[sdiy] Digital noise generation

[Scott Gravenhorst]

Magnus Danielson <cfmd at bredband.net> wrote:
>From: Antti Huovilainen <ajhuovil at cc.hut.fi>
>Subject: [sdiy] Digital noise generation
>Date: Tue, 8 Mar 2005 18:20:35 +0200 (EET)
>Message-ID: <Pine.OSF.4.61.0503081817450.2284 at kosh.hut.fi>
>
>> Hello
>
>Hi Antii,
>
>> I recently did some tests in matlab emulating a feedback shift register 
>> noise generator. However, the noise sounded quite bad with frequent 
>> "snaps", mostly independent of shift register length. I tried both using 
>> the noise as-is (clocked at 44khz) or downsampled (clocked between
>> 200khz - 1mhz).
>
>I assume you meant 200 kHz to 1 MHz.
>
>> What methods do you people use to generate noise digitally? (to avoid 
>> selecting transistors and worrying about interference with high gain amps)
>
>What polynomials did you use?

This may be very important.  I've listened to 24 and 32 bit _maximum_length
LFSR noise generators, personally, I found even the 24 bit to be as smooth as
silk.  Shhhhhhhhhhhhhhhhhh...

>If you dig in the Synth-DIY archive I published a list up to length of 64.

And I have a list on my site up to 168 stages.  (c:

http://home1.gte.net/res0658s/electronics/LFSRtaps.html

- though it appears to be down right now...  grrrr...

>
>Did you use a parallelized version?
>I can help you with that if you need.
>
>Did you amplitude weigthed the outputs of the parallelized version? 
>Again, I can help you, but it is really a quick-and-dirty approximation of
>filtering with a one-pole lowpass filter. Exponential drop-off in amplitude
>should work the "older" the bit is supposed to be.

Heh, I did it the cheesey way, the output is one bit which is then fed to a
low pass filter.  It works for me...


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