Re: [AH] Nonlinear or pseudorandom modulators

From Brian Willoughby
Sent Tue, Feb 6th 2018, 21:25

One does not feed digital noise /into/ an LFSR, the LFSR /is/ digital =
noise.

Let me modsplain this for you.

The term, LFSR, stands for Linear-Feedback Shift Register. The content =
of the LFSR itself is used as feedback and fed into the binary input. =
Typically, the feedback is the eXclusive OR product of two or more bits =
from close to the output. The only catch is that the LFSR cannot start =
out with all zero bits, or it will always remain zero. The longer the =
bit chain, the longer the pattern before it repeats. One can also use =
just a single output bit at the end of the chain, or consume multiple =
bits, although adjacent bits are not really unrelated, thus the =
pseudo-random nomenclature.

If you were to feed digital noise into an LFSR, it wouldn=E2=80=99t =
really be an LFSR any more.

The nice thing about the LFSR is that it can be implemented in software. =
If you want it to be predictable, that is a better choice, because you =
can shift only once for each new bit needed. If you want the LFSR output =
to be unpredictable, then run it in hardware with its own bit clock so =
that the sequence is not tied to any outside signal. As you probably =
know, there have been chips like the MM5837 implementing this since the =
seventies, if not earlier.

Brian

On Feb 6, 2018, at 12:41 PM, Kylee Kennedy <xxxxxxxxx@xxxxx.xxx> wrote:
> Also was thinking you were going to mention the Turing Thing from Tom =
Whitwell. I have a buddy that uses four of them in his eurorack for just =
that sort of psuedo-randomness.=20
>=20
> Another thing to look at is the Buchla voice of uncertainty which is =
digital noise into an LFSR. So it become more psuedo-random and more =
musical.
>=20
> I also like the Lorenz Affect LFO from Non-Linear Circuits called =
'Sloth' for slow evolving LFOs that do weird phase shifts. It's not =
exactly what you are asking for but I use it similarly to a =
psuedo-random source.
>=20
>=20
> Kylee