% Demodulation demodulated_signal = modulated_signal .* c; % Low-pass filter (to remove high-frequency components) cutoff_freq = 2 * fm; % Cut-off frequency [b, a] = butter(6

modulation

demodulation

[Sulaymon Eshkabilov]

Can you elaborate your question a bit more with details?

Answer 1

[William Rose]

@eman,

Suppose the original signal is a 17 Hz sine wave plus a 23 Hz cosine wave, 0.2 seconds long, plus an offset so it is always positive:

fs=1e4;                 % sampling rate (Hz)
t=0:1/fs:0.2;           % time (s)
y=2+sin(2*pi*17*t)+cos(2*pi*23*t);    % original signal

Suppose the carrier wave frequency is 1 kHz.

fcarrier=1000;          % carrier freq (Hz)
c=cos(2*pi*fcarrier*t); % carrier wave 

Choose a modulation index, M, so that the modulating signal stays non-negative:

M=1/max(abs(y));        % modulation index

Use y to amplitude-modulate the carrier. Call the AM signal x.

x=y.*c*(1+M);           % A.M. signal

Plot what you have so far:

figure; subplot(211); plot(t,y,'-r'); grid on; title('Original Signal')
subplot(212); plot(t,x,'-g'); grid on; title('A.M. Signal'); xlabel('Time (s)')

Your original signal and carrier frequency will be different, but the code above gets you started.

Now let's try to extract y, from x. In other words, demodulate the AM signal.

You can take it from here. Define a lowpass filter with an appropriate cutoff frequency. Rectify y (i.e. take its absolute value) and then apply the lowpass filter to it. You could use Matlab's filter() or filtfilt(). Read the help for filter() or filtfilt(). Good luck.