[sdiy] Advices for ringmod and noise source

[ASSI]

Two more ideas for noise sources:

Cross modulation of HF-VCO with subsequent downsampling and A/D 
conversion is sometimes used to produce "almost" random numbers. Given 
sufficiently high sampling rate this should sound pretty white without 
any further processing.

The binary cellular automatons in "A new Kind of Science" by Stephen 
Wolfram look pretty interesting as well and should be easily 
implemented in both software and CPLD/FPGA. If you follow the right 
links on http://atlas.wolfram.com/ you'll even find the spectral 
properties for many of the generators. See also the summary on RNG on
http://www.wolframscience.com/nksonline/page-974b-text?firstview=1

I think http://www.ciphersbyritter.com/ has already been mentioned in 
this thread. Highly recommended reading.

Last but not least, if you use an LFSR for creating random numbers more 
than a single bit wide (or multiple independent single-bit streams), 
you need to clock the shift register once for each bit you take out of 
the stream - otherwise the short-term correlation of the sequence is 
terrible. In an FPGA you can "roll out" the circuit (just like loop 
unrolling works in software), so that you get the same effect as a 
pre-set number of clocks with just a single clock event. With little 
effort you can get these generators up to about 280MHz on relatively 
cheap FPGA platforms and they won't repeat during your lifetime either.


Achim.
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