need help for understanding audio compression code using dct

THIS IS THE CODE:

clc
clear
close all
%%
[A,fs]=audioread('1 (1).wav');

%% Sender
C=[];
A=A';
for i=512:512:numel(A)
    B=dct(A(i-511:i));
    C=[C, B(1:128)];
end
%% Reciever
A2=[];
for i=128:128:numel(C)
    S=[C(i-127:i),zeros(1,384)];
    S=idct(S);
    A2=[A2,S];
end
%% Evaluation
dis=numel(A)-numel(A2);
A2=[A2,zeros(1,dis)];
PSNR=psnr(A2,A)
MSError=mse(A2,A)
SNR=snr(A2,A)
%% plot
figure,
plot(A);
hold on
plot(A2,'r')
plot((A2-A),'y')
legend('Original' , 'Recieved','Difference')
title('Audio Compression By DCT');
xlabel('Samples');
ylabel('Amp.');
grid();

Answer 1

[Jonas]

it shows, how a sound could sound if you transmit only 128 dct values per block instead of 512 sound values per block

% read audio file
[A,fs]=audioread('1 (1).wav');

% Sender
C=[];
A=A';

% compute dct for block of data a 512 samples and use only the first 128 resulting dct values 
for i=512:512:numel(A)
    B=dct(A(i-511:i));
    C=[C, B(1:128)];
end

% Send those 128 per 512 actual sample data to the receiver
% Reciever
A2=[];
for i=128:128:numel(C)
    
    % receiver takes each 128 dct values, adds zeros behind to get a 512
    % sample block
    S=[C(i-127:i),zeros(1,384)];
    
    % restore a sound part from inverse dct from 128 dct values and 384
    % zero values
    S=idct(S);
    
    % append the block to the overall result vactor
    A2=[A2,S];
end
%% Evaluation
dis=numel(A)-numel(A2);
A2=[A2,zeros(1,dis)];
PSNR=psnr(A2,A)
MSError=mse(A2,A)
SNR=snr(A2,A)
%% plot
figure,
plot(A);
hold on
plot(A2,'r')
plot((A2-A),'y')
legend('Original' , 'Recieved','Difference')
title('Audio Compression By DCT');
xlabel('Samples');
ylabel('Amp.');
grid();

% listen to both sounds, first the original, then the version where each
% 512 sample block was restored from only 128 dct values
soundsc(A,fs)

pause(numel(A)/fs +0.5);

soundsc(A2)