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I am trying to simulate DPCM in order to understand how it works. I am trying to implement the simplest version of DPCM which has a system scheme like in figure 1.Figure 1

Sorry for the quality of the picture but this was the best I could get. Let me do some explaining of the system.

System should do a DPCM modulation like in the figure, encode it, send it via a binary symmetric channel, decode it and demodulate it.

Here is what I tried:

fs = 15000;
input_data = getaudiodata(recObj);
input_data = input_data/max(abs(input_data));

quant_steps = linspace(-14/16, 14/16, 15);

quant_vals = linspace(-15/16, 15/16, 16);

epsilon = 0.3;
delayed = 0;

for ii = 1:length(input_data)
    y = input_data(ii) - delayed;
    [quants(ii), y_hat] = quantiz(y, quant_steps, quant_vals);
    delayed = y_hat + delayed; 
end

binary_sequence = de2bi(quants, 'left-msb');
binary_sequence = binary_sequence';
binary_sequence = binary_sequence(:)';

%BSC

received_sequence = binary_sequence;

for ii = 1:length(input_data)
    if rand < epsilon
        received_sequence(ii) = mod((received_sequence(ii) + 1), 2);
    end
end

%Demodulator

decoded = zeros(1, length(received_sequence)/4);
demodulated = zeros(1, length(received_sequence)/4);
reconstructed_signal = zeros(1, length(received_sequence)/4);

delayed = 0;

for ii = 1:length(decoded)
    decoded(ii) = bi2de(received_sequence((ii - 1) * 4 + 1 : ii*4), 'left-msb');
    demodulated(ii) = decoded(ii);
    reconstructed_signal(ii) = (demodulated(ii) * 1/8 - 1)  + delayed;
    delayed = reconstructed_signal(ii);
end

MSE = (1/length(reconstructed_signal)) * sum((reconstructed_signal' - input_data).^2);

With this code, unfortunately, reconstructed signal increases constantly.

Waiting for your help.

As always,

Have a nice day!

Furkan Küçük
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  • Hi! There are functions that you do not define. Either define them or use explicit coding instead. – Fat32 Jul 25 '18 at 23:35
  • Hello Fat32. Nice to have you there. Actually I don't have functions to define. All things are variables or the functions MATLAB already has. If you specify the functions you diddn't encounter before, you can whether ask me or easily find with the help page. – Furkan Küçük Jul 27 '18 at 07:08
  • Hmm ok. I thought those functions *getaudiodata*, *quantiz* , and *de2bi* existed as your own functions, but now I checked and i can see that MATLAB already has them. Sorry for that... Seems I've never used them before! – Fat32 Jul 27 '18 at 07:36

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