add Task2

function Freq_cochlea = cochlea(Freq_low, Freq_high, N)

    % divide the band

    d1 = log10((Freq_low/165.4)+1) / 0.06;

    d2 = log10((Freq_high/165.4)+1) / 0.06;

    d = linspace(d1, d2, N+1);

    Freq_cochlea = 165.4*(10**(0.06*d)-1);

end

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% Project 1, Task 2, Task 3

% load

[samples, Freq_sample] = audioread('C_01_01.wav');

% define

lpf = [20 50 100 400];

Freq_low = 200;

Freq_high = 7000;

% Set the number of bands N=4

N = 4;

figure();

hold on

for i in 1:length(lpf):

    yn = tone_vocoder(N, Freq_low, Freq_high, Freq_sample, lpf[i], sample)

    yn = fftshift(fft(yn));

    yn_abs = abs(yn);

    %plot yn_abs

    subplot(4, 1, i);

    xn = linspace(-Freq_sample/2, Freq_sample/2, length(sample))

    plot(xn, yn_abs);

    title(sprintf('N = 4, f_{lpf}=%d Hz, Tone Vocoder',lpf(i)))

    xlabel('Frequency(Hz)');ylabel('Magnitude');

    audiowrite(sprintf('task2_N=4_lpf=%d.wav',lpf(n)),yn,Freq_sample);

    saveas(figure1,sprintf('task2_lpf%d.png',lpf(i)));

    grid on

end

function yn = tone_vocoder(N, Freq_low, Freq_high, Freq_sample, Freq_filter, sample)

    % divide the frequency

    Freq_cochlea = cochlea(Freq_low, Freq_high, N);

    yn = zeros(length(sample))

    for band_index = 1:N

        band = [Freq_cochlea[band_index] Freq_cochlea[band_index+1]]

        % Design band-pass filter at band i

        band_pass = butter(4, band/(Freq_sample/2));

        % Do band-pass filtering at band i

        sample_band = filter(band_pass, sample);

        % Do full-wave rectification

        % apply low-pass filtering to get the envelope at band i

        low_pass = butter(2, Freq_filter/(Freq_sample/2), 'low');

        filtered_sample_band = filter(low_pass, sample_band)

        % Generate a sinewave, whose frequency equals to the center frequency of the i-th bandpass filter

        fm = (band[1]+band[2])/2

        xn = 1:length(sample)

        sin_wave = sin(2*pi*fm*xn/Freq_sample)

        % Multiply the envelope signal and sinewave

        yn = yn + sin_wave .* filtered_sample_band

    end

    yn = yn/norm(yn)*norm(sample);

    yn = yn.';

end

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