How to do embedding of watermark using PN sequence in audio signal using frequency hopping technique ?

Question

vaishali passi on 6 Feb 2014

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https://se.mathworks.com/matlabcentral/answers/115035-how-to-do-embedding-of-watermark-using-pn-sequence-in-audio-signal-using-frequency-hopping-technique

Answered: korada pavankumar on 12 Feb 2019

For my project I am refering to the paper in which these lines are mentioned:-

Watermark embedded into the audio signal in frequency domain is the pseudorandom noise sequence. Depending on which bit is to be embedded the process varies :– if bit 1 is to be embedded the value of the coefficient with lower frequency is set to be k dB above the mean value, while the value of the other coefficient is set to be k dB below the mean value. If bit 0 is to be embedded the operation is done vice versa. The fact that mean value of each sub-band remains unchanged is used when removing the watermark.

For this I have written a code but I am not getting the unchanged mean. mean is getting changed. what should be done for getting unchanged mean.

[f Fs]=wavread('timbaland.wav');
N = length(f);
slength = N/Fs;
nf=1024; %number of point in DTFT
Y = fft(f,nf);
x = randn(1, N) + cos(2*pi*100/1024*(0:N-1));
x(1024*ceil(end/1024)) = 0;
z = reshape(x, 1024, []);
y = 20*log10(abs(fft(z)));
one_count = 0;
zero_count = 0;
G=1023; % Code length
sd1 =randsrc(1,10,[0 1]); % Initial state of Shift register
PN1=[];
j=0;
sd(1)=0;
% First m-sequence
for j=1:G
PN1=[PN1 sd1(5)];
if sd1(1)==sd1(4)
temp1=0;
zero_count = minus_count + 1;
else temp1=1;
one_count = one_count + 1;
end
sd1(1)=sd1(2);
sd1(2)=sd1(3);
sd1(3)=sd1(4);
sd1(4)=sd1(5);
sd1(5)=sd1(6);
sd1(6)=sd1(7);
sd1(7)=sd1(8);
sd1(8)=sd1(9);
sd1(9)=sd1(10);
sd1(10)=temp1;
%disp(j);
%disp(sd1);
end
%disp(one_count);
%disp(zero_count);
g=autocorr(PN1);
%key
key=PN1;
[m,n]=size(y);
vks = [];
for i=1:1
temp= y(:,i);
mean1= mean(temp);
%disp(mean1);
low = 0; high = n;
index1 = low + (high-low) * rand;
index1 = round(index1);
if index1==0
index1 = 1;
end
%fprintf('Index1 = %d',index1);
%disp(strcat('Index1 = ', num2str(index1)));
fv= temp(index1,1);
low = 0; high = n;
index2 = low + (high-low) * rand;
index2 = round(index2);
%fprintf('Index2 = %d',index2);
if index2==0
index2 = 1;
end
fsv= temp(index2,1);
diff= mean1-20;
K= (diff/mean1)*100;
lf = 0;
hf = 0;
if (fv < mean1)
lf = fv;
hf = fsv;
loc1 = index1;
loc2 = index2;
else
lf = fsv;
hf = fv;
loc1 = index2;
loc2 = index1;
end
addi = 0;
subst = 0;
for j=1:length(key)
if(key(j) == 1)
disp('Bit is 1');
lf = mean1 + K;
hf = mean1 - K;
addi = addi + lf;
subst = subst + hf;
end
if(key(j) == 0)
disp('Bit is 0');
lf = mean1 - K;
hf = mean1 + K;
addi = addi + hf;
subst = subst + lf;
end
temp(loc1) = lf;
temp(loc2) = hf;
end
mean2= mean(temp);
%disp(mean2);
vks(:,i) = temp;
end

please tell me where i have done the mistake in this code. and also tell me that after doing fft only we get the coefficients ? Please reply as earliest possible. Thanks in advance. Link for whole document from where above lines are taken : Frequency Hopping Method for Audio Watermarking