[time-nuts] T.I. questions

Hal Murray hmurray at megapathdsl.net
Fri Feb 6 20:42:36 UTC 2015


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.


-- 
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