[Discuss-gnuradio] Scrambler - Descrambler

[devin kelly]

Hello,

I'm a little confused on how to use the scrambler and descrambler, it looks like the descrambler always preprends a byte to my stream.  When I run this flowgraph I get this from the message ports (the first message is the original message, the second message is from the descrambler):

* MESSAGE DEBUG PRINT PDU VERBOSE *
()
pdu_length = 10
contents =
0000: d0 22 e7 d5 20 f8 e9 38 a1 4e
***********************************
* MESSAGE DEBUG PRINT PDU VERBOSE *
()
pdu_length = 10
contents =
0000: 06 d0 22 e7 d5 20 f8 e9 38 a1
***********************************

I'm not totally sure what do about this, I've tried adding a delay block but that screws up my packet_len tags. 

Here's the original work function:

{  
const unsigned char *in = (const unsigned char*)input_items[0];
unsigned char *out = (unsigned char*)output_items[0];

for(int i = 0; i < noutput_items; i++) {
    out[i] = d_lfsr.next_bit_descramble(in[i]);
}  

return noutput_items;
}

I've also tried this work function (my approach here is to ignore the first byte out of the descrambler and then put junk into the last byte which would then be ignored on the next call to work() and yes I realize this isn't a general solution)

{   

const unsigned char *in = (const unsigned char*)input_items[0];

unsigned char *out = (unsigned char*)output_items[0];

      unsigned char junk;

      for(int i = 0; i < noutput_items + 8; i++) {

          if (i < 8) {

            junk = d_lfsr.next_bit_descramble(in[i]);

          } else if (i > noutput_items) {

            out[i - 8] = d_lfsr.next_bit_descramble(0xff);

          } else {

            out[i - 8] = d_lfsr.next_bit_descramble(in[i]);

          }   

      } 

return noutput_items;
}

I run the flowgraph and now my last byte is junk (argh!)

***********************************

* MESSAGE DEBUG PRINT PDU VERBOSE *

()

pdu_length = 10

contents = 

0000: d0 22 e7 d5 20 f8 e9 38 a1 4e 

***********************************

* MESSAGE DEBUG PRINT PDU VERBOSE *

()

pdu_length = 10

contents = 

0000: d0 22 e7 d5 20 f8 e9 38 a1 3d 

Does anyone have any advice on how to use to the scrambler and descrambler?  Would writing my own message blocks that use the LFSR be a better approach?

Thanks for any help,

Devin

scrambler.png
Description: PNG image