Re: [Discuss-gnuradio] Scrambler Mask, Seed and Lenght

[Marcus Müller]

HI Thanasis,
first things first: That is gr::digital::scrambler_bb, which has doxygen documentation [1].
The length is really just the length of the underlying shift register -- which ought to be at least (order of generator poly)+1, ie. floor(log2(mask))+1
I recommend having a quick glance at its source code [2], revealing that it's actually but a block wrapper around gr::digital::lfsr[3].
If I read lfsr's code correctly, you're right, the highest bit of the mask will always be ignored, because it [4] will always be ANDed with a 0:
     unsigned char newbit = (popCount( d_shift_register & d_mask )%2)^(input & 1);
since d_shift_register never sees the 8th bit being set:
    d_shift_register = ((d_shift_register>>1) | (newbit<<d_shift_register_length));

So, yes, I think I agree, and length should at least be 8 here (your polynomial has degree 7), and it's a bit uncommon to see a LFSR being used without the x^0 coefficient.

Best regards,
Marcus


[1] http://gnuradio.org/doc/doxygen/classgr_1_1digital_1_1scrambler__bb.html#details
[2] https://github.com/gnuradio/gnuradio/blob/master/gr-digital/lib/scrambler_bb_impl.cc
[3] http://gnuradio.org/doc/doxygen/classgr_1_1digital_1_1lfsr.html
[4] https://github.com/gnuradio/gnuradio/blob/master/gr-digital/include/gnuradio/digital/lfsr.h#L128

On 05/15/2015 08:24 AM, Thanasis Balafoutis wrote:

Hi,

the default values for the parameters of the scrambler block are the following:

Mask: 0x8A

Seed: 0x7F

length:7

Is this a 7-bit scrambler? If yes, why the mask corresponds to an 8-bit number? (0x8A = '10001010')

What is the polynomial that corresponds to this mask?

According to this link (which I found as reference): http://wiki.spench.net/wiki/GNU_Radio_Notes#GLFSR_Source

the mask 0x8A is related with the polynomial x^8+x^4+x^2+1

Is that correct? Do I miss something or is there an error on default values?

Thank you!

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