[sdiy] Syncable digital noise source.

[Scott Jaeger]

On Jun 30, 2008, at 4:31 PM, Tom Wiltshire wrote:

> Hi all,
>
> 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 
> ).
>
> 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.
>
> Does anyone have a Nord Lead 2? Do you use the sync'ed noise much?  
> Do you like the sound? Is this a simply aesthetic dislike on my part  
> or does it just sound nasty?!

When I owned an NL2, the noise sync was one of my favorite features. I  
remember that in that mode, the osc 2 freq knob only gave you about 8  
discrete flavors of synced noise. I don't have any idea of how they  
implemented it in that instrument, but it sounds a lot like what's  
done in the Ricoh RP2A03G's sync mode: the standard mode is a maximal  
length 16-bit LFSR, but the alt mode uses a different 2-tap  
configuration (can't remember which bit) to produce sequences that  
were only 93 bits long, which is where the really special metallic  
stuff happens. Not at all the same technique but messing around with  
lesser tap configurations should get you in the aesthetic ballpark.

The Harvestman Zorlon Cannon (PIC18 based) closely follows the Atari  
TIA/POKEY method by ANDing the head bit of the LFSR with a square  
wave  - there's a whole world of good sounds to be discovered by  
playing around with LFSR length, tap configuration, throw some divide  
by Ns in there and it gets even weirder. Best results with 4 and 5 bit  
lfsr's, of course.
>
-Scott