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Topic: "Reading a Binary Header" for dummies (Read 3827 times) previous topic - next topic
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"Reading a Binary Header" for dummies

I've had enough programming experience to be able to find a hex value in a file, but I'm absolutely virginal when it comes to finding the 11 set bits in a file that signify a header.  Bit level programming is Greek to me.

I'm trying to plug away at some source code others have written, but as yet it just isn't clicking with me.

Help!

"Reading a Binary Header" for dummies

Reply #1 –
This is real programming  Just have to get practiced with the operators,
shifts >>, >>>, <<
and & or |, xor ^
A reference book comes in handy, plenty of googleables 'logical operators shift programming...etc'
To find 11 bits on? first find a full byte then see whats around it.

gl'
no conscience > no custom

"Reading a Binary Header" for dummies

Reply #2 –
Quote
To find 11 bits on? first find a full byte then see whats around it.

[a href="index.php?act=findpost&pid=337270"][{POST_SNAPBACK}][/a]



Does the first bit really have to be aligned on the beginning of a byte? Then I doubt he would have had to ask...


Edit: previous lookup table solution i suggested was faulty and removed now.

"Reading a Binary Header" for dummies

Reply #3 –
Quote
Quote

To find 11 bits on? first find a full byte then see whats around it.

Does the first bit really have to be aligned on the beginning of a byte? Then I doubt he would have had to ask...

you can look either side *

pseudocode:
loop pos{
if array[pos]=0xff
{ if (array[pos-1] has bit 3=0 and bits2,1,0=1 )
  {return pos*8-3;}
  if (array[pos-1] has bit 2=0 and bits 2,1=1 AND array[pos+1] has bit 8=1)
{return pos*8-2}
  if etc..
  }//end fullbytefound
}//end search

Quote
To find 11 consecutive set bits anywhere in a byte stream, I'd make a table of of possible byte combinations....

possible, whatever works does to get on with 

[span style='font-size:8pt;line-height:100%']Edit: *warning - Eric has pointed out the possibiity of sync word falling between two bytes showing this approach is incomplete[/span]
no conscience > no custom

"Reading a Binary Header" for dummies

Reply #4 –
Quote
I've had enough programming experience to be able to find a hex value in a file, but I'm absolutely virginal when it comes to finding the 11 set bits in a file that signify a header.  Bit level programming is Greek to me.


If you are talking about the MPEG Audio header, it is 12 bits set to "1". And the start is on a byte boundary.
So you only have to search for hex FFF

"Reading a Binary Header" for dummies

Reply #5 –
Quote
If you are talking about the MPEG Audio header, it is 12 bits set to "1".

But only 11 for mpeg 2.5 streams

"Reading a Binary Header" for dummies

Reply #6 –
Quote
...the start is on a byte boundary.
So you only have to search for hex FFF
[a href="index.php?act=findpost&pid=337321"][{POST_SNAPBACK}][/a]


Can someone confirm/refute this?  Does every header sync always start on a byte boundary?

"Reading a Binary Header" for dummies

Reply #7 –
yes, it always starts at a byte boundary

"Reading a Binary Header" for dummies

Reply #8 –
Good - that simplifies things considerably.  Thanks to everyone.