[sdiy] Psych Tone

[rsdio at audiobanshee.com]

LFSRs are actually quite poor for cryptography because they repeat in a predictable pattern. The sequence is only pseudo-random, not truly random. They’re good if you want a sequence that progresses in a seemingly random order, never repeating until the end of the cycle at which time it repeats the entire sequence again, exactly.

The number of bits determines the repeating pattern, so the 6-bit LFSR of the Psych Tone has a very short cycle (unless you change the taps on the fly, of course).

Besides the length of the Shift Register, another critical factor is the selection of the taps that drive the feedback. This can have a significant effect on the patterns generated.

By the way, vinyl sync uses an LFSR because you can put the needle down anywhere, look at the local pattern, and determine exactly where in the sequence the needle landed. This is related to the problem where public WiFi could be compromised by sniffing packets until the pseudo-random sequence is determined and then predicted. The vinyl sync folks turned a cryptography failure into a feature!

Brian


On Jan 5, 2019, at 1:26 PM, Ralph Moonen <ralph at tink.org> wrote:
> Love this!
> 
> lfsr’s are used a lot in cryptography where they need to be as random as possible. in music that might be different. Has anyone here built (not-so-random) lfsr sequencers for melodic generation? Something I will ponder over the weekend :-)
> 
> 
> On 5 Jan 2019, at 18:06, Donald Tillman <don at till.com> wrote:
>> Hiya,
>> 
>> I just whipped up a simulation of the Psych Tone in JavaScript.  Please enjoy:
>> 
>>   http://till.com/articles/psychtone/
>> 
>> (It's still a little rough.)
>> 
>> "WTF is a Psych Tone?"
>> 
>> The Psych Tone composes and plays musical sequences based on a Linear Feedback Shift Register.  Don Lancaster invented and published (and named) it in the February 1971 issue of Popular Electronics magazine.
>> 
>> The original article is here:
>> 
>>   https://www.tinaja.com/glib/psyctone.pdf
>> 
>> As hobbyist electronic music projects go, it's pretty impressive.  A 6-bit LFSR with variations, VCO, envelope generator, voltage controlled attenuator, and some logic to introduce rests, all in one package.
>> 
>> ---
>> 
>> Previously I wrote a JavaScript simulation of Marvin Minsky's Triadex Muse.  That's here:
>> 
>>   http://till.com/articles/muse/
>> 
>> Interesting update: I've been contacted by the Barbican Centre in London.  They're putting together an exhibition for this summer called "AI - More than Human" and it will feature a physical Muse, under glass, with my software simulation running on an iPad next to it.  This way museum visitors can see the actual Muse, and try out the simulation, and even "take it home".
>> 
>> -- Don
>> --
>> Donald Tillman, Palo Alto, California
>> http://www.till.com
>