Hal,
On 02/06/2015 09:42 PM, Hal Murray wrote:
[email protected] 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
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