[sdiy] VERY cool/bizarre: Buchla noise

[Magnus Danielson]

From: harrybissell <harrybissell at prodigy.net>
Subject: Re: [sdiy] VERY cool/bizarre: Buchla noise
Date: Wed, 29 Sep 2004 22:34:42 -0700
Message-ID: <415B9AF1.FF73DDA5 at prodigy.net>

Gene and Harry where getting onto one of my favorite subjects:

> Gene... those are excellent ways of putting it.  I'm being picky to explain
> WHY the MM5837 sucks (for a synthesizer noise source) although as a
> source for (let's say) a room equalizing instrument it is actually VERY good.
> 
> In the first case... it is obviously NOT truly random...
> in the second case.. it might as well be truly random
> 
> well said !

It's not random, but it's sufficiently pseudo-random to behave as true random
for the application. This is where we want to go.

For most applications it is very cheap to generate a sufficiently long random
generator for it to behave as true random numbers. I've published a list of
polynomials that should get you started.

On the same time is the LACK of true randomness in pseudo-random generators
also used EXTENSIVELY. In GPS is the C/A code 1023 bits/chips long and
repeating 1000 times a second (1.023 Mchips/s). Each satelite has it's own
unique pattern and the "randomness" allows for them to become spread-spectrum.
A civilian GPS receiver locks onto this pseudo-random pattern and eventually
tracks on a fractional bit resolution. The 50 b/s data is synchronously XORed
onto the signal, so it creates 20 ms of same polarity. The data contains all
the details about satelite orbits etc. So, the randomness allows for the
satelites to share spectrum (154*10.23 MHz is the common center frequency for
L1) where as the lack of randomness is used for separation and sub-bit
correlation tracking. The P-code (in practice encrypted to the military Y-code
by XORing with another pattern) has a chip rate of 10.23 MHz and it's random
noise pattern lasts a week. Same thing but much more "random".

In room acoustics/speaker measurements I used to use the MLSSA system which
uses a pseudo-random pattern that when correlating the returning signal popped
out the impulse responce. Again is both the pseudo-randomness both random and
not random at the same time and being a property that one really want.

So, pseudo-random isn't random or non-random - it's BOTH!

Cheers,
Magnus

> H^) harry
> 
> Gene Stopp wrote:
> 
> > 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
> > >
> > >
>