[sdiy] VERY cool/bizarre: Buchla noise

[Gene Stopp]

OK philosphy students... this is a good one frankly. Whenever I try to
explain PRBS stuff to people (usually customers, usually in regards to
internet traffic data patterns or framesize distributions) I end up using
phrases like "it's not really random but it's pretty darn close" and
"subjectively random" and "it might as well be truly random" and "but it
will repeat after a while" and "it's generated by a digital machine so it's
not truly random" etc. etc. Looks like the definition of "random" is what's
in question here.

If you keep adding bits to the shift register length you will quickly reach
a point at which the repeat interval (assuming you put the feedback taps in
the right place) is far beyond a single human lifespan. If we can all agree
that that is random, then the cutoff point between psuedo-random and random
is somewhere in between.

Although it's certainly out there past 17 bits :)

- Gene


-----Original Message-----
From: owner-synth-diy at dropmix.xs4all.nl
[mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of Harry Bissell Jr
Sent: Wednesday, September 29, 2004 3:12 PM
To: Rainer Buchty; Metrophage
Cc: synth-diy at dropmix.xs4all.nl
Subject: Re: [sdiy] VERY cool/bizarre: Buchla noise


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



--- 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
> 
>