# Communication System and AM modulation convert to Coherent demodulation

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Furkan Korkmaz on 3 May 2020
Answered: Samatha Aleti on 6 May 2020
This is the code for AM Modulation. I can not upload the ''Whale sound'' here. Can you write Matlab code also for AM coherent demodulation. Do not use ammode and amdemod command. You can use diff, filter and integral command in matlab code. Pls give answer.
%% Communication systems I, Spring 2020
% FFT in Matlab, Part I.
clc;
close all;
clear all
file = 'Whale_sound1.mp3'; % this our voice signal
[signal,Fs] = audioread(file);% Fs is already given. We do not need to touch it.
Ts=1/Fs; % sampling interval in time domain
signal=signal(:,1)';
N0 = length(signal);
sound(signal,Fs)
%Plot Sound File in Time Domain
t=0:Ts:Ts*(N0-1); %length of signal in seconds
figure();
subplot(2,2,1)
plot(t,signal)
xlabel('Time (s)');
ylabel('Amplitude');
title('Signal in Time Domain');
% Fourier Transform (One-Sided Spectrum)
n = 2^nextpow2(N0); %FFT point is power of 2 to easier implementation
nfft=n;
signalScaled = Ts.*signal; % this is energy signal so we need to multiply with Ts. For power signal the scaling factor will be the number of samples.
S=fft(signalScaled,nfft); % Take FFT (you can take the fft number with a power of 2)
SignalShifFFT = fftshift(S); % two side spectrum both negative and positive frequencies
mx=abs(SignalShifFFT); % Take the absulte spectrum of the signal
freqm = (-nfft/2:nfft/2-1)/(nfft*Ts);
%Plot Sound File in Frequency Domain
subplot(2,2,2)
plot(freqm ,mx); %plot the absolute spectrum
xlabel('Frequency (Hz)')
ylabel('Amplitude')
title('Two-Sided Spectrum of Signal')
%k=0:length(S)-1; % In frequency spectrum, there will be N0 points
%w=k/(length(S)*Ts); % frequency seperation
%% AM Modulation
A=0.5; %carrier amplitude;
fc= 10*10^3; %carrier frequency 10 KHz
CarrierSignal= cos(2*pi*fc*t);
ModulatedSignal = [A + signal].*CarrierSignal;
subplot(2,2,3)
plot(t,ModulatedSignal); %plot the absolute spectrum
xlabel('Time')
ylabel('Amplitude')
title('AM Modulated Signal in Time')
% Find the spectrum Modulated Signal by taking FFT
ModulatedSignalFFT=fft(ModulatedSignal,nfft);
ShifFFT = fftshift(ModulatedSignalFFT);
ABSModulatedSignalFFT=abs(ShifFFT)/N0;
freqm = (-nfft/2:nfft/2-1)/(nfft*Ts);
subplot(2,2,4)
%PlotSpectrum of the Modulated Signal
plot(freqm,ABSModulatedSignalFFT); %plot the absolute spectrum
xlabel('Frequency (Hz)')
ylabel('Amplitude')
title('Two-Sided Spectrum of AM Modulated Signal')

Samatha Aleti on 6 May 2020
Hi,