[sdiy] VERY cool/bizarre: Buchla noise

[Scott Gravenhorst]

Harry Bissell Jr <harrybissell at prodigy.net> wrote:
>I still don't agree... It is NOT truly random
>as it does not have an infinite number of states...
>it has only as many as the length of the register.
>
>Think of a register with four bits... the
>lowest frequency it can generate would be
>1111000011110000  etc...
>the highest is
>1010101010101010
>
>The all ones (or zeros depending on the logic) state
>will latch the device and it must be avoided or
>recovered from.
>
>It approaches 'random' and over the short term
>(compared to other methods) is a good way to generate
>noise - as long as you don't recognise that it
>IS NOT RANDOM !!!
>
>(but it is good enough for rock 'n roll :^)
>
>H^) harry

I've used this - a 4 bit LFSR - as a waveform generator.  The
XOR network is switchable to any combination of the 4 bits,
the circuit is clocked by a VCO.  I use the fact that it
repeats quickly as a feature here.

I believe in LFSR noise generators, as long as they are at
least 32 bits long.  If my math is right, that's around 4300
seconds (71 minutes) when clocked at 1 MHz.  I've found that
clocking them at far less than that still yields what sounds
indistinguishable (to me) from real white noise.  Of the
features of this method are no popcorn noise, no need for
selection/rejection of components and a predictable, large
output level without need for amplification.  And it's a
cheap thing to build and has no critical layout needs.  The
more bits the better.  I have a Xilinx PDF with a table
detailing XOR taps for maximum length LFSRs up to 168 bits.




>
>
>--- Rainer Buchty <rainer at buchty.net> wrote:
>
>> >I think you've nailed it right there! It is not
>> random.
>> 
>> *No* PRNG (i.e. noise generated by a feed-back shift
>> register) is true
>> random but shows a periodic behavior. That's why you
>> should use at least
>> 22, 23 Bits so that pattern repeat takes place in
>> the domain of a few
>> minutes rather than seconds.
>> 
>> Rainer
>> 
>> 
>

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