How to convert bitstream to packets and bitxor function

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Toan 2014-11-14

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https://ww2.mathworks.cn/matlabcentral/answers/162717-how-to-convert-bitstream-to-packets-and-bitxor-function

评论： Guillaume 2014-11-17

I have to convert a bit stream file such as name `BL.264` source file . Now I want to parse the file into packets with packet size is 50 bytes. After that, I want to implement bit XOR between first packet and second packet. To do it, I will use `fread` and `bitxor` functions. However, I have some issues:

If size of file is not divisible by packet sizes, we must add some padding at the end of final packet, *Right?* Let see my implementation, is it correct?
The bitxor supports xor bitwise that have limit length. In my case, packet size is 50 bytes, it is impossible to bitxor two packet. How to resolve that problem.

Let see my code. If have any problem, let me know

function packet=bitstream2packet(file,psize)
      %%file='BL.264';
      %%psize=50;
      fid=fopen(file,'r');
      bytelist=fread(fid,'*uint8')';    
      bitexpanded = dec2bin(bytelist(:), 8) - '0';     
      bitstream = reshape( bitexpanded.', [],1);
      %%Add padding
      remain=rem(size(bitstream,1),psize);
      bitstream(end+psize-remain)=0;
      packet = reshape( bitstream.', [],50);
      xorPacket=bitxor(packet(1,:),packet(2,:))
      fclose(fid);
end

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Answer 1

Guillaume 2014-11-14

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You're storing your bitstream as a logical array. In that case, it's a logical operation that you want to perform, not a bitwise xor. Therefore use xor instead of bitxor.

Compare

n1 = 8; n2=32
n1logicalbit = dec2bin(n1, 8) - '0';  %gives [0 0 0 0 1 0 0 0]
n2logicalbit = dec2bin(n2, 8) - '0';  %gives [0 0 1 0 0 0 0 0]
logicalxor = xor(n1logicalbit, n2logicalbit); %gives [0 0 1 0 1 0 0 0]
binaryxor = bitxor(n1, n2); %gives 40
isequal(logicalxor, dec2bin(binaryxor, 8)-'0') %return true

Otherwise your code is fine. There's a few unnecessary transpositions, you could simply do:

bytelist = fread(fid, '*uint8'); %no transpose
bitexpanded = dec2bin(bytelist, 8) -'0'; %no colon which in your case just transposed

I would also move the fclose to just after you've done the fread. This way if your code stops further down because of an error or the user interrupting it, the file won't be left open.

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Toan 2014-11-15

编辑：Toan 2014-11-15

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Thank Guillaume. I modified. It work well. However, I want to recovery these packet to orginal file (BL.264). This is my modification code for bitstream2packet. And second is packet2file. Let look at the second code and let me know is it correct?

function [packet remain]=bitstream2packet(file,psize)  
    fid=fopen(file,'r');
    bytelist=fread(fid,'*uint8');   
    fclose(fid);
    bitexpanded = dec2bin(bytelist, 8) - '0';   
    bitstream = reshape( bitexpanded.', [],1);
    %%Add padding
     remain=rem(size(bitstream,1),psize);%%bytes
    bitstream(end+psize-remain)=0;
    packet = reshape( bitstream.', [],psize);          
end

The second code to write packet to file is implemented by two steps:

Remove the padding at the end of last packet
Convert packet to bin array and use bin2dec function

function packet2file(file_out, bitstream,psize,remain)
        %%Write to file
        fwid=fopen(file_out,'w');
        bitstream_re = reshape( bitstream, [],1);
        bitstream_re(end-(psize-remain)+1:end)=[];%%Remove padding
        bitstream_8re=reshape(permute(reshape(bitstream_re,size(bitstream_re,2),8,[]),[2 1 3]),8,[]);
        bitstream_8re=bitstream_8re';
        dPacket = {};
        errorPacket=ones(1,8)*-1;
        for i_packet=1:length(bitstream_8re)
            if isequal(bitstream_8re(i_packet,:),errorPacket)
                fwrite(fwid,'NULL');
                continue;
            end
            dPacket =  bin2dec( sprintf('%d',bitstream_8re(i_packet,:)'));             
            fwrite(fwid, char(dPacket));        
        end
        fclose(fwid);
end

It works. However, it take long time due to scanning all packet and write. Besides, I my recover packet includes error packet denote by [-1 -1...]. Hence, I will write NULL instead of these pakets. Is it no problem when I want to compute PSRN between orignal file and recovery file?

Guillaume 2014-11-15

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I'm not sure what you're doing in your packet2file, but it looks overly complicated.

Once you've removed the padding, I would reshape to a n*8 matrix, convert it to char (by adding '0', the same way you subtracted '0' to go from char to binary) and call bin2dec. At this point you can replace your error packet by whatever you want:

bitstream_8re = reshape(bitstream_re, [], 8);
bitstream_char = char(bitstream_8re + '0');
bitstream_byte = bin2dec(bitstream_char);

I must say I don't particularly understand your error packet. How can it be [-1 -1 -1 -1 -1 -1 -1 -1] since bits are either 0 or 1?

errorpacket = '11111111'; %or whatever
errorrows = any(bsxfun(@eq, bitstream_char, errorpacket), 2);%find rows with errorpacket
bitstream_byte(errorrows) = 0; %or whatever you want to replace it with.

You can then write your bitstream as bytes. Note that I assume you want to write bytes not chars (which are two bytes each), and certainly not the string 'NULL'. I don't know what PSRN stands for.

fwrite(fwid, bitstream_byte);

Toan 2014-11-17

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Thank Guillaume. I am very impressive with your scheme. However, my packet size is 50bytes=400bits (8 x 50) , instead of 50 bits as your code. I try to modify it by

bit400 = fread(fid, Inf, 'ubit400=>uint64');

It has error

Error using fread
Invalid precision.

How to use your scheme in this case. BTW, I need to convert the bit400 to binary representation, so I will use

bitexpanded = dec2bin(bit400(:), 8) - '0';

Is it correct? The reason why I must convert to binary representation is that my algorithm is developed for bit level. Thank you

Guillaume 2014-11-17

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Now, I'm very confused. your original code split the file in packets of 50 bits, not bytes.

If you're operating on bytes, then you don't need to read the file in bits unit, just read it as a 50*Inf array of bytes:

byte50 = fread(fid, [50 Inf], '*uint8')'; %again it will automatically pad the last packet

Even if you're operating on bits afterward, you probably don't need to convert to binary. Certainly not for xoring packets. The bit* functions should be enough.

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