. As you see y axis is in range (1,3) but I want it in range (-1,1) like the orginal signal. I want just to deduct the Ac I added in (Ac + m), to get the original, but with amdemod() I don't get that result.

Answer 1

[Sebastien Dupertuis]

I think that by removing the mean (DC offset) of your demodulated signal in figure 3, you will then get your signal within the range (-1,1):

m = m-mean(m);

I would write an amDemod function like this:

function m = amDemod(s,fc,fs,n)
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%
% Function amDemod()
% Goal  : Demodulation of an AM signal
% IN    : - s : AM modulated signal
%         - fc: Carrier signal frequency
%         - fs: Sampling frequency
%         - n : Number of carrier signal periods
% IN/OUT: -
% OUT   : - m : Original demodulated signal
%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%

  % Rectify the modulated signal
  s = abs(s);
  % Set the number of samples on which to do the moving average
  k = round(n*fs/fc);
  % Take the moving average
  m = movmean(s,k);
  % Remove the DC offset
  m = m-mean(m);
end

Hope this helps.

Answer 2

[SINETHEMBA MNGUNI]

Hi, I need to modulate and demoduate an Audio.wav signal, how can I go about it?