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John Larkin[_3_] John Larkin[_3_] is offline
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Default "Random" Circuit Needed.

On Thu, 23 Apr 2015 19:59:43 -0500, John Fields
wrote:

On Thu, 23 Apr 2015 09:02:02 -0700, John Larkin
wrote:

On Sun, 19 Apr 2015 02:15:07 -0500, John Fields
wrote:

On 19 Apr 2015 03:14:30 GMT, Jasen Betts wrote:

On 2015-04-18, David Eather wrote:
On Thu, 02 Apr 2015 20:42:50 +1000, Jasen Betts wrote:


I was wondering about that myself... I'll see if there's a cure.

r=(75*r+74)%65537 visits 0-65535 with no gaps.

not that i'd want to build it using 74LS logic.


That is an absolute turd. It screws up if the cycle tries to repeat more
than once - it not longer visits 0 - 65535 without gaps (it outputs a
665536 which needs 17 bits) and will miss a 16 bit number every cycle
after the first, OR if the 17-th bit is ignored it will produce an excess
number of zeros.

No, that is absolute bull****.

it's this:
http://en.wikipedia.org/wiki/Lehmer_...mber_generator

except offset by -1 so that the Lehmer zero state (which is
disallowed) is excluded and the maximal state fits in 16 bits.

---
If the all-zeroes state is disallowed, then there'll always be a
bias on the output.

The circuit I posted includes the all-zeroes state and, in fact, all
of its dflops are/can be cleared in order to initialize it.

John Fields


We are just finishing up a waveform generator box that includes two
programmable-bandwidth Gaussian analog noise generators. We used 47
and 49 bit maximal-length shift registers, clocked at 64 MHz. We just
peek at 18 bits of each register whenever we want a random number. An
asymmetry of one code out of 2^48 is not a big concern.


---
But, regardless of your machinations, it still isn't truly random,
is it?

John Fields


Each 18-bit sample is taken at a time determined by the rollover of a
64-bit DDS. All the samples are piped through a 128-tap digital FIR
filter, which stirs them up pretty well. The result goes through a
16-bit DAC and an analog lowpass filter. It looks like bandlimited
Gaussian noise, and I don't think my customers are going to demand
their money back.


--

John Larkin Highland Technology, Inc
picosecond timing laser drivers and controllers

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com