%loading the signal library in octave
pkg load signal
clear all
close all

%reading of the orignial signal in data
[y,fs]=audioread('./data/modulator22.wav');

%calculation of the time from the sampling frequency
t=0:1/fs:length(y)/fs-1/fs;

%showing the original signal
##figure;
##set(gcf, 'name','Signal of origin');
##plot(t,y);
##xlabel('time(s)');
##ylabel('amplitude(n.u.');

%save a speed down version of the original signal
audiowrite('./data/modulator22_speedown.wav', y , fs/2);

%save the speed down version's signal and sampling frequency  and calculate it's new time duration
[y_speedown,fs_speedown]=audioread('./data/modulator22_speedown.wav');
t_speedown=0:1/fs_speedown:length(y_speedown)/fs_speedown-1/fs_speedown;

%showing the speed down version's signal
##figure;
##set(gcf, 'name','Speed down of the signal of origin');
##plot(t_speedown,y_speedown);
##xlabel('time(s)');
##ylabel('amplitude(n.u.');

%Apply the DFT(flase) and FFT(true) to the speed down signal
%frequencySpectrum(y_speedown, fs_speedown, false);
%frequencySpectrum(y_speedown, fs_speedown, true);

%Illustrate the spectogram of the speed down version at 2 windows values (1 for a precise frequency read and 1 for a precise time domain)
spectrogram(y_speedown, fs_speedown, 5, 30);
spectrogram(y_speedown, fs_speedown, 5, 5);

%Lowering the sampling frequency using the downsample() function
##y_downsampled = downsample(y, 2);
##fs_downsample = fs/2;
##audiowrite('./data/modulator22_downsample.wav', y_downsampled , fs_downsample);
##t_downsample=0:1/fs_downsample:length(y_downsampled)/fs_downsample-1/fs_downsample;

%Lowering again the sampling frequency but with the decimate() function (applying a LP filter at the start )
##y_decimated = decimate(y, 2);
##fs_decimate = fs/2;
##audiowrite('./data/modulator22_decimate.wav', y_decimated , fs_decimate);
##t_decimate=0:1/fs_decimate:length(y_decimated)/fs_decimate-1/fs_decimate;

%Comparing them both on the same graph to see the effect of the LP filter on the signal itself
##figure;
##set(gcf, 'name','Decimate vs Downsample');
##title('Decimate vs Downsample');
##plot(t_downsample,y_downsampled,'b');
##hold on;
##plot(t_decimate,y_decimated,'g');
##xlabel('time(s)');
##ylabel('amplitude(n.u)');
##legend('downsample()','decimate()',"location","southeastoutside");

%Illustrating them with their frequencySpectrum to see the effect of the LP filter
##frequencySpectrum(y_decimated, fs_decimate, true);
##frequencySpectrum(y_downsampled, fs_downsample, true);

%%%IIR and FIR LP FILTER IMPLEMENTATION %%%%
%creating a low pass filter FIR 
N=30;
fc=1000;
bfir=fir1(N,fc/(fs/2));
freqz(bfir,1);

%filter the signal
y_FIR_filter=filter(bfir,1,y);

%plot of the signal FIR
##figure;
##plot(y_FIR_filter,'g');

%creating a low pass filter IIR
N=8;
fc=1000;
[biir,a]=butter(N,fc/(fs/2));
freqz(biir,a);
Z=roots(biir); %zeros of the transfer fct
P=roots(a); %poles fo the transfer fct

%representation of zeros/poles in complex plan
##figure;
##zplane(Z,P);
##title('Zeros and poles of the transfer function of the IIR filter');
##legend('zeros','poles');
##grid on;

%filter the signal
y_IIR_filter=filter(biir,a,y);

%plot of the signal IIR
##figure;
##plot(y_IIR_filter,'b');

%downsample of the IIR signal
y_IIR_downsample = downsample(y_downsampled, 2);
fs_IIR_downsample = fs/2;
t_IIR_downsample=0:1/fs_IIR_downsample:length(y_IIR_downsample)/fs_IIR_downsample-1/fs_IIR_downsample;

figure;
set(gcf, 'name','Downsample of IIR');
plot(t_IIR_downsample,y_IIR_downsample,'b');
xlabel('time(s)');
ylabel('amplitude(n.u)');
legend('downsample()',"location","southeastoutside");