[sdiy] Pseudo randoms in software

[Seb Francis]

wow, it just gets more interesting!

I can see I might end up implemented several algorithms and adding some changeable parameters :)

Seb


Grant Richter wrote:

> >
> > Magnus Danielson wrote:
> >
> >>> Grant Richter wrote:
> >>>
> >>>> Another method is to send a high frequency clock into a counter with
> >>>> rollover. i.e. spin a counter like slot machine wheels. Then use a slow
> >>>> process to sample to counter like freezing the slot machine wheels. If the
> >>>> two clocks are unsynchronized (like a RC oscillator feeding a pin) the
> >>>> difference in speed should produce random results.
> >>>
> >>> This is actually a very nice idea, requiring minimum external components.
> >>> With a nice fast external oscillator this should be really quite random.
> >>> Just as I thought I was set on an idea .. now more options! ;)
> >>
> >> Well, it is not random enought. I even think it sound very repetitious. It
> >> "looks" random at first thought.
> >>
>
> That is the process used to select stages for the "random" setting of the
> ARP 1613 sequencer.
>
> >
> > Remember I'm trying to get a source for a random LFO, not digital noise.
> >
>
> Welcome to the land of Information Entropy. Random sources can be
> characterized Claude Shannon's measure of "Entropy".
>
> The calculation involves summing the "bins" of the histogram multiplied by a
> weighing factor. If the log base 2 is used then the output is in "bits".
>
> Maximal entropy is when every possible value is used once and only once.
> This is typical of pseudo random shift registers which are simply counters
> that count in an unknown order. Because they use each possible value only
> once (per repetition) they represent maximal entropy for the shift register
> length. But they will repeat the exact same sequence each time unless the
> initial shift register value is changed each time.
>
> Music has relatively low entropy because it is repetitious and self similar
> over short time periods.
>
> For more information on analog sources with variable entropy you can see:
>
> http://www.musicsynthesizer.com/WoggleBug/WoggleBug.html
>
> http://www.wiard.com/1200/Noise_Ring.html
>
> and Mike Murphy's motherlode of designs at:
>
> http://www.musicsynthesizer.com/MIC/mic.html
>
> all of which are designed to produce "non-optimal" random sequences with
> entropy numbers closer to music rather than noise.