[sdiy] Syncable digital noise source.

[ASSI]

On Dienstag 01 Juli 2008, Tom Wiltshire wrote:
> I mentioned the noise source on the Nord Lead 2 a couple of weeks
> ago . This allows you to reset the LFSR to a known start state. This
> means you can sync the noise to an oscillator. According to Clavia, a
> given LFSR sequence gives fixed formants in the frequency response.
> They've got a nice diagram of it on page 40 of the manual (http://
> www.clavia.se/products/manuals.htm).

In case anybody is wondering, these plots are in the Nord Lead 2X 
manual, not the Nord Lead 2....

> Anyway, since then, I've tried putting a reset input on my PIC noise
> source. Then I fed a squarewave from an SH101 into the reset input.
> The result was not worth the limited effort, in my opinion. Whilst
> the formant-shaping produces noise with certain "flavours", it didn't
> produce much that sounded good to my ears. Obviously you can start
> the LFSR in many different places and get many different flavours, so
> perhaps I didn't explore enough. I wasn't encouraged to by the
> results I got. The sound isn't pitched enough to be really useful,
> and as unpitched noise goes, pure white or pink have a smoother
> quality whilst the synced noise often sounded "spattery" and uneven.

That's because you implemented a real pseudo-random generator and Clavia 
most likely did not - there are no strong formants even on 
sub-sequences of a good pRNG unless you make the sub-sequence very 
short, on the order of the length of the LFSR.  The reason why it could 
perhaps work like described in the Clavia manual is when they use 
the "state" of the pRNG as their noise wave and not the single bit 
produced each time it is stepped.  This produces strong 
auto-correlation in the noise signal as each random bit will produce an 
exponential pulse and such sequences can have strong formants that are 
related to the length of the shift register and the number of bits you 
take out for each step; the order and spacing of bits also plays into 
this.  The result is largely unpredictable, but once you find a good 
combination of parameters, they can be reproduced with ease.  It is 
also possible that in this mode they use non-maximum length sequences 
which can be coloured even more strongly.


Achim.
-- 
+<[Q+ Matrix-12 WAVE#46+305 Neuron microQkb Andromeda XTk]>+

Factory and User Sound Singles for Waldorf Blofeld:
http://Synth.Stromeko.net/Downloads.html#WaldorfSounds