[time-nuts] T.I. questions

[Magnus Danielson]

Hal,

On 02/06/2015 09:42 PM, Hal Murray wrote:
>
> magnus at rubidium.dyndns.org said:
>> The typical noise generator chips uses a PRNG based on DFFs and XOR
>> gate(s). A typical weakness is that the chain of DFFs is to short,  causing
>> a relatively high rate of cycling, which hearable as a beating. However, for
>> some uses, that is OK.
>
> The buzzword for the typical PRNG is LFSR - Linear Feedback Shift Register.
>
> Many years ago, Xilinx published a good app-note on this topic.  There is
> also a section in Art of Electronics.
>
> With the right generating polynomial, you get a sequence of bits that doesn't
> repeat until 2^N-1 bits.  The math geeks like to collect them.  Each 1 bit in
> the polynomial turns into an XOR gate, so you will also find collections of
> polynomials with fewest bits.
>
> It's hard to imagine serious problems with too-short.  Each FF doubles the no-repeat length.

Polynomials happens to be prime numbers for a good reason.

This makes them scarse, only create a few limited sequences. Combing two 
or more can overcome that limit, as seen in a navigation system near us. :)

Cheers,
Magnus