[sdiy] Non maximal-length LFSR

[ASSI]

Hi Richie,

On Thursday 03 March 2016, 19:55:59, Richie Burnett wrote:
> What my intended algorithm does instead, is implement sixteen identical
> LFSRs in parallel at the same time using an XOR instruction acting on
> 16-bit words in a circular buffer.  But these 16 bits aren't 16 bits of
> one Linear Feedback Shift Register, they are all the same bit but across
> sixteen different LFSR instances.  This is how I propose to generate the
> 16-bit words ensuring de-correlation between the bits, by initialising
> the 16 different shift registers to different initial states.  The taps
> are currently one bit apart but I can move them anywhere I want.

You are still using bits from the same sequence (each LFSR, a bitslice of 
your 48 registers, has the same structure), so you need to carefully 
initialize the LFSR states to have a large enough distance.  You reduce the 
correlation length by a factor of 16, which is inconsequential if you keep 
the sequences the maximum of 2^44 cycles apart, but could have unintended 
results if they are quite close together.  If you really want just 16 random 
bits per cycle, one of the pseudo-random generators that are used elsewhere 
might actually be much less of a headache.  You don't need one that's 
cryptographically secure, so there are lots of existing examples around.


Regards,
Achim.
-- 
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