How to get last two bits except for LSB from audio signal in MATLAB?

i have a code in which i am getting LSB values of audio signal now i am want to get last two bits except for LSB..Here is code:

    fid1=fopen('ash.wav','r'); 
    header=fread(fid1,40,'uint8=>char');
    data_size=fread(fid1,1,'uint32');
    [dta,count]=fread(fid1,inf,'uint16');   
    fclose(fid1);
    lsb=1;

    identity=[1 0 1 0 1 0 1 0]';

    dta(index)=bitset(dta(index),lsb,identity(1:8));
    dta(index)=bitset(dta(index),lsb,m_bin(1:10));
    index=([indices(19):indices(28)]);
    dta(index)=bitset(dta(index),lsb,n_bin(1:10));                              
    index=([indices(29):indices(28+len)]);
    dta(index)=bitset(dta(index),lsb,msg_bin(1:len)');

how to do it?

[michael]

Please write the complete code in a code format, so that it would be readable

[ayesha jabeen]

This is a code:

fid1=fopen([pathname filename],'r');

%first 40 bytes make wav header,store the header

header=fread(fid1,40,'uint8=>char'); 

%41st byte to 43rd byte,length of wav data samples 

data_size=fread(fid1,1,'uint32');

%copy the 16 bit wav data samples starting from 44th byte

[dta,count]=fread(fid1,inf,'uint16');   

%close the file only wav data samples are sufficient to hide the text 

fclose(fid1);

lsb=1;

msg='Hello how are you?';     

%run aurecover.m to recover this message from new2.wav file

msg_double=double(msg);     
  %convert it to double

msg_bin=de2bi(msg_double,8);  %then convert message to binary

[m,n]=size(msg_bin);          %size of message binary

msg_bin_re=reshape(msg_bin,m*n,1);  %reshape the message binary in a column vector   

m_bin=de2bi(m,10)';

n_bin=de2bi(n,10)';

len=length(msg_bin_re);       %length of message binary 

%len=m*n

len_bin=de2bi(len,20)';       %convert the length to binary

%hide identity in first 8 wav data samples.

identity=[1 0 1 0 1 0 1 0]';

dta(1:8)=bitset(dta(1:8),lsb,identity(1:8));

%hide binary length of message from 9th to 28 th sample 

dta(9:18)=bitset(dta(9:18),lsb,m_bin(1:10));

dta(19:28)=bitset(dta(19:28),lsb,n_bin(1:10));                              

%hide the message binary starting from 29th position of wave data samples

dta(29:28+len)=bitset(dta(29:28+len),lsb,msg_bin(1:len)');

%open a new wav file in write mode

fid2=fopen('new2.wav','w');

Answer 1

[Walter Roberson]

bitset(YourArray, bitset(YourArray, 2, NewBit2Array), 3, NewBit3Array)

[ayesha jabeen]

Sir newBit2array and Newbit3Array which value stored??

[Walter Roberson]

Those arrays should be the same size as YourArray, and they should hold the new value to go into bit 2 for each corresponding location, and the new value to go into bit 3 for each corresponding location.

If you are only changing a single location, you still need to use the two bitset() calls. bitset() is not able on its own to realize that you want to change multiple bits in a single location, so you need to call it once for each bit position to be changed at the location.

[Walter Roberson]

Opps, should be

    bitset(bitset(YourArray, 2, NewBit2Array), 3, NewBit3Array)

Answer 2

[Geoff Hayes]

ayesha - if

 lsb=1;

and

 dta(index)=bitset(dta(index),lsb,msg_bin(1:len)');

sets the least significant bit (lsb), then to set the bits before this one, you would just bitshift lsb to the left to modify the two bits prior to lsb. For example,

 % bit shift once to to modify the bit prior to lsb
 bitToModify = bitshift(lsb, 1);

 % bit shift twice to modify the bit that is two away from lsb
 bitToModify = bitshift(lsb, 2);

[ayesha jabeen]

Sir i want to hide data in last two bits.In this situation how can i am use bitToModify variable?In above situation i am use lsb in bitset function now how i am use this modified bits?

[Geoff Hayes]

ayesha - if you have three bits of your message that you want to hide in the last three bits of a single byte in your audio data, you would use bitset three times as

 audioByte = bitset(audioByte,1,bitOne);
 audioByte = bitset(audioByte,2,bitTwo);
 audioByte = bitset(audioByte,3,bitThree);

EDIT

Code corrected as per Walter's comment.

[Walter Roberson]

bitset() returns a value rather than modifying the array in place, so you need to make those into assignments.

Also bitshift(lsb,2) would be 4 rather than 3 so you would not be storing into bits 1, 2, 3, but instead 1, 2, 4. The second parameter should just be a bit number in natural numbers.

[ayesha jabeen]

Thank you so much Sir i got the idea...Now i am try to do it...